51. ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES AND ADJACENT STRATA FROM DEEP SEA DRILLING PROJECT SITE 603, OUTER HATTERAS RISE 1 Keith W. Dunham, Philip A. Meyers, and Pamela L. Dunham, The University of Michigan 2 ABSTRACT Organic matter contents of black shales from the Cretaceous Hatteras and Blake Bahama formations have been compared to those from surrounding organic poor strata using C/N ratios, δ 13 C values, and distributions of extractable and nonsolvent extractable, long chain hydrocarbons, acids, and alcohols. The proportion of marine and land derived organic matter varies considerably among all samples, although terrigenous components generally dominate. Most black shales are hydrocarbon poor relative to their organic carbon concentrations. Deposition of the black shales in Hole 603B evidently occurred through turbiditic relocation from shallower landward sites and rapid reburial at this out er continental rise location under generally oxygenated bottom water conditions. INTRODUCTION Occurrences of dark colored layers of Cretaceous rocks having relatively high concentrations of organic matter have been found in numerous locations studied as part of the Deep Sea Drilling Project (DSDP). The distribu tion of such occurrences in the North Atlantic Ocean has been discussed by Arthur (1979), Tucholke and Vogt (1979), Thierstein (1979), Graciansky et al. (1981), Weis sert (1981), and Waples (1983), among others, with the intent of identifying the paleoceanographic factors in volved in the formation of these unusual strata, com monly called "black shales." Improved preservation of organic matter, increased contribution of continental or ganic matter to oceanic basins, and enhanced produc tion of marine organic matter are some of the factors that have been suggested. Because these three possibili ties affect the character of the organic content of black shales, the organic matter in North Atlantic examples has been investigated; these investigations are summa rized by Tissot et al. (1980), Summerhayes (1981), Katz and Pheifer (1982), and Graciansky et al. (1982). Vary ing proportions of marine and terrigenous organic con stituents are found in sediments deposited at different times and locations in the Cretaceous Atlantic Ocean. A comparison of the organic matter contained within black shales with that of the adjacent organic carbon poor li thologies further contributes to this information. In this chapter, we describe comparisons of analyses done on black shales and closely bedded strata from the Hatteras and Blake Bahama formations in the western North At lantic Ocean. Methods Twenty five Hole 603B samples from different ages in the Creta ceous were selected on board Glomar Challenger for this study. These van Hinte, J. E., Wise, S. W., Jr., et al., Init. Repts. DSDP, 93: Washington (U.S. Govt. Printing Office). 2 Address: Oceanography Program, The University of Michigan, Ann Arbor, MI 48109. were augmented with postcruise sampling from frozen core sections. Five sample groups contain closely bedded organic carbon rich and organic carbon lean strata. Hatteras Formation samples consist of one Turonian sample, six Cenomanian samples, and eight Albian samples. Three samples of Barremian age, five Hauterivian, and one Valangini an sample are from the Blake Bahama Formation. All samples were frozen immediately after collection and remained frozen until analysis began. The samples were freeze dried for determination of their total car bon contents with a Hewlett Packard 185B CHN Analyzer. Residual carbon was measured after HC1 dissolution of carbonates and was considered to represent the total organic carbon content. Percent cal cium carbonate was calculated from the difference between initial and residual carbon contents. Organic matter atomic C/N ratios were de termined from residual carbon values. Organic carbon contents of the samples were calculated on a dry weight basis (%) for the original, carbonate containing sediment. Stable carbon isotope ratios of the organic carbon content of these samples were determined on carbonate free samples using a VG Mi cromass 602 mass spectrometer calibrated with NBS 20 (carbonate) and NBS 22 (petroleum) standards. Data are corrected for 17 O and are presented relative to the PDB standard. A two stage extraction procedure was used to obtain the geolipid contents. Soxhlet extraction with toluene methanol yielded the easily extractable, or free, lipids. A second extraction with 0.5N KOH in methanol toluene provided the hydrolyzable, or bound, geolipids. Both fractions were treated with methanolic boron trifluoride to convert fatty acids to their methyl esters. Geolipid subfractions were separated by column chromatography on alumina over silica gel. The subfrac tions obtained contained alkanes and alkenes, aromatic hydrocarbons, fatty