High Resolution Iron Isotope Study of Late Pleistocene Sapropels (S5, S7) Ayelet Benkovitz This thesis was submitted for the degree "Master" to the senate of the Hebrew University of Jerusalem. The study was carried out under the supervision of: Prof. Alan Matthews, Institute of Earth Sciences, the Hebrew University of Jerusalem, Israel. Dr. Mira Bar-Matthews, Geological Survey of Israel. Dr. Nadya Teutsch, Geological Survey of Israel. Report GSI/23/2016 Jerusalem, July 2016 Geological Survey of Israel Ministry of National infrastructures Energy and Water Resources
Transcript
High Resolution Iron Isotope Study of Late
Pleistocene Sapropels (S5, S7)
Ayelet Benkovitz
This thesis was submitted for the degree "Master" to the senate of the Hebrew University of
Jerusalem.
The study was carried out under the supervision of:
Prof. Alan Matthews, Institute of Earth Sciences, the Hebrew University of Jerusalem, Israel.
Dr. Mira Bar-Matthews, Geological Survey of Israel.
Dr. Nadya Teutsch, Geological Survey of Israel.
Report GSI/23/2016 Jerusalem, July 2016
Geological Survey of Israel Ministry of National infrastructures
Energy and Water Resources
II
Abstract
Iron rapidly reacts in oxic waters to form Fe-oxides. Its isotopic composition is preserved
during processes such as weathering or deposition of clastic sediments, and no significant
isotopic fractionation occurs. However, once O2 becomes depleted and reducing conditions
prevail, there are various pathways in which iron isotopically fractionates before being
incorporated into sediments. Due to its different possible oxidation states, Fe is suitable for
reconstructing redox conditions in low-temperature marine environments.
The primary aim of this work is a study of the iron isotope composition of two Eastern
Mediterranean (EM) Sea sapropels (S5 and S7) south of Cyprus (site ODP967 at 2550m water
depth) and Nile Fan sapropel S1 (core 9509 at 900m water depth) and their comparison with
previous results on sapropel S1 (ODP967) and organic–carbon rich Black Sea sediments.
Sapropels are organic-rich sediments that deposited in association with precession cycle and
global climate events i.e., insolation maxima, higher rainfall, and strong monsoons, which lead
to stratification and cessation of thermohaline water circulation in the EM sea. A consequence
was O2-suffocation of water column and the development of reducing conditions which
enabled sapropel formation. Together with organic matter enrichment, sapropels are
characterized by elevated concentrations of Fe and S (mostly reflecting pyrite formation),
enrichments in Ba, Ni and redox sensitive trace elements (V, Mn, Mo and U); all of which are
observed in depth profiles for the two strong sapropels S5 and S7. Reducing conditions
evolution was studied by comparing the enrichment factor (EF) variations of Mo and U of
sapropels S5 and S7 and their enclosing sediments. Enclosing sediments formed under sub-
oxic to anoxic conditions, whereas at their peak both S5 and S7 acquired Mo/U ratios of the
seawater implying sulfidic (euxinic) bottom water conditions. In contrast, previously studied
sapropel S1 (ODP967) was found to deposit in mild sulfidic bottom conditions, whereas Nile
Fan sapropel S1 deposited in sub-oxic conditions.
Iron isotope enrichment in sapropels S5 and S7 follow the “Benthic iron shuttle” model, which
was developed for Black Sea euxinic sediments; In the oxic environment of the continental
shelf Fe occurs as Fe(III). When it is mobilized and transported from the shelf, Fe passes the
chemocline (Fe(III)-Fe2+ boundary), where it is reduced and isotopically fractionated. The
isotopically light soluble Fe2+ is exported towards the seafloor and, in euxinic conditions,
reacts with H2S to form syngenetic pyrite. An inverse relation of Fe enrichment (presented
Fe/Al) and isotope depletion (δ56Fe) is observed. Both sapropels S5 and S7 show strong
III
inverse trends of Fe/Al vs. δ56Fe (minimum values of δ56Fe=-0.72‰ whereas Nile Fan
(δ56Fe=0.09±0.1‰) and no isotopic fractionation was observed with Fe enrichment. The
Fe/Al vs. δ56Fe plots for sapropel S5 showed the strongest trend of δ56Fe depletion with
respect to Fe enrichment comparable to Black Sea sediments, while sapropels S7 and sapropel
S1 from the same site (ODP967) showed a weaker but similar trend. Even though sapropels
S7 and S1 at ODP 967 site showed similar Fe/Al vs. δ56Fe slopes, maximum δ56Fe depletion for
S7 was much greater than found for S1 (minimum δ56Fe=-0.28‰), implying that reducing
conditions during sapropel S1 were less extensive than those developed during sapropel S7.
The obtained chemical and Fe isotope data show that combination of δ56Fe depletion vs. Fe
enrichment trends with minimum δ56Fe values may be used to estimate the intensity of past
marine redox conditions. Moreover, the results of this study show that both S5 and S7 are
strongly developed sapropels (stronger than S1) that formed in well-developed euxinic
bottom water conditions.
IV
Acknowledgments
Firstly, I would like to thank my advisers Prof. Alan Matthews, Prof. Mira Bar- Matthews and
Dr. Nadya Teutsch for their patience and willingness to answer my questions, advise and help
when needed, and ensure that the work was always done in a pleasant atmosphere.
Thank you also to Dr. Ahuva Almogi-Labin for her guidance and comments.
Thank you to Ms. Olga Berlin for assistance with chemical analyses, Mr. Raanan Bodzin for
operating the SEM, and to Mr. Yevgeni Zakun for his assitance with operating the MC-ICP-MS
instrument.
A special graditude to Mr. Ofir Tirosh for all the direction in the clean lab and the many hours
of chemical analysis and support during difficult hours.
And last but not least to my loving family, who even with no clue as to what I am doing and
working on (though they did try to understand), they always showed their interest and gave
support.
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Table of Contents
Abstract ............................................................................................................................................... II Acknowledgments .............................................................................................................................. IV Figure list ............................................................................................................................................. 3 Table list .............................................................................................................................................. 4 1. Introduction ..................................................................................................................................... 5
1.1. Sapropel Formation .................................................................................................................. 6 1.1.1. Periodicity and climatic conditions ................................................................................... 6 1.1.2. Factors affecting sapropel deposition ............................................................................... 6 1.1.3. Mechanism of sapropel formation during stratification ................................................... 9
1.3. Iron as a proxy for sapropel redox conditions........................................................................ 15 1.3.1. Iron isotope fractionation ............................................................................................... 15 1.3.2. Iron in the sea .................................................................................................................. 16 1.3.3. The Benthic Iron Shuttle .................................................................................................. 17
1.4. Sapropels studied in this work ............................................................................................... 19 1.4.1. Sapropel S1 ...................................................................................................................... 20 1.4.2. Sapropel S5 ...................................................................................................................... 21 1.4.3. Sapropel S7 ...................................................................................................................... 21
1.5. Research aims ......................................................................................................................... 23
4.1.1. Productivity and sapropel boundaries ............................................................................ 44 4.1.2. Environmental conditions during sapropels S5 and S7 formations ................................ 45 4.1.3. Iron in the sediment ........................................................................................................ 49
4.2 Iron isotopes ............................................................................................................................ 50 4.2.1. Sapropels S5 and S7 at OD967 ........................................................................................ 50 4.2.2. Sapropel S1 site 9509 ...................................................................................................... 52 4.2.3. Comparison with Sapropel S1 at site ODP967 ................................................................ 52 4.2.4. Reconstruction of redox conditions using Fe/Al vs. δ56Fe correlations .......................... 54
7. Supplementary .............................................................................................................................. 63 Table S1: Chemical composition of sapropels S5 and S7. ............................................................. 64 Table S2: Chemical composition of sapropels S5 and S7 normalized to Al (X/Al) ......................... 66 Table S3: TOC and age data for sapropels S5 and S7 .................................................................... 68 SEM pictures of sapropels S5 and S7 ............................................................................................ 69
Figure 1.2: Cross-section of the Mediterranean Sea…………………………………………………….......... 12
Figure 1.3: Present day Mediterranean Sea circulation………………………………………………........... 12
Figure 1.4: Schematic model of water stratification during sapropel formation period………… 13
Figure 1.5: Model of enrichment patterns and changes in authigenic Mo/U ratios………………. 18
Figure 1.6: Depth profile of δ98/95Mo in sapropel S1 ODP967D…………………………………….......... 19 Figure 1.7: δ56Fe vs. S content of Gotland Deep bulk sediment samples from the Baltic Sea……………………………………………………………………………………………………………………………………..…. 21
Figure 1.8: “The Benthic Iron Shuttle” model………………………………………………………………………... 22
Figure 1.9: δ18O depth profile of core RC9-181 south to Crete Island………………………………….… 26 Figure 1.10: Reconstructed sea surface temperatures (SST) during the deposition of ODP967 sapropels S1, S5 and S7……………………………………………………………………………………………………….... 26
Figure 2.1: Location map for studied cores………………………………………………………………….......... 28
Figure 2.2: Schematic presentation of sample processing and analysis………………………………… 31 Figure 2.3: Average δ57Fe vs. δ56Fe with 2SD of the different standards analyzed along with the studied samples………………………………………………………………………………………………………….…… 34
Figure 3.1: Depth profiles of Ba/Al, TOC and Ni/Al for sapropels S5 and S7………………………….. 36
Figure 3.2: Depth profiles of sensitive redox elements (RSTE) for sapropels S5 and S7……….… 37
Figure 3.3: Depth profiles of Fe/Al and wt.% S for sapropels S5 and S7………………………........... 39
Figure 3.4: Scanning electron microscope images of pyrite………………………………………….......... 41
Figure 3.5: Depth profiles of Fe/Al and δ56Fe for sapropels S5 and S7…………………………………... 45
Figure 3.6: Depth profiles of Fe/Al and δ56Fe for sapropel S1 from core 9509………………………. 46
Figure 4.1: Comparison of Al (wt.%) and Ba(ppm) of sapropels S5 and S7…………………………….. 48
Figure 4.2: Enrichment Factor (EF) plots of Mo vs. U for sapropels S5 and S7……………………….. 50
Figure 4.3: Fe (mole) vs. S (mole) in sapropels S5 and S7……………………………………………........... 53
Figure 4.4: Fe pyrite, Fe excess and Fe total in sapropels S5 and S7…………………………………….… 54
Figure 4.5: Fe/Al vs. δ56Fe and S content vs. δ56Fe for sapropels S5 and S7…………………………… 55
Figure 4.6: Fe/Al vs. δ56Fe for the Black Sea sediments………………………………………………........... 56
Figure 4.7: Fe/Al vs. δ56Fe for sapropel S1 from core 9509…………………………………………........... 56 Figure 4.8: Enrichment factor plot of Mo vs. U for sapropel S1 (<63µm fraction) at ODP967……………………………………………………………………………………………………………………............. 57 Figure 4.9: Depth profiles of Fe/Al and δ56Fe for sapropel S1 (<63 µm fraction) at ODP967……………………………………………………………………………………………………………………………….… 57
Figure 4.10: Fe/Al vs. δ56Fe for sapropel S1 (<63µm fraction) at ODP967……………………………… 58 Figure 4.11: Fe/Al vs. δ56Fe for within sapropels S7, S5, S1 (ODP967) boundaries, sapropel S1 core 9509 and the Black Sea………………………………………………………………………………………………….. 58
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Table list
Table 1: Technical data of studied samples……………………………………………………………………………. 29
Table 2: Iron isotope data of standards measured during the five sapropel analytical sessions 35 Table 3: Iron isotope data of standards and sapropel samples according to session chronology ………………………………………………............................................................................... 43
Table 4: Average δ56Fe and 2SD for replicated samples…………………….………………………….……….. 46
Table S1: Chemical composition of sapropels S5 and S7………………………………………………………… 68
Table S2 : Chemical composition of sapropels S5 and S7 normalized to Al (X/Al)…………………… 70
Table S3: TOC and age data for sapropels S5 and S7……………………………………………………………… 72
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1. Introduction
Sapropels are organic-rich marine sedimentary deposits (Muerdter et al., 1984; Ten Haven et
al., 1987; Rohling and Hilgen, 1991; de Lange et al., 2008). Organic carbon concentrations in
sapropels commonly exceed 2 wt.%, and deposits that contain 0.5%-2% organic carbon are
also referred to as sapropelites (Kidd et al., 1978; Ten Haven et al., 1987). Sapropel deposits
are characterized by dark color, absence of benthic fauna and enrichment of redox sensitive
elements such as iron (Fe), sulfur (S), manganese (Mn), molybdenum (Mo), vanadium (V) and
uranium (U) (Fig.1.1; Thomson et al., 1999; Calvert and Fontugne 2001; Gallego-Torres et al.,
2010; Azrieli –Tal et al., 2014; Tachikawa et al., 2015). Sapropels form during anoxic events,
(defined as O2<2µM/l; Algeo and Tribovillard, 2009), and frequently in euxinic conditions
where dissolved sulfide (H2S), the respiration by-product of bacterial sulfate reduction (BSR),
is present in the water column. Sapropels are heterogeneous sediments and frequently consist
of dark grey to off-white colored layers, indicating changes in depositional environmental
conditions during their formation. Sapropels are characteristic sediments of semi-restricted to
closed basins such as the Eastern Mediterranean (EM) Sea and the Black Sea (Ten Haven et.
al., 1987; Severmann et al., 2006; Lyons et al., 2009; Almogi-Labin et al., 2009), but are also
characteristic of the western Mediterranean Sea and past Oceanic Anoxic Events (OAE)
sediments worldwide. Plio-Pleistocene sapropel formation in the Mediterranean Sea is
periodic and strongly tied to astronomical cyclicity. The youngest (Holocene) sapropel is
referred to as S1, with numbers sequentially increasing with age.
Fig.1.1 Core ODP967C. Dark sections represent organic and sulfur-rich sediment. The orange ellipse encloses sapropel S5 (967C-1H-5), one of the two major sapropels studied in this work.
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1.1. Sapropel Formation
1.1.1. Periodicity and climatic conditions
Sapropel deposition has been recorded back to the middle Miocene (Miocene; 23-5.3Ma)
(Kidd et al., 1978; Muerdter et al., 1984; Kroon et al., 1998; Rohling and Thunell, 1999), but
they mainly date back to the Early Pliocene (Pliocene; 5.3-2.6Ma). A core obtained in the
Hellenic trench south of Cyprus (ODP967) showed a record of 80 sapropels going back
3.15My (Kroon et al., 1998) commencing with the youngest sapropel S1.
Sapropels are predominantly interglacial sediments, forming during warm climate periods. A
notable exception is the more recent sapropel S6, which deposited during the previous glacial
period MIS6 (Rossingol-Strick, 1985). Sapropels periodic deposition is associated with the
~21ky precession cycle; one of the three Milankovitch Cycles driving global climate. At times
when precession is minimal, insolation in the northern hemisphere is at its maximum and
winter insolation becomes reduced, leading to increased seasonal contrast and intensified
monsoons and rainfall over the EM region (Rossingol-Strick, 1985; Kroon et al., 1998; Rohling
and Thunell, 1999; Bar-Matthews et al., 2000; Larrasoaña et al., 2003; Almogi-Labin et al.,
2009; Bar-Matthews et al., 2014). Hence, sapropels are more developed in the EM region than
at the west Mediterranean (Ten Haven et al., 1987; Rohling et al., 2015). The peak of organic
carbon accumulation is calculated to occur about 3ky after the initiation of a strong monsoon
event (Kroon et al., 1998; Rohling and Thunell, 1999; Emeis et al., 2000; Gallego-Torres et al.,
2010), although this point is in debate since the calculations were only made for the youngest
sapropel S1 (Rohling et al., 2015). Sea sediment records show a decrease in sapropel
deposition frequency from the end of the Pliocene, ~2.6 Ma (Kroon et al., 1998; Emeis et al.,
2000). At this time, climate contrast increased leading to wetter winters and drier summers
possibly related to the uplift of the Tibetan plateau. (Rohling and Hilgen, 1991; Rohling and
Thunell, 1999).
1.1.2. Factors affecting sapropel deposition
Two main hypotheses are proposed for sapropel formation (Rossingol-Strick, 1985; Ten
Haven et. al., 1987; Rohling and Hilgen, 1991; Calvert and Fontugne, 2001; de Lange et al.,
2008; Gallego-Torres et al., 2010).
a. Enhanced productivity and free oxygen (O2) deficiency in bottom waters resulted in the
accumulation and preservation of organic carbon. The enhanced productivity was brought
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about by large fresh water inputs into the Mediterranean Sea from the Nile River and
North African rivers, following strong monsoonal rainfall over the Ethiopian highlands.
The incoming river water was rich in nutrients that induced planktonic blooms and
consumed oxygen. Excess production of organic matter resulted in depletion of bottom
water O2, thus allowing preservation of undecomposed organic matter.
b. Stratification and inhibition of Mediterranean Sea deep water formation during sapropel
periods due to enhanced fresh water inputs (both riverine and rainfall).
For a better understanding of this latter process, a closer look at the present day
Mediterranean Sea circulation and its water masses is required.
The Mediterranean Sea is a semi-restricted water basin with one connection to the
Atlantic Ocean through the Straits of Gibraltar. A cross-section of the present day
Mediterranean Sea reveals the different water layers and their movement direction,
salinities and seafloor geomorphology (Fig.1.2). The sea water column can be divided into
three main layers (Malanotte-Rizzoli and Bergamasco, 1989; Rohling et al., 2015):
i. Surface layer, up to 200m depth (Fig.1.3a). In this layer, an equilibrated state exists
with the atmosphere gases and the seawater is saturated with O2. Most sea life
inhabits this layer since it is the photic zone.
ii. Deep water layer at depths of 600-3000m. This layer forms in two specific sites; one in
the western basin and the other in the eastern Mediterranean Sea. In the east, deep
water forms in the Adriatic and Aegean Sea (EMDW - Eastern Mediterranean Deep
Water). In the west, deep water is formed along the French seashore in the Gulf of Lion
(WMDW - Western Mediterranean Deep Water). New deep water forms every 100 ±
20 years (Rohling et al., 2015).
iii. Levantine Intermediate water (LIW) at depths of 200-600m. This layer is crucial for
deep water formation and is the driving force of water circulation in the
Mediterranean Sea. It forms between Rhodes to Cyprus, but can be found throughout
the entire basin (Fig.1.3b). These waters were originally at the surface, but significant
evaporation by dry cold Arctic winds cooled this surface water in winter to
temperatures of 15-16 ⁰C and increasing salinity to 39-39.2%. The resulting density
increase caused them to sink below 200m to form the intermediate water layer.
