A multi-proxy approach to the paleoceanographic variability...

A multi-proxy approach to the paleoceanographic variability within the last glacial cycle offshore Morocco Cindy De Jonge 2009-2010

Promotor: Dr. David Van Rooij (UGent) Co-promotor: Dr. Alina Stadniskaia (Royal NIOZ) Guidance: Lies De Mol (UGent) Thesis submitted to obtain the degree of Master in Marine and Lacustrine sciences. (Faculty of Science)

De Jonge Cindy - A multi-proxy approach to the paleoceanography within the last glacial cycle offshore Morocco.

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1. TABLE OF CONTENTS

ABSTRACT____________________________________________________________________PAGE 3 1. INTRODUCTION ______________________________________________________________PAGE 3 2. SETTING____________________________________________________________________PAGE 4

2.1. GEOGRAPHY AND GEOLOGY 2.2. HYDROLOGY

3. MATERIAL AND METHODS ______________________________________________________PAGE 6 3.1.ON-BOARD MEASUREMENTS 3.2. SUBSAMPLING 3.3. GRAIN-SIZE ANALYSIS 3.4. STABLE ISOTOPE STRATIGRAPHY 3.5. LIPID EXTRACTION AND PREPARATION 3.6. ANALYSIS AND IDENTIFICATION OF LIPID BIOMARKERS 3.7. AGE MODEL AND MASS ACCUMULATION RATE

4. RESULTS ___________________________________________________________________PAGE 7

4.1. FRAMEWORK 4.2. TERRIGENOUS FRACTION 4.3. BIOGENIC FRACTION 4.4. MOMENT PARAMETERS 4.5. SEA SURFACE TEMPERATURES (TEX86) AND (U

K’37).

5. DISCUSSION_________________________________________________________________PAGE 12

5.1. TERRIGENOUS INPUT 5.2. CURRENT REWORKING 5.3. MD08-3227 AGE MODEL 5.4. PRIMARY PRODUCTION 5.5. SEA SURFACE TEMPERATURES 5.6. SEDIMENTARY EVIDENCE OF SHORTER-TIMESCALE FLUCTUATIONS.

6. CONCLUSIONS _______________________________________________________________PAGE 18 ACKNOWLEDGMENTS ___________________________________________________________PAGE 19 REFERENCES__________________________________________________________________PAGE 19


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ABSTRACT

A marine sediment core (MD08-3227) taken off-shore Morocco was analysed to determine the main climatic drivers on the sedimentary composition in the Pen Duick Escarpment, a mud volcano province in the Gulf of Cadiz. A multi-proxy approach, based on XRF (Fe/Al, K/Al, and K/Ti), grain size distribution (mean grain size, sorting, skewness, and kurtosis) and biomarker (TEX86, U

K’37 and BIT-index) analyses was used for this

paleoceanographical study. This contribution comprises the Holocene, the last glacial and the former interglacial, up to 130 ky ago. Glacial stages and Heinrich events are characterised by an enhanced aeolian input, coeval with a lower contribution of fluvial input. Stadial biogenic production points to the presence or enforcement of an upwelling system at the coring site. Also, the hypothesis of an enhanced bottom current during these stadials in the southern part of the Gulf of Cadiz is not rejected. The two sea surface temperature reconstructions do not proceed in parallel, but show a larger offset during stadials. This could be related to lateral transport or subsurface production of the alkenones. On shorter timescales, a large variability is observed that is probably related to Dansgaard-Oeschger oscillations.

Keywords: Gulf of Cadiz; Heinrich events; biomarker; SST (sea surface temperature); paleoclimate

_____________________________________________________________________________________ 1. INTRODUCTION

Understanding the mechanisms and the causes of abrupt climate change is one of the major chal-lenges in paleoclimate research today. Results from cores in the Atlantic Ocean at latitudes close to the Gulf of Cadiz (GoC) and the Alboran Sea in the Western Mediterranean (e.g. Sánchez-Goñi et al., 2002, 1999; Cacho et al., 1999, 2002; Moreno et al., 2002; Martrat et al., 2004) indicate a strong sensitivity of these regions to record millennial climatic and oceanographic changes. This part of the European and African continental margins in the Northeast Atlantic Ocean is of special interest for climate reconstructions as the polar front advanced southwards into this region during the Last Glacial Maximum (McIntyre et al., 1972). As the East Atlantic Boundary current delivered water with a North Atlantic signature as far as the Iberian Peninsula (Zahn, 1994), these surface waters are very sensitive to oceanographic changes at higher latitudes.

Several studies regarding late Quaternary climate have been conducted in the northern part of the GoC. Terrigenous input was observed to be independent of temperature fluctuations, but linked to the arid versus humid conditions on the continent (e.g. Zazo et al., 2008). In the Lower Pliocene already, summer insolation maxima are found to coincide with humid continental condi-tions and high fluvial run-off (Sierro et al., 2000). The insolation-dependent North Atlantic Oscillation (NAO)-index is known to cause rapid hydrological variations in the Mediterranean Sea (Goni et al., 2002; Moreno et al., 2004), and its imprint on GoC sediments is confirmed by Mertens (2009). Sea surface temperature, turbidity and paleoproductivity changes in the northern GoC during the last 50 ka are attributed to Heinrich events and Dansgaard-Oeschger oscillations (Cacho et al., 2001; Colmenero-

Hidalgo et al., 2004). In the southern part of the GoC (roughly from 35° until 34°N), paleoceanographic studies are rare (Marret and Turon, 1994; Mertens, 2009). It is very hard to generalise conclusions obtained from the northern part, as the oceanography of the site is very different. There is at the moment no presence of Mediterranean Outflow Water (MOW) measured (Van Rooij et al., submitted), with surface waters more dominated by a trade wind controlled upwelling system. However, the southern part of the GoC is unique as its harbours cold water corals (Wienberg et al., 2009). The development of a drift-sequence betrays the presence of a strong bottom current (Van Rooij et al., submitted). A recent study of Mertens (2009), performed a paleoceanographic reconstruction down to Heinrich event 4 on a core 16.3 km to the NW of our study site. NW African continental climate conditions are reconstructed from marine sediment cores by Hooghiemstra et al. (1992) representing the period from about 250 ka to 5 ka BP (between 38° and 29°N). Most paleoclimatic studies offshore NW Africa focus on more subtropical latitudes (generally between about 36° to 24°N). Our study site is thus situated in between two intensively studied areas.

Now, a 33m long core taken near the Pen Duick escarpment (PDE) in the Gulf of Cadiz, offshore Morocco, was studied. The core was sampled at a low-dynamic depression, about 70m below a mud volcano ridge. A multi-proxy approach is used, combining geological (grainsize analysis), geochemical (elemental measurements) and state-of-the-art lipid biomarker analysis techniques. The latter was used to reconstruct paleo sea surface temperatures and terrestrial input applying the TEX86-/ U

K’37 and the BIT-indices respectively.


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Within the Pen Duick Escarpment, the establishment of a background paleo-environmental reconstruction is necessary in the light of the further development of research on the cold-water coral mounds. This study will moreover enable the comparison within other cores from the PDE.