acid methyl esters, and hydroxy lipids including sterols and al kanols. Hydroxy compounds were silylated with bistrimethylsilyltri fluoroacetamide (BSTFA) prior to gas chromatography. Splitless injection gas liquid chromatography was employed to de termine the types and amounts of components present in the geolipid sub fractions. A Hewlett Packard 5830 FID gas chromatograph equipped with a 20 m SE54 fused silica capillary column was used with hydrogen as the carrier gas. Quantification was achieved through the use of known amounts of internal standards added to each sample before column chromatography. Individual compounds are tentatively identified by retention times in this preliminary survey. Reported values have been corrected for mass discrimination over the wide molecular weight range reported. LITHOLOGIC SETTING Cretaceous strata in the North Atlantic have been di vided into the Plantagenet Formation, the Hatteras For mation, and the Blake Bahama Formation on the basis 1195
Transcript
51. ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES ANDADJACENT STRATA FROM DEEP SEA DRILLING PROJECT SITE 603,
OUTER HATTERAS RISE1
Keith W. Dunham, Philip A. Meyers, and Pamela L. Dunham, The University of Michigan2
ABSTRACT
Organic matter contents of black shales from the Cretaceous Hatteras and Blake-Bahama formations have beencompared to those from surrounding organic-poor strata using C/N ratios, δ1 3C values, and distributions of extractableand nonsolvent-extractable, long-chain hydrocarbons, acids, and alcohols. The proportion of marine and land-derivedorganic matter varies considerably among all samples, although terrigenous components generally dominate. Mostblack shales are hydrocarbon-poor relative to their organic-carbon concentrations. Deposition of the black shales inHole 603B evidently occurred through turbiditic relocation from shallower landward sites and rapid reburial at this out-er continental rise location under generally oxygenated bottom-water conditions.
INTRODUCTION
Occurrences of dark-colored layers of Cretaceous rockshaving relatively high concentrations of organic matterhave been found in numerous locations studied as partof the Deep Sea Drilling Project (DSDP). The distribu-tion of such occurrences in the North Atlantic Oceanhas been discussed by Arthur (1979), Tucholke and Vogt(1979), Thierstein (1979), Graciansky et al. (1981), Weis-sert (1981), and Waples (1983), among others, with theintent of identifying the paleoceanographic factors in-volved in the formation of these unusual strata, com-monly called "black shales." Improved preservation oforganic matter, increased contribution of continental or-ganic matter to oceanic basins, and enhanced produc-tion of marine organic matter are some of the factorsthat have been suggested. Because these three possibili-ties affect the character of the organic content of blackshales, the organic matter in North Atlantic exampleshas been investigated; these investigations are summa-rized by Tissot et al. (1980), Summerhayes (1981), Katzand Pheifer (1982), and Graciansky et al. (1982). Vary-ing proportions of marine and terrigenous organic con-stituents are found in sediments deposited at differenttimes and locations in the Cretaceous Atlantic Ocean. Acomparison of the organic matter contained within blackshales with that of the adjacent organic-carbon-poor li-thologies further contributes to this information. In thischapter, we describe comparisons of analyses done onblack shales and closely bedded strata from the Hatterasand Blake-Bahama formations in the western North At-lantic Ocean.
Methods
Twenty-five Hole 603 B samples from different ages in the Creta-ceous were selected on board Glomar Challenger for this study. These
van Hinte, J. E., Wise, S. W., Jr., et al., Init. Repts. DSDP, 93: Washington (U.S.Govt. Printing Office).
2 Address: Oceanography Program, The University of Michigan, Ann Arbor, MI 48109.
were augmented with postcruise sampling from frozen core sections.Five sample groups contain closely bedded organic-carbon-rich andorganic-carbon-lean strata. Hatteras Formation samples consist of oneTuronian sample, six Cenomanian samples, and eight Albian samples.Three samples of Barremian age, five Hauterivian, and one Valangini-an sample are from the Blake-Bahama Formation. All samples werefrozen immediately after collection and remained frozen until analysisbegan.
The samples were freeze-dried for determination of their total car-bon contents 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-cium carbonate was calculated from the difference between initial andresidual carbon contents. Organic-matter atomic C/N ratios were de-termined from residual carbon values. Organic-carbon contents of thesamples were calculated on a dry-weight basis (%) for the original,carbonate-containing sediment.