When normal (present-day) anti-estuarine circulation operates, fresh and oxygenated
Atlantic Ocean water enters the Mediterranean Sea from the west through the Straits of
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Fig.1.2 Cross-section of the Mediterranean Sea showing its present day water masses. Numbers in white rectangles are drilling sites. This study focuses on site ODP967 located at a depth of ~2550m in the present day EMDW. (Emeis and Sakamoto, 1998)
Fig.1.3 Present day Mediterranean Sea circulation patterns of a. Surface water. The circulation is anti-estuarine. Water enters through the Straits of Gibraltar heading east. b. Levantine Intermediate water (LIW). The LIW is formed in the EM between Rhodes and Cyprus and flows eastwards and westwards. (Pinardi and Masetti, 2000)
a
b
Gibraltar, flowing eastward to deep-water formation sites while evaporating at the
surface. Cold winds cool the water enabling it to descend and mix with the underneath
high-salinity LIW layer. The mixed end product of these two water masses is denser than
its components, allowing it to sink further to the deep sea (Malanotte-Rizzoli and
Bergamasco, 1989; Pinardi and Masetti, 2000; Rohling et al., 2015).
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Fig.1.4 Schematic model of water stratification during sapropel formation periods. AIW- Adriatic Intermediate Water
AeWI- Aegean Intermediate Water
1.1.3. Mechanism of sapropel formation during stratification
During strong monsoon events, there was intensified discharge of fresh water from the Nile
and North African rivers. Contemporaneous with the monsoons’ intensification and enhanced
river flow, there was increased rainfall precipitation over Greece, Turkey and the Levant basin
(Rohling and Hilgen, 1991; Bar-Matthews et al., 2000; 2014; Almogi-Labin et al., 2009).
Melting of regional ice-sheets also contributed to raising sea level (Emeis et al., 1998; Almogi-
Labin et al., 2009). All these fresh water inputs lead to low Eexcess, which is defined as (Eq.1):
Eexcess = E – P – R
where E is evaporation, P is precipitation and R is runoff from land. Today, evaporation
exceeds incoming freshwater and the water column is well ventilated (Haven et. al., 1987;
Emeis et al., 1998). During sapropels formation incoming fresh water into the Mediterranean
Sea exceeded evaporation and Eexcess appeared to be low. During these low Eexcess periods,
evaporation was not significant enough to create LIW. Winter winds cooled surface water,
which sank only to 300-400m depth. Hence, deep water formation was hindered and water
stratification developed. With no operating circulation, O2 did not reach the deep sea and
anoxic water conditions evolved that favored preservation of organic matter and the
formation of sapropels.
The Mediterranean Sea water column and water layer flow directions during sapropel
deposition are illustrated in Fig.1.4. As can be seen, surface water in the Adriatic, Aegean and
open EM Seas sank half way to create intermediate water masses (AIW and AeWI) and flowed
on top the denser O2-deficient old deep water (ODW).
Deep sea stagnation by itself cannot explain the high organic matter concentrations found in
sapropel sediments, and therefore the first hypothesis (1.1.2.a) is also needed to complete the
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picture (Kroon et al., 1998). The bloom of organic carbon sank towards seafloor while
consuming O2 during decomposition. Once all O2 pool in bottom waters was consumed, organic
carbon deposited to form sapropels (Rohling and Hilgen, 1991; Emeis et al., 2000; Calvert and
Fontugne, 2001).
Existence of benthic fauna is not possible under anoxic pore and bottom water conditions and
therefore these faunas are absent from sapropel layers (Rohling and Hilgen, 1991; Calvert and
Fontugne 2001; de Lange et al., 2008). If O2 is only partially depleted, not reaching fully anoxic
conditions, specific benthic foraminifera may survive due to adaptation to low O2
concentrations. It was also suggested that deep infaunal species will move up the diagenetic
sedimentary column towards the water-sediment boundary where O2 concentrations may be
higher, and eventually will replace the original existing to less-tolerant low O2 conditions
fauna (Melki et al., 2010).
1.2. Geochemical and mineralogical proxies of sapropel formation
1.2.1. Oxygen isotopic composition (δ18O)
Sea surface temperature (SST) has an important role in understanding sapropel formation. It
reflects the amount of insolation and global climate, and it is an important factor in O2
solubility and deep water formation (Emeis et al., 1998; Lyons et al., 2009). Increase in SST is
correlated with depletion in δ18O values in planktonic foraminifera found in sapropel samples
(Muerdter et al., 1984; Emeis et al., 1998; 2000; Almogi-Labin et al., 2009). A study of stable
isotope oxygen (δ18O) of planktonic foraminifera from Ionian and the Levant Basins sapropels
(ODP964 and ODP967) shows that there is a significant and rapid decrease of δ18O values in
planktonic foraminifera Globigerinoides ruber at presapropel-sapropel boundary, ranging
from 3.4‰ to 0.7‰ (Emeis et al., 1998). They explained this depletion due to temperature
increase, combined with surface salinity decrease and global ice-volume melting.
Remarkable contemporaneous negative shifts in δ18O were also found in land deposits –
speleothems. Bar-Matthews et al. (2000; 2003) found a close match between decreases in
δ18O values of speleothems and δ18O records of EM sea cores, implying that climate is
recorded on land as well in the form of enhanced rainfall.
1.2.2. Bioproductivity
An indicative element for organic carbon productivity is biogenic Ba, with significant
enrichments occurring in sapropel layers (van Santvoort et al., 1996; Calvert and Fontugne,
11
2001; Gallego-Torres et al., 2010; Azrieli –Tal et al., 2014). As with most trace and minor
elemental concentrations, Ba abundances are expressed as ratios normalized to Al (X/Al) in
order to minimize the effects of fluctuations in CaCO3 or opal content in the sediments (Calvert
and Pedersen, 1993; Thomson et al., 1999). Also for most sediments Al is considered as an
indicator of the alumino-silicate fraction (clays, detrital feldspars) with very little ability to
undergo diagenetic mobilization.
Barium abundances expressed as Ba/Al ratios are considered to be more reliable indicators of
the original thickness of sapropel layers (i.e., the full paleoproductivity event leading to
organic carbon deposition) rather than organic carbon concentrations (Thomson et al., 1999;
Gellegro-Torres et al., 2010). Commonly, organic carbon in the upper few centimeters of the
sapropel is oxygenated by diffusion once O2 returns to deep waters after the cessation of
sapropel forming conditions (reventilation). Barium, on the other hand, is not affected by
reventilation and therefore, Ba/Al ratios preserve the original sapropel length (Ten Haven et
al., 1987; Thomson et al., 1999; de Lange et al., 2008).
Nevertheless, Tribollivard et al. (2006) note that Ba abundance must be treated with caution
as a paleoproductivity proxy, particularly in sediments characterized by intense sulfate
reduction, which could lead to biogenic barite dissolution and Ba migration through pore
waters (van Os et al., 1991; van Santvoort et al., 1996). Thus, Tribollivard et al. (2006) suggest
that the effective use of Ba as a paleoproductivity proxy may be limited to marine sediments
deposited in portions of the ocean with low to moderate productivity. Alternatively, they
suggest that elements such as Cu and Ni, which are delivered to the ocean in association with
organometallic complexes, can serve as a marker for sediments with a high organic matter
flux. Since these elements are mostly hosted by pyrite (FeS2) in strongly reduced sediments,
they may be preserved in sediments while organic matter may be remineralized by bacterial
activity. Consequently, Ni and Cu may 'speak to the original presence of organic matter, even if
it is partially or totally lost after deposition’ (Tribollivard et al., 2006).
1.2.3. Trace elements
Redox sensitive trace element (RSTE) concentrations or ratios are widely used proxies for
redox conditions in marine sediments (Calvert and Pedersen, 1993; Crusius et al., 1996;
Morford et al., 2001; Algeo and Maynard, 2004; Brumsack, 2006; Tribollivard et al., 2006).
RSTE enrichments are marked in organic-rich sediments, particularly those formed in euxinic
conditions, whereas well oxygenated low carbon sediments show scant enrichments. Several
12
factors account for this enrichment pattern as detailed in Algeo and Maynard (2004): 1) Many
RSTE have different oxidation states and the reduced forms that exist in low oxygen waters
are more readily complexed with organic acids, and can be taken into solid solution with
authigenic sulfides, or precipitated with insoluble oxyhydroxides. 2) RSTE are strongly
influenced by processes that operate under low oxygen conditions, such as Mn/Fe redox
cycling, availability of organic carbon substrates, and H2S presence at sediment redox
boundaries or in an euxinic water column. Enrichments of Fe and S and RSTE such as V, Mo
and U relative to background sedimentary values are typical of sapropels (Thomson et al.,
1999; Calvert and Fontugne 2001; Gallego-Torres et al., 2010; Azrieli –Tal et al., 2014;
Tachikawa et al., 2015). Profiles of RSTE across sapropels are semi-quantitative tracers,
providing a relative picture of the prevailing redox conditions during sapropel deposition.
In contrast to other RSTE whose concentrations rise in the sapropel, Mn concentrations are
frequently depleted (van Santvoort et al., 1996; Thomson et al., 1999). Manganese in
oxygenated seas is mainly in the insoluble form of MnO2 and MnOOH, with absorbed particles
including trace elements as Mo and V. Under mildly reducing conditions it is reduced to
soluble Mn2+ while releasing the absorbed particles. Mn2+ migrates out of the sediment by
diffusion but re-precipitates as Mn-oxides once it is reintroduced to O2 or trapped in solid-
phase Mn-carbonates (van Santvoort et al., 1996; Tribovillard et al., 2006; de Lange et al.,
2008). Thus, Mn enrichments typically exist at the sapropel upper boundaries.
1.2.4. Mineralogical proxies
Pyrite (FeS2) is a mineral frequently occurring in high concentrations in sapropels (Muerdter
et al., 1984; Raiswell and Berner, 1985; Rohling, 1994; Lyons and Severmann, 2006; Azrieli-
Tal et al., 2014). Both syngenetic and diagenetic pyrite is found. Syngenetic pyrite is formed
when Fe reacts with H2S in the water column before it reaches the sediment-seawater
interface (Lyons and Severmann, 2006; Raiswell and Canfield, 2012). In euxinic conditions it
forms during relatively short time periods. Scanning electron microscope (SEM) studies show
that syngenetic pyrite forms as small (typically 5-6m mean diameter) spherical framboidal
crystal aggregates of sub-micron-size crystals (Wilkin et al., 1996; Passier et al., 1999).
Diminutive grain sizes are typical of framboids that form currently just below the O2–H2S
interface in the Black Sea and other euxinic basins, where they reside for few months in the
water column as they migrate to the sea floor (Muramoto et al., 1991; Wilkin et al., 1996;
Lyons, 1997). Diagenetic pyrite in sapropels on the other hand, is formed by upward diffusion
13
of Fe out of the underlying sediment where it can react with H2S in pore water (Passier et al.,
1996; 1999; Tribollivard et al., 2006; Poulton and Canfield, 2011). This type of pyrite can be
formed during sub-oxic and anoxic conditions enabling much longer formation time and
hence larger and often irregular framboidal aggregate size, together with overgrowths of
euhedral pyrite grains (Passier et al., 1999; Wilkin et al., 2006).
The pyrite formation pathway is represented by three equations (Eq.2; Raiswell and Canfield,
2012):
a. 2CH2O + SO42– → H2S + 2HCO3
–
b. 2FeOOH + 3H2S → 2FeS + So + 4H2O
c. FeS + So → FeS2
a. CH2O represents organic matter as a simple carbohydrate. Bacteria reducing sulfate
respiration produces H2S b. FeOOH (reactive Fe) react with H2S to create Fe-monosulfide
(FeS). Reactive Fe is the Fe that would react with H2S in short time periods (with half-lives
less than a month; Raiswell and Canfield, 1998) like Fe-oxides, as opposed to Fe in silicates
which reacts poorly with H2S even if introduced to H2S for a long time period (105 years;
Canfield et al., 1993). c. Further reaction with S forms pyrite.
At sapropel boundaries, where reducing conditions are less intense, it is possible to find the
other crystallographic forms of FeS2 such as marcasite (Muerdter et al., 1984).
1.2.5. Molybdenum and uranium enrichment factors
A relatively new tool for studying the intensity and type of redox conditions is the ‘enrichment
factor’ (EF) variation pattern of Mo and U (Algeo and Tribovillard, 2009). The EF of an
element is defined as the ratio of any element/Al in a sample to an accepted value for the
same element/Al ratio in a reference material (e.g., shale; Eq.3):
EF=(X/Al)sample/(X/Al)reference.
By using the patterns of EF variations on a MoEF vs. UEF diagram, it is possible to define
conditions of authigenic Mo/U uptake at the sediment–seawater interface and the type of
marine basin in which anoxic sediments form (Fig.1.5). This tool was used to show changes in
MoEF vs. UEF before, during and after sapropel deposition; Azrieli-Tal (2012; et al., 2014)
showed that sapropel S1 formed in open marine type anoxic to sulfidic redox variation
conditions, whereas the non-sapropel sediments formed in sub-oxic conditions. Anoxic-
sulfidic conditions are characterized by strong enrichments of Mo and U, whereas sub-oxic
14
conditions in the surrounding sediments are marked by U enrichment. Uranium uptake is
possible in sub-oxic conditions (beginning at the Fe(III)/Fe2+ redox boundary), whereas Mo
reacts only at the presence of H2S.
1.2.6. Molybdenum isotope composition (δ98/95Mo)
Molybdenum isotopes are recognized as a powerful tool for defining redox conditions during
marine sedimentation (Arnold et al., 2004; Poulson et al., 2006; Lyons et al., 2009; Brucker et
al., 2009). In an oxygenated sea, Mo occurs as molybdate MoO42- and its concentration is
~105nM (Anbar and Rouxel, 2007). Isotopic fractionation occurs when Mo is adsorbed to Mn-
oxides and receives values of δ98/95Mo=~-0.7‰ in the sediment. In euxinic conditions a large
isotopic fractionation occurs during the reaction of MoO42- with S2- to form thiomolybdate
species (MoOxS4-x2-). However, when euxinic conditions are strong ([H2S] > 11M) quantitative
uptake of all reactive thiomolybdate by particulate matter results in net transfer of all
seawater molybdate to the euxinic organic-carbon rich sediments, which acquire the seawater
Mo isotopic composition (δ98/95Mo = 2.3‰; Arnold et al., 2004). Azrieli-Tal et al. (2014)
showed however, that when euxininc conditions are mild ([H2S] <11M), transformation to
thiomolybdate species is incomplete and the δ98/95Mo value of the lower part of sapropel S1
was controlled by the large negative isotope fractionation between molybdate and
thiomolybdate species (Fig.1.6).
Fig.1.5 Model of enrichment patterns and changes in authigenic Mo/U ratios in response to environment redox change. The dotted lines represent sea water (SW) Mo/U molar ratio (~7.5-7.9) and fractions of SW ratios. (Algeo and Tribollavard, 2009)
15
1.3. Iron as a proxy for sapropel redox conditions
In oxic waters, iron reacts rapidly and quantitatively to form Fe-oxides and therefore no
significant isotopic fractionation occurs during processes such as weathering or deposition as
clastic sediments (Anbar and Rouxel, 2007). Once O2 becomes depleted and reducing
conditions prevail, there are various pathways in which iron fractionates isotopically before
being incorporated into sediments. Due to its different possible oxidation states, Fe is suitable
for reconstructing redox conditions in low-temperature marine environments. Iron (as Fe/Al
ratios) and its isotopic composition have been shown to be powerful recorders of redox
conditions in euxinic basins, where reduced Fe is being shuttled from the continental shelf
into the euxinic basin (Severmann et al., 2008). Iron/Al vs. δ57Fe variations were studied in
sapropel S1 by Azrieli-Tal et al. (2014), who showed a weak trend of decreasing δ57Fe with
increasing Fe/Al. The study of Fe isotopic composition of two sapropels considered to form at
stronger euxinic conditions than S1, forms a major objective of this research and the following
paragraphs briefly describe the background.
1.3.1. Iron isotope fractionation
Iron has four stable isotopes 54Fe [5.58%], 56Fe [91.95%], 57Fe [2.18%], 58Fe [0.30%] (Bullen,
2011). The isotopic composition of Fe is given using the delta notation (Eq.4):
a. δ56Fe = ([56Fe/54Fe] sample/[56Fe/54Fe] std – 1)*1000 (‰)
b. δ57Fe = ([57Fe/54Fe] sample/[57Fe/54Fe] std – 1)*1000 (‰)
c. δ57Fe =1.5* δ56Fe
Fig.1.6 Depth profile of δ98/95
Mo in sapropel S1 ODP967D. Light Mo isotopic composition is found in the lower part of the sapropel, with a δ
98/95Mo = −0.94‰ peak at 123cm depth.
(Azrieli-Tal et al., 2014)
16
where the normalizing standard (std) is commonly IRMM-014 (Institute for Reference
Materials and Measurements, Belgium).
Isotopic fractionation between two phases is defined by the equation (Eq.5):
56Fe(A-B) = δ56FeA - δ56FeB
Iron composition in igneous rocks is δ56Fe = 0.09±0.1‰ relative to IRMM-014 (Beard et al.,
2003; Supplementary1). This isotopic composition is similar to that of continental-shelf rocks
implying that weathering has a negligible fractionation effect (Beard et al, 2003; Matthews et
al., 2004; Severmann et al., 2008; Johnson et al., 2008).