The aim of this study is i) to evaluate variations in terrigenous sediment fraction in a glacial/interglacial framework, based on XRF data and grainsize analysis, ii) to register surface water conditions by studying upwelling proxies (XRF data as proxy for the opal and calcium carbonates), iii) to record paleotemperatures using the UK’37 (alkenones) and TEX86 (GDGTs) indices and evaluate the different processes governing their signals. 2. SETTING 2.1. Geography and Geology

The GoC is located between 9°W to 6°45′W and 34°N to 37°15′N, enclosed by the Iberian Peninsula and Morocco (fig. 2A). The Gibraltar arc domain is characterized by a westward directed trusting during Oligocene through Miocene times, creating a structure resembling an east-dipping subduction accretionary complex (Gutscher, 2002). The bedrock is covered by a sedimentary layer of variable thickness, ranging from 0.2 to 2 km of Late Miocene to Plio-Quaternary age (Zitellini et al., 2009). The active tectonic strike-slip and compressional context is witnessed by the presence of mud volcanoes, salt diapires and fluid escape features. Locally, extensional tectonics can create large rotated blocks bound by lystric faults (Flinch, 1996). In the El Arraiche mud volcano field these rotated blocks are expressed as two subparallel ridges, Vernadsky Ridge and Renard Ridge, both with steep fault escarpments, as exemplified by the PDE on Renard Ridge (Figure 1). The PDE, a fault-bounded cliff, reveals a series of elongated mounds and mound clusters. The mounds occur in water depths between 500 and 600m and can measure up to 60m in height. The south-west part of the cliff, facing the coring site, has a height of 65 m above the sediments and an average slope gradient varying between 15 and 20° (Foubert et al., 2008).

A sparker seismic survey of the site shows that the upper sequence U4, with an average thickness of 50 ms TWT, contains continuous and parallel reflections(fig. 1B) (Van Rooij et al., submitted). The core MD08-3227 sampled only this layer, yielding a continuous record.

Figure 1 – (A) Bathymetric map of the Pen Duick Escarpment (contour lines every 5 m), with indication of the seismic profile. The red mark indicates the location of coring site MD08-3227, and B) seismic profile (SSW-NNE), crossing Alpha mound and coring site MD08-3227 (after Van Rooij et al., submitted).


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2.2. Hydrology

Current circulation patterns in the GoC established at the end of the Miocene, when the strait of Gibraltar opened (Maldonado and Nelson, 1999) (fig. 2A).

Northeast Trade Winds are a persistent feature of the wind forcing along the coast of Northwest Africa and generate upwelling. The wind direction is favourable for upwelling all year long (Nykjaer and Van Camp, 1994). If upwelling occurs in summer, destabilising the thermocline, an effect on the phytoplankton is noticed.

The water upwelling is Eastern North Atlantic Central Water (ENACW) (e.g. Hernández-Guerra and Nykjaer, 1997 and Pelegrí et al., 2005). ENACW, which occupies the depths of the permanent thermocline, has two memebers, with either a subtropical (ENACWst) or a subpolar origin (ENACWsp) and depending on the wind strength either type can be upwelled (Fiuza et al, 1998). The poleward flowing ENACWst, is lighter, relatively warmer and saltier than the southward flowing subpolar branch and contains less nutrients. According to Van Rooij et al. (submitted) the El Arraiche mud volcano field is at depth only influenced by the subsurface North Atlantic Central Water (NACW), flowing southward. The typical signature of the Mediterranean Outflow Water (MOW) is not recorded in the study area (fig. 2B). Along the Moroccan margin, Pelegri et al. (2005) reported the presence of the MOW at 800 m water depth, reaching as far as the Canary Islands.

Associated with the coastal upwelling are surface currents. The surface Canary current is part of the eastern boundary current of the subtropical North Atlantic gyre and flows southward along the NW African continental shelf and slope (e.g. Knoll et al., 2002). It is the last meander of the Azores current, whose location can vary seasonally and interanually (Klein and Siedler, 1989), under influence of the North Atlantic Oscillation. In spring the Canary Current weakens, coeval with the trade winds. The summer brings about further weakening of the trade winds, and this reduces the water inflow from the north. This water inflow from the North is mainly fed by the southward flowing Portugal Coastal Countercurrent (PCCC). It transports fresh modes of well-ventilated ENACW, originating west of 10.33°W. During summer the southward flowing Portugal Current induces an anticyclonic circulation within the Gulf of Cadiz. In winter, the northward flowing Portugal Coastal Current (PCC) dominates the surface waters (Perez et al., 2001; Martins et al., 2002). It entrains subtropical waters polewards, originating from the Azores front between 35 and 36°N (e.g., Peliz et al., 2005). A winter-time cyclonic (cold core) surface current system in the northern GoC would advect material from the southern part. These chaotic wa

ter bodies cause the GoC to separate the Iberian upwelling from the upwelling off northwest Africa (Lafuente and Ruiz, 2007).

Figure 2 - Present hydrography in the Gulf of Cadiz: A) surface and deep-water currents (after Foubert et al., 2008), sites of cores used for correlation are indicated and B) the water properties as measured over this site, with the depth range of the Pen Duick Escarpment indicated (after Van Rooij et al, submitted). displaced North-westerlies Enhanced humidity in the Mediterranean re


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2 3. MATERIALS AND METHODS

Within the framework of the ESF EuroDIVERSITY MiCROSYSTEMS project, several cores were acquired within the vicinity of the PDE (Renard Ridge), off Morocco. This study is based on a 30.3 m long calypso core, collected at 35°16.28’N and 6°47.89’W, at a water depth of 642 m during the R/V Marion Dufresne 169 cruise in 2008. 3.1. On-board measurements

On-board, magnetic susceptibility and gamma-ray attenuated density were measured with a 2 cm resolution with the Geotek Multi-Sensor Core logger. In addition, reflectivity measurements were made every 4 cm allowing colour analysis (CIELAB L*, a* and b* values). Afterwards, the AVAATECHX-ray Fluorescence (XRF) Core Scanner (Royal Netherlands Institute for Sea Research, Texel, The Netherlands) was used to obtain rapid and non-destructive determination of the chemical composition. The sample resolution was 1 cm and the whole core was measured at 10 as well as 30 kV (with an irradiation time of 30s). Additionally, a line scan was performed of the core data, allowing accurate imagery of the core surface. 3.2. Subsampling

Subsamples have been taken at irregular intervals, in order to document (a) the baseline variations of the Ca/Fe curve, and (b) in order to investigate higher-frequency variability (with focus on MIS3). First, the samples for grainsize and oxygen isotopes were taken (1 cm above and below depth bsf indicated), after which the remainder of the sediment was used for biomarker analysis (max. 2 cm above and below depth bsf indicated). Samples were weighed before and after drying at 50-60°C in order to calculate the water content (%), porosity and dry density (g/cm³). Then, sediments were sieved in wet conditions with distilled water. Sieves of 63 µm and 150 µm were used to subdivide the sample in three size fractions for further analyses. The coarse and finest fraction were dried and weighed.

3.3. Grain-size analysis

One gram of the subsampled sediment was reserved for bulk grain-size analysis. Organic matter and the carbonate fraction were removed with a double acetic acid treatment (10% concentration). The acid was diluted twice with 150 mL distilled water, which was removed from the settled sediment. Prior to the Malvern Hydro 2000S (Marine Biology section, Ghent University)

analysis, samples (kept in a 10% calgon solution) are centrifuged (20 rpm) for 24 hours, to obtain a homogeneous solution. Grain-size distribution data was processed using the software Gradistat (Blott and Pye, 2001).

