-
32. ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS GREEN AND
BLACKCLAYSTONES FROM HOLE 530A IN THE ANGOLA BASIN1
Philip A. Meyers, Oceanography Program, Department of
Atmospheric and Oceanic Science,Thomas W. Trull,2 Senior Honors
Project, Department of Chemistry
andOrest E. Kawka,3 Oceanography Program, Department of
Atmospheric and Oceanic Science, The University of
Michigan, Ann Arbor, Michigan
ABSTRACT
Three pairs of Upper Cretaceous black shales and adjacent green
claystones from Hole 53OA were analyzed to com-pare types and
amounts of organic matter and lipids and to seek information about
their environments of deposition.The organic-carbon-rich black
shales have C/N ratios nearly seven times those of the
organic-carbon-lean green clay-stones. The lipid content of organic
matter in the black shales is about ten times less than in adjacent
green layers. Or-ganic matter in both types of rocks is thermally
immature, and distributions of alkanoic acids, alkanols, sterols,
and al-kanes contain large amounts of terrigenous components.
Pristane/phytane ratios of less than one suggest that
youngerTuronian sediments were laid down under anoxic conditions,
but ratios greater than one suggest that older TuronianCenomanian
deposits accumulated in a more oxic environment. Closely bedded
green and black layers have very similartypes of lipid
distributions and differ primarily in concentrations, although
black shales contain somewhat largeramounts of terrigenous lipid
components. Geochemical and stratigraphic evidence suggests much of
the organic matterin these samples originated on the African
continental margin and was transported to the Angola Basin by
turbidityflow. Rapid reburial of organic-carbon-rich sediments led
to formation of the black shales.
INTRODUCTION
Black shales containing high amounts of organicmatter were
deposited in deep waters of the South At-lantic during the
Cretaceous period. Occurrences ofblack shale formation are found in
the Barremian-Ap-tian at Deep Sea Drilling Project (DSDP) Site 361
in theCape Basin and at Sites 327 and 330 on the FalklandPlateau,
in the Albian at Site 363 on the northern flankof Walvis Ridge, in
the Turonian-Santonian at Site 356on the Sao Paulo Plateau, and in
the Aptian-Albian andTuronian-Santonian at Site 364 in the Angola
Basin(Bolli, Ryan et al., 1978). Concentrations of organiccarbon
reach as high as 24% dry weight of Aptian sedi-ment at Site 364
(Foresman, 1978). The conditionswhich led to the formation of these
organic-carbon-richlayers are not understood, although some
combinationof low availability of oxygen and abundant supply
oforganic matter seems likely.
Primary among the scientific goals of DSDP Leg 75was to
investigate the Cretaceous paleoceanographicconditions in which
black shales were formed. Hay et al.(1982) summarize information
obtained from Site 530 inthe Angola Basin where black shale layers
were found inthe Albian to early Coniacian. They conclude that
or-ganic-carbon-rich sediments accumulated in an oxygenminimum zone
of varying intensity on the African con-tinental slope and were
redeposited in deep waters by
1 Hay, W. W., Sibuet, J . -C, et al., Init. Repts. DSDP, 75:
Washington (U.S. Govt.Printing Office).
2 Present address: Woods Hole Oceanographic Institution, Woods
Hole, Massachusetts.3 Present address: School of Oceanography,
Oregon State University, Corvallis,
Oregon.
turbidity flows and slumping. Preservation of organicmatter in
the deep-water environment was enhanced byrapid burial in the
bottom.
Organic geochemistry can provide information whichwill help
answer some of the questions about blackshales. Evidence of
biological sources and of deposi-tional environments is often
preserved in the elementaland molecular composition of organic
matter in rocksand sediments. In the case of the Aptian to
Coniacianrocks at Site 530, relatively thin layers of black
shalesare interbedded with thicker layers of organic-carbon-lean
green and red claystones (Site 530 summary, thisvolume). The
differences in bulk organic contents andin color indicate changes
in depositional conditions andpossibly variations in supply of
organic matter. Analy-ses of total organic carbon, C/N atomic
ratios in organ-ic matter, and lipid composition of samples from
closelybedded green and black layers were done to compare
theorganic matter character of these different rocks.