Isotopic fractionation may occur through biotic and abiotic processes. Microbial processes
produce the largest fraction of light Fe isotope species in nature while abiotic processes
produce a wider range of Fe isotope compositions (Matthews et al., 2004; Teutsch, et al., 2005;
Johnson et al., 2008; Teutsch, et al., 2009). In anaerobic sediments microbes are able to
generate energy through dissimilatory iron reduction (DIR). In this process Fe(III) is the
electron acceptor during bacterial respiration, producing light isotopic aqueous Fe2+ by
isotopic exchange with surface oxide Fe to yield δ56Fe =~-3‰ to -1‰ (Crosby et al., 2007).
These lower values are in the range of abiotic Fe exchange in room temperature equilibrated
state; ΔFe(II)-Fe(III)=-2.75±0.15‰ (Johnson et al., 2002). This suggest that Fe fractionation
during DIR lies mainly in isotopic exchange between the two different Fe phases and not in
the bacteria species or Fe-oxide type (Crosby et al., 2007). In further stages when Fe
precipitates, kinetic fractionation takes place too and the precipitant is found to be even more
Fe light, leaving the source fluid isotopically heavier (Johnson et al., 2002; Dauphas and
Rouxel, 2006).
1.3.2. Iron in the sea
The sources for oceanic Fe are varied and include dust, rivers, hydrothermal activity and
recycling of continental Fe bearing minerals (Poulton and Canfield, 2011). Iron in most
minerals is in the divalent form Fe(II) (ferrous compound), but once exposed to O2 it oxidizes
to Fe(III) (ferric compound). Around 90% of the newly entering Fe into seas forms amorphous
or poorly crystalline Fe-oxyhydroxides, and the rest bonds to organic complexes (Taylor and
Macquaker, 2011; Raiswell and Canfileld, 2012). Mobilization of Fe(II) in the ocean occurs
when intermediate states between oxic and euxinic conditions prevail, i.e., low to zero O2
concentrations but non-sulfidic conditions (Poulton and Canfield, 2011). In an oxygenated sea,
Fe2+ is only found in very low concentrations of <2nM (Anbar and Rouxel, 2007). Once H2S is
17
Fig.1.7 δ56
Fe vs. S content of euxinic Gotland Deep bulk sediment samples from the Baltic Sea. The plot shows that elevated sulfur contents in the sediment are correlated with light δ
56Fe values.
(Fehr et al., 2010)
present, dissolved Fe (Fe2+) will bond to form insoluble pyrite with a light δ56Fe (or δ57Fe)
signature (Raiswell and Canfield, 1998; Severmann et al., 2006; Johnson et al., 2008; Fehr et
al., 2010; Azrieli-Tal et al., 2014). A potential example of this is found in the present day deep
Baltic Sea where elevation in S content in organic-rich sediments is accompanied by δ56Fe
depletion (Fig.1.7). At times of high SO42- bacterial reduction resulting with a sulfidic water
column, transformation of isotopically light Fe from continental-shelf to seafloor occurs. The S
reacts with Fe to form either diagenetic or syngenetic pyrite. Sulfur, like Fe, is isotopically
fractionated during the reduction process yielding light pyrite (Wilkin et al., 1996; Lyons,
1997; et al., 2009; Johnson et al., 2008). If S is in excess it will be bond into organic complexes
or into other sulfide containing minerals.
1.3.3. The Benthic Iron Shuttle
The Fe/Al ratios in oxygenated sea sediments are usually about the crustal value of 0.55 and
do not exceed 0.6 (Raiswell and Canfield, 2012), but anoxic and euxinic sediments exhibit
higher concentrations (Fe/Al >0.6; Severmann et al., 2006; 2008). This observation led to the
development of a model which explains how increased amounts of Fe reach the sea floor
during these anoxic events (the Benthic Iron Shuttle; Severmann et al., 2008; Fehr et al.,
2010).
The benthic isotope shuttle is illustrated in Fig.1.8. Iron in deep sea sediments is divided into
three types: scavenged (FeScav), reactive (FeRea) and non-reactive (FeUnr). As previously noted
(section 1.2.4) non-reactive Fe is found in minerals such as silicates, which scarcely react in
the presence of H2S, while reactive Fe has a strong affinity for sulfide. However, not all the
reactive Fe bonds to sulfide to form pyrite. The fraction of FeRea that does is classified as
scavenged Fe.
18
Fig.1.8 “The Benthic Iron Shuttle” model. Iron(III) minerals are dissolved on the oxic shelf and reduced to
Fe2+
. Below the redoxcline Fe2+
is mobile and exported with Fe-oxides to the euxinic basin. Above the redoxcline most Fe
2+ will oxygenate back to immobile Fe(III). Also shown are the relative amounts of different
Fe fractions in the weathering input, oxic shelf and euxinic basin. FeRea is the detrital supplied (mostly oxide) Fe with the potential to rapidly react with H2S; FeUnr is the unreactive (mostly silicate) detrital Fe; FeScav is the additional reactive Fe that is scavenged from the exported Fe during the syngenetic pyrite formation. (Lyons et al., 2009).
The ratios between these Fe types vary in the sediment and are controlled by concentrations
of Fe and H2S, sedimentary deposition rates, and the Fe-bearing mineral type and its
concentration in the source rock.
As evident from the diagram, the transition from oxic deposits on the shelf to euxinic deposits
in the basin is accompanied by a significant decrease in δ56Fe values. The slightly heavier Fe
composition of continental-shelf compare to the detrital input is due to the fractionation
processes that occur in pore waters and transport the light Fe to the water column.
There are two mechanisms, or shuttles, by which Fe is exported from the oxic shelf to deep
sea basin; an oxic shuttle and an anoxic shuttle. In the oxic shuttle, fine-grained reactive
Fe(III)-oxyhydroxides from the continental shelf and a smaller amount of Fe2+, are
19
transported toward the deep sea basin. Once Fe(III)- oxyhydroxides reach the Fe(III)/Fe2+
redoxcline, they are reduced to mobile Fe2+, but most of this ferrous ion will reoxidized back
into Fe(III)-oxyhydroxides, and only a small percentage of it will succeed crossing the
redoxcline. This reoxidation process is accompanied by the isotope fractionation of
Δ56Fe(Fe(III)-Fe2+)= 2.75±0.15‰ (Johnson et al., 2002), leaving the residual Fe2+ even lighter
(the benthic export flux in Fig.1.8). When the water column is euxinic, Fe2+ reacts with free
sulfide to form FeS, which in further reactions will precipitate as syngenetic pyrite (see
section 1.2.4). The anoxic shuttle contributes much less Fe to the seafloor than the oxic
shuttle. In this mechanism, reduced Fe2+ from continental-shelf pore water is directly
exported to the Fe(III)/Fe2+ redoxcline and reacts with H2S if present.
The net effect of the export of Fe to the euxinic basin is an overall increase in Fe/Al due to the
additional reactive Fe that is entrapped as pyrite and the decrease in δ56Fe due to the export
of isotopically light Fe. This leads to the observed inverse relationship between Fe/Al and
δ56Fe (Fig.1.8) observed in Black Sea anoxic sediments (Severmann et al., 2008).
1.4. Sapropels studied in this work
The sapropels studied in this work are EM sapropels S5 and S7 from ODP core 967. Sapropel
S1 from the same core (ODP967) was studied by Azrieli-Tal et al. (2014) for both Fe and Mo
isotopic compositions and its Fe isotope data will be compared with the new data on S5 and
S7 from this study. This study also includes new Fe isotope measurements on sapropel S1
from core 9509 located at ~900m depth in the Nile Fan (Almogi-Labin et al., 2009). A brief
description of the three sapropels S1, S5 and S7 follows.
The three sapropels, S1, S5 and S7 were deposited during interglacial marine isotope stages
found in a core south to the island of Crete (core RC9-181; Rossingol-Strick, 1985), showing
the clear alignment between δ18O and MIS along the core. The three sapropels, S1, S5 and S7,
correspond to peaks of light δ18O of the biofauna. Both sapropels S5 and S7 deposited during
particular warm climate and are considered to be the most intense sapropels, although
reducing conditions for sapropel S7 were less extreme than during sapropel S5 (Gallego
Torres et al., 2010). Hence, they provide a contrasting view with the less developed sapropel
S1, and can be compared to each other for redox conditions study.
20
1.4.1. Sapropel S1
The recent and most studied Holocene sapropel S1 was dated by using 14C. The measured age
differs slightly with sapropel location and water depth. Larrasoaña et al. (2003) dated
sapropel S1 ODP967 to ~9.9-7.3ka. Almogi-Labin et al. (2009) used a combination of 14C
dating and comparison of the marine record with the land speleothem δ18O record to date
sapropel S1 that formed at depths of ~1000m at site 9501 south-west of Cyprus to ~9.5-
8.2ka, and 9509 in the Nile Fan to ~9.5-7.5ka. An extensive study of S1 sapropels formed at
water depth >1800m was made by de Lange et al. (2008), determining an age range ~9.7 to
5.7ka. These data suggest resumption of O2 to the deep sea sapropels was later than for
shallow depths and hence the longer time formation of deep water sapropels. A prominent
reventilation event at ~8ka is noted in many of these sapropels (Almogi et al., 2009; Azrieli
Tal et al., 2014).
The δ18O values of planktonic foraminifera from different locations show distinct variations:
δ18O of sapropel S1 ODP967 exhibits a range of -0.66‰ to +0.22‰ with an outstanding peak
at ~9ka of δ18O=0.92‰ (1SD=0.59‰; Emeis et al., 1998). At a different site near Cyprus
(core 9501), δ18O is heavier by up to ~0.9‰ than sapropel S1 located at the Nile fan (core
9509) (Almogi-Labin et al., 2009). The high rainfall evident for this period (termed the
African Humid period; de Menocal et al., 2000) from ~12.5-8ka and therefore the heavy water
influx into the Levant basin can explain this δ18O difference which is equivalent to 4⁰C in
terms of temperature offset (Rossingol-Strick, 1985; Almogi-Labin et al., 2009). Data derived
from ODP967 (Emeis et al., 1998), calculated that average SST during sapropel formation was
~18.5⁰C with a maximum of ~19.5⁰C (Fig.1.10a).
The onset of stagnation that lead to the sapropel S1 deposition commenced at the end of
Heinrich event 1 at ~15.5ka (H1, 18–15.5ka), a period characterized by ~3⁰C global warming
and rise of sea water level by ~100m of the North Atlantic (Grimm et al., 2015). Full deep sea
O2 depletion was reached 5.5kyr later, but a continuous record of benthic fauna during
sapropel S1 deposition at shallow depths suggests that only intermittently anoxic conditions
persisted in the mild depths of the EM sea, and that some water circulation took place during
sapropel formation (de Lange et al., 2008; Grimm et al., 2015). Reventilation during the
transition from sub-oxic to oxic conditions at sapropel termination removed the uppermost
few centimeters of the organic carbon layer (Thomson et al, 1999; de Lange et al., 2008;
Azrieli-Tal, 2012; et al., 2014).
21
1.4.2. Sapropel S5
Sapropel S5 formed during the warmest substage of MIS5, MIS5e (Shackleton, 1969), was
characterized by large changes in ice-volume and greenhouse gas concentrations (Roucoux et
al., 2008) and is considered to be deposited as a single event (Gallego-Torres et al., 2010).
About 3ky before its deposition, melting icebergs and intensification of African monsoons
arrested normal Mediterranean Sea circulation and deep water formation. Anoxic conditions
were strongly developed, reaching the lower part of the photic zone, ~200m deep (Rohling et
al., 2015). Based on correlations between calculated ages of planktonic δ18O values from
different sites, anoxic conditions initially developed in the western part of the sea, followed a
few hundred years in the EM.
Sapropel S5 was dated to ~124-119ka by comparing the δ18O record of the dated Soreq Cave
speleothems to those of Mediterranean Sea cores (Bar-Matthews et al., 2000). These dates are
similar to Emeis et al. (1998) results of ~125-118.5ka. Additional dating for this sapropel
using 238U isotopes gave the age of ~121-116ka (Severmann and Thomson, 1998).
Data for planktonic foraminifera in ODP967 core show a -1.91‰ to - 0.03‰ range of δ18O for
this sapropel which is considerably lighter than values for sapropel S1 from the same core.
Average temperature during sapropel deposition was ~21⁰C with maximum temperature of
22.9⁰C at its termination (Fig.1.10b; Emeis et al., 1998). Thus water temperatures during S5
were several degrees warmer than for sapropel S1.
1.4.3. Sapropel S7
Compared to sapropel S1 and even S5, relatively little is known about sapropel S7. The δ18O
range for this sapropel at ODP967 is more variable than in sapropel S5, and ranges between
-1.94‰ to +1.47‰. Sapropel S7 was interpreted to deposit in less warm and more moderate
climate than S5 with an average temperature of ~20⁰C, but not exceeding 20.1⁰C (Fig.1.10c)
(Ten Haven et al., 1987; Emeis et al., 1998). This sapropel was dated to ~207-201ka (Emeis et
al., 1998).
During MIS7, insolation changes were more dominant and considered to show the highest
amplitude over the last ~900ky (Roucoux et al., 2008). Data on sediments that deposited just
before sapropel S7 from the Ioannina basin (core I-284, north-west Greece; Roucoux et al.,
2008) show abnormal abundance in pollen and increasing insolation that reached a maximum
at ~200ka. Increase in pollen indicates greater erosion following enhanced vegetation growth
due to evolving warmer climate and moisture availability over the area. This observation is
22
supported by low δ18O found in speleothems from Pekiin Cave during ~250-185ka, implying
enhanced rainfall over the EM area (Bar-Matthews et al., 2003). A rise in δ18O of both
planktonic and benthic foraminifera during ~208-200ka suggests a cooling event. This event
can be observed in the SST profile for this sapropel (Fig.1.10c; Emeis et al., 1998).
Fig.1.10 Reconstructed sea surface temperatures (SST) during the deposition of ODP967 sapropels a. S1 b. S5 and c. S7 .Open circles represent SST temperatures and closed circles alkenones concentrations. During the pre-sapropel stage, there is a rise in temperature which decreases after sapropels termination. (Emeis et al., 1998).
S5
S7
S1
a
c
b
Fig.1.9 δ18
O depth profile of core RC9-181 south to Crete Island. The three sapropels discussed in this work are outlined by orange ellipses. (Rossingol-Strick, 1985).
23
1.5. Research aims
This research primarily focuses on Fe behavior and its isotopic composition (δ56Fe) in Eastern
Mediterranean sapropels, and how climate change affects its record in the sediment. Seasonal
signals are stronger in the Mediterranean Sea than in other ocean sediments (Malanotte-
Rizzoli and Bergamasco, 1989) and therefore make Mediterranean sediments particularly
suitable for paleoclimate studies.
Iron isotope records from two different sapropels, S5 and S7, from the same core were
studied and S1 from core 9501 (Almogi-Labin et al., 2009). Those sapropels formed during
interglacial at times of peak insolation. However, sapropel S7 deposited in a more moderate
and less warm climate than sapropel S5 (Ten Haven et al., 1987), and thus, the two sapropels
present contrasting aspects on sapropel formation.
The studied samples are from ODP967 core, which has been the subject of previous studies
(Emeis et al., 1998; Kroon et al., 1998; Larrasoaña et al., 2003; Azrieli-Tal et al., 2014). Azrieli-
Tal et al. (2014) study of sapropel S1 from this site show that weak euxinic conditions
occurred during its formation, but only during its earlier stages. This study will compare the
redox conditions for sapropels S5 and S7 with those developed in S1. This comparison will be
achieved through a combination of studies involving Fe isotopes, MoEF –UEF covariations, and
RSTE profiles.
A major goal of this work is to determine whether Fe isotopes can be used to reconstruct the
intensity of past redox conditions. . This aim is achieved by comparing Fe/Al vs. δ56Fe trends
of sapropels S5 and S7 with: previous results on sapropel S1 from the same site (Azrieli-Tal
2012; et al., 2014), sapropel S1 from core 9509 located in Nile Fan (new measurements in this
study), and organic carbon rich sediments of the Black Sea (Severmann et al., 2008). Sapropel
S1 from 9509 was studied for its Fe isotope composition since it represents a sapropel with
low TOC content formed under non-euxinic (probably sub-oxic to anoxic pore water
conditions) and therefore potentially provides a baseline for the benthic shuttle model.
24
Fig.2.1 Location map for studied cores. Core ODP967 at water depth ~2550m in the Hellenic Trench south of Cyprus (top marker). Core 9509 from water depth ~900m in the Nile Fan (bottom marker)
ODP 967 ~2550m water depth
9509 ~900m water depth
2. Methods
2.1. Sapropel sampling
Depth profiles across the three sapropels S5, S7 and S1 (Table 1) were studied for their Fe
isotopic composition from two different locations (Fig.2.1).
a. Sapropel S7: 27 bulk samples of the sapropel and immediate enclosing sediments from
the same core as sapropel S5. According to Emeis et al. (1998), sapropel S7 deposited in
two pulses: a major pulse between 130cm (section 2) and 4cm (section 3) corresponding
to 199.83 ‒ 207.74ka according to the chronology of Emeis et al. (1998) and a second
earlier and weaker pulse between 23cm and 30cm (section 3; 216.83 to 219.73ka). No
TOC data is available for this earlier pulse and this work only covers the major sapropel
* C and D refer to two different drills taken at the ODP967 site. The distance between the two drill locations is about 20m, and therefore they are considered to give an equivalent record.
25
S7 pulse. Samples were taken at 1cm resolution in the sapropel and up to 5cm resolution
in the enclosing sediments.
b. Sapropel S5: 26 bulk samples of the sapropel and the immediate enclosing sediments,
from a drilling core at the Hellenic Trench south of Cyprus (ODP967C). The core was
sampled on April 1995 during Leg 160 of the Oceanic Drilling Program (ODP) under the
scientific direction of Prof. Kay-Christian Emeis. Samples were obtained from the ODP
drilling core storage facility at Bremen, Germany. Samples were taken at 2cm resolution
in the sapropel and up to 5cm resolution in the enclosing sediments.
c. Sapropel S1: 19 samples of sapropel from core 9509. It is a long, continuous, undisturbed
core collected by R/V Marion Dufresne at depths of 884m during February 1995 under
Dr. Martine Paterne’s supervision, and stored in CNRS Gif sur Yvette (France). High
resolution sampling (every 2-5cm) was made throughout the core and at 1-2cm
resolution across the sapropels by M. Bar-Matthews and A. Almogi-Labin. Core 9509 lies
directly in the pre-Aswan plume of suspended sediment discharged during the annual
Nile flood. Geochemical data for this sapropel and enclosing sediments are given in
Azrieli-Tal (2012).