3.4. Stable isotope stratigraphy

In order to obtain a stable isotope (δ18O and δ

13C) stratigraphy, 3-10 specimens of the planktonic foraminifera Globigerina bulloides were picked from 64 samples within the size fraction between 250-315 µm. After picking, these samples were cleaned in a methanol bath after 5 second sonication. Analyses were performed at LSCE (Laboratoire des Sciences du Climat et l'Environnement, Gif-sur-Yvette, France) using a VG Optima mass spectrometer, equipped with a Kiel device. Unfortunately, due to a delay in measuring time, the results were not available for this study.

3.5. Lipid extraction and preparation

50 ml of sediment was subsampled from slots of no more than 4 cm broad, carefully methanol cleaning the utensils in-between two samples. They were stored at 4 °C in sterilised tubes. After freeze-drying, at Royal NIOZ, 64 samples were powdered with a mortar, assuring homogenisation. Extraction of about 20 g of the freeze-dried sediment was performed by an accelerated solvent extractor (ASE 2000, Dionex), using methanol (MeOH) and dichloormethane (DCM) solvent mixture (9/1 v/v) at 1000 psi and 100 °C. The obtained total lipid extracts (TLEs) were rotary evaporated at 600 mbar. A first cleaning of the TLEs was performed over a small pipette column filled with Na2SO4 as a stationary phase and DCM as an eluent. Elemental sulfur was removed after an overnight reaction with cupper, activated after reaction with 1N HCl solution. The resulted CuS was removed using a Na2SO4 column and DCM as an eluent. To enhance the analytical power, the extract was separated in three fractions with different polarity, flushing it with three solvent mixtures with an increasing polarity (n-hexane/DCM (9/1, v/v- apolar fraction), n-hexane/DCM (1/1, v/v – keto fraction) and DCM/methanol (1/1, v/v-total polar fraction)) over a small column of activated alumina oxide (Al2O3 3 h at 150°C).

3.6. Analysis and identification of lipid biomarkers

3.6.1. Analysis of alkenones

After being diluted in ethyl acetate (1 mg/mL), the keto fractions were analyzed for long-chain alkenones. Their presence was revealed using gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). GC was performed


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using a Hewlett Packard 6890 gas chromatograph equipped with an on-column injector and a flame ionization detector. A fused silica capillary column (CP Sil5 50 m 0.32mm, df=0.12 µm; ref no: 9211431) with helium as a carrier gas was used. The samples were injected at 70 °C. The GC oven temperature was subsequently raised to 130 °C at a rate of 20 °C/min, and to 320 °C at 4 °C/min. The temperature was then held constant for 25 min. The data acquisition was done by Atlas software. GC–MS was conducted using a Thermofinnigan TRACE gas chromatograph coupled with a Thermofinnigan DSQ quadrupole mass spectrometer, scanning a mass range of m/z (mass/charge) 50-800 Da at three scans per second and with an ionisation energy of 70 eV using the same GC conditions as for GC. 3.6.2. Analysis of intact isoprenoidal and branched glycerol dialkyl glycerol tetraethers.

To determine the distribution and composition of intact glycerol dialkyl glycerol tetraethers (GDGTs), total polar fractions were dried under stream of nitrogen and re-dissolved in a mixture of hexane/isopropanol (99/1, v/v). The dissolved polar fraction was first filtered through a 0.4 µm PTFE filter attached to a 1 mL syringe, dried under the nitrogen and re-diluted in hexane/isopropanol (99/1, v/v) at a concentration of 2 mg/mL. Prepared samples were analyzed using a high performance liquid chromatography–mass spectrometry (HPLC-MS) method for their direct analysis (Hopmans et al., 2000). GDGTs were analyzed using an Agilent 1100 series / 1100 MSD series instrument, with auto-injection system and HP-Chemstation software. Injection volume was 10 µL. The column and temperature program used was a Prevail Cyano column (150 x 2.1mm; 3 um on 30°C) and 5 min hexane/propanol (99:1) with a gradual increase to obtain a hexane/propanol (98:2) after 45 minutes. The flow rate was 0.2 ml/min. The column was cleaned (back-flushing) for 10 minutes with hexane/propanol (90:10). Analyses were performed in selective ion monitoring mode in order to increase sensitivity and reproducibility. Agilent Chemstation software was used to integrate peak areas in the protonated molecule and its isotope ([M+H] + and [M+H]++1) ion traces.

3.6.3. Calculation and calibration of UK’37

UK’ 37 values were calculated using the Prahl and Wakeham (1987) equation UK’ 37 = (C37:2)/(C37:2 + C37:3), where the relative abundance of C37:2 was expressed as the uncorrected area of the 37 carbon containing n-alkenone with 2 double bonds. C37:3 is defined equal but the molecule has three double bonds. Calibration to sea surface temperatures was performed in two ways, as formulated in Conte et al. (2006), where T = 29.876* UK’ 37-1.334 and as in

Muller et al. (1998), where T = (UK’ 37-0.069)/0.033. Five samples were subsequently analysed in a single quadropole GC/MS system, to check the purity of some of the peaks. 3.6.4. Calculation and calibration of TEX86 values and calculation of BIT indices

TEX86 values were calculated using the equation described in Schouten et al. (2002) as; TEX86 = (GDGT2 + GDGT 3 + GDGT 5’)/(GDGT1 + GDGT2 + GDGT 3 + GDGT 5’). Temperature estimates were obtained using the Kim et al. (2008) calibration, where T = -10.78 + 56.2*TEX86. For the temperature estimates, only TEX86 values were used where all the isomers of GDGTs could be detected in sufficient abundances. BIT values were calculated using integration values obtained with the same method, also ensuring only values with a response in area of at least 10exp4. The BIT index, based on branched tetraethers (Hopmans et al, 2004), equals (GDGT I + GDGT II + GDGT III)/ (GDGT I + GDGT II + GDGT III + GDGT 5), ranging from 0 (only marine material) to 1 (no crenarchaeol). 3.7. Age model and mean accumulation rate

The stratigraphy of core MD08-3227 is based on 8 tie points with core GeoB 9064-1 (Mertens, 2009) and 18 tie points with core MD95-2042 (Shackleton, 2000) through correlation with both XRF Ca/Fe records (Fig. 6). Sites of cores used depicted on figure 2 . All GeoB 9064-1 tie points are within MIS1 and MIS2. The tie points with MD95-2042 are distributed over MIS2 to MIS6. The record (spanning 17m) ranges from present to at 132 ky BP.

Linear sedimentation rates (cm/ka) were derived between the 18 tie points, with the surface as an additional point that is assumed to have an age of 0 ky. Multiplying these values with the dry density (g/cm³) of the samples yields the mass accumulation rates (MARs). 4. RESULTS 4.1. Framework

It is well established that calcium carbonate records in the Atlantic Ocean can be related to glacial-interglacial cycles, with higher carbonate concentrations during interglacial periods. Preliminary stratigraphic interpretations can be based on downcore XRF records of Ca and Fe. In order to divide our core in marine isotopic stages, we compared the Ca/Fe record with the GRIP δ18O record of the past 122 ky (Johnsen et al., 2001).