Stable carbon isotope ratios of the organic-carbon content of thesesamples were determined on carbonate-free samples using a VG Mi-cromass 602 mass spectrometer calibrated with NBS-20 (carbonate)and NBS-22 (petroleum) standards. Data are corrected for 1 7O and arepresented relative to the PDB standard.
A two-stage extraction procedure was used to obtain the geolipidcontents. Soxhlet extraction with toluene-methanol yielded the easilyextractable, or free, lipids. A second extraction with 0.5N KOH inmethanol-toluene provided the hydrolyzable, or bound, geolipids. Bothfractions were treated with methanolic boron trifluoride to convertfatty acids to their methyl esters. Geolipid subfractions were separatedby column chromatography on alumina over silica gel. The subfrac-tions obtained contained alkanes and alkenes, aromatic hydrocarbons,fatty acid methyl esters, and hydroxy lipids including sterols and al-kanols. Hydroxy compounds were silylated with bistrimethylsilyltri-fluoroacetamide (BSTFA) prior to gas chromatography.
Splitless injection gas-liquid chromatography was employed to de-termine the types and amounts of components present in the geolipid sub-fractions. A Hewlett-Packard 5830 FID gas chromatograph equipped witha 20-m SE54 fused silica capillary column was used with hydrogen asthe carrier gas. Quantification was achieved through the use of knownamounts of internal standards added to each sample before columnchromatography. Individual compounds are tentatively identified byretention times in this preliminary survey. Reported values have beencorrected for mass discrimination over the wide molecular-weightrange reported.
LITHOLOGIC SETTING
Cretaceous strata in the North Atlantic have been di-vided into the Plantagenet Formation, the Hatteras For-mation, and the Blake-Bahama Formation on the basis
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K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM
of earlier DSDP sampling (cf. Jansa et al., 1978; Sheri-dan et al., 1983; Summerhayes and Masran, 1983). Therocks comprising these formations at Site 603 are de-tailed in the Site 603 chapter (this volume) and are onlybriefly described here. Black shales were found in rocksranging in age from Santonian to Berriasian at Hole603B on the outer Hatteras Rise in the North AmericanBasin (Fig. 1). Neither the lithologic settings nor the or-ganic-carbon contents of these deposits are uniform overthis considerable time span, although the common pres-ence of turbidites and of bioturbation show some impor-tant similarities existed in the depositional environments.Santonian to late Turonian rocks consist of variegatedclaystones with sparse black shales and correspond to thePlantagenet Formation. The early Turonian to Aptian sec-tion contains abundant black shales interspersed amongred and green claystones and corresponds to the Hat-teras Formation. Limestones and sandstones with blackclaystone turbidites make up the Aptian-Berriasian Blake-Bahama Formation. Cenomanian black shales containthe highest concentrations of organic carbon (Herbin etal., this volume; Meyers, this volume), and all black shales
generally exist as thin strata surrounded by organic-car-bon-poor rocks.
RESULTS
Organic Carbon, C/N Values, and Carbon Isotopes
Hatteras Formation
The seven black shales of the Hatteras Formation havea significantly higher concentration of organic carbonthan the adjacent lighter-colored strata. Organic carbonaverages 2.71% in the black shales and 0.28% in the ad-jacent strata (Table 1). Mclver (1975) compiled an aver-age value of 0.3% organic carbon for ancient deep-oceansediments from DSDP Legs 1 through 33. This valuemay be considered the background level for normal deep-ocean sediments. The high concentrations found in theblack shales, therefore, indicate unusual circumstancesfor their deposition.
C/N ratios of Hatteras Formation black shales haveaverages significantly higher than adjacent green and redclaystones. Based upon a survey of marine sediments,
45°N
40'
35'
r i i i i i i I i i
DSDP SitesLegs 2, 11, 43,44, 76, 93
80° 75° 70° 65°
Location of DSDP Site 603 in relation to other DSDP sites in the western North Atlantic Ocean.
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ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES
Table 1. General descriptions of Cretaceous samples selected for organic geochemical comparison, Hole
Premuzic et al. (1982) suggest that C/N ratios less than8 indicate mostly marine organic matter, and values greaterthan 15 show a predominance of land-derived material.With increasing time of burial, however, C/N ratioschange as a result of diagenesis. Waples and Sloan (1980)report a gradual decrease in C/N values from about 10in Quaternary sediments to about 4 in Miocene samples,followed by increases in older sediments. Furthermore,C/N values tend to be high in sediments where marineorganic matter is well preserved, such as in Cenomanianblack shales from the Angola Basin (Meyers et al., 1984).