METHODS
Six samples were collected on board D/V Glomar Challenger
fromfreshly opened core sections from Hole 530A for the purpose of
thiscomparative study. Two younger Turonian samples, one green
clay-stone and one black shale, were selected from Section
530A-95-5.These samples were halved longitudinally and shared
between thisstudy and the kerogen study described by Deroo et al.
(this volume).Four older Turonian samples from Section 530A-97-1
consist of agreen claystone overlying a black shale layer, the top
of the black shalelayer, the bottom of the black shale layer, and
the underlying greenclaystone. The samples were immediately frozen
in heat-sealed Kapakbags and kept frozen until analyzed.
Samples were first freeze-dried, and then their total carbon
contentwas measured with a Hewlett-Packard 185B CHN analyzer.
Residualcarbon was measured after HC1 dissolution of carbonates and
wasconsidered to represent the total organic carbon content.
Percent cal-
1009
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P. A. MEYERS, T. W. TRULL, O. E. KAWKA
cium carbonate was calculated from the difference between
initial andresidual carbon contents. Atomic C/N ratios of organic
matter weredetermined from residual carbon CHN values. Percent
organic carboncontents of the samples were calculated on a
dry-weight basis for theoriginal carbonate-containing sediment.
A two-stage procedure was used to extract lipids from the
freeze-dried samples. Soxhlet extraction with toluene/methanol
yielded theeasily extracted materials. A second extraction with
0.5N KOH inmethanol/toluene provided the hydrolyzable fraction of
components.These are called the free and bound fractions,
respectively, in thischapter and are comparable to the free and
bound fractions of Bras-sell et al. (1980). Both fractions were
treated with methanolic borontrifluoride to convert fatty acids to
their methyl esters, and then lipidsubfractions were separated by
column chromatography on silica gel.The classes so obtained
contained aliphatic hydrocarbons, fatty acidmethyl esters, and
hydroxy lipids, including sterols and alkanols.Hydroxy compounds
were silylated with BSTFA prior to gas chroma-tography (GC).
Splitless injection gas-liquid chromatography was employed to
de-termine the types and amounts of compounds comprising the üpid
sub-fractions. A Hewlett-Packard 5830 FID gas chromatograph
equipped-with a 20 m SE54 glass capillary column was used with
hydrogen ascarrier gas. Quantification was accomplished through the
use of knownamounts of internal standards added to each sample
before columnchromatography and includes corrections for GC
response differencesand laboratory contaminants. Individual
compounds were tentativelyidentified by GC retention times in this
preliminary survey of the lipidcharacter of these rocks.
RESULTS AND DISCUSSION
Organic Carbon and C/N Values
Black shale samples have much higher concentrationsof organic
carbon and much higher C/N ratios than dothe green shales (see
Table 1). These differences in theamount of organic matter and in
its elemental composi-tion are consistent with those found in a
large assort-ment of samples from layers of black shales and red
andgreen claystones in Cretaceous rocks from Hole 53OA(Site 530
summary; Meyers et al., this volume).
Concentrations of organic carbon of the green clay-stones
average 0.53 % and are only slightly higher thanthe averages of
0.2% found in modern, deep-ocean sedi-ments (Degens and Mopper,
1976) and of 0.3% com-piled by Mclver (1975) from analyses of older
marinedeposits from DSDP Legs 1 through 31. The organicmatter in
these layers probably was buried in sedimentsdeposited under
oxygenated bottom waters, a situationwhich does not favor organic
matter preservation (Hin-ga et al., 1979; Cobler and Dymond, 1980;
Demaisonand Moore, 1980). This material may have experiencedfurther
postdepositional destruction as documented byWaples and Sloan
(1980) in samples from DSDP Leg 58.In contrast to the green
claystones, the black shalesamples contain concentrations of
organic carbon which
Table 1. General descriptions of samples selected for organic
geochem-ical comparison of green and black layers, Hole 53OA.
Core-Section(interval in cm)
95-5, 0-2195-5, 38-5097-1, 87-9197-1, 91-9597-1, 99-10597-1,
105-110
Sub-bottomdepth
(m)
101410141027102710271027
Age
Younger Turon.Younger Turon.Older Turon.Older Turon.Older
Turon.Older Turon.