2.2. Sample processing
The entire sample processing is described in Fig.2.2. Along with the sapropel samples, two
geological reference materials, BHVO-1 and IF-G, were fully processed and analyzed for Fe
isotope composition. In this study bulk samples were processed, in contrast to Azrieli-Tal et
al. (2014) in which the <63 micron-size fraction was utilized in order to make Fe speciation
studies in future work. Studies of Box et al. (2011) and Azrieli et al. (2014) showed however
that both the bulk and the <63 micron-size fraction accurately record the sapropel events.
2.2.1. Sample digestion
In order to dissolve the sapropel material, a heating step preceded the acid digestion:
- sample was freeze-dried.
- ~1g sample was ground to fine powder.
- ~100mg of ground sample was heated in an 800⁰C oven for 12h to volatilize organic
matter (Owens et al, 2012; Azrieli-Tal et al., 2014).
- the combusted sample was transferred to a Teflon beaker with 1ml concentrated
HNO3 and 1ml HF, and fluxed on a hot plate set to ~150⁰C for 12h.
26
- the solution was evaporated to dryness and redissolved with 4ml of 1:1 concentrated
HCl and H2Ox2, and evaporated to dryness .
- sample was dissolved once again in 5ml 6M HCl with 0.01% H2O2. Addition of H2O2
ensures oxidation of all Fe to the ferric form for the column separation.
2.2.2. Chromatographic separation
Iron purification is necessary for Fe isotopic analysis (Teutsch et al., 2005; Severmann et al.,
2006). The following procedure was adopted based on the protocol developed at ETHZ,
Zurich. Chromatographic separation was conducted using 10ml plastic column (Muromac®)
containing about 1ml resin AG 1-X4 Resin (200-400 mesh, Cl form, Bio-Rad). Each sample was
processed through the column twice for complete separation of Fe as follows:
- preconditioning of the resin was with 2 aliquots of 1ml 6M HCl +0.01% H2O2.
- 100µl sample (in 6M HCl +0.01% H2O2, containing 30-150µg of Fe) was loaded on the
resin.
- matrix elution with 2x1mL and 3x0.5mL 6M HCl + 0.01% H2O2.
- 0.5x1ml 1M HCl was passed through the column and discarded to reduce high
concentration acid in sample with no loss of Fe.
- elution and sample collection into a Teflon beaker with 6x0.5mL 1M HCl.
2.2.3. Preparation for chemical and isotopic analysis
Chemical analysis was performed before and after column separation. The pre-column
analysis included redox sensitive elements (Ba, Fe, S, Mn, Ni, V, Mo and U) and Al: dilution of
150µl (in 6M HCl) into 5ml with H2Ox2. Post-column chemical analysis was made to ensure
purification and full recovery of Fe after the column chemistry. Full recovery assures that no
Fe isotopic fractionation occurred during column chemistry. For chemical and isotopic
analyses after column chromatography, the eluted sample in 1M HCl was first converted into a
nitrate matrix as follows:
- the eluted Fe solution was evaporated to dryness.
- sample was redissolved with ~200µl concentrated HNO3 and evaporated to dryness.
This stage was repeated twice.
- final dissolution with 4ml 0.1M HNO3.
Post column chemical analysis was made on solution prepared by dilution of 200µl of the final
nitrate solution to 2ml 0.1M HNO3..
27
For the isotopic analysis, each sample was diluted to create a solution of 2mg/L Fe in 0.1M
HNO3. The amounts taken for dilution were based on the post-column Fe chemical analysis of
the final nitrate solution.
2.2.4. Scanning Electron Microscope (SEM) analysis
Three sapropels S5 samples and four sapropel S7samples in powder form (before chemical
digestion) were analyzed for pyrite using Scanning Electron Microscope (SEM). The analyzed
samples were at maximum Fe peaks within the sapropels and low Fe content sediments at
sapropel boundaries and enclosing sediments.
2.3 Geochemical and Fe isotope analysis
Chromatographic separation for Fe was carried out in the clean laboratory of the Institute of
Earth Sciences at the Hebrew University (HUJI). Major elemental analysis of bulk sapropel S5
samples were performed at HUJI with Inductively Coupled Plasma Optical Emission
Spectrometry (ICP-OES, Perkin Elemer Optima 3000), and for sapropel S7 with a similar
instrument (ICP-OES, Perkin Elemer Optima 3300) at the Geological Survey of Israel (GSI).
Concentrations of Mo and U were measured with Inductively Coupled Plasma Mass
Spectrometry (ICP-MS, Agilent 7500cx) at HUJI. Certain modifications to chemical analytical
procedures were made during the course of the study. It was found that analyzing bulk
Chemical analysis
4. digestion
Powder
1. drying
2. grinding
3. combustion
Bulk Sample
Sample in 6M HCl 5. chromatographic separation
6. transferring sample to
matrix 0.1M HNO3
Sample in
0.1M HNO3
Isotopic analysis
Fig.2.2 Schematic presentation of sample processing and analysis
SEM analysis
28
sample solutions in a 0.18M HCl matrix, rather than the previously used 0.1M HCl matrix,
produced more reliable results for Al. The internal standard for ICP-OES analysis was changed
from the commonly used scandium (Sc) to lutetium (Lu) because Sc was found to be present
in some of sapropel S5 samples (probably due to contamination from the furnace). Scanning
Electron Microscope (SEM, FEI Quanta 450) studies were done at the GSI.
Analytical reproducibility was checked for chemical procedure; three sapropel S7 bulk
samples were prepared in duplicate (including ashing and digestion) and analyzed. Maximum
measurement differences for Fe were 3%, 10.5% for other major elements (Al, Mn, Mg, Ca and
S) and 5.5% for trace elements (Cr, Co, V, Ni, Zn, Mo and U). Barium however showed a larger
difference range of 13-29% for the duplicate samples. This Ba difference probably reflects
inhomogeneous grinding of biological skeletons in the samples.
All chemical composition data measured during the course of this study are presented in
supplementary Tables S1 and S2. Maximum reproducibility errors on single values for major
and trace elements were ±2%, except for Ni which received error of ±7%.
Isotopic analysis was performed at the GSI using a High Resolution Multi Collector Inductively
Coupled Plasma Mass Spectrometry (HR-MC-ICP-MS, Nu plasma II, Nu Instruments).
Samples were measured using sample–standard bracketing with the metal standard IRMM-
014. In every six samples, the ETH Fe-salt standard (FeCl2) was measured. The IRMM-014
standard and FeCl2 solutions were also processed through column chemistry as another check
of the column chemistry procedure.
The Fe isotopic composition of these together with SRMs BHVO-1 and IF-G, processed
alongside sapropel samples during five sessions of the HR-MC-ICP-MS Fe isotopic analysis, are
detailed in Table 2. Accuracy and precision of the Fe isotope analysis during each session was
obtained by analyzing the ETH Fe-salt standard (FeCl2) throughout each analysis session (as
noted above). The isotopic compositions obtained for the FeCl2 (δ56Fe=-0.70 ±0.05‰, δ57Fe=
-1.04 ±0.09‰; n=87 2SD; 5 sessions in 14 months) are in excellent agreement with previous
measurements from different laboratories (δ56Fe=-0.73 ±0.10‰, δ57Fe=-1.07 ±0.15‰, n=89
0.10‰, n = 216 2SD, HR-MC-ICP–MS, Nu1700, Nu Instruments, Kiczka et al., 2011).
Duplicates of samples for isotopic analysis were fully processed including column chemistry
to check reproducibility of Fe isotope. Results obtained for both Fe solutions after column
29
chemistry are within instrumental error and validate there is no problem with fractionation of
Fe during column chemistry.
Iron isotopic compositions (δ56Fe and δ57Fe) of standard reference materials (BHVO-1 and IF-
G) were identical within uncertainties (except for session 2 for IF-G) to the values reported in
previous studies, and mostly fall within the Craddock and Dauphas (2011) recommended
values. The IF-G data in session 2 significantly exceeded the accepted values. However, based
on the accuracy of BHVO-1, FeCl2 and repeated analysis of previously measured sapropel
samples during this session, it seems the analytical problem was specific to IF-G, and sapropel
samples results were treated to be true. The importance of processing these two standards is
to verify the accuracy and reproducibility of the entire procedure including digestion, column
chromatography and isotopic analysis. The good reproducibility and comparability of FeCl2
and SRMs data validate the accuracy and precision of the entire procedure including the HR-
MC-ICP-MS analyses of the sapropel samples.
All average Fe isotopic results of the standards analyzed during the five analytical sessions
(Table 2; except for IF-G of session 2) are presented on a δ57Fe vs. δ56Fe plot (Fig.2.3). As can
be seen, the data closely plot along the mass dependent fractionation line, indicating that
there are no problems of isobaric interference effects associated with the Fe isotope analyses.
Sapropel S1 samples from cores ODP967D (<63 m fraction) and 9509 (bulk and <63 m)
were previously studied by Azrieli –Tal (2012) for Mo and Fe isotopes (Fe isotopes only for
ODP967D), and therefore already chemically processed. Thus, chemical data for sapropels
from both sites and Fe isotope data for sapropel from ODP967D are taken from Azrieli–Tal
(2012; et al., 2014). In this study, sapropel S1 9509 samples were only purified and measured
for Fe isotopes. Samples used were the dried Fe powders obtained during the chemical
processing for Mo by Azrieli-Tal (2012).
Carbon concentrations content of sapropels (TOC) presented in this study were taken from
Emeis et al. (1998) (Table 1 in article and Supplementary data Table S3).
30
Fig.2.3 Average δ57
Fe vs. δ56
Fe with 2SD of the different standards analyzed along with the studied samples (Table 2). Solid line is the theoretical mass dependent relationship between the two isotopes: δ
Table 2 Iron isotope data of standards measured during the five sapropel analytical sessions
*The accepted isotopic data for FeCl2 is the average calculated results from Fehr et al. (2008), Teutsch et al. (2009) and Kiczka et al. (2011). BHVO-1 and IF-G data are taken from Craddock and Dauphas (2011). 2SD values were calculated for the presented values in the articles with the equation sqrt (2SD1
2 + 2SD2
2 +…+ 2SDn
2), whereas 2SDn is the error of single measurement.
.
31
3. Results
All elements, except for TOC and S are presented normalized to Al as X/Al weight ratios (Table
S2). Total organic content (TOC) and S are presented as weight percent (wt.%). For data
presentation, Ba, Ni, V, Mo, U and Mn are multiplied by 104.
3.1. Chemical depth profiles
3.1.1. Productivity and sapropel boundaries
It is important to set accurate sapropel boundaries for a more precise interpretation of the
environmental conditions in which the sapropels and their background sediment formed. As
noted in the introduction (section 1.2.2.), Ba/Al ratios have been extensively used to define
the original boundaries of the sapropel (van Santvoort et al., 1996; Thomson et al., 1999; de
Lange et al., 2008; Azrieli-Tal et al., 2014). In all these studies the Ba/Al profile exhibited
quasi-Gaussian profiles. Nevertheless, Tribollivard et al. (2006) have indicated that Cu and Ni
may provide more reliable indicators of a productivity event characterized by high organic
matter flux. Within both sapropels S5 and S7, TOC, Ba/Al, and Ni/Al are elevated relative to
background sediments (Fig.3.1). Hence, the upper and lower sapropel boundaries are defined
using TOC, Ba/Al and Ni/Al ratios, respectively, according to their enrichments above
background levels (after Thomson et al., 1999).
For sapropel S5 both the lower and upper Ba/Al boundaries (defined by dashed lines) match
TOC enrichments (yellow rectangular areas) (Fig.3.1a,c). The upper boundary of sapropel S7
however, was more difficult to determine since Ba/Al shows a minimum at the upper limit of
TOC enrichment (133cm), followed by a minor peak at ~130cm (Fig.3.1b). It is thus not clear
if the sapropel cessation occurred at the first Ba/Al minimum at 133cm, or if this minimum
possibly represents a reventilation event that was followed by a brief return to sapropel
productivity that terminated at about 127cm.
The boundaries for sapropel S5 defined by Ba/Al and TOC closely fit the enrichment of Ni/Al
in the sapropel. However, the upper boundary for Ni/Al for sapropel S7 fits the TOC data and
the Ba/Al data defining the boundary at 133cm, but not 127cm. In the following diagrams, the
upper and lower sapropel S5 boundaries are set at 74cm and 103cm, respectively, and for
sapropel S7 boundaries are set at 133cm and 152cm. The issue of the discrepancy between
the boundary set at 133cm and the potential boundary at 127cm defined by Ba/Al will be
presented in the discussion.
32
The age models for the sapropels in black font are taken from Emeis et al. (1998) and red are
after Bar-Matthews et al. (2000).
Fig.3.1 Depth profiles of Ba/Al, TOC and Ni/Al for sapropel S5 (a, c and e) and sapropel S7 (b, d and f). Upper and lower sapropel boundaries and zone of TOC enrichment are defined by dashed lines and the yellow rectangular areas. Age models and positions of the sapropel boundaries are described in the text. TOC data is taken from Emeis et al. (1998). The age models for the sapropels in black font are taken from Emeis et al. (1998) and red are after Bar-Matthews et al. (2000).
0
90
180
270
360
120 130 140 150 160 170
S7
Ba*1
04/A
l
Mean Depth [cm]
200 202 203 206 207
Age [ka]b
0
2
4
6
8
40 60 80 100 120 140
S5
wt.
% T
OC
Mean Depth [cm]
118 120 122 124 126
119 124Age [ka]
c
0
90
180
270
360
40 60 80 100 120 140
S5
Ba*1
04/A
l
Mean Depth [cm]
118 120 122 124 126
119 124Age [ka]
a
0
2
4
6
8
120 130 140 150 160 170
S7
wt
% T
OC
Mean Depth [cm]
200 202 203 206 207
Age [ka]d
0
20
40
60
120 130 140 150 160 170
S7
Ni*
10
4/A
l
Mean Depth [cm]
f200 202 203 206 207
Age [ka]
0
20
40
60
40 60 80 100 120 140
S5
Ni*
10
4/A
l
Mean Depth [cm]
e
118 120 122 124 126
119 124Age [ka]
33
3.1.2. Redox Sensitive Trace Elements profiles
The RSTE V, Mo, and U are enriched in both sapropels compared to background sediments,
(Fig.3.2a-f). The most distinct feature of sapropel S5 profiles is the gradual rise in V/Al
background values of ~20 to a maximum value of ~110 at 86-78cm (i.e., close to the sapropel
termination; Fig.3.2a). Similarly, Mo/Al and U/Al show an initial rise from background values,
but maximum values of ~40 and 10, respectively, are attained somewhat earlier at 92-78cm
(Figs.3.2c,e). The U/Al profile (Figs.3.2e) shows an outstanding peak at 86.5cm, which is not
seen in other elements for this sapropel. All three profiles show a sharp decrease to
background levels from 78-74cm indicating a falloff in the intensity of reducing conditions at
the sapropel termination. The intensity of RSTE maxima in the S5 profiles is indicated by high
magnitude compared to peak values from sapropel S1 at site ODP967 of 60 (V/Al), 9 (Mo/Al)
and 3.5 (U/Al) (Azrieli Tal et al., 2014). Sapropel S7 is similarly characterized by gradual
increase in RSTE ratios with sapropel deposition; Mo/Al and U/Al reach a maximum at
140.5cm (Figs3.2d,f), whereas V shows a maximum plateau at 144 -136cm (Fig.3.2b). The
maximum values are comparable to S5, though generally slightly lower for V/Al and U/Al.
There is evidence, particularly in the Mo/Al values, but also in U/Al, of an earlier smaller
maximum at 149.5cm. Such a double maximum separated by a trough has been recognized in
sapropel S1 at ODP967 (Azrieli-Tal et al., 2014) and attributed to a weakening in sapropel
redox conditions, possibly due to a reventilation event. Following the maximum values at
140.5cm for Mo and U, there is a systematic decline to background levels at the sapropel
boundary.
In contrast to these RSTE enrichments, Mn shows low Mn/Al values within both sapropels S5
and S7 but shows well defined peaks at boundaries (Figs.3.2g,h). The Mn/Al peak are
observed at both boundaries for sapropel S5, but is not present at the upper boundary of S7.
0
35
70
105
140
40 60 80 100 120 140
S5
V*1
04/A
l
Mean Depth [cm]
a
0
35
70
105
140
120 130 140 150 160 170
S7
V*1
04/A
l
Mean Depth [cm]
b
34
3.1.3. Iron and Sulfur profiles
The Fe and S profiles show distinct increases in the sapropels (Fig.3.3). Iron/Al shows a
distinct rise from background values of ~0.7 in the pre and post-sapropel sediments to
maximum values of ~1.3 and 2.4 in sapropels S5 and S7, respectively. . The background Fe/Al
Fig.3.2 Depth profiles of sensitive redox trace elements (V, Mo, U, and Mn normalized to Al) for sapropel S5 (a, c, e and g) and sapropel S7 (b, d, f and h). Legend as in Fig.3.1. In both sapropels, all trace elements (except for Mn) concentrations start to rise near the bottom sapropel boundary and decrease sharply with decline of the sapropels. Manganese behaves oppositely with concentration peaks near the sapropel boundaries and low concentrations within the sapropels.