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Figure 3 – Terrigenous proxies of core MD08-3227 plotted against depth (cm, bsf). A) XRF-derived Fe (per total counts); B) XRF-derived Fe/Al ratio; C) Grain-size analysis based mean grain-size (µm); D) Fines/coarses ratio, where fines = 0-8 µm and coarses = 8-31 µm); E) XRF-derived K/Al ratio; F) BIT-index; G) XRF-based K/Al ratio; H) Mass accumulation rate (g.cm-².ka-1). Grey areas denote Heinrich events, plain vertical lines denote the marine isotopic stages, dotted vertical lines denote marine isotopic substages 5a-5e.

The complete set of proxies was divided according to its informative value regarding terrigenous input (fluvial and aeolian), biogenic production and sea surface temperature (SST).

The key results of this study are a series of anomalous events, delined using the terrigenous component of the sediments. In successive paragraphs they will be continue to be referred to as the YD, and Heinrich events 1-6.

4.2. Terrigenous fraction

On figure 3 we can see that the mass accumulation rate during MIS1 is low. The Fe/Al ratio (figure x) shows no distinct peaks, contrasting with the fines/coarse ratio that shows three evenly spaced peaks. Where the mean grain-size peaks, the fines/coarse ratio drops, coinciding with a fairly high BIT-ratio. The drop in the K/Al ratio is not clearly expressed. This will be defined as the YD.

The Fe curve in MIS2 shows a pronounced dip, from 560-530 cm bsf. The mass accumulation rate


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is high. Fe/Al shows two strongly expressed peaks (400-450 and 500-620 cm bsf). Only the first one is reflected in a high mean grain-size and a low fines/coarse ratio. The third one (HE2) is, unlike the events described earlier, not characterised by a large grain-size. Also, fines/coarse ratios are fairly high in this peak.

MIS3 is characterised by a constant Fe value, with some minor negative peaks. The mass accumulation is very variable. High Fe/Al values were observed between 670 and 740 cm bsf (HE3), a small peak between 770 and 790 (HE4), a broad elevation between 800 and 940 cm bsf (HE5a) and a peak between 980 and 1015 cm bsf, that can be related to two separate events (HE5b and HE6).

The mean grain-size decreases across this unit, peaking at the same depths as the Fe/Al ratio, but also between HE3 and HE5. The fines/coarse ratio is low above HE5a, increasing sharply at this depth. It peaks coeval with Fe/Al peaks, but also on smaller scale (cyclicity spanning 38 – 22 cm bsf). BIT-values in MIS3 show a sequence of short cycles that are especially clear in the upper part of MIS3. The cycles span depth ranges between 30 and 64 cm and are not coeval with cycles observed in fines/coarse ratios.

Fe counts are high throughout MIS4. The mass accumulation rate follows this pattern. HE6 is recognised in high Fe/Al values and a coarser mean grain-size. Fines/coarse ratios are relatively low, as also observed in the K/Al values. After HE6, the Fe/Al ratio shows no pronounced peaks in this stage. The BIT-index is sampled at a resolution too low to infer any cyclicity. Its value does rise within this stage (from 3.4 to 5.5).

Fe cps form 2 peaks in the MIS5b and 5d. Compared with 5a and 5e, 5c still has a high Fe value. Fe/Al shows clearly that the surplus of Fe over Al is distributed over 3 cycles within 5a-5d, and is low during 5e. The mean grain size mirrors the Fe/Al distribution, with peaks at the top of MIS5a, in MIS5b and MIS5d. The fines/coarse ratio also shows a high variability, with the lowest value this time in MIS5b and d. K/Al and BIT-values show no clear trend throughout MIS5, next to an increase from the top of MIS5d until the base of MIS6.

Mass accumulation rates peak in MIS5e and MIS6. Fe peaks in MIS6this area, with (going upcore) a very steep increase at the base of this stage and after a peak value of only 25cm broad, a very gradual increase until the low values in MIS5e are reached. The mean grain size is low in this stage, with a very high fines/coarse ratio. K/Al increases strongly towards the base of MIS6 and the one BIT-value calculated in this stage is very high (10.6).

Fe cps are low in MIS7. Fe/Al shows no distinct peak, but the mean grain size and fines/coarse ratio form a number of distinct peaks. K/Al and the BIT-index are comparably low. 4.3. Biogenic fraction

Figure 5 shows that in the most recent stage, the biogenic silicious and carbonate fraction correlate well, with Ca showing a relative higher peak in the most recent sediment layers, also in the YD (up to 150 cm bsf). This is unlike the MIS2 where Si shows higher peaks, relatively to the carbonate fraction, at the depths of HE1 and HE2.

During the MIS3, the opal content was fairly constant, with some minor peaks. The most prominent ones were 10 cm above HE3, 10 cm above HE4 and 15 cm above HE6. The carbonate fraction was high at the base of MIS3, but drops gradual, showing a periodicity, ranging from 40 to 11 cm sediment. Ca/Fe peaks during all HEs. Where the span between two peaks is long, often a series of minor peaks is observed, that span no more than 5-7 cm sediment.

MIS4 was characterised by a low biogenic production, in both the carbonate and opal proxies. Both the Ca and Si record show three clearly defined peaks in MIS5. Going upcore, the values of biogenic production drop in MIS5b and MIS5d. They peak in the next substage, after a steady increase. The first two peaks are coeval, but the Ca peak in MIS5e is lead by the Si peak, that rather occurs in MIS5d.

The values of the biogenic proxies drop at the base of MIS6, increasing slowly towards the high value of MIS5e.


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Figure 4 – Grain-size moment parameters, plotted against depth (cm, bsf). A) Sorting; B) Skewness; C) Kurtosis. Grey areas denote HE, plain vertical lines denote marine isotopic stages, dotted vertical lines denote marine isotopic substages 5a-5e. 4.4. Moment parameters

The moment parameters are depicted in figure

4. Sorting and skewness show a large variability in MIS1, where a high value of skewness coevals a low value of sorting. Kurtosis only peaks during the YD, coeval with a peak in sorting and a low value in skewness. Sorting and kurtosis tend to increase together (r² = 0.585, p = 6.374E-13) throughout the core.

Sorting increases towards the base of MIS2, with a mediocre value during HE1 and a high value at the depth of HE2. On the contrary, kurtosis shows no response in HE2, but peaks in HE1. The skewness drops during both events, but to a much higher degree in HE1 than HE2. Kurtosis shows a sequence of smooth variations (mean value of depth spanned = 70 cm) in the upper part of MIS1, where sampling frequency is high enough.

During MIS3, which was sampled on a higher resolution, kurtosis shows a number of short, nicely defined trends. They are present throughout the entire unit and they seem to be associated with the HEs. Over the course of HE3 and HE6, a smooth rise and drop of kurtosis is observed. This also seems to happen over HE4, but is not present in HE5a. The sorting is also very variable within this stage and seems to be associated HEs 5a and 6. Throughout MIS3, it follows a cyclicity that range between 45 and 28 cm of sediment. Skewness show less variability, being generally low during HE and high in-between.

After HE6, the kurtosis in MIS4 anticorrelates

with the sorting. They are stable throughout most of the unit. Now the skewness is less stable. MIS5 sorting and skewness seems to vary according to the subdivision in d18O stages. Sorting shows no general trend across this unit, whilst the skewness decreases towards the base of MIS5. The kurtosis shows little affinity with the substages. At the boundary between MIS5 and throughout MIS6, where the sample resolution is higher, we see two clearly defined cycles of 105 and 114 cm sediment wide.