Lithology
Green claystoneBlack shaleGreen claystoneBlack shaleBlack
shaleGreen claystone
CaCO3(%)
< l< l< l< l< l< l
Corg
w0.20
12.950.584.89
10.010.80
C/N
4.536.06.8
38.140.3
5.7
are unusually high for most marine sediments. Suchhigh values
have been reported for only a few modernlocations, generally under
anoxic or poorly oxygenatedbottom waters as summarized in Demaison
and Moore(1980). An uncommon combination of abundant supplyof
organic matter and of exceptional preservation insediments is
required for these organic-carbon-rich de-posits to accumulate.
The contrast in C/N ratios between the green clay-stones and
black shales may reflect different amounts ofdiagenesis of organic
matter, different sources of organ-ic matter, or some combination
of these two possibil-ities. Atomic C/N ratios of the green
claystones average5.7 (Table 1), a value similar to those found in
DSDPLeg 58 sediments (Waples and Sloan, 1980). Such lowratios
reflect greater diagenetic loss of organic carbonthan of nitrogen,
because surficial marine sedimentscommonly have C/N ratios in the
range of 9 to 18(Stevenson and Cheng, 1972; Muller, 1977). In
organic-carbon-lean samples like the green claystones, C/N ra-tios
can be affected by the presence of inorganic am-monium and by
organic nitrogen compounds sorbed toclay minerals (Muller, 1977),
and thus they may not bereliable indicators of sources of organic
matter. In thecase of the organic-carbon-rich black shales,
however,the high concentrations of organic matter override
theeffects of these minor nitrogen contributions. The atom-ic C/N
ratios average 38.1 in the black shales and mayindicate an
important land-plant contribution to thesediment organic matter
(cf. Muller, 1977). Carbon iso-topic analyses of DSDP Leg 40
samples also suggest thepossibility of terrigenous material in
black shales depos-ited during Cretaceous times in the Angola Basin
(Fores-man, 1978; Simoneit, 1978a). However, Hinga et al.(1979)
report elemental analyses of diatom debris fromsediment traps in
the North Atlantic Ocean which yieldatomic C/N ratios of 28 to 54,
and Dean et al. (1981)show a correlation between high
organic-carbon contentand greater depletion in 13C in
mid-Cretaceous marinelimestones samples from the Hess Rise during
DSDPLeg 62. Evidently, much remains to be learned aboutthe effects
of diagenesis upon elemental and isotopiccompositions of organic
matter, and C/N ratios andcarbon isotopic data must be interpreted
with caution.
Free and Bound Lipid Components—General
Representative chromatograms of lipid fractions arepresented in
Figure 1. Identification and quantificationof individual lipid
components are based upon inte-grated peak areas from such
chromatograms with cor-rections for mass discrimination over the
fairly broadmolecular weight range surveyed. Identifications mustbe
considered tentative until verified by combined
gaschromatography-mass spectrometry analyses now under-way.
Laboratory contamination of samples was minor,and corrections have
been made for it in our data. Noobvious evidence of shipboard
contamination was ob-served (cf. Doran and Johnson, 1979).
Lipid contents of the green claystones and blackshales analyzed
in this chapter are summarized in Table2. In general, free lipids
are at higher concentrations
1010
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CRETACEOUS CLAYSTONES
Table 2. Summary of geolipid contents of samples of green
claystones and black shales, Hole 53OA.
Geolipid component
Hydrocarbons
Total, µg/g sedimentTotal, mg/g C o r gn-alkanes, µg/g
sediment/i-alkanes, mg/g C o r gPristane/phytanePristane/n-Ci7
«-C29/π-c17CPI (16-32)
Fatty acids
Total, µg/g sedimentTotal, mg/g C o r gn-alkanoics, µg/g
sedimentn-alkanoics, mg/g C o r g«-C26/«-Ci6CPI (16-32)
Alkanols and sterols
Total, µg/g sedimentTotal, mg/g C o r gn-alkanols, µg/g
sedimentn-alkanols, mg/g C o r gn-C28/n-Ci6CPI (16-32)Sterols, µg/g
sedimentSterols, mg/g C o r g
95-5,
Green
Free
0.86
3.41.72.11.00.274.2
0.400.200.0660.0330.19
310.0260.013
0-21
claystone
Bound
1.8
2.81.41.71.30.127.1
0.760.380.100.0511.37.30.0690.035
95-5.