0
5
10
15
20
25
40 60 80 100 120 140
S5
U*1
04/A
l
Mean Depth [cm]
e
0
20
40
60
40 60 80 100 120 140
S5
Mo
*10
4/A
l
Mean Depth [cm]
c
0
500
1000
1500
40 60 80 100 120 140
S5
Mn
*10
4/A
l
Mean Depth [cm]
g
0
500
1000
1500
120 130 140 150 160 170
S7
Mn
*10
4/A
l
Mean Depth [cm]
h
0
20
40
60
120 130 140 150 160 170
S7
Mo
*10
4/A
l
Mean Depth [cm]
d
0
5
10
15
20
25
120 130 140 150 160 170
S7
U*1
04/A
l
Mean Depth [cm]
f
35
0.5
1
1.5
2
2.5
40 60 80 100 120 140
S5
Fe/A
l
Mean Depth [cm]
a
0.5
1
1.5
2
2.5
120 130 140 150 160 170
S7
Fe/A
l
Mean Depth [cm]
b
ratios of ~0.7 are higher than crustal values of 0.55 (Raiswell and Canfield 2012), but are
similar to Fe/Al values of the sediments enclosing sapropel S1 at ODP967 (Azrieli-Tal et al,
2014) and Site 9509 in the Nile plume (Azrieli-Tal, 2012). These higher values in the EM
sediments are attributed to the high input of detrital iron minerals from the Nile run off
(Azrieli-Tal et al., 2014).
Generally, Fe/Al values are higher in S7. It is possible that the very high Fe/Al values in S7
represent anomalous detrital Fe mineral input. Though the correspondingly high S
concentrations (wt.%) for this period in S7 may also suggest that the high Fe/Al could have
enhanced authigenic Fe-sulfide mineral formation.
Sulfur concentrations (Fig3.3c) show distinct variations in sapropel S5. Lower S in the four
lowest samples of sapropel S5 match the lower Fe/Al values for these samples. Higher S is
also observed in the samples with Fe/Al >1 in the upper part of S5, but there is also a strong
variability in the S concentrations that is not apparent in the Fe/Al values. Sulfur
concentrations for sapropel S7 are higher than those for sapropel S5 and the peaks seem to
match those for Fe/Al. Notable in both Fe/Al and S profiles for sapropel S7 are the double
peaks (maxima); a smaller peak at 149.5cm and the major peak at 140.5cm. These double
peak profiles thus match the Mo/Al profile for sapropel S7 (Fig. 3.2d).
Additional notable features are the rises in Fe/Al and S in the sediments immediately
underlying both sapropels (Fig.3.3). The rises are also observable in the Mo/Al and V/Al
profiles (Figs.3.2a-d). These pre-sapropel enrichments have been recognized in a number of
sapropels (Thomson et al., 1999; Azrieli-Tal et al., 2014) and have been termed
protosapropels. They are thought to reflect either initial sapropel formation (Murat and Got,
1987; Troelstra et al., 1991) or post-depositional pyritization resulting from diffusion of
aqueous sulfide from the sapropel into the underlying sediment, and its reaction with upward
diffusing Fe2+ (Passier et al., 1996, 1999; Thomson et al., 1999).
36
3.2. Scanning Electron Microscope studies
SEM images of representative samples from sapropels S5 and S7 are presented in Fig.3.4.
Pyrite in the back scatter (BSE) images appears as bright crystals and the darker surrounding
matter is mainly clay and fossil remains.
Consistent with elevated Fe and S contents, pyrite is found in high abundance in both
sapropels but in different forms. Sapropel S7 at maximum Fe peak (140.5cm) consists of many
well-shaped pyrite framboidal aggregates, mainly spherical. Some of these aggregates are as
big as ~10µm diameter, composed of ~1µm sized pyrite crystals. Traces of fine-coating
(unidentified membrane), possibly organic, coating small portions of the pyrite suggest that
crystals were interrupted during formation process (Fig.3.4a). Big framboidal aggregates are
found in the Mn-rich pre-sapropel sample (153.5cm) and even under the Mn-rich layer
(155.5cm; Fig.3.4b). Additionally, this sample also contains ~8µm euhedral pyrite crystals.
These crystals are not smooth, since some of the crystal faces are characterized with
dissolution pits. At the lower earlier Fe peak (149.5cm), the framboids, however, are poorly
arranged and are also covered by a film of thin coating (membrane). The minimum trough in
Fe/Al at 146.5cm following this mild peak was scanned, but no pyrite was found. The fossil
remains, where framboids are usually assembled, are free of pyrite. However, barite crystals
are detected in the samples at 146.5cm, suggesting redeposition of barite due to newly
introduced O2 to the water column. As noted in the introduction (section 1.2.2), barite will
dissolve once SO42- concentrations are low. A reventilation event would have elevated barite
concentration enabling barite reprecipitation.
Fig.3.3 Depth profiles of Fe/Al and S (wt.%) for sapropel S5 (a,c) and sapropel S7 (b,d). Legend as in Fig.3.1. Both elements are enriched within sapropel boundaries compared to enclosing sediments.
0
5
10
40 60 80 100 120 140
S5
wt.
% S
Mean Depth [cm]
c
0
5
10
120 130 140 150 160 170
S7
wt.
% S
Mean Depth [cm]
d
37
In contrast to sapropel S7, the maximum sapropel S5 Fe peak (92.5cm; Fig.3.4c) is
characterized with up to ~10µm fine-shaped octahedral crystals and much reduced amounts
of small framboidal aggregates about the same size. Nevertheless, framboids are the
commonly observed form of pyrite in sapropels and not individuals crystals as seen here. In
Mn-rich sediment near sapropel bottom boundary both forms are also found, but in less
abundance than within the sapropel (102.5cm). Most crystals and framboids are smaller than
a b
c d
Fig.3.4. Back scattered scanning electron microscope images. a. Framboid aggregate at sapropel S7peak (140.5cm). There are traces of thin fine coating layer (membrane) on the pyrite, possibly organic matter. b. Euhedral pyrite crystal in pre-sapropel S7 sediment (155.5cm). c. Euhedral shaped and framboidal aggregate pyrite at maximum Fe peak in sapropel S5 (93cm). d. Small euhedral pyrite crystals at Mn-rich bottom sapropel boundary (103cm).
38
in sapropel Fe peak. Overlying sediment further away from the sapropel boundary is absent
of any pyrite (56.5cm).
3.3. Iron isotope profiles
3.3.1. Iron isotope data
Iron/Al and δ56Fe data on samples are presented according to analysis sessions in Table 3.
The data includes a number of samples that were remeasured in more than one session to
check for reproducibility. In addition, samples denoted by i(ii,iii) refer to digested samples
that were chromatographically separated in replicate, and samples which were replicated by a
complete repeat of the whole chemistry procedure (including ashing and digestion) are
denoted by a(b). Depth profiles of iron isotopes appearing in this section present the average
value if more than one measurement was valid (Table 4). Sapropel S7 samples are taken
from two sequential cuts (2 and 3 sections) from core 967D-2H. Depths of samples of cut 3 are
given relative to the top of the cut and total depth is given by the bracketed number.
Fig.3.5 Depth profiles of Fe/Al (blue circles) and δ56
Fe (orange circles) for a. sapropel S5 and b. Sapropel S7. Legend as in Fig.3.1. A distinct inverse relationship between Fe/Al to δ
56Fe values exists
for the two sapropels, with minimum δ56
Fe values correlating to maximum Fe/Al.
Fig.3.6 Depth profiles of Fe/Al and δ56
Fe for sapropel S1 from core 9509. Legend as in Figs.3.1 and 3.5. The Fe/Al varies slightly from ~0.75 to 1 (data from Azrieli-Tal, 2012), while δ
56Fe appears as an almost
constant value around ~0‰.
0.5
1
1.5
2
2.5
-1
-0.5
0
0.5
1
120 130 140 150 160 170
S7
Fe/Al
56
Fe
Fe/A
l d5
6Fe
Mean Depth [cm]
b
43
As detailed in the introduction, samples from sapropel S1 at site 9509 in the Nile delta were
also analyzed for their δ56Fe in order to provide a baseline for Fe/Al vs. δ56Fe change in
sapropels formed in non-euxinic conditions. The Fe/Al data from Azrieli-Tal (2012) is
charcaterized by a double-peak profile that was attributed to varying amounts of detrital iron
minerals being delivered in the Nile run off. Despite these variations in Fe concentrations
within the sapropel, there are no significant variations of δ56Fe, which span a narrow range
between -0.05‰ and 0.08‰, with an average of 0.02 ±0.07‰. This ~0 ‰ value throughout
the profile is consistent with varying detrital mineral inputs, since these minerals (mainly
igneous iron titanium oxides) would have δ56Fe values close to zero.
44
4. Discussion
4.1. Sapropel chemistry
4.1.1. Productivity and sapropel boundaries
Notably, sapropel S7 exhibits higher TOC than sapropel S5, whereas Ba/Al concentrations in
S7 are significantly lower, though Ni/Al levels are similar in both sapropels (Fig.3.1). Several
reasons can be proposed to account for these differences. 1) Elevated concentrations of Al in
sapropel S7 sourced by enhanced detrital input into the sea could have resulted in low Ba/Al
ratios. 2) Bioproductivity in sapropel S5 was greater than in sapropel S7, but TOC in the
sapropel S7 was better preserved. To examine between these two explanations, a
concentration comparison of Al (wt.%) and Ba(ppm) between the two sapropels was made
(Fig.4.1).
Aluminum concentrations (closed circles) within both sapropels are similar (4-6 wt.%) and
without any outstanding peaks. Barium (open circles) on the other hand, is much higher in
sapropel S5 than in sapropel S7. Dymond and Collier (1996) showed similar trends of high
organic carbon with relatively low Ba concentrations compared to expected values based on
Corg-Ba relationship in >1200m equatorial Pacific sediments. In a previous study on
Mediterranean Sea sapropels S5 and S7 (Gallegro-Torres et al, 2010; ODP964 and 969) the
same Corg-Ba relationship was observed. Dymond and Collier (1996) suggested that the
process by which Ba is transported and/or scavenged to the deep ocean is based on
decomposing organisms, but overall it is less efficient at times of high organic carbon flux.
Alternatively, Paytan and Griffith (2007) suggested that high productivity lowers organic
Fig.4.1 Comparison of Al (wt.%) and Ba(ppm) in sapropels a. S5 and b. S7. Aluminum (closed circles) in both sapropels behaves similarly whereas Ba concentrations (open circles) are significantly higher in sapropel S5 than in S7. Legend as in Fig.3.1.
0
2
4
6
8
10
0
500
1000
1500
2000
40 60 80 100 120 140
S5
wt.
% A
l
Ba
[pp
m]
Mean Depth [cm]
a
0
2
4
6
8
10
0
500
1000
1500
2000
120 130 140 150 160 170
S7
wt.
% A
l
Ba [p
pm
]
Mean Depth [cm]
b
45
matter decomposition rates, and hence, it has less time to form the paleoproductivity mineral
barite. Moreover, barite in low sulfate environments tends to dissolve in the sediment and
migrate upwards to the water column. Remineralization of barite will happen when sulfate is
not depleted, and will create ‘barite fronts’ (van Os et al., 1991; Paytan and Griffith, 2007;
Schoepfer et al., 2015). Thus, the later elevation in Ba/Al found after the cessation of TOC
deposition, occurs simultaneously with a decrease in S concentrations to background values
(Fig.3.3d), suggesting that it might be a barite front. The Ni/Al depth profile for sapropel S7
(Fig.3.1f) shows a decrease corresponding to that of TOC, implying that there is no organic
matter oxidized in the upper part of sapropel and which is missing from the record.
Interpolation according to Ba/Al alone will imply for an oxidized upper layer, as was observed
for sapropel S7 from ODP964, 969 and 966 (Gallegro-Torres et al., 2010).
Consequently, as noted in the results, the upper boundary of sapropel S7 was set according to
TOC boundary and Ni/Al ratio (133cm). It is proposed that the small post-sapropel
enrichment of Ba/Al represents a Ba front. In sapropel S5, Ni/Al is coordinated with the
decline of TOC but also with Ba/Al ratio.
4.1.2. Environmental conditions during sapropels S5 and S7 formations
The geochemical profiles presented in section 3.1.2 and 3.1.3 show that both Fe and S and the
trace elements Mo and V are strongly elevated in the sapropel but also show enrichments
before TOC rises at the base of the sapropel. This enrichment zone (the protosapropel) has
been interpreted to imply either anoxic conditions developing at the sapropel onset or post-
sapropel pyrite formation due to downward diffusion of sulfide into the sediment and Fe
mobilization upward from the sediment. Interestingly, these protosapropel elevations occur
at the same time as elevations in Ni/Al (Fig.3.1e,f) consistent with a productivity event prior
to sapropels, and which might have contributed to the development of sulfidic conditions.
A more detailed look into the evolution of redox conditions throughout the sapropel period is
provided by the EF plot of Mo vs. U (Fig.4.2). As noted in the introduction (section 1.2.3), Mo
vs. U EF plots show distinct trends dependent on the environmental redox conditions (Algeo
and Tribollivard, 2009). Particularly, both Mo and U are enriched in the sediment when
euxinic conditions prevail, but in sub-oxic to anoxic non-sulfidic conditions only U will be
elevated at the Fe(III)/Fe2+ redox boundary. This distinction is very useful when examining
redox conditions evolution.
46
Fig.4.2 Enrichment Factor (EF) plot of Mo vs. U for sapropels a. S5 and b. S7. Evolution and termination of sulfidic conditions for sapropel S5 is less steep than those for sapropel S7. Uranium peak of sapropel S5 (87cm) is not included in this plot because it yields an anomalously low Mo/U value (see text). The plots are presented on a logarithmic scale. SW represents sea water Mo/U ratio.
Saproepl S5 exhibits an outstanding U/Al peak at 86.5cm. High authigenic U were obtained in
other Mediterranean S5 sapropels with association to high TOC deposition (Gallegro-Torres et
al., 2010). The TOC and other RSTE depth profiles (Figs.3.1c, 3.2a,c) at this site do not indicate
an outstanding productivity event at ~87cm. Furthermore, MoEF/UEF for this sample will yield
a low ratio suggesting existence of anoxic to sub-oxic conditions, which is not evident in other
S5 depth profiles. The reason behind this peak is not an analytical error as remeasurements
yielded similar results, and the possibility of sample being polluted with U is low. Therefore,
the reason for this anomalous U/Al peak is not yet understood and thus is not included in the
EF plot (Fig.4.2a).
At peak sapropel conditions, characterized by highest RSTE values, both sapropels plot along
the sea water line (SW), indicating that uptake of Mo and U into the sediment is equal to Mo/U
ratio (~7.9) of the sea. Such profiles are consistent with strongly sulfidic bottom water
conditions (Fig.1.5; Algeo and Tribollivard, 2009), but not a strongly restricted water body
like the Black Sea where the Mo/U change would become flatter due to Mo depletion in the
water body (Fig.1.5). Pre-sapropel sediments (green circles) show enrichments in MoEF that
trends upward towards SW line. This trend is particularly well defined for sapropel S7
sediments where two of the samples (those immediately below the sapropel) lie on the SW
1
10
100
1000
1 10 100
S5
pre-sapropel sediments
sapropel
post-sapropel sediments
Mo
EF
UEF
SW
a
strong sulfidic
conditions
decline in sulfidic
conditions
developing sulfidic
conditions
1
10
100
1000
1 10 100
S7
pre-sapropel sedimentssapropel
post-sapropel sediments
Mo
EF
UEF
SW
b
evolution of
sulfidic
conditions
strong sulfidic
conditions
collapse of
sapropel
sulfidic
conditions
sub-oxic
47
line. This trend is less well defined for sapropel S5 where the data show greater dispersion
(nevertheless two data points also plot on the SW line). These upward MoEF/UEF trends in the
pre-sapropel sediments are consistent with evolving sulfidic conditions prior to the full
sapropel development.
The decline from peak sapropel euxinic conditions in S7 commences within the sapropel (at
~140cm) and are defined by blue data points showing a trend of decreasing MoEF/UEF values
downwards (away) from the SW line (Fig.4.2b). These data point to a gradual collapse of
sulfidic water conditions to non-sulfidic anoxic –sub oxic conditions within the sapropel. The
post sapropel sediments (red circles) are in the range of low Mo/U ratios regarded as only
sub-oxic (Fig.1.5). In contrast to this gradual collapse of sulfidic conditions in S7, all sapropel
S5 samples plot along the SW line and all the post sapropel sediments are in the anoxic to sub-
oxic range of values. This suggests that sulfidic conditions persisted in S5 right up to its
termination.
The considerations above allow drawing conclusions regarding the TOC preservation in both
sapropels. Preservation of organic matter in sapropel S5 (Fig.3.1c) was possible due to anoxic
conditions existing immediately after sapropel cessation, with more sub-oxic conditions only
developing later. Such anoxic and non-sulfidic conditions suggest that water circulation was
slow and it took time until O2 was reintroduced to the deep sea. Sediments deposited on top of
S5 sapropel during this anoxic stage may have allowed O2 to reach only the uppermost part of
the sapropel and oxidize it, as possibly suggested by the sharp drop from 2% to almost zero in
TOC values at upper sapropel.
The interpretation of the redox conditions for sapropel S7 is not as apparent as in sapropel S5.
The strong shift from sulfidic to anoxic conditions in the sapropel and then to sub-oxic
conditions in the post-sapropel sediments (as observed in the EF plot Fig.4.2b) suggests that
TOC could have oxidized at the upper sapropel boundary, but was not preserved along the full
sapropel length. However, as noted before (section 3.1.1), the upper TOC boundary matches
Ni/Al profile and argues against the possibility of TOC oxidation.
In addition to elevated Mo and U found in pre-sapropel sediments, pyrite is commonly found
at sapropel bases (Passier et al., 1999). This pyrite is proposed to form during pore water
diagenesis by locally produced sulfide (Calvert and Fontugne, 2001). Downward diffusion of
sulfide into underlying sapropel sediments would be possible only after reacting out of all
reactive Fe existing in the sapropel (Berner, 1969). In less extreme sulfide production
48
environments, upwards diffusion of Fe2+ from pre-sapropel sediments could occur and enrich
the sapropel in pyrite (Passier et al., 1999). Thus, in addition to pyrite formed in the water
column, additional pyrite could form during diagenesis. This may be the case in sapropels S5
and S7.