Sorting follows a rising trend in MIS6, with substantial superimposed variability. Skewness seems to anticorrelate with sorting somewhat. MIS7 shows a clear anticorrelation between sorting and skewness. Sorting shows three peaks (and skewness thus three valleys). Kurtosis correlates with the sorting, and thus also shows three peaks.


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Figure 5 – SST and biogenic parameters plotted versus depth (cm, bsf). A) SST reconstruction based on TEX86; B) SST reconstruction based on UK’ 37; C) Si/Al/Fe, a proxy for biogenic opal; D) Ca/Fe, a proxy for calcium carbonates. Grey areas denote HE, plain vertical lines denote the marine isotopic stages, dotted vertical lines denote marine isotopic substages 5a-5e. 4.5. Sea surface temperatures (TEX86) and (U

K’37).

Based on data in figure 5A and 5B, TEX86-

based SST were found to be 2.5 °C warmer than UK’ 37-based SSTs. Temperatures in MIS1 and MIS2 decline almost parallel, but the offset grows bigger towards the base of MIS2. Both temperatures drop at the depths defined as YD and HE1. During MIS3, the TEX86-based SST varies with a cyclicity that spans about 45 cm of sediment. UK’ 37 also has a cyclicity that spans 45 cm of sediment (especially present near the base of MIS3). In both parameters, some cycles are better expressed than others, and they are not completely coeval.

Distinct cooling of UK’ 37 based SST happened at

the top of HE3, HE4 and HE6. This is not linked to a drop in TEX86 SST.

The stadials MIS4, MIS5b, MIS6 and MIS8 show the largest offset between UK’ 37 and TEX86 based temperatures (between 4.6 and 6°C). Adversely, the interstadials 5a and 5e have an offset of less than 1°C. The stronger the glacial climate, the larger the difference observed. This is in line with observations made in MIS 3, MIS2 and MIS1.


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5. DISCUSSION

Sediments in low-energy marine environments are composed of both biogenic and terrigenous materials. The first includes the remains of organisms, which reconstruct a record of past climate and ocean circulation (e.g. Peterson et al., 2000). The nature and abundance of terrigenous material mainly provides a record of humidity-aridity variations on the continents and the direction and strength of winds and other modes of transport to and within the oceans (e.g. Lamy et al., 2000; Haug et al., 2001). Local hydrodynamic variations and bottom current variations in the study area also have an influence on the sediment composition (McCave and Hall, 2006). 5.1. Terrigenous input

During the glacial periods, Heinrich Events had a large imprint on the global climate. The melting of armadas of icebergs, calving off the Pan-Atlantic ice sheets caused a drop in surface water salinity, leading to reduced deep water formation and a sluggish deep water circulation (e.g. Clark et al., 2002; Schönfeld et al., 2003). South of the IRD-belt (Ruddiman, 1977), distinct layers of dropstones are absent. The significant impact of Heinrich events on these depositional environments are a result of atmospheric, hydrologic, and oceanographic connections.

Here, we defined the HE imprint in the sediments based on anomalous events in terrigenous input. Continental crust has a different chemical composition as the oceanic environment. We can thus assay the biogeochemical imprint of sediment particles to reveal their source.

In order to trace aeolian input, we trace the distribution of Fe, normalised over Al. Fe supply is associated with hematite, which is linked with increased eolian input (Rogerson et al., 2006). An increase in Fe/Al suggests a greater proportion of aeolian transport sourced from the northern Sahara (Yarincik, 2000). Another reconstruction of the effect of HE on the terrigenous component in the southern part of the GoC (Mertens et al., 2009) attributes enhanced provenance of Fe and Ti in the sediments to aeolian input, using an end-member analysis.

In addition to the geochemical imprint, the grainsize distribution can be used to derive relative wind strengths, if more vigorous winds can transport larger grain sizes from the source area. Aeolian transport mostly affects particles with a larger grain size and is often traced with Si/Al, Ti/Al or Zr/Al ratio (Calvert and Pedersen, 2007). As the Si/Al ratio correlates poorly with other ratios, we assume that the Si content is partly influenced by biogenic silicium and will use this proxy rather as a biogenic proxy. As Fe/Al correlates very well with the XRF-based grain size

proxies ((r²= 0.972 with Ti/Al; r²= 0.901 with Zr/Al ) only this proxy will be plotted. K/Al is a proxy for the amount of chemical weathering, and thus of the amount of rainfall on the continent and used as a proxy for fluvial input (e.g. Yarincik, 2000).

While the average continental crust is rich in K, basaltic sources have a typical high Ti signature. The K/Ti ratio thus becomes a proxy for basaltic sources (encountered at high latitudes) versus acidic sources, average continental crust. It was used earlier at higher latitudes (The Faroer Islands), by Richter et al. (2006), indicating increased contribution of continentally derived material, possibly through ice rafting from Fennoscandia, during stadial periods and, especially, Heinrich events.

The terrigenous fraction is a composite, assembled by various sources and different transport processes. This is reflected in the grainsize distribution of the sediment. Holz (2004) separated offshore NW African sediments in three subpopulations, a fluvial sourced group and two aeolian derived populations, using endmember analysis. According to deep-sea sediment studies along the NW African coast (e.g. Moreno et al., 2002; Stuut et al., 2002), terrigenous sediments with a grain size mode larger than 6 µm are transported as aeolian dust, whereas fluvial transport is assumed for particles smaller than 6 µm. This agrees with median grain sizes of transported Saharan dust over NW Africa to consist of rather fine-grained particles between 8 and 18 µm (Stuut et al., 2005). As no endmember analysis is performed on these samples, the percentage of clay + very fine silt + fine silt (denoted as fines) (2-8 µm) to approximate the fine-grained endmember. We use the percentage of medium silt + coarse silt (denoted as coarse) as a surrogate for the coarse endmember (8-31 µm). The plotted ratio of these two (fines/coarses) will denote the relative importance of fluvial versus aeolian input.

As carbonates were dissolved prior to grainsize analysis, the results obtained only concern the siliclastic fraction. However, care has to be taken regarding trends in these small grainsizes. The Mastersizer 2000 is ideally not used to analyse these small grainsizes and the tap water might have caused the clay particles to coagulate and be counted in larger grain size classes. Still, downcore trends might be visible.

The organic component of sediments can also yield information on the amount of terrestrial material transported. The BIT-index is based on terrestrial branched tetraethers that are produced in soils and mainly transported by rivers. So, this index is often used to evaluate changes in fluvial input.


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Figure 6 – Correlation of B) the XRF-based Ca/Fe curve of MD08-3227 with C) δ18O (per mil PDB) values of core MD95-2042 and A) the XRF-based Ca/Fe curve of GeoB 9064-1, with the tie points indicated.