Black
Free
0.83
6.70.0524.80.0370.632.1
5.10.039
38-50
shale
Bound
3.80.0292.70.0210.332.0
2.00.0150.120.0011.33.10.200.001
97-1,
Green
Free
6.91.21.80.311.62.16.50.94
2.30.401.50.260.374.6
1.60.280.120.0211.3
a0.160.028
Sample(interval in cm)
87-91
claystone
Bound
0.840.140.280.0480.750.507.80.95
1.40.240.690.120.0836.2
1.50.260.130.0224.0
170.0430.007
97-1,
Black
Free
450.928.40.171.73.24.01.6
6.50.134.00.0821.082.6
4.60.0940.200.0041.7
160.610.013
91-95
shale
Bound
1.70.0350.220.0043.19.40.0720.001
97-1,
Black
Free
1801.8
270.271.32.51.51.8
11.00.115.10.0510.742.2
8.60.0860.590.0067.72.71.40.014
99-105
shale
Bound
0.850.0080.350.0030.910.678.31.1
2.90.0292.10.0210.242.4
2.90.0290.230.0022.04.30.210.002
97-1,
Green
Free
354.45.70.711.22.40.560.66
3.50.442.30.290.4103.7
3.00.380.460.0585.9
160.320.040
105-110
claystone
Bound
0.880.110.320.400.900.770.690.73
1.70.211.00.130.0744.5
1.00.130.0630.0080.666.20.0170.002
Note: Blank space = not determined.a No odd-chain n-alkanols
were detected.
than are bound lipids. This is especially true for
thehydrocarbon fraction of total, or free plus bound,lipids.
Free-to-bound ratios of concentrations range be-tween 0.5 to 3.8
for fatty acids and alcohols and be-tween 8 and over 200 for
hydrocarbons. No consistentdifference in this ratio exists between
green and blacklayers. Concentrations of each lipid fraction per
weightof dry rock are generally greater in black shales than
ininterbedded green claystones. However, the converse isfound when
concentrations are expressed in milligramslipid per gram organic
carbon, showing that the organ-ic-carbon-rich black shales are lean
in lipid componentsrelative to other forms of organic matter.
Degradationof nonlipid materials is the probable cause for
greenclaystones to have higher proportions of lipid compo-nents.
Among the lipid components, hydrocarbons havethe greatest
concentration, fatty acids are intermediate,and alcohols have the
smallest concentration. Withinthe alcohol fractions, sterols and
n-alkanols generallyhave roughly similar total concentrations.
Aliphatic Hydrocarbons
Ratios of biomarker hydrocarbons extracted fromsediments and
sedimentary rocks are commonly used toprovide information about
sources of organic matter,paleoenvironmental conditions of
sedimentation, anddiagenesis of organic matter. Pristane-to-phytane
ratiosless than one suggest anoxic depositional environments
whereas ratios greater than one indicate oxic conditions(Didyk
et al., 1978). The values in Table 2 from Section530A-95-5 are
about 0.85 for free hydrocarbons and 1.8for bound hydrocarbons, and
those from Section 53OA-97-1 are between 1.2 and 1.7 in the free
hydrocarbon ex-tracts and 0.75 to 0.91 in the bound extracts.
Adjacentgreen and black samples have essentially the same
pris-tane/phytane ratios. Because free hydrocarbons havehigher
concentrations than do the bound fractions, theirmarker compounds
are probably more representative ofthe total extractable organic
matter in these samples.
The low pristane/phytane ratios in the free hydrocar-bon
fractions of Samples 53OA-95-5,0-21 cm, and 53OA-95-5, 38-50 cm
suggest that the organic matter in boththese Turonian samples was
buried under anoxic condi-tions. However, the green layer (Sample
53OA-95-5, 0-21 cm) was bioturbated, and so lack of oxygen musthave
been confined to the sediments. Other investiga-tors have reported
low pristane/phytane ratios in Creta-ceous rocks from the Atlantic.