Iron-monosulfide concentration in water column and pore waters plays a critical role in pyrite
morphology. In supersaturated environments framboids are likely to be formed, whereas
during slower growth at lower supersaturation levels, larger sized framboids and finer
euhedral crystals will be found. With increasing degree of saturation, also crystal shape
changes from cubic to octahedral as seen here (Fig.3.4b-d), and then to spherulitic (Passier et
al., 1999). According to this criteria and the pyrite crystal morphology shown in Fig.3.4, even
though Fe was in excess in both sapropels (Fe/S ratios are discussed in section 4.1.3), the
growing pyrites were of different form. Sapropel S7 at its peak deposited in a highly
supersaturated environment and pyrite formed fast, while sapropel S5 pyrite formed slower
at lower supersaturations (but nevertheless overall strong euxinic conditions as for sapropel
S7; Fig.4.2) sufficient to form octahedral-shape crystals as well as framboidal aggregates.
Enrichment in Mn in the sediment is an indication for O2 resumption to bottom water
inducing the redeposition of Mn2+ as Mn-oxides. The Mn peak at the upper boundary of
sapropel S5 has been observed in a number of sapropel studies and has been attributed to a
change towards more oxidizing conditions at the cessation of sapropel growth (Thomson et
al., 1995; 1999; van Santvoort et al., 1996). However the two Mn peaks located at the base of
both sapropels S5 and S7 (Fig.3.2g,h) are more difficult to explain by post-sapropel oxidation,
since they would require the oxidation front from above to penetrate through the entire
sapropel. Also, such Mn peaks are not evident in depth profiles for Mediterranean Sea
sapropels S5 and S7 from other sites (ODP964, 966 and 969; Gallegro-Torres et al, 2010). On
the other hand, a similarly Mn peak is evident in sapropel S1 at site 9509 (Azrieli-Tal, 2012).
Local changes in redox conditions that crossed the Mn-oxides -Mn2+ transition in the sediment
near the lower sapropel boundary could possibly account for these Mn peaks, but further
research is required to understand the relationship between Mn occurrence and redox
conditions in these sapropels.
49
4.1.3. Iron in the sediment
Iron, as other reductive sensitive elements in both cores, shows enrichments before TOC
accumulation. These ‘protosapropel’ enrichments apparently occur prior to the sapropel
formation, rather than post-sapropel deposition. This could occur either by Fe2+ diffusion
from underlying anoxic pore waters upwards or/and diffusion of exported continental shelf
soluble Fe2+ downward into the sediment.
On a Fe (mole) vs. S (mole) plot (Fig.4.3), both sapropels S5 and S7 samples (blue circles) and
their enclosing sediments (green circles) are plot above the defining Fe/S ratio of pyrite
(black solid line). Thus, Fe/S mole ratios in the sapropels are greater than the ratio in pyrite
(Fe/Spyrite=0.5), pointing to excess Fe. The excess of Fe (Feexcess=Fetotal-Fepyrite) that is not in
pyrite most probably consists of Fe-oxyhydroxides and silicates. An estimation of the amount
of Fe bound into pyrite (Fe pyrite) was made by assuming that all S is taken up into pyrite. The
amount of Fe in pyrite is thus given by Fe pyrite =0.5* S (mole). The remaining Fe (Fe excess)
is the non-sulfide bound Fe. Profiles of Fe total, Fe pyrite and Fe excess are plotted in Fig.4.4.
As anticipated, the Fe pyrite in the sapropel is a major fraction of total Fe. Minor amounts of
pyrite occur in pre-sapropel sediments, as shown by small elevation in Fe pyrite, while post-
sapropel sediments are close to zero. As expected, Fe excess is the dominant component in
background sediments.
0
0.05
0.1
0.15
0.2
0 0.05 0.1 0.15 0.2 0.25
S5
sapropel
background
sediments
Fe [
mo
l]
S [mol]
pyrite
a
0
0.05
0.1
0.15
0.2
0 0.05 0.1 0.15 0.2 0.25 0.3
S7
sapropel
background
sediments
pyrite
Fe [
mo
l]
S [mol]
b
Fig.4.3 Fe (mole) vs. S (mole) in sapropels a. S5 and b. sapropel S7. Both sapropels and their enclosing
sediments show Fe/S mole ratios greater than those of pyrite (Fe/Spyrite=0.5).
50
Note that Fe in sapropel S7 shows a wider variation than in S5. It is assumed that the
enhanced erosion and riverine runoffs into the EM sea during sapropel S7 (section 1.4.3) is
the main reason for the higher and variable Fe, compared to sapropel S5 Fe. This concept is
applicable only to sapropels located in proximity to land (like ODP967) but not for mid-sea
sediments where its Fe supplement is mainly from dust (Passier et al., 1999).
Some correlation exists between Fe total and Fe pyrite, consistent with pyrite being a major
component of sapropel Fe. This is particularly evident in S7 where the peaks in Fe pyrite are
matched by the peaks in Fe total. Iron excess amounts in S7 are lower within the sapropel
than in the background sediments. This also appears true for sapropel S5 when compared to
Fe excess levels in the overlying sediment, whereas underlying pre-sapropel sediments show
similar Fe excess to the sapropel. Variable amounts of Fe pyrite were calculated for both
sapropels, especially for sapropel S5 (Fig.4.4). Rohling et al. (2006) found fluctuations in δ13C
and δ18O for Mediterranean Sea S5 sapropels and suggested variability in the depth/intensity
of winter mixing leading to change in water column stability. Such instability could affect the
position of Fe(III)/Fe2+ redox boundary and hence lead to variable pyrite production.
4.2 Iron isotopes
4.2.1. Sapropels S5 and S7 at OD967
The applicability of the Benthic iron shuttle model is based on the inverse relation of δ56Fe
relative to Fe/Al (Severmann et al., 2008) and S content (Fehr et al., 2008; 2010) (Fig.4.5).
Fig.4.4 Fe pyrite (blue circles), Fe excess (green circles) and Fe total (dashed line) in sapropels a. S5 and b. S7. Legend as in Fig.3.1
0
4
8
12
120 130 140 150 160 170
S7
Fe pyrite
Fe total
Fe excess
wt.
% F
e
Mean Depth [cm]
b
0
4
8
12
40 60 80 100 120 140
S5
Fe pyrite
Fe total
Fe excess
wt.
% F
e
Mean Depth [cm]
a
51
Background sediments (green circles) exhibit Fe/Al values of ~0.7 to 0.75 typical of other EM
sapropels (Thomson et al., 1999; Azrieli-Tal et al., 2014) and δ56Fe values (mostly 0‰ to
0.3‰) characteristic of oxic marine sediments (Beard et al., 2003). Three of the pre-sapropel
S5 sediments have δ56Fe values >0.4‰. Such high values are not typical of modern day oxic
sediments, and possibly may represent reductive removal of isotopically light Fe (as Fe2+) and
some localized post-sapropel oxidation event. Sapropels (blue circles) on the other hand,
show a close inverse correlation between Fe/Al increase and heavy isotope depletion, which
matches the benthic iron shuttle model.
The slopes of the inverse Fe/Al vs. δ56Fe trends differ for the two sapropels; the slope for
sapropel S5 is m=-0.51 while sapropel S7 slope is m=-1.7. Nevertheless, they both exhibit
same maximum δ56Fe depletion (δ56Fe= -0.72‰).
The calculated slope of the Fe/Al vs. δ56Fe relationship for euxinic Black Sea sediments is
m=-0.69 (Fig.4.6; Severmann et al., 2008, supplementary2). The slope for sapropel S5 is thus
Fig.4.5 Fe/Al vs. δ56
Fe for sapropels a. S5 and b. S7, and S content vs. δ56
Fe for sapropels c. S5 and d. S7. Curve fits are presented for within sapropel samples only (i.e., not including background sediments). Clear inverse relationships exists between Fe/Al and δ
56Fe for both sapropels and for S vs.
δ56
Fe in sapropel S7. A weaker correlation (R2=0.3) is found between S and δ
56Fe for sapropel S5.
0.5
1
1.5
2
2.5
-1 -0.5 0 0.5 1
S7
sapropel
background sediments
y = 0.72 - 1.7x R2= 0.81
Fe
Fe/A
l
b
0
2.5
5
7.5
10
-1 -0.5 0 0.5 1
S5
sapropel
background sediments
y = 2.7 - 3.4x R2= 0.3
Fe
wt.
% S
c
0
2.5
5
7.5
10
-1 -0.5 0 0.5 1
S7
sapropel
background sediments
y = 2.4 - 6.4x R2= 0.72
Fe
wt.
% S
d
0.5
1
1.5
2
2.5
-1 -0.5 0 0.5 1
sapropel
background sediments
y = 0.92 - 0.51x R2= 0.73
Fe
/Al
Fe/A
l
Fe
S5a
52
0.7
0.8
0.9
1
1.1
-1 -0.5 0 0.5 1
S1 9509
sapropel
background
sediments
Fe
Fe
/Al
comparable with the Black Sea, one of the most euxinic basins in present-day marine
environments.
4.2.2. Sapropel S1 site 9509
Sapropel S1 core 9509 depth profile situated in the Nile plume shows two peaks of Fe/Al but
relatively constant values of δ56Fe across the sapropel and its enclosing sediments (Fig.3.6).
Based on the match between Fe/Al and Ti/Al peaks in the 9509 profile, these peaks were
interpreted to reflect periods of increased detrital iron-titanium mineral input (e.g., ilmenite,
FeTiO3) from the Nile River into the EM Sea (Azrieli-Tal, 2012). The continental-shelf δ56Fe
values of this sapropel samples are in correspondence to the non-sulfidic conditions, but yet
sub-anoxic, that existed in the Nile plume during sapropel formation (Bayon et al., 2013).
Consequently there is no inverse trend in Fe/Al vs. δ56Fe (Fig.4.7), since the euxinic basin
conditions for the formation of isotopically light pyrite did not exist.
Fig.4.6 Fe/Al vs. δ56
Fe for the Black Sea sedimetns. As in sapropels S5 and S7, a strong inverse relationship exists. (Severmann et al., 2008)
Fig.4.7 Fe/Al vs. δ56
Fe for sapropel S1 from core 9509. Sapropel and enclosing sediments have oxic δ56
Fe values (~0‰) implying that enrichment in Fe is not accompanied by its isotopic fractionation. Fe/Al data are from Azrieli-Tal (2012).
53
4.2.3. Comparison with Sapropel S1 at site ODP967
As opposed to the sub-oxic conditions at the Nile plume (core 9509), weak sulfidic conditions
prevailed during the peak of sapropel S1 at site ODP967 south to Cyprus (Azrieli-Tal et al.,
2014). The EF plot for this sapropel (Fig.4.8) shows that MoEF/UEF has not completely reached
the SW line and only the samples with the highest Mo/U ratios can be considered sulfidic. This
is in direct contrast with sapropels S5 and S7 where the sapropels plotted along the SW line.
The Fe isotope profile for sapropel S1 was only measured in the lower part of the sapropel
where a distinct peak in Fe/Al occurs in the depth profile (Fig.4.9). Similar peaks in S, Mo/Al,
U/Al and V/Al in the lower part of the sapropel point to maximum sapropel conditions. The Fe
isotope data shows a decrease to a minimum of δ56Fe = -0.28‰ at maximum Fe/Al (~126cm;
Fe/Al=1.3).
On the Fe/Al vs. δ56Fe plot, the sapropel shows a weak trend of δ56Fe with Fe/Al enrichment,
(Fig.4.10). This weak trend was interpreted to reflect the benthic iron shuttle, where weakly
sulfidic bottom water conditions developed at the Fe/Al peak, allowing the formation of
isotopically depleted pyrite to form.
Fig.4.10 Fe/Al vs. δ
56Fe for sapropel S1 (<63µm
fraction) at ODP967. The sapropel shows a weak inverse relationship of δ
56Fe depletion with
increasing Fe/Al concentrations. Fe/Al data and isotopic data are taken from Azrieli-Tal (2012).
Fig.4.9 Fe/Al and δ56
Fe profile for sapropel S1 (<63 µm fraction) at ODP967 taken from Azrieli-Tal (2012; et al., 2014). Legend as in Fig.3.5. Iron maximum value is correlated with minimum measured δ
56Fe for this sequence,
at 126cm (Fe/Al=1.33; δ56
Fe= -0.28‰).
Fig.4.8 Enrichment factor plot of Mo vs. U for sapropel S1 (<63µm fraction) at ODP967 site. (Azrieli-Tal et al., 2014)
0.6
0.8
1
1.2
1.4
-1 -0.5 0 0.5 1
S1
sapropel
y = 0.77 - 1.8x R2= 0.66
Fe
Fe/A
l
0.6
0.8
1
1.2
1.4
-1
-0.5
0
0.5
1
80 100 120 140 160
S1
Fe/A
l d5
6Fe
Mean Depth [cm]
54
0.5
1
1.5
2
-1 -0.5 0 0.5
Fe/A
l
δ56Fe (‰)
reconstructing redox conditions
sapropel S1 9509
sapropel S1
Black Sea
sapropel S5
sapropel S7
4.2.4. Reconstruction of redox conditions using Fe/Al vs. δ56Fe correlations
In order to assess whether reconstruction of past redox conditions is possible by Fe isotopic
study, the Fe/Al vs. δ56Fe trends for all the sapropels discussed in this study are plotted in
Fig.4.11, together with the original Black Sea trend of Severmann et al. (2008).
The data for sapropel S1 at site 9509 sets a baseline for non-euxinic bottom water conditions.
All other sapropels show inverse Fe/Al vs. δ56Fe trends. The strongest trends in isotopic
teems (i.e., maximum depletion in Fe isotopes for lowest Fe/Al change) are those for S5 and
the Black Sea. Thus, in terms of the benthic iron shuttle, euxinic conditions in sapropel S5
were equal to those of the Black Sea. The longer trend of S5 most probably reflects much
larger supply of Fe from the EM continental shelf compared to the highly restricted Black Sea
with a narrow shelf area. As noted by Raiswell and Canfield (2012), the flux of benthic iron is
dependent on shelf area relative to that of the basin. Sapropel S7 also shows a strong euxinic
Fig.4.11 Fe/Al vs. δ56
Fe for within sapropels S7, S5, S1 (ODP967) boundaries, sapropel S1 core 9509 and the Black Sea. Data for S7, S5 and S1 (9509) are from this study; sapropel S1 at ODP967 from Azrieli-Tal et al. (2014); the Black Sea data taken from Severmann et al (2008).
55
trend, though the steeper negative slope suggests that the euxinic conditions were weaker
than S5 or the Black Sea where lighter Fe isotopic compositions are found for similar values of
Fe/Al. The high values of Fe/Al in S7 suggest that the supply of Fe to the EM was much higher
during this sapropel event, particularly at its peak. Remarkably, S1 at ODP967 site follows the
same trend as S7, but only overlaps with the weaker parts (low δ56Fe) of the S7 signal,
consistent with the proposal that only weaker euxinic conditions took place.
Thus, it appears that the benthic iron shuttle model works well for sapropels and enables
euxinic conditions to be defined during sapropel growth. Moreover, the slope and extent of
the linear inverse relation between Fe/Al and δ56Fe enables assessment of the relative
strength of the euxinic conditions.
56
5. Conclusions
Two main conclusions summarize the findings of this study:
1. Marine redox conditions during sapropels S5 and S7 formation
Based on the elevated TOC, Fe and S and RSTE (except Mn) concentrations compared to
background levels in enclosing non-sapropel sediments, and the trends of MoEF vs. UEF cross
plots, well-developed anoxic-sulfidic bottom water conditions existed during the deposition of
sapropels S5 and S7. The specific characteristics of these reducing conditions, however, differ
between the two sapropels. Sapropel S7 shows a gradual development of reducing conditions
from sub-oxic to anoxic conditions in the underlying sediments to peak euxinic conditions in
the upper sapropel (~140cm), followed by a collapse from peak conditions within the
sapropel, until final cessation of sapropel formation at ~133cm. This sapropel collapse
represents a genuine weakening of S7 in the uppermost third of the sapropel and does not
represent post-sapropel reventilation. The pyrite in this sapropel is mainly in the form of
framboids indicating relatively short time formation. On the other hand, the pyrite found in
sapropel S5 is mostly euhedral crystals which suits a longer time for pyrite mineral
formations. In contrast to S7, the cessation of maximum reducing conditions is sharper for
this sapropel, occurring in the topmost few centimeters, and is consistent with a more
extensive period of bottom water euxinia.
2. Iron and its isotopic composition in the different sapropels
Iron is found in elevated amounts in sapropels (ODP967 S1, S5 and S7 and core 9509 S1). In
deep water (~2550m) sapropels from core ODP967, Fe enrichment is simultaneous with
δ56Fe decrease and follows the benthic iron shuttle model of Severmann et al. (2008). In the
intermediate depth (~900m) Nile Fan sapropel S1 (core 9509) sub-oxic to anoxic, but not
sulfidic, conditions persisted and hence it only bears an oxic continental-shelf Fe isotopic
composition (δ56Fe=~0‰). Both sapropels S5 and S7 give same minimum δ56Fe value (δ56Fe=
-0.72‰) at their Fe/Al peak, though the sapropel S7 peak (Fe/Al =2.4) is about 1.7 times that
of sapropel S5 Fe peak (Fe/Al = 1.3). This weaker depletion trend for sapropel S7 indicates
weaker reducing conditions than those that existed during sapropel S5 formation. Moreover,
sapropel S1 from same location shows a similar trend to the one of S7 but exhibits higher
δ56Fe (δ56Fe=-0.28‰), implying for even weaker reducing conditions compared to sapropels
S5 and S7. The euxinic sediments of the Black Sea, considered to be among the most strongly
57
sulfidic developed in today's oceans show a similar Fe/Al vs. δ56Fe trend to that of S5,
emphasizing the fact that sapropel S5 is the most strongly developed of recent sapropels and
an analog for intense organic carbon rich sediment formation.
58
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63
7. Supplementary
1. Beard et al., (2003) reports Fe isotope data in reference to terrestrial igneous rocks with
(δ56Feir= 0.00 ±0.05‰), whereas IRMM-014 is δ56Feir= -0.09 ±0.05‰ relative to the
reference. Conversion of data was done by adding 0.09. SD was calculated with the
equation sqrt (SD12 + SD2
2 +…+ SDn2).
2. Data for samples is from supplementary information to Severmann et al., (2008) article.
δ56Fe values were given normalized to igneous rocks and converted to IRMM-014 by
adding 0.09‰.