HE are recognised in high Fe/Al and mean grain size values, pointing to a high aeolian input and low fines/coarse, K/Al and BIT-indices, the fluvial input proxies. For both sets of proxies, all major variations are linked to a HE. This can be attributed to an increasing continental aridity during HEs (Iberia; Goni et al., 2002; NW-Africa; Lèzine and Casanova, 1991). This dust was further transported, as a weakened thermohaline circulation led to a stronger hemispheric temperature gradient coupled with intensified trade winds (Vidal et al., 1997; Jullien et al., 2007). A second consequence was a southward shift of the ITCZ (Jennerjahn et al., 2004), that reduced the monsoonal precipitation over NW-Africa (deMenocal et al., 2000). Using these proxies, we delined 6 Heinrich events (HE), 1-6 (Bond and Lotti, 1995) and the Younger Dryas (YD) (fig. 3). This delineation is based on careful selection of the most pronounced peaks in all proxies. Subsequent plotting of obtained depths with ages from Hemming (2004) fitted within in the age model.

The observation that the aeolian input is most expressed in MIS2 and the end of MIS3 is accounted for by the effect that during glacial times and especially during the LGM, tropical NW Africa was dry and atmospheric circulation was enhanced (e.g. Sarnthein, 1978; Zühlsdorff et al., 2007). The absence of a parallel pattern in the fluvial proxies can point to a more stable climate in the Middle Atlas, the catchment of Moroccan rivers. Another possibility is that the coarser mean grain size in these periods might have been further boosted by

more vigorous bottom currents during glacial periods (Van Rooij et al., submitted). 5.2. Current reworking

Holz et al. (2004), reported no significant effect of current reworking along Cape Blanc (NW Africa) and could thus attribute all changes in grainsize to different transport mechanisms. To evaluate if that applies for us, the moment parameters sorting, kurtosis and skewness were plotted; derived from the grainsize analysis. The skewness and kurtosis describe the degree of symmetry in the distribution and the relative peakedness/flatness of a distribution respectively. The kurtosis thus compares sorting in the central portion of the population with that in the tails. High sorting values on the other hand, can be explained by the influence of several sources while sorted low values of sorting will indicate either a single source or significant bottom current action (Blott and Pye, 2001).

Knowing that most distributions only have one mode, the mean values indicate that the ‘peak’ of the grainsize distributions is within the sortable silt range. The smaller the mean grainsize, the less effect a sorting current can have. A low value of sorting during warm periods can thus be explained by the absence of grains on who sorting can act.

A high value of sorting coeval with a negative skew during cold periods as Heinrich events can be explained as an input of larger grains or the input of larger grains affected by a bottom current.


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Grainsize distributions are not easily related to

climatological modes, as the moment parameters have a very large variability. The warm periods are characterised by a low kurtosis and sorting, confirming a dominant supply of fine-grained materials. The skewness mainly varies with mean grain size. A negative correlation between skewness and mean grain size predicts that the presence of larger particles drives the skewness of the distribution.

During very cold periods –MIS2 and MIS4- arid conditions are not indicated by a higher kurtosis. The HE2 is particular not only in this respect, but it is characterised by a very low mean grainsize, Fe count and a high fines/coarse ratio. The low kurtosis is indicative of a large contribution of fine-sized particles. This is expressed in a low sorting rate. The low Fe is coeval with a very high Ca content, what indicates that the unit of HE2 could coincide with the LGM. A visual inspection of the core reveals a strong concentration of biogenic carbonates, thus diluting the siliclastic component.

A low mean grainsize during and after HE2 is also reported in Mertens (2009), pointing to a regional signal. During the LGM, erosion was more extensive, as vegetation retreated (Hooghiemstra et al., 2006). In surplus, the low sea level would cause the rivers to erode their bedding and extend further seawards, thus delivering higher amounts of clay and fine silt to the continental shelf. This higher fluvial input is corroborated by the high BIT-values. To affirm this, the other cold period in our core (MIS4) should have a similar signature. This indeed is the case for most of the proxies. As the K/Ti ratio is higher, the extent of ice rafting is derived to be minor. The Fe cps is high (and thus the Ca amount is low), pointing to a much lower biogenic production in this unit.

The kurtosis reaches its highest value at the top of MIS3, MIS5d and during Heinrich events, as expected under dusty atmosphere conditions. The presence of an enhanced bottom current during stadials is a hypothesis currently posed. Llave et al. (2006) reported an amplification of the deep branch of the MOW during cool stages, such as the HE. The MOW at this study site flows below the NACW. A denser and colder flow would plunge even deeper, especially as the NADW underlying this water body will be less dense in a weak thermohaline circulation system. This phenomenon is thus not likely to directly have contributed to the coarser grainsizes encountered at our site. To the contrary, Foubert et al. (2008) postulated that the MOW could have reached the PDE due to a possible enhanced meddy activity. In general, the moment parameters do not refute nor support the hypothesis that bottom current intensity at the PDE was stronger during cold HE.

5.3. MD08-3227 age model

Figure 7 - Age model of core MD08-3227. Diamonds; tie points with GeoB9064 (Mertens, 2009). Triangles; tie points with MD95-2042 (Shackleton et al., 2000). Circles; Heinrich events from this core.

The age model (fig. 7) and derived mass accmulation rates (fig. 3H) show to be very variable, with up to 8 times more sedimentation during MIS1 and 6 times more mass sedimenting in MIS4 compared to the baseline. The high terrigenous input in the sediments during glacials was noticed earlier, but the high sedimentation rate early in MIS1 is not easily explained. It is probably caused by a mistake in correlation. 5.4. Primary production during HE

The Ca/Fe ratio is indicative of the presence of carbonates, produced by coccolithophores and planktonic foraminifera. Fairly constant Si/Al-ratios in the hemipelagic unit imply that Si is mainly derived from terrigenous aluminosilicates, whereas higher and more variable ratios in deposits suggest a significant contribution from biogenic opal. To compare the production of these silica-producing organisms with Ca/Fe, we create the Si/Al/Fe ratio.

The amount of biogenic production depends on the amount of nutrients in the surface waters. This is determined by the amount of nutrients added with fluvial input, or by the amount of nutrient-rich subsurface water upwelled. In order to shed light on the extent of the paleocirculation in the southern GoC, we can determine the units with high biogenic components in the sediments (in our case opal, and calcium-bearing calcite and aragonite) and try to asses when upwelling occured.

The Ca/Fe proxy reveals a heavy imprint of glacialogical forcing, as it parallels the K/Ti proxy almost completely. As Fe and Ti correlate almost perfectly, this indicates a strong link between the


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basaltic input of K and the production and/or conservation of Ca. The opal proxy also shows a positive response to HE. This could be attributed to several mechanisms. First, a link in the ocean system can cause a link between these two proxies. A cold and less-saline surface water layer followed the southward flow of NASW along the Iberian margin (Bard et al., 2000; Colmenero-Hidalgo et al., 2002; de Abreu et al., 2003), measured in the Alboran Sea as well (Cacho et al., 1999). However, the presence of a less-saline water surface layer will most often lead to a reduced production, as higher density differences will slow down the upwelling system. A seasonal, more pulsed supply of organic matter during HEs (Melki et al., 2009) can cause a higher conservation of produced organic matter.

A second point of view implies an atmospheric link between the K/Ti input and Ca and Si production and conservation. The enhanced dust influx discussed earlier can fertilize the surface waters, but the enhanced production observed during glacials along NW Africa (e.g. Pokras and Mix, 1987) is mostly attributed to changes in upwelling. Trade wind related coastal upwelling is restricted to a narrow 5 km wide zone along the coast, observed in the southern Gulf of Cádiz as filaments south of the Strait of Gibraltar (Vargas et al., 2003). As our study site is about 6 km offshore, it is possibly influenced by changes in the extent of this upwelling system. This was observed in the Gulf of Cádiz (Colmenero-Hidalgo et al., 2002, 2004) and is confirmed in the study of Mertens et al. (2009).