For example, ratios of0.6 and 0.3 have been measured in an Aptian
blackmudstone from DSDP Site 364 in the Angola Basin (Si-moneit,
1978a) and for a Cenomanian black shale fromDSDP Site 367 (Didyk et
al., 1978), respectively, and in-dicate anoxic conditions
accompanying deposition ofthese other organic-carbon-rich
samples.
In contrast to Section 53OA-95-5, the free hydrocar-bon
pristane/phytane ratios suggest at least mildly oxy-
1011
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P. A. MEYERS, T. W. TRULL, O. E. KAWKA
14
14
Sample 530A-97-1,105-110 cmFree hydrocarbons
29 31
Sample 530A-97-1,105-110 cmFree acids
16 17I.S. 18I
22 24 26
20
Sample 530A-97-1, 105-110 cmFree alcohols
II I.S.
28
1/
30
Figure 1. Representative chromatograms of hydrocarbon, fatty
acid (as methyl esters), and fatty alcohol plus sterol (as BSTFA
ethers)fractions of free and bound extracts. (GC conditions: 20m
SE54 glass capillary column, hydrogen carrier gas at 2 ml/min., 70
to270°C at 4°C/min. Major straight-chain components are identified
by carbon numbers.)
1012
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CRETACEOUS CLAYSTONES
17 19 21
PR PH
23
25
Sample 530A-97-1,105-110 cmBound hydrocarbons
SQ
27
29
31
33
14 16 17I.S. 18
18:1
22
20
^-üW-'v^>~A~
Sample 530A-97-1,105-110 cmBound acids
30
14
16
Figure 1. (Continued).
18
VA/—Λ/
22
I.S.(5αCholestane)
Sample 530A-97-1,105-110 cmBound alcohols
Cholesterol
1013
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P. A. MEYERS, T. W. TRULL, O. E. KAWKA
genated depositional environments during Cenomaniantimes for
both the green claystones and the black shalesin Section 530A-97-1.
However, this interpretation con-flicts with the abundance of
sulfur present as pyrite inthe black shales (Hay et al., 1982; Site
530 summary,this volume), the presence of porphyrins indicated
bystrongly colored red bands observed in column chroma-tography of
the extracts from black shales, and the highconcentrations of
organic carbon in the black shales, allof which suggest anoxic
conditions within the sediments(Didyk et al., 1978). The
discrepancy between the pris-tane/phytane ratios and other
anoxic/oxic indicators inSection 530A-97-1 may result from thermal
enhance-ment of this ratio similar to that observed by Simoneit
etal. (1981) in Cretaceous black shale samples from near adiabase
intrusion at DSDP Site 368. However no intru-sions are known at
DSDP Site 530, and other organicmatter parameters remain to be
examined to test thispossibility.
A high proportion of land-derived organic mattermay dilute lipid
input from marine algae and result inhigher values of the
pristane/w-heptadecane ratio inocean sediments (Didyk et al.,
1978). The high values inthe free hydrocarbon fractions in rocks
from Section530A-97-1 (Table 2) suggest large contributions of
ter-rigenous organic matter to both green claystones andblack
shales. Bound hydrocarbon fractions have ratiosbelow one and
evidently contain proportionately morealgal and microbial organic
matter.
Another measure of terrigenous vs. aquatic hydrocar-bon origin
is the ratio of C2 9/C1 7 w-alkanes. Land plantscommonly have
w-alkane distributions dominated byC2 7, C29, and C3i hydrocarbons,
whereas algae containmostly shorter-chain w-alkanes (Simoneit,
1978b); there-fore, high C29/Q7 ratios may indicate large
terrigenouscontributions of organic matter. In Samples
530A-97-1,87-91 cm and 530A-97-1, 91-95 cm, this ratio
suggestsmajor amounts of land-derived components in the
hy-drocarbon contents of the green claystone and underly-ing black
shale. Sample 530-97-1, 99-105 cm is from thebase of a black shale
layer 14-cm thick and may containabout equal portions of
terrigenous and aquatic hydro-carbons. The underlying green
claystone sample, Sam-ple 530A-97-1, 105-110 cm, has a C2 9/C1 7
ratio whichindicates predominance of algal hydrocarbons. How-ever,
preferential losses of n-C17 relative to longer-chainrt-alkanes
(cf. Giger et al., 1980) may result in the C29/C1 7 ratio
presenting an exaggerated indication of theoriginal proportion of
land-derived material.