64
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3 2
.48
4.0
1 -
93
3 1
46
36
3 1
08
55
1
48
14
8 2
62
10
1 8
4
38
.9
52
3
3.6
4
5.9
7
.8
10
6 1
07
10
6.5
7
.1
5.6
7
.2
1.8
1 0
.69
1.5
9 2
.61
5.0
-
99
2 1
57
49
5 1
04
47
.0
13
7 1
48
23
1 1
00
91
2
7.5
5
2
21
.2
42
.5
7.0
11
1 1
12
11
1.5
4
.66
3.7
0 1
4.9
2
.09
0.9
8 1
.13
1.1
8 1
.05
- 1
52
9 2
18
16
31
94
3
4.2
1
25
10
3 2
59
11
4 2
12
10
.2
31
.0
5.0
4
1.0
5
.7
11
6 1
17
11
6.5
4
.58
3.2
9 1
6.7
1
.68
0.6
8 0
.68
2.6
0 1
.59
- 1
68
0 1
59
19
10
85
3
6.3
9
2
10
0 1
83
12
6 1
06
10
.8
16
.2
4.4
1 4
3.1
6
.4
12
1 1
22
12
1.5
4
.30
3.0
0 2
0.5
2
.06
0.4
84
0.4
87
3.4
1 0
.86
- 2
23
4 1
32
33
46
76
1
2.6
8
1
83
2
02
12
6 1
03
6.1
1
7.5
2
.55
45
.0
6.8
12
6 1
27
12
6.5
3
.44
2.6
1 2
1.1
1
.80
0.1
58
0.4
27
3.2
7 1
.10
- 1
56
7 9
3
38
13
62
2
8.1
4
8
96
1
38
12
6 8
9
16
.0
14
.4
4.0
5 4
5.8
7
.7
13
1 1
32
13
1.5
4
.14
2.8
9 1
6.5
2
.09
0.6
6 0
.82
2.3
0 1
.10
- 1
59
6 1
21
73
0 8
3
23
.0
10
1 8
8
16
2 1
25
14
4 1
6.1
2
0.5
7
.2
44
.9
3.4
6
Table S1. Chemical composition of sapropels S5 and S7. .
Ta
ble
S1
.
Ch
emic
al c
om
po
siti
on
of
sap
rop
els
S5 a
nd
S7
.
65
a(b
) re
plic
ate
s o
f sa
mp
les
that
wer
e ch
emic
ally
dig
est
ed s
ep
arat
ely.
*
ele
men
ts m
easu
res
wit
h IC
P-M
S . A
ll o
ther
ele
men
ts w
ere
mea
sure
d w
ith
ICP
-OES
.
to
p
de
pth
b
ott
om
d
ep
th
m
ean
d
ep
th
Al
Fe
C
a
Mg
N
a K
P
S Ti
M
n
B
a Sr
Cr
C
o
Ni
V
B
Zn
Cu
*
As*
Rb
*
Mo
*
Pb
*
U*
cm
cm
cm
wt.
%
wt.
%
wt.
%
wt.
%
wt.
%
wt.
%
wt.
%
wt.
%
wt.
%
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
pp
m
Sap
rop
el S
7 O
DP
96
7D
-
12
4 1
25
12
4.5
8
.1
5.4
1
1.7
2
.07
1.8
4 1
.62
- 0
.313
5
.6
91
1 4
05
55
1 1
36
38
.5
95
1
36
- 1
17
80
2
.90
65
1
.66
15
.8
3.9
7
12
9 1
30
12
9.5
6
.6
4.4
4 1
5.5
1
.82
1.7
5 1
.40
- 0
.423
4
.91
15
61
64
7 6
40
11
6 4
8.1
1
01
14
5 -
10
7 1
09
3.6
3 5
3
1.2
4 1
3.3
6
.8
13
1a
13
2 1
31
.5
7.4
4
.86
14
.4
2.0
8 1
.83
1.5
2 -
0.7
3 5
.1
11
53
81
1 6
12
12
6 4
6.8
1
03
15
0 -
11
8 9
2
4.2
4 6
9
1.2
6 1
3.6
7
.6
13
1b
“
“ 7
.0
4.6
8 1
4.1
1
.85
1.7
0 1
.47
- 0
.76
5.0
1
12
0 6
72
59
8 1
22
48
.7
10
5 1
43
- 1
09
10
4 3
.41
53
1
.18
12
.9
7.5
13
3 1
34
13
3.5
5
.3
3.8
9 1
7.3
1
.58
1.7
3 0
.63
- 3
.57
3.6
6 1
11
8 2
45
67
9 1
13
51
1
17
18
3 -
98
1
09
11
.1
20
.9
3.2
4 4
.25
10
.0
13
4 1
35
13
4.5
6
.0
4.9
9 1
8.4
1
.95
2.1
6 0
.57
- 2
.04
3.8
3 1
18
7 3
13
76
6 1
27
60
2
06
34
4 -
11
0 1
17
28
.9
16
.6
12
.3
2.2
0 1
7.1
13
5 1
36
13
5.5
5
.6
5.1
1
6.8
1
.96
2.2
1 0
.55
- 3
.45
3.4
6 1
16
3 5
91
67
0 1
19
70
2
06
39
7 -
10
5 1
04
41
.8
17
.1
31
.7
2.7
9 2
0.8
13
6 1
37
13
6.5
4
.71
5.1
1
8.0
1
.79
1.8
6 0
.48
- 6
.7
2.8
7 1
13
3 7
99
66
3 1
06
65
2
09
40
6 -
91
8
1
46
.3
14
.4
54
1
.02
21
.9
13
7 1
38
13
7.5
5
.3
9.6
1
5.7
2
.08
2.3
7 0
.55
- 3
.56
3.1
1 1
64
3 6
36
62
4 1
23
89
2
57
37
7 -
10
7 9
9
80
1
6.0
1
23
1.2
5 3
5.8
13
8 1
39
13
8.5
4
.28
7.3
1
6.1
1
.64
1.9
0 0
.430
-
6.3
2
.54
12
36
72
8 6
03
96
6
6
18
0 3
10
- 9
4
91
8
7
12
.5
13
6 2
.46
28
.4
13
9 1
40
13
9.5
4
.80
8.2
1
5.6
1
.85
2.0
1 0
.63
- 6
.3
2.8
1 1
22
0 8
37
58
3 1
07
66
2
20
32
2 -
10
8 1
11
99
1
8.7
1
84
2.0
2 3
1.7
14
0 1
41
14
0.5
4
.05
9.7
1
5.2
1
.56
1.7
8 0
.83
- 8
.5
2.3
9 1
09
6 4
56
55
7 9
0
38
.7
13
9 2
81
- 9
2
78
1
77
26
.3
20
3 5
.26
30
.4
14
1 1
42
14
1.5
4
.95
7.1
1
6.8
1
.87
2.0
3 0
.53
- 6
.4
2.8
7 1
33
5 2
43
65
7 1
02
51
1
38
33
7 -
98
9
9
71
1
3.9
1
63
1.4
3 3
2.8
14
2 1
43
14
2.5
4
.82
6.7
1
6.3
1
.81
2.0
5 0
.62
- 5
.8
2.8
2 1
41
5 3
20
65
8 1
00
41
.2
13
8 3
05
- 1
00
88
7
4
19
.8
14
6 3
.62
38
.0
14
3 1
44
14
3.5
5
.5
6.6
1
6.8
2
.05
2.3
8 0
.58
- 4
.28
3.1
7 1
71
9 2
61
70
2 1
13
58
1
57
36
9 -
11
0 9
3
59
1
5.6
1
40
0.6
1 3
3.6
14
4 1
45
14
4.5
5
.7
6.0
1
6.2
2
.10
2.3
7 0
.64
- 2
.71
3.3
2 1
61
4 7
68
71
7 1
19
46
.7
15
4 4
12
- 1
14
12
7 4
4.8
1
7.0
1
51
1.8
0 3
3.9
14
5a
14
6 1
45
.5
6.2
5
.8
16
.5
2.1
1 2
.50
0.7
0 -
2.8
7 3
.71
16
05
47
5 7
18
12
0 4
2.6
1
52
38
3 -
10
9 1
06
33
.1
23
.3
10
7 3
.53
29
.6
14
5b
“
“ 6
.1
5.7
1
6.0
2
.08
2.4
8 0
.66
- 2
.73
3.6
4 1
58
4 3
15
70
1 1
16
51
1
54
37
3 -
10
8 9
3
32
.4
20
.8
10
9 2
.59
28
.9
14
6 1
47
14
6.5
6
.1
5.5
1
6.7
1
.99
2.3
0 0
.66
- 2
.49
3.7
4 1
62
2 3
37
70
4 1
25
45
.2
14
5 3
08
- 1
07
87
3
2.0
2
0.2
8
9
4.3
4 2
3.9
14
7 1
48
14
7.5
5
.5
5.0
1
7.9
1
.79
1.9
3 0
.55
- 4
.19
3.2
8 1
37
8 2
86
71
7 1
06
46
.3
12
7 2
27
- 9
1
11
4 2
5.2
1
6.3
6
9
1.8
1 2
0.5
14
8 1
49
14
8.5
5
.6
5.3
1
8.0
1
.81
2.1
1 0
.58
- 3
.64
3.3
5 1
48
8 6
41
75
2 1
10
40
.9
13
3 2
64
- 9
4
80
4
2.3
1
6.2
8
1
1.2
3 2
0.9
14
9 1
50
14
9.5
5
.1
6.4
1
6.7
1
.57
1.8
1 0
.68
- 5
.7
2.9
8 1
25
2 1
00
65
4 9
0
43
.6
12
2 2
31
- 8
5
77
9
8
27
.7
91
7
.4
24
.5
15
0 1
51
15
0.5
6
.7
6.4
1
3.9
1
.90
2.1
0 1
.15
- 2
.73
3.9
6 1
30
0 3
42
62
9 1
17
49
.0
15
7 2
91
- 1
01
11
2 9
5
45
.9
99
1
0.8
1
7.1
15
1 1
52
15
1.5
7
.0
6.2
1
4.2
2
.06
2.2
3 1
.16
- 2
.48
4.0
7 1
41
2 3
00
66
9 1
29
50
1
44
37
6 -
10
5 9
6
77
4
4.0
7
6
6.6
1
7.7
15
2 1
53
15
2.5
6
.8
5.9
1
3.7
2
.00
2.1
4 1
.05
- 2
.93
4.0
0 5
97
3 2
29
65
5 1
70
47
.6
12
6 4
19
- 1
06
10
9 6
1
41
.1
41
.0
9.8
1
2.2
15
3a
15
4 1
53
.5
7.2
7
.0
10
.7
1.9
9 1
.78
1.3
4 -
3.2
8 5
.2
216
31
21
1 4
93
14
9 4
5.9
9
8
25
6 -
11
0 9
1
78
6
1
45
.6
11
.3
9.7
15
3b
“
“ 7
.3
6.8
1
0.9
2
.00
1.8
1 1
.39
- 2
.83
5.4
22
313
3
18
50
6 1
50
50
9
8
27
2 -
11
4 8
6
66
5
7
44
.4
12
.2
9.9
15
5 1
56
15
5.5
9
.6
7.0
3
.35
1.9
4 2
.00
1.7
0 -
1.2
6 8
.2
13
45
23
4 2
26
13
9 5
9
92
2
07
- 1
20
65
2
1.1
8
0
23
.9
13
.2
10
.6
16
0 1
61
16
0.5
9
.2
6.9
4
.24
1.9
5 1
.98
1.6
5 -
1.4
8 8
.7
14
79
25
6 2
68
13
8 4
8.4
9
5
18
9 -
12
2 5
8
21
.4
77
1
5.9
1
3.2
9
.7
16
5 1
66
16
5.5
8
.3
6.7
4
.86
1.8
1 1
.90
1.5
8 -
1.3
0 9
.0
15
01
28
8 3
00
13
6 6
0
89
1
95
- 1
11
56
1
9.6
5
2
7.6
1
2.3
7
.3
66
top
d
ep
th
bo
tto
m
de
pth
m
ean
d
ep
th
cm
cm
cm
Fe
Ca
Mg
Na
K
P
Ti
Mn
B
a†
Sr
†
Cr†
C
o†
Ni†
V†
B†
Zn†
Cu†
As†
Rb†
Mo†
Pb†
U†
Sap
rop
el S
5 O
DP
96
7C
-
51
5
2
51
.5
0.6
9 1
.2
0.2
9 0
.12
0.2
5
0.3
1 -
11
4 2
5
92
1
6
6.0
2
1
20
3
7
16
2
0
0.6
6
.8
0.2
0 8
.0
0.5
8
56
5
7
56
.5
0.7
0 0
.50
0.2
1 0
.11
0.2
1
0.2
9 -
70
1
8
37
1
5
4.5
1
8
16
3
5
13
8
.8
0.4
5
.6
0.1
6 5
.9
0.5
2
61
6
2
61
.5
0.7
7 0
.39
0.1
9 0
.10
0.1
8
0.3
1 -
60
1
7
26
1
6
4.5
1
8
18
3
9
12
7
.9
0.6
3
.7
0.2
1 5
.0
0.5
5
66
6
7
66
.5
0.7
1 0
.24
0.1
9 0
.11
0.1
9
0.2
8 -
62
2
0
21
1
4
6.1
1
8
20
3
3
11
6
.9
3.0
6
.0
0.9
9 5
.1
0.8
1
71
7
2
71
.5
0.8
9 1
.8
0.3
4 0
.20
0.2
7
0.5
1 -
39
3 3
5
12
5 1
9
6.0
2
8
24
4
8
20
1
9
2.3
8
.3
1.1
7
.3
1.5
76
7
7
76
.5
1.1
1
.3
0.2
9 0
.27
0.2
0
0.4
0 -
14
3 1
20
70
1
6
5.2
3
8
71
6
1
16
1
8
12
8
.1
18
5
.9
5.1
78
7
9
78
.5
1.2
3
.2
0.3
8 0
.50
0.1
7
0.5
1 -
23
8 2
55
15
5 2
1
10
4
7
12
4 6
8
26
4
0
18
4
.1
37
1
1
9.1
80
8
1
80
.5
1.2
2
.7
0.3
6 0
.43
0.1
6
0.4
1 -
23
6 2
05
13
7 2
1
8.7
4
5
11
0 6
4
24
3
1
14
3
.1
28
8
.5
8.4
82
8
3
82
.5
1.0
2
.5
0.4
1 0
.48
0.1
5
0.1
6 -
23
1 3
48
13
1 2
3
11
4
4
11
6 6
1
24
4
6
10
2
.6
25
8
.1
9.2
84
8
5
84
.5
1.1
2
.5
0.3
8 0
.37
0.2
0
0.2
3 -
20
5 8
0
11
4 1
9
7.8
3
6
70
6
0
21
4
1
19
6
.9
38
7
.6
6.9
86
8
7
86
.5
1.1
2
.6
0.3
9 0
.40
0.1
7
3.0
4 -
22
8 2
44
14
1 2
2
9.0
4
1
10
3 6
1
25
3
2
9.1
4
.0
30
8
.1
21
88
8
9
88
.5
1.0
2
.3
0.4
2 0
.39
0.1
3
0.3
0 -
19
8 6
6
11
7 2
1
9.5
3
8
71
6
2
21
3
9
7.8
2
.8
31
7
.9
8.4
90
9
1
90
.5
1.0
2
.3
0.3
6 0
.40
0.1
9
0.5
5 -
19
7 2
18
11
9 2
1
9.1
4
3
60
4
8
22
2
8
7.2
4
.6
35
7
.7
8.4
92
9
3
92
.5
1.3
1
.9
0.3
4 0
.40
0.2
3
0.1
9 -
18
1 2
79
10
7 2
0
8.9
4
3
61
6
8
19
3
5
12
6
.8
44
8
.3
7.9
94
9
5
94
.5
0.8
3 1
.3
0.2
9 0
.27
0.2
5
0.3
4 -
19
4 1
52
86
1
8
6.7
3
0
50
5
9
17
2
0
2.8
6
.5
14
5
.9
4.6
96
9
7
96
.5
0.8
3 2
.2
0.3
1 0
.31
0.1
6
0.3
8 -
18
6 2
14
11
9 1
8
8.9
4
0
54
4
8
20
2
8
4.4
3
.8
24
6
.8
6.1
98
9
9
98
.5
1.0
2
.3
0.4
0 0
.33
0.1
8
0.5
6 -
24
3 2
03
14
7 2
2
8.4
4
5
52
7
5
25
2
9
7.4
4
.6
24
7
.4
5.8
10
0 1
01
10
0.5
0
.93
1.7
0
.33
0.2
5 0
.23
0
.23
- 1
76
10
3 1
28
19
8
.6
38
5
8
48
1
9
32
1
2
7.1
2
1
6.9
3
.6
10
2 1
03
10
2.5
1
.0
1.8
0
.35
0.2
1 0
.28
0
.09
- 5
91
42
1
28
26
9
.3
35
4
6
70
2
1
46
8
.9
8.2
7
.6
7.7
1
.9
10
4 1
05
10
4.5
0
.82
0.7
2 0
.25
0.0
8 0
.23
0
.33
- 1
24
19
4
8
14
7
.3
20
2
0
35
1
3
11
5
.2
6.9
4
.5
6.1
1
.0
10
6 1
07
10
6.5
0
.79
1.0
0
.26
0.1
0 0
.22
0
.37
- 1
40
22
7
0
15
6
.6
19
2
1
33
1
4
13
3
.9
7.3
3
.0
6.0
0
.99
11
1 1
12
11
1.5
0
.79
3.2
0
.45
0.2
1 0
.24
0
.25
- 3
28
47
3
50
20
7
.3
27
2
2
56
2
4
46
2
.2
6.7
1
.1
8.8
1
.2
11
6 1
17
11
6.5
0
.72
3.6
0
.37
0.1
5 0
.15
0
.57
- 3
67
35
4
17
19
7
.9
20
2
2
40
2
8
23
2
.3
3.5
1
.0
9.4
1
.4
12
1 1
22
12
1.5
0
.70
4.8
0
.48
0.1
1 0
.11
0
.79
- 5
19
31
7
78
18
2
.9
19
1
9
47
2
9
24
1
.4
4.1
0
.59
10
1
.6
12
6 1
27
12
6.5
0
.76
6.1
0
.52
0.0
5 0
.12
0
.95
- 4
55
27
1
10
8 1
8
8.2
1
4
28
4
0
37
2
6
4.7
4
.2
1.2
1
3
2.2
13
1 1
32
13
1.5
0
.70
4.0
0
.50
0.1
6 0
.20
0
.56
- 3
85
29
1
76
20
5
.5
24
2
1
39
3
0
35
3
.9
4.9
1
.7
11
0
.83
Ta
ble
S2
Ch
emic
al c
om
po
siti
on
of
sap
rop
els
S5 a
nd
S7
no
rmal
ized
to
Al (
X/A
l).