Carbon isotope data provide further information aboutthe sources of organic matter in these samples. In gen-eral, land-derived organic matter is more depleted in 1 3Cthan is marine organic matter, although carbon isotoperatios appear to be sensitive to diagenetic modificationin black shale deposits and hence should not, by them-selves, be considered absolute determinants of source(Dean, Claypool, et al., 1984; Meyers et al., 1984). Hat-teras Formation black shales average -25.9%o and rangefrom -23.8 to -27.7%o. Adjacent green and red clay-stones have a wide range of δ1 3C values, from - 22.0 to-29.9%o.
Blake-Bahama Formation
Organic-carbon percentages of the five Blake-BahamaFormation black shales average 1.92% (Table 1). Lime-stones and sandstones adjacent to the black shales aver-age slightly lower values, with the exception of lime-stone Sample 603B-66-2, 123-126 cm, which has a valueof 0.04% organic carbon.
C/N ratios in the Blake-Bahama Formation are rela-tively high, averaging 59.3. There are two large devia-tions from this average. Sample 603B-71-5, 135-139 cmhas an undetectable amount of nitrogen; therefore, theC/N ratio is reported as greater than 100. Sample 603B-66-2, 68-70 cm has a C/N value of 6.3 (Table 1).
Blake-Bahama black shales have δ1 3C values averag-ing -25.O%o. Adjacent lithologies once again show awide range of values from - 23.6 to - 28.7%o (Table 1).
Extractable Alkanes, Fatty Acids, and Alkanols
Hatteras Formation
Concentrations of extractable (free) and bound geolip-ids are given in Tables 2 and 3 in terms of parts per mil-lion of dry sample weight and also relative to the organ-ic-carbon concentrations. In comparison to samples fromDSDP Site 530 in the Angola Basin, the organic-car-bon-lean samples contain about the same amounts of n-alkanes, but are richer in alkanoic acids and alkanols(Meyers et al., 1984). The black shale samples, in con-trast, contain substantially lower concentrations thanthe black shales from the Angola Basin (Meyers et al.,1984) and often less than adjacent strata. It is evidentthat these black shales of the Hatteras Formation aregeolipid-lean.
The free and bound geolipid distribution of the blackshales and adjacent strata are compared in Figures 2through 9. Long-chain-length geolipids, such as C27-C33
odd-numbered rt-alkanes and C24-C32 even numbered n-alkanoic acids and n-alkanols, are present in all samples
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K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM
Table 2. Concentrations of solvent-extractable (free) and nonsolvent-extractable (bound) geolipid fractions obtained from Hole 603Bsamples from the Hatteras and Blake-Bahama formations.
Black shaleLimestoneBlack shaleBlack shaleLimestoneBlack shaleBlack shaleSandstoneLimestone
n-Alkanes
Free
2.9
2.9
1.63.3
0.8
1.7.2.5
2.2
1.91.51.90.5
0.87.7
5.0
1.7
4.1
3.82.8
4.0
2.6
2.9
2.4
Bound
_
0.7
0.4
0.5
1.20.7
3.4
10.40.27.9
0.2
13.2-
0.2
0.1
-1.00.20.5
0.4
0.8
0.5
0.5
π-Alkanoicacids
Free
24.018.610.410.115.3
5.1
9.9
8.8
6.5
9.21.1
4.4
8.9
38.8
22.710.617.118.09.0
32.922.519.819.1
Bound
7.8
4.8
1.8
5.5
3.2
2.8
1.7
3.1
1.1
0.4
1.30.4
7.0
3.6
4.1
1.7
2.0
4.7
1.1
6.2
2.2
2.8
1.2
π-Alkanols
Free
11.924.236.211.218.5_
15.113.8
2.7
10.111.965.6
28.714.017.817.128.425.524.024.315.4
Bound
3.2
3.2
2.1
2.2
2.9—
1.30.7
1.60.6
11.93.3
6.7
1.40.7
1.4
1.2
1.6
0.6
1.2
1.4
Note: All concentrations given in micrograms of geolipid components, as measured by gas chro-matography, per gram dry weight of sample. Dash indicates that the concentration was notmeasured.