The relative higher peaks of Si during MIS2, is most straightforwardly explained by the changed oceanographic conditions during the LGM. There is generally a lack of biogenic silica in the southern part of the GoC (Abrantes, 1988). Often used as an upwelling indicator, its strong increase during the LGM can be attributed to the influence of upwelling on the PDE. 5.5. Sea surface temperatures

The Tetraether Index of lipids with 86 carbon atoms (TEX86) is based on the number of cyclopentane rings in specific sedimentary membrane lipids (Schouten et al, 2002). These membrane lipids, isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), are produced exclusively by Archaea, which tune the chemical characteristics of their membrane lipids to different temperatures. Alkenones’ production seems to be restricted to a handful of species of coccolithophores, Haptophyte green algae (Volkman, 2000). The algae adapt to the water temperature by changing the degree of unsaturation in the alkenones. The ratio UK’ 37 was developed, using the degree of unsaturation of alkenones (ketones).

Two observations are to be explained: i) GDGT-based SST reconstruction yielding a higher temperature than the reconstruction based on alkenones and ii) The offset between the two proxies enlarging during glacial periods, with very distinct dips in UK’ 37 values at the top of HEs. The offset between the two temperature archives can be attributed to a number of mechanisms.

The first observation is remarkable as the alkenone-producing organisms are phytoplanktonic organisms, thus restricted to the photic zone. The surface waters in which they thrive are warmer than the deeper-dwelling Crenarcheaota, whose concentration is highest below the deep chlorophyll maximum in temperate coastal waters (Massana et al., 1997). The depth effect can be significant, as the offset between TEX86 calculated temperature and in situ temperature increases from 0 to 20 °C from surface waters to 500 m (Turich et al., 2007). In an upwelling regime, the permanent stratification is affected, mixing the upper water layers. A destabilised water column can experience a deep water chlorophyll maximum. Subsurface production of alkenones will lead to colder subsurface temperature reconstruction (Ohkouchi et al, 1999). Parallel, a forced displacement of subsurface-dwelling Archaea can cause a warm imprint on the GDGT distribution.

In addition, it was suggested the alkenone signal

produced could be biased by nutrients and light stress (e.g. Epstein et al., 1998; Prahl et al., 2005), as they get mixed in cold, dark water layers during stronger upwelling. UK’ 37 values decrease under nutrient stress and increase under prolonged dark stress (Prahl et al., 2005). To explain a decrease in temperature, surface layers should have been low in nutrients. This is not typical for upwelling waters.

Extensive lateral transport from surface waters experiencing other temperature regimes is also a possible cause. The positive offset of GDGTs should be derived from water at lower latitudes, or from warm shelf waters (Criado-Aldeanueva et al., 2006). As no northward flowing currents are present at the site, and these temperatures are only encountered south of the Azores, the first option seems unlikely. Alkenones on their turn are more likely to be derived from subpolar waters. If the hydrological situation was parallel as today, stronger trade winds (as during current winter) enhanced the water inflow from the north. The Eastern Boundary Current penetrated at latitudes as far as Portugal during glacials (e.g. Zahn, 1994). As the surface currents are fed by waters over the Portuguese shelf, an alkenone contamination might be possible. Although the hydrological situation is much more complex than depicted here, this mechanism would also account for the different


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impact on GDGTs and alkenones, as Mollenhauer et al. (2008) and Shah et al. (2008) indicate that GDGTs are less likely to be derived from lateral currents than alkenones. Kim et al. (2009) confirmed that long-distance (>1000km) lateral advection is relatively low.

Upwelling of NACW of a subpolar origin, now present between 500 and 750 meter water depth might also account for the colder alkenone temperatures, but not for the relatively lower impact on GDGTs.

The mechanism of as alkenone as GDGT export is strongly linked to the primary production in surface waters. Active food webs give rise to a major flux of faecal pellets and marine snow (Schouten et al., 2002; Huguet et al., 2006; Wuchter et al., 2006). When primary production in surface waters is seasonal, this could explain the observation of a seasonal, rather than an annual, reflection of sea surface temperatures (e.g. Weijers et al., 2007). The seasonality of both proxies changes between places and trough times. To shed some light on this issue, we compare the most recent temperature estimates with present SSTs. UK’ 37 temperatures range between 17.74 and 20.25 °C in the Holocene. Current SST at 35.2°N and 6.4°W are 16.1 °C in February and 22.3° C in September (Schweitzer, 1997). This suggests that the UK’ 37-based SST in fact represents a mean annual SST and bears no clear seasonal signal in the current oceanographic regime. TEX86 temperatures reconstructions are a lot higher. They range from 24.2 to 24. 9 °C. This can’t be derived from current SSTs and thus has to be an overestimation of the temperatures. This is off course no sound result and in order to gain insight in this, seasonal sediment traps should evaluate the exported pulses and its temperature signature in the GoC.

After being produced in the surface waters, the distribution of molecules in the sediments can still be skewed by degradation. Isoprenoid GDGTs are present in terrestrial soils (Weijers et al., 2005) and can affect TEX86 values of marine sediments in both directions, especially when OM input is high. This effect is enlarged in oxic conditions, as terrestrial organic matter (OM) is less degraded in

marine environments than its autochthonous equivalents (De Lange, 1998; Huguet et al., 2008 for GDGTs). This is probably not the case at this site though, as the BIT-values throughout the core are very low. In alkenone-based temperatures, degrading processes can introduce a ‘warm’ bias (Prahl et al., 2001; Gong and Hollander, 1999; Hoefs et al., 1998). An enhanced oxidation of bottom waters during interglacials could cause a more positive value, what could explain our colder values during stadials. This hypothesis is not confirmed by the BIT-index, as most of its fluctuations can be attributed to fluvial input during glacials (fig. 3) and not to preferential degradation of the marine isoprenoids.

Next to the offsets, UK’ 37 and TEX86 values can correlate during cold events as well, as witnessed by the parallel drop in temperatures during HE3. Aberrant features associated with this HE could give a clue concerning the mechanisms acting on SST proxies in the southern part of the GoC.

The SST of a nearby site (17 km to the NW), described by Mertens et al. (2009) shows no similar trends in offset between TEX86 and U

K’37 though.

This might imply that the phenomenon we observe is related to the more near-shore position of our site, pointing to an influence of the upwelling system as main driver of the offset. Our cores might indeed be on the boundary of the last glacial upwelling system, as Mertens et al. (2009) detected no enhanced silica values during the LGM. 5.6. Sedimentary evidence of shorter-timescale fluctuations.

Next to providing a general overview, this study focuses on the HEs in MIS3, and the Dansgaard-Oeschger oscillations preceding them. They are recognised in the same set of parameters as Heinrich events, as they take the form of rapid warming episodes, typically in a matter of decades, each followed by gradual cooling over a longer period. In general, Heinrich events only occur in the cold spells immediately preceding D-O warmings, leading some to suggest that D-O cycles may cause the events, or at least constrain their timing (Bond & Lotti 1995).