Evidence of alteration of w-alkanes can be given bycarbon
preference indices (CPI) of hydrocarbon ex-tracts from rocks (Bray
and Evans, 1961). In general,hydrocarbons in young marine sediments
have relativelyhigh CPI values and those in thermally mature
rockshave values close to one. Hunt (1979) notes that in
situa-tions where essentially no land-derived organic matter
ispresent the CPI value of young sediment can be as lowas one, but
this is rarely found and does not seem to bethe case at Site 530.
The CPI's of the green claystonesfrom Section 530A-97-1 are quite
low and may reflectconsiderable hydrocarbon diagenesis in these
samples.The black shales from this section have somewhat high-
er CPI values, indicating less hydrocarbon maturationor
alteration. This difference is consistent with the dif-ference in
overall preservation of organic matter in thetwo types of rocks as
evidenced by their organic carbonconcentrations.
Distributions of free n-alkanes of samples from Sec-tion
530A-97-1 are bimodal (Fig. 2), showing both algal(C17 to C19) and
land plant (C27 to C31) inputs. Boundn-alkanes do not have strongly
bimodal distributions. Inthe green claystones, the bound
distributions have max-ima at shorter chain lengths than in the
correspondingfree fraction. The distribution of bound H-alkanes
inSample 530A-97-1, 99-105 cm, a black shale, is skewedtowards
longer chain-length components, and in generalall of the n-alkane
distributions of the black shale sam-ples seem to contain more land
plant materials than dothe green claystones.
Fatty Acids
As indicated by low values of the C2 6/C1 6 n-alkanoicacid
ratios (Table 2), the major constituents of fatty aciddistributions
are shorter-chain /i-alkanoic acids. Themajor component in five of
the six free acid fractionsand in all of the bound fractions is
«-C16 acid (Fig. 3).Bimodal distributions containing algal (C16,
C18) andterrigenous (C^ to C32) acids make up the free fractionsand
resemble the fatty acid distribution of an Aptianblack mudstone
from DSDP Site 364 in the Angola Ba-sin (Simoneit, 1978a). Bound
fractions have smallercontributions of land-plant long-chain acids
than do thefree fractions. A similar difference between
distribu-tions of free and bound acids is reported by Brassell
etal. (1980) in Pleistocene-to-Miocene samples fromDSDP Site 440 in
the Japan Trench. This differencesuggests a more autochthonous
origin for the boundfractions of lipids in DSDP samples. Peaks
having re-tention times corresponding to those of unsaturated
andbranched acids were present in most of the fatty
acidchromatograms, but identification of these acids awaitsfurther
analysis.
Distributions of acids in the green claystones and inthe
adjacent black shales are not the same. Black shalescontain
relatively larger proportions of long-chain ter-rigenous π-alkanoic
acids. Because postdepositionaldegradation would preferentially
destroy shorter-chainacids (cf. Matsuda and Koyama, 1977), rocks in
whichorganic matter is poorly preserved should retain fewerof these
acids than would rocks in which degradationhas been retarded. If
the original acid contents of thegreen and black layers had been
similar, those of the or-ganic-carbon-lean green claystones should
now have rel-atively small contributions of C1 6 and C1 8
components,and this is not the case. On this basis, the difference
indistributions probably reflects the input of more land-plant
acids to the black shales than to the green clay-stones rather than
being a diagenetic result. These ter-rigenous materials may have
been originally depositedon the African continental margin and
become trans-ported to their present location by turbidity
flow.
CPI values of free acids average 4.2 for the greenclaystones and
2.3 for the black shales. In the boundfractions, the respective
mean CPI values are 5.9 and
1014
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CRETACEOUS CLAYSTONES
Sample97-1,87-91 cm
15 17 19 21 23 25 27 29 31 33 35 15 17 19 21 23 25 27 29 31 33
35
Sample97-1,105-110 cm
15 17 19 21 23 25 27 29 31 33 35Alkane chain length
15 17 19 21 23 25 27 29 31 33 35Alkane chain length
Figure 2. «-Alkane distributions in free (open bars) and bound
(hachured bars) extracts of hydrocarbons fromgreen claystones
(left) and black shales (right).