Dat
a so
urc
e Ta
ble
S1
67
top
d
ep
th
bo
tto
m
de
pth
m
ean
d
ep
th
cm
cm
cm
Fe
Ca
Mg
Na
K
P
Ti
Mn
B
a† Sr
† C
r†
Co†
Ni†
V†
B†
Zn†
Cu†
As†
Rb†
Mo†
Pb†
U†
Sap
rop
el S
7 O
DP
96
7D
-
12
4 1
25
12
4.5
0
.67
1.4
0
.26
0.2
3 0
.20
-
0.6
9 1
12
50
6
8
17
4
.8
12
1
7
- 1
4
9.8
0
.36
8.0
0
.21
1.9
0
.49
12
9 1
30
12
9.5
0
.67
2.3
0
.28
0.2
6 0
.21
-
0.7
4 2
35
98
9
7
17
7
.3
15
2
2
- 1
6
16
0
.55
8.0
0
.19
2.0
1
.0
13
1a
13
2 1
31
.5
0.6
5 1
.9
0.2
8 0
.25
0.2
0
- 0
.69
15
5 1
09
82
1
7
6.3
1
4
20
-
16
1
2
0.5
7 9
.2
0.1
7 1
.8
1.0
13
1b
"
" 0
.67
2.0
0
.26
0.2
4 0
.21
-
0.7
1 1
59
96
8
5
17
6
.9
15
2
0
- 1
6
15
0
.48
7.6
0
.17
1.8
1
.1
13
3 1
34
13
3.5
0
.73
3.3
0
.30
0.3
2 0
.12
-
0.6
9 2
10
46
1
28
21
9
.6
22
3
4
- 1
8
20
2
.1
3.9
0
.61
0.8
0 1
.9
13
4 1
35
13
4.5
0
.83
3.1
0
.33
0.3
6 0
.10
-
0.6
4 1
98
52
1
28
21
1
0
34
5
7
- 1
8
19
4
.8
2.8
2
.1
0.3
7 2
.8
13
5 1
36
13
5.5
0
.92
3.0
0
.35
0.4
0 0
.10
-
0.6
2 2
09
10
6 1
20
21
1
3
37
7
1
- 1
9
19
7
.5
3.1
5
.7
0.5
0 3
.7
13
6 1
37
13
6.5
1
.1
3.8
0
.38
0.3
9 0
.10
-
0.6
1 2
41
17
0 1
41
22
1
4
44
8
6
- 1
9
17
9
.8
3.1
1
2
0.2
2 4
.7
13
7 1
38
13
7.5
1
.8
3.0
0
.40
0.4
5 0
.10
-
0.5
9 3
12
12
1 1
19
23
1
7
49
7
2
- 2
0
19
1
5
3.0
2
3
0.2
4 6
.8
13
8 1
39
13
8.5
1
.7
3.8
0
.38
0.4
4 0
.10
-
0.5
9 2
89
17
0 1
41
22
1
6
42
7
3
- 2
2
21
2
0
2.9
3
2
0.5
8 6
.6
13
9 1
40
13
9.5
1
.7
3.2
0
.38
0.4
2 0
.13
-
0.5
8 2
54
17
4 1
21
22
1
4
46
6
7
- 2
3
23
2
1
3.9
3
8
0.4
2 6
.6
14
0 1
41
14
0.5
2
.4
3.8
0
.39
0.4
4 0
.21
-
0.5
9 2
71
11
3 1
38
22
9
.6
34
6
9
- 2
3
19
4
4
6.5
5
0
1.3
7
.5
14
1 1
42
14
1.5
1
.4
3.4
0
.38
0.4
1 0
.11
-
0.5
8 2
70
49
1
33
21
1
0
28
6
8
- 2
0
20
1
4
2.8
3
3
0.2
9 6
.6
14
2 1
43
14
2.5
1
.4
3.4
0
.38
0.4
2 0
.13
-
0.5
9 2
94
66
1
37
21
8
.6
29
6
3
- 2
1
18
1
5
4.1
3
0
0.7
5 7
.9
14
3 1
44
14
3.5
1
.2
3.1
0
.37
0.4
3 0
.10
-
0.5
8 3
13
48
1
28
21
1
1
29
6
7
- 2
0
17
1
1
2.8
2
5
0.1
1 6
.1
14
4 1
45
14
4.5
1
.0
2.8
0
.37
0.4
1 0
.11
-
0.5
8 2
82
13
4 1
25
21
8
.2
27
7
2
- 2
0
22
7
.8
3.0
2
6
0.3
1 5
.9
14
5a
14
6 1
45
.5
0.9
3 2
.7
0.3
4 0
.40
0.1
1
- 0
.60
25
8 7
6
11
5 1
9
6.8
2
4
61
-
18
1
7
5.3
3
.7
17
0
.57
4.8
14
5b
"
" 0
.93
2.6
0
.34
0.4
0 0
.11
-
0.5
9 2
58
51
1
14
19
8
.4
25
6
1
- 1
8
15
5
.3
3.4
1
8
0.4
2 4
.7
14
6 1
47
14
6.5
0
.91
2.7
0
.33
0.3
8 0
.11
-
0.6
2 2
68
56
1
16
21
7
.5
24
5
1
- 1
8
14
5
.3
3.3
1
5
0.7
2 3
.9
14
7 1
48
14
7.5
0
.92
3.3
0
.33
0.3
5 0
.10
-
0.6
0 2
52
52
1
31
19
8
.5
23
4
2
- 1
7
21
4
.6
3.0
1
3
0.3
3 3
.7
14
8 1
49
14
8.5
0
.94
3.2
0
.32
0.3
7 0
.10
-
0.6
0 2
64
11
4 1
33
20
7
.3
24
4
7
- 1
7
14
7
.5
2.9
1
4
0.2
2 3
.7
14
9 1
50
14
9.5
1
.3
3.3
0
.31
0.3
5 0
.13
-
0.5
8 2
45
20
1
28
18
8
.5
24
4
5
- 1
7
15
1
9
5.4
1
8
1.4
4
.8
15
0 1
51
15
0.5
0
.95
2.1
0
.28
0.3
1 0
.17
-
0.5
9 1
93
51
9
3
17
7
.3
23
4
3
- 1
5
17
1
4
6.8
1
5
1.6
2
.5
15
1 1
52
15
1.5
0
.89
2.0
0
.29
0.3
2 0
.17
-
0.5
8 2
01
43
9
5
18
7
.2
21
5
4
- 1
5
14
1
1
6.3
1
1
0.9
4 2
.5
15
2 1
53
15
2.5
0
.88
2.0
0
.30
0.3
2 0
.16
-
0.5
9 8
84
34
9
7
25
7
.0
19
6
2
- 1
6
16
9
.0
6.1
6
.1
1.4
1
.8
15
3a
15
4 1
53
.5
0.9
8 1
.5
0.2
8 0
.25
0.1
9
- 0
.73
30
07
29
6
9
21
6
.4
14
3
6
- 1
5
13
1
1
8.4
6
.3
1.6
1
.3
15
3b
"
" 0
.93
1.5
0
.28
0.2
5 0
.19
-
0.7
4 3
06
5 4
4
69
2
1
6.9
1
3
37
-
16
1
2
9.0
7
.9
6.1
1
.7
1.4
15
5 1
56
15
5.5
0
.73
0.3
5 0
.20
0.2
1 0
.18
-
0.8
6 1
40
24
2
4
14
6
.2
9.6
2
2
- 1
3
6.8
2
.2
8.3
2
.5
1.4
1
.1
16
0 1
61
16
0.5
0
.75
0.4
6 0
.21
0.2
1 0
.18
-
0.9
4 1
60
28
2
9
15
5
.2
10
2
1
- 1
3
6.2
2
.3
8.4
1
.7
1.4
1
.0
16
5 1
66
16
5.5
0
.81
0.5
9 0
.22
0.2
3 0
.19
-
1.1
1
82
35
3
6
16
7
.3
11
2
4
- 1
3
6.8
2
.4
6.3
0
.93
1.5
0
.88
† E
lem
en
ts w
ere
div
ide
d b
y 1
04
68
sample name TOC Age
cm wt.% ka
Sapropel S5 ODP967C-
1H-5, 68-69 0.7 117.13
1H-5, 69-70 0.6 117.35
1H-5, 70-71 0.5 117.58
1H-5, 71-72 0.3 117.81
1H-5, 72-73 0.8 118.03
1H-5, 73-74 0.6 118.26
1H-5, 74-75 3.2 118.49
1H-5, 75-76 2.9 118.71
1H-5, 76-77 3 118.94
1H-5, 77-78 2 119.17
1H-5, 78-79 3.4 119.39
1H-5, 79-80 3.4 119.62
1H-5, 80-81 3.2 119.85
1H-5, 81-82 3.3 120.07
1H-5, 82-83 3.4 120.3
1H-5, 83-84 3 120.52
1H-5, 84-85 2.1 120.75
1H-5, 85-86 2.7 120.98
1H-5, 86-87 2.5 121.2
1H-5, 87-88 4.3 121.43
1H-5, 88-89 3.2 121.66
1H-5, 89-90 3.4 121.88
1H-5, 90-91 3 122.11
1H-5, 91-92 3.2 122.34
1H-5, 92-93 4.2 122.56
1H-5, 93-94 4.4 122.79
1H-5, 94-95 2.5 123.02
1H-5, 95-96 4.5 123.24
1H-5, 96-97 3.8 123.47
1H-5, 97-98 2.9 123.69
1H-5, 98-99 3.1 123.96
1H-5, 99-100 2.8 124.22
1H-5, 100-101 2.1 124.49
1H-5, 101-102 2 124.76
1H-5, 102-103 1.7 125.02
1H-5, 103-104 1 125.29
1H-5, 104-105 0.6 125.29
1H-5, 105-106 1.1 125.29
1H-5, 106-107 0.6 125.29
1H-5, 107-108 0.4 125.29
sample name TOC Age
cm wt.% ka
1H-5, 108-109 0.3 125.29
1H-5, 109-110 0.3 125.29
Sapropel S7 ODP967D-
2H-2, 130-131 0.8 199.83
2H-2, 131-132
200.16
2H-2, 132-133 1.1 200.5
2H-2, 133-134
200.83
2H-2, 134-135 4.2 201.17
2H-2, 135-136
201.5
2H-2, 136-137 5.4 201.84
2H-2, 137-138
202.17
2H-2, 138-139 4.9 202.51
2H-2, 139-140
202.86
2H-2, 140-141 6.1 203.21
2H-2, 141-142
203.73
2H-2, 142-143
204.25
2H-2, 143-144 7.1 204.59
2H-2, 144-145 7.1 204.92
2H-2, 145-146
205.27
2H-2, 146-147 5.6 205.62
2H-2, 147-148
205.98
2H-2, 148-149 3.2 206.33
2H-2, 149-150
206.53
2H-3, 0-1 2.8 206.73
2H-3, 1-2 2.4 207.07
2H-3, 2-3 1.2 207.4
2H-3, 3-4 0.9 207.74
2H-3, 23-25
216.8
2H-3, 25-27
217.79
2H-3, 27-28
218.74
2H-3, 29-30
219.73
Table S3 TOC and age data for sapropels S5 and S7 used in this study. Data from Emeis et al. (1998).
69
SEM pictures of sapropels S5 and S7
a. Pyrite from sapropel S5 at Fe peak (93cm). This pyrite does not have a defined shape but rather like emerging sub-crystals from a big pyrite crystal at their base. b. Pyrite crystals (bright figures) from sapropel S5 at Fe peak (93cm). This sapropel is abundant with big euhedral crystals. In sapropels the framboids are the major form and not well- shaped big crystals as seen here. c and d. Diagenetic pyrite crystals at Mn-rich underlying-sapropel S5 sediment (103cm).
a
c
b
d
70
e. A framboid coated with membrane from sapropel S7 at early small Fe peak (149.5cm). The pyrite growth was interrupted in the middle and has not completed its formation. f. A framboid from sapropel S7 at Fe peak (140.5). Not all framboids are spherical-shaped. A skeleton fossil from sapropel S7 at a minimum point between two Fe peaks (146.5cm). g. Pyrite is usually formed in the remained skeletons but this fossil does not contain pyrite. An inhibition in pyrite formation may be due to weakening of reducing conditions fallowing a reventilation event.
f e
g
המוביליערכי -דוה ו,הוא עובר למצב (Fe(III)/Fe2+ redox boundaryחיזור )-ומגיע לגבול החמצון
(Fe2+במעבר .) המחומצן הינו , כאשר הברזל בין צורוני הברזל מתרחשת פרקציונציה איזוטופית זה
הקל איזוטופית עושה את דרכו לכיוון קרקעית הים. בתנאים המומס הכבד איזוטופית ואילו הברזל
ליצירת פיריט סינגנטי ושקע בסדימנט, ונמצא כי H2Sם בהם עמודת המים היתה מחזרת, הברזל הגיב ע
מראים ערך S7 -ו S5מתקיים יחס הפוך בין העשרת ברזל להרכבו האיזוטופי הקל. שני הספרופלים
δ56Fe ( מינימליδ56Fe=-0.72‰ עבור מקסימום העשרה בברזל בסדימנט, בעוד ש )ספרופל לS1 מדלתת
טופיים של ברזל במדף יבשת לערכים איזומתאימים ה δ56Fe =0.09±0.1‰( ערכי 9509הנילוס )קידוח
.ןמחומצ
, והיא S5מגמת פרקציונצית הברזל ביחס להעשרתו בסדימנט התגלתה כחזקה ביותר עבור ספרופל
( הראו מגמה דומה ODP967) S1 -וS7 דומה למגמה שנמצאה בסדימנטים מהים השחור. ספרופלים
ונמוך S5זהה לזה של S7עבור δ56Fe , למרות שהערך המינימלי של S5ספרופל אך פחות חזקה מזו של
מלמד כי S1(. הערך הכבד יחסית עבור ספרופל δ56Fe=-0.28‰) S1באופן משמעותי מזה של ספרופל
.S7התנאים המחזרים בזמן היווצרות הספרופל היו פחות עוצמתיים מאלו של
מפותחיםשקעו תחת תנאים מחזרים S7 -ו S5ני הספרופלים תוצאות המחקר מלמדות כי למרות שש
.S7היו עוצמתיים יותר מאשר התנאים שהשתררו בזמן היווצרות S5בים העמוק, התנאים עבור
בסדימנט, מאפשרים לנו להעריך כמו כן, המחקר מראה כי מידת העשרתו של הברזל והרכבו האיזוטופי
את מידת עוצמתם של תנאים מחזרים בסביבות ימיות קדומות.
תקציר:
ים מחומצנים היטב ברזל קשור לתחמוצות ברזל, והרכבו האיזוטופי נשמר בתהליכי בליה והשקעה -במי
מחודשת בסדימנט. בתנאים של ריכוז חמצן נמוך לעומת זאת, יש מספר דרכים דרכם ברזל עובר
ממצבי החמצון השונים כתוצאהוזאת ,פרקציונציה איזוטופית לפני שהוא נשמר ברקורד הסדימנטרי
ברזל מהווה בשל כך, מצון. חהרחשת בעת המעבר בין מצבי תפית המשל ברזל והפרקציונציה האיזוטו
. כלי גיאוכימי יעיל ללימוד תנאים מחזרים של סביבות ימיות קדומות בעלות טמפרטורה נמוכה
שכבות ספרופליות י תבשהמטרה העיקרית בעבודה זו היתה ללמוד את המערכת האיזוטופית של ברזל
מטר( מדרום לקפריסין 2550עומק מים מODP967 מקידוח S7-ו S5ם ממזרח הים התיכון )ספרופלי
מטר(, והשוואתם לעבודה 900ומק מים של בע 9509מקידוח S1 וספרופל מדלתת הנילוס )ספרופל
.עשירים בחומר אורגני מהים השחור ולסדימנטים ,(ODP967)מקידוח S1קודמת על ספרופל
ספרופלים הם סדימנטים עשירים בחומר אורגני אשר שקיעתם קשורה לפרמטרים אורביטליים ובעיקר
מינימום נקיפה ובמקסימום בזמן ;', אחד ממעגלי מילנקוביץPRECESSATIONלמעגל הנקיפה
בהמיספרה הצפונית. בעקבות כך התפתחו תנאי אקלים oN 65 קרינת השמש במהלך הקיץ בקו רוחב
קיצוניים שהתבטאו במונסונים חזקים באפריקה התיכונה וירידת גשמים מסיבית באזור מזרח הים
התיכון דרך נהרות וביניהם נהר הנילוס, וגרמו לעצירת הסירקולציה -יםההתיכון. המים התנקזו אל
ים התיכון ולריבוד שכבות המים. כמו כן, המים שהגיעו מהנהרות היו עשירים ההתרמוהלינית של
שהאיצו ותרמו לפריחות פלנקטוניות, אשר צרכו חמצן ממאגר עמודת המים בעת טיםנבנוטריי
מאגר החמצן במים העמוקים לא ,קעית. בעקבות העדר סירקולציההתפרקותם תוך כדי שקיעתם לקר
תנאים אנאוקסיים תהתפתחומצב שהוביל להתחדש ומאגר החמצן אזל עם התפרקות החומר האורגני,
אשר אפשרו השקעת חומר אורגני והיווצרותם של הספרופלים.
פליות מועשרות בברזל, גופרית, (, שכבות ספרוCorg>2%מלבד אחוז החומר האורגני הגבוה בספרופלים )