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Figure 8 – Short-term fluctuations between 23 and 71 ky BP. A) del18O data from the GRIP ice-core plotted versus years BP; B) Ca/Fe from MD08-3227 plotted versus ky BP, as all other proxies hereafter; C) mean grain size (µm); D) BIT-index; E) TEX86-based SSTs (°C); F) U

K’37-based SSTs (°C); G) Sorting; H) Kurtosis; I) XRF-derived K/Ti.


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Cacho et al. (2000) recognised 11 Dansgaard-Oeschger events in the Alboran Sea, based on δ18O and alkenone-based SSTs. The succession of DO cycles in this core is somewhat obscured, probably due to the sampling resolution that was too low, or a superimposed climate effect. Still, some cycles are characterised, based on the aeolian and fluvial fluxes, analogue with the Heinrich events. We will focus on the cycles best resolved, preceding HE4 and 5.

Preceding HE3, HE4 and HE5, 4 cycles were recognised, what coincides with the four cycles described in Cacho et al. (2000). Still, large errors in interpretation could be due to mistakes in the pinpointing of the HEs, what could be ameliorated using δ18O-values.

The correlation between proxies within these cycles is indistinct.

Alkenone-based temperatures show a very variable cyclicity, spanning from 1.5 to 5.6 ky. All cycles recognised comprise of a multitude of 1.4 – 1.8 ky, what could indicate a DO-imprint (Bond et al., 1999). TEX86-cycles show a more consistent pattern, with cycles of 1.5 and 2.5 ky. The mechanism behind this is not clear at the moment. Where TEX86-derived values often peaks in the middle of a cycle, UK’ 37-based SST will dip in the beginning of the cycle, and restore to a higher value by the time TEX86-based temperatures dip. This discrepancy could be explained if a time lag was present between the response of both temperature records. Or, it could be the same mechanism that was discussed to be operating on a longer timescale, causing an offset between these temperature records during stadials.

The presence of a meltwater flux along the Iberian coast during DO-oscillations is confirmed by a study of Pailler and Bard (2002), who state that periods of short-term establishment of subpolar conditions may be associated to the southward migration of the polar front and to meltwater influx, and coincide with HE and the cold stage of DO-cycles. Ca/Fe anti-correlates with the mean grainsize on long timescales as on small, indicating a dominant effect of Fe on the Ca/Fe distribution.

It is remarkable that UK’ 37 values follow a less clear pattern than TEX86 values. This can be due as to the variability of the lateral transport mechanism as to an upwelling system of variable strength. Also, integration errors are larger on alkenones, as their peaks were less clearly expressed.

The BIT-values tend to peak within a DO-cycle but show a very complex short-term cyclicity, with temperature spans ranging from 7.41 ky to 1 ky. On a longer term though, they shows clear peaks, associated with HE, separated by 6.9 and 7.4 ky.

The boundaries of the DO-oscillations are partly determined by the aeolian input, thus the response

of aeolian transport (as inferred from the mean grain size) is clear, showing a clear dip on the interstadials. The shortest fluctuations in mean grain size vary between 1.3 and 1.4 ky, indicating a DO-imprint. On a longer term they also show a cyclicity, that seems to be independent of the fluvial input, as they are not coeval nor anti-correlating. They are seperated by 6.9 and 11.9 ky.

Sorting and kurtosis both posses a cyclicity, varying between 1.4 and 2 ky per cycle. They seem to be low in the lower part of a DO-unit and higher in the upper part preceding HE3, but this pattern is not continual. 6. CONCLUSION The comparison of geochemical, sedimentological and biomarker proxies yielded information concerning continental climate and oceanic conditions.

Four modes seem to be discernable. One regime corresponds to the coldest periods, during MIS2 and MIS4, with cold SSTs and a very high productivity associated with strong upwelling conditions, as indicated by silicious upwelling indicators. This regime was also characterised by a larger sediment input trough rivers, probably related to sea-level changes and continental aridity. The warmest periods of MIS 5 and 1 represent the second regime. It corresponds to a low-intensity upwelling with warm SST and low productivity. Terrigenous components suggest humid continental conditions. A third regime is characterised by very distinct peaks in terrigenous input parameters and was interpreted as being Heinrich events. Associated with this regime are very distinct cold spells in the UK’ 37 SST reconstructions. Large aeolian and low fluvial input points to arid continental conditions. Enhanced biogenic production and lower SSTs indicate an increase in upwelling. Switches between this short-lived mode and its interstadials are quickly and not easily resolved in this data set. The short-lived oscillations in-between the HE (known as Dansgaard-Oeschger oscillations) leave an imprint on SSTs and terrigenous input proxies, but relations in-between proxies were no readily inferred. These observations are in good agreement with many ‘‘classical’’ studies describing the northern NW African upwelling and trade winds-system. Our multi-proxy approach enables us now to link oceanic conditions with continental climate, without the problem of correlation that is encountered when using separate archives. To decipher the different drivers and the leading and lagging components, a spectral analysis of the data set could be carried out. A stable isotope stratigraphy, including δ18O and δ13C is also indispensable for pinpointing the climate modes


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with more certainty. Especially at shorter time scales, there is a knowledge gap concerning the time relationship between these processes. This is important, since this information may offer clues for understanding the forcing mechanisms that triggered the D/O cycles.

Another feature of this study is the comparison between two sea surface temperature reconstructions. The oceanographic conditions in the GoC don’t only cause the TEX86 to report temperatures significantly higher than UK’ 37-based reconstructions, but also render the offset larger in cold periods. Our speculations involve lateral transport of alkenones, derived from subpolar waters off-shore the Iberian Peninsula and subsurface production of alkenones during stronger upwelling. The significant offset between the SST reconstructions and the fact that this offset is not constant, indicates that the interpretation of these proxies has to be done with care and preferably in the framework of a paleohydrological reconstruction. ACKNOWLEDGMENTS

I am very thankful to my supervisors, David Van Rooij, Alina Stadniskaia and Lies De Mol for supporting me throughout this last year, as their help was invaluable. This piece of scientific work would not have been possible without the input of their time and expertise.

I would also like to thank the other researchers at the Renard Centre of Marine Geology from the UGent and the department of organic biogeochemistry from the Royal NIOZ. For supporting me through my first steps in the sedimentary lab, I would like to thank Danielle Schram. I’m grateful for all the help (or mere pleasant company) during the lab work of Anita Abarzua, Dries Boone, Jasper Moernaut, Katrien Heirman, Koen De Rycker, Maarten Van Daele and Rindert Janssens and all those people I only met briefly. I am very grateful for the wonderful cooperation with the NIOZ, in the department of organic biogeochemistry, lead by Jaap Damsté. I received the most excellent input and indispensable help in the lab at the Royal NIOZ from Anchelique Mets, Ellen Hopmans, Jord Blokker, Jort Ossebaar, Marcel van der Meer, Marianne Baas and Michiel Kienhuis and Stefan Schouten. I would like to thank the colleagues in the NIOZ for always being welcome: Angela Pitcher, Claudia Zell, Darci Rush, Francien Peterse, Isla Castenada, Laura Villanueva, Nicole Bale, Petra Schoon, Robert Gibson, Raquel Lopes dos Santos, Ronald van Bommel, Sabine Lengger, Veronica Willmot and all those I did not get to meet in person. Your friendship was wonderful and I look forward to continue working with you.

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