16 18 20 22 24 26 28 30 32 34
16 18 20 22 24 26 28 30 32 34
16 18 20 22 24 26 28 30 32 34Fatty acid chain length
16 18 20 22 24 26 28 30 32 34
16 18 20 22 24 26 28 30 32 34
16 18 20 22 24 26 28 30 32 34Fatty acid chain length
Figure 3. n-Alkanoic acid distributions in free (open bars) and
bound (hachured bars) extracts of fatty acidsfrom green claystones
(left) and black shales (right).
1015
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P. A. MEYERS, T. W. TRULL, O. E. KAWKA
2.3. Although the higher CPI's of the green claystonesreflect
the large contributions of n-C16 and «-C18 acids,more odd-chain
alkanoic acids appear in the black shaledistributions than in those
of the green claystones (Fig.3). These odd-chain components are
especially evidentfrom «-C19 to Λ-C2 3 and may be diagenetically
derivedfrom longer-chain acids.
Alkanols
Most distributions of «-alkanols contain large contri-butions of
C1 6, C22, and C2 8 (Fig. 4). The ratio of n-C2$to «-C16 is used in
Table 2 as an indicator of land plantversus aquatic alkanols.
Although this ratio makes nodistinction between black shale and
green claystone norbetween free and bound alkanols, it does show an
im-portant contribution of terrigenous material in most ofthe
samples. This conclusion is supported by the abun-dance of n-C26,
n-C2$, and /2-C3O in these samples (Fig.4). The alkanol
distributions of the black shales in par-ticular differ from those
found in Pleistocene-to-Mio-cene samples from DSDP Site 362 (Boon
et al., 1978)and from DSDP Site 440 (Brassell et al., 1980) in
whichn-C22 and n-C^ dominate. As also seen in the fatty acid
contents, the alkanols indicate more terrigenous mate-rial in
the black shales than in the green claystones.
Alkanol CPI values in Table 2 are higher than CPI'sof alkanes or
alkanoic acids from the same samples.Green claystones have higher
CPI's than do their adja-cent black shales, and free fractions
usually have higherCPI's than found for bound alkanols. The lower
valuesof the black shales is similar to the lower alkanoic acidCPI
values and may also be the result of diagenetic pro-duction of
odd-chain components from longer-chainprecursors. Because few
reports of n-alkanol contentsof ancient rocks exist, it is
difficult to interpret theseCPI values. However, the strong
predominance of even-chain components in both the black shales and
the greenclaystones seems remarkable. It may be that diagenesisof
alkanols does not form shorter-chain homologs butinstead decreases
the overall amount of alkanols whilenot greatly modifying the
original biogenic character.
Sterols
Contributions of C2 7, C2 8, and C2 9 components to thetotal
concentrations of sterols were compared by thescheme of Huang and
Meinschein (1979). For this com-
i π
if1ül
Sample95-5, 0-21 cm
I p'P pp f
Sample95-5, 38-50 cm
16 18 20 22 24 26 28 30 32 16 18 20 22 24 26 28 30 32
Sample97-1,87-91 cm
ill k • f
Sample97-1, 91-95 cm
16 18 20 22 24 26 28 30 32 16 18 20 22 24 26 28 30 32
[i.k
Sample97-1, 105-110 cm
16 18 20 22 24 26 28 30 32Alkanol chain length
Sample97-1, 99-105 cm a
16 18 20 22 24 26 28 30 32Alkanol chain length
Figure 4. n -Alkanol distributions in free (open bars) and bound
(hachured bars) extracts of alcohols fromgreen claystones (left)
and black shales (right).
1016
-
CRETACEOUS CLAYSTONES
parison, all diene and monoene stenols and stanols hav-ing the
same carbon number were tentatively identifiedby GC retention times
and their contributions added to-gether. The comparison (Fig. 5)
shows that C28 sterolsare not abundant in any of these Cretaceous
samplesand that C27 and C28 sterols contribute about equally tothe
totals. Except for the free sterol fractions of twogreen clay
stones, most of the samples have similar com-positions in both the
free and bound fractions. How-ever, these sterol compositions do
not resemble thosereported by Huang and Meinschein in modern
sedi-ments, indicating that the original sterol contents ofthese
sedimentary rocks have been modified. It is possi-ble that sterane
distributions, which remain to be de-termined, will provide better
source information, asdid those found in the Cretaceous Viking
Formation ofCanada (Huang and Meinschein, 1979).
Ratios of C29/C27 stenols range from 0.07 to 2.3 inthe free
fraction and from 0.08 to 1.1 in the bound.They show no
relationship to green or black sample col-or, nor do the ratios of
the free and bound fractions ofan individual sample appear to be
related. Stenol/stanolratios are between 1.3 and 35 for C27 sterols
and 0.09and 2.8 for C29 sterols. The large proportion of
un-saturated sterols in many of these rocks is surprising inview of
the rapid decrease in stenol/stanol ratios usuallyfound in young
marine sediments (Huang and Mein-schein, 1978). Although it is
tempting to conclude thatthe presence of these unsaturated
molecules in the blackshales results from exceptional preservation
of organic
matter, their existence in the organic-carbon-lean
greenclaystones is paradoxical. These preliminary sterol
iden-tifications evidently require further investigation.
CONCLUSIONS
Upper Cretaceous black shales contain higher con-centrations of
organic carbon and have higher organicmatter C/N ratios than do
adjacent green clay stonelayers. Lipid contents of both types of
rocks consist ofsimilar distributions of marine and terrigenous
compo-nents, but black shales have somewhat larger propor-tions of
terrigenous n-alkanes, /z-alkanoic acids, andw-alkanols than are
found in green claystones. The lipidcontribution to total organic
matter is less in the blackshales than in the organic-carbon-lean
green claystones,indicating enhanced preservation of nonlipid
matter inthe black shales and general thermal immaturity of
thesesamples.
The rocks in this study were deposited in the earlyAngola Basin
as turbidites whose source is believed tobe on the African
continental margin (Hay et al., 1982;Site 530 summary, this
volume). The interbedding ofthin layers (1 to 15 cm) of black
shales in thicker layersof green and red claystones and the
presence of abun-dant bioturbation rules out the possibility of
perma-nently anoxic bottom waters during Turonian time. Sed-iments
rich in organic matter may have accumulatedwhere an extended and
intensified oxygen minimumzone impinged on the ocean bottom in
midwater depths(Demaison and Moore, 1980; Dean et al., 1981).
Rede-
Green Claystones1. Sample 530A-95-5, 0-21 cm3. Sample 530A-97-1,
87-91 cm6. Sample 530A-97-1, 105-110 cm
Black Shales2. Sample 530A-95-5, 38-50 cm4. Sample 530A-97-1,
91-95 cm5. Sample 530A-97-1, 99-105 cm
100 100
Figure 5. Total C27, C28, and C29 sterol distributions in free
and bound extracts from green claystones andblack shales.
1017
-
P. A. MEYERS, T. W. TRULL, O. E. KAWKA
position of these sediments in deeper waters could thenhave
occurred by turbidity flow, and subsequent preser-vation of organic
matter could have been achieved byrapid burial in a nominally oxic
environment.
Differences between the green and black layers sug-gest that
variability existed in depositional conditions. Ifthese turbidites
came from the same source, then theremust have been major
fluctuations in the oxygen mini-mum zone in response to changes in
the supply of ter-restrial and/or aquatic organic matter. If they
had dif-ferent sources, then the organic-carbon-rich sedimentscame
from a site which received more land-derived ma-terial. Organic
geochemical data from this investigationsuggests that the black
shales contain more terrestrialorganic matter than do the green
claystones. Finally,younger Turonian sediments (Section 530A-95-5)
mayhave been deposited under anoxic conditions on the ba-sis of
pristane/phytane ratios, whereas older Turonian.sediments (Section
530A-97-1) were laid down undermildly oxic conditions. This
difference points out thatthe conditions leading to black shale
formation need nothave been the same at all times or at all
locations in theoceans.
ACKNOWLEDGMENTS
We thank W. E. Dean, M. J. Leenheer, J. Rullkötter, and B. R.
T.Simoneit for suggesting improvements to the manuscript. We
aregrateful to the Deep Sea Drilling Project (International Phase
ofOcean Drilling) for making possible the participation of P. A.
Meyerson board D/V Glomar Challenger during Leg 75.
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Date of Initial Receipt: June 22, 1982
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