and are characteristic of land plant waxes (Simoneit,1978). Short-chain-length w-alkanes, such as C1 7-C2i, arefound more often in the bound fraction. These short-chain-length /7-alkanes can be indicative of algal input(Simoneit, 1978). Comparison of the seven black shalesamples reveals one dominant geolipid feature. A largeproportion of terrigenous components is present in boththe free and bound fractions. The importance of land-derived geoliopids is especially conspicuous in the n-al-kane distributions, which in this regard resemble distri-butions reported for black shales sampled from the Hat-teras and Blake-Bahama formations at DSDP Sites 391and 534, farther south of Site 603 in the North Ameri-can Basin (Deroo et al., 1978; Erdman and Schorno,1978; Stuermer and Simoneit, 1978; Herbin et al., 1983).The ratio of the isoprenoid hydrocarbons, pristane andphytane, is generally less than one in all of the HatterasFormation samples.
Both the free and bound fatty acid distributions aredominated by «-C16, which is a ubiquitous componentcharacteristic of all biota (Figs. 2 through 9). Distribu-tion of «-fatty acids between black shales and adjacentstrata are very similar. The black shales show a strongland-derived geolipid signature. These distributions aresimilar to a black shale at Site 391, also from the Hat-teras Formation (Cardoso et al., 1978).
Alkanol distributions in the Hatteras Formation showlittle variation between black shales and adjacent greenand red claystones. Common to both the free and boundfractions in all of the samples is the dominance of the
Table 3. Geolipid concentrations from Hatteras and Blake-Bahama formation Hole603B samples relative to organic carbon concentrations.
Black shaleLimestoneBlack shaleBlack shaleLimestoneBlack shaleBlack shaleSandstoneLimestone
M-Alkanes
Free
0.70.44.81.51.76.8
13.22.2
10.05.02.32.52.62.7
1.63.22.51.9
70.04.01.42.01.2
Bound
_2.10.21.14.83.73.5
54.70.79.61.0
42.6
—
0.10,20.60.1
12.50.40.40.30.3
n-Alkanoicacids
Free
6.22.6
31.54.6
32.620.452.1
9.034.230.7
1.322.028.713.4
7.220.010.29.2
225.033.212.113.69.6
Bound
2.00.75.52.56.8
11.28.93.25.81.31.62.0
22.61.2
1.33.2
1.22.4
27.56.3
1.21.90.6
n-Alkanols
Free
3.13.4
109.75.1
39.4———79.546.0
3.350.538.422.7
9.126.410.78.7
710.025.812.916.67.7
Bound
0.80.56.41.06.2———6.82.32.03.0
38.41.1
2.12.6
0.4
0.7
30.01.60.30.80.7
Note: Solvent-extractable (free) and nonsolvent-extractable (bound) concentrations in parts per mil-lion are divided by percent organic carbon for each geolipid fraction. Dash indicates that theconcentration was not measured.
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ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES
Free fraction100
5 0
603B-33, CC,1-18 cm
l ! I , l l I I I I , • • l • • . i • l . l I15 17 19 21 23 25 27 29 31 33 14 16 18 20 22 24 26 28 30 14 16 18 20 22 24 26 28 30
100
50 -
603B-34-2,85-87 cm
n i l I i l l I I II I i l l i l l I • . i . i i i . 1. . . i
Figure 2. Distributions of free (solvent-extractable) geolipids from three Hatteras Formation black shales. Relative abundances are normal-ized to the major component in each sample. Isoprenoid hydrocarbons pristane and phytane are presented as dotted and dashed lines,respectively.
Bound fraction100
50
603B-33, CC,1-18 cm
II I I 1,1,1.1 I • i •15 17 19 21 23 25 27 29 31 33 14 16 18 20 22 24 26 28 30 14 16 18 20 22 24 26 28 30
Figure 4. Distributions of free (solvent-extractable) geolipids from three closely spaced Cenomanian samples from the Hatteras Formation.See Figure 2 legend for details.
100
50
Section 603B-37-4, bound fraction
119-123 cmGreen
I I I111 . . . I . I . . l l I i l l15 17 19 21 23 25 27 29 31 33 14 16 18 20 22 24 26 28 30 14 16 18 20 22 24 26 28 30
100
> 50 -
123-1 26 cmBlack
1 1 1 1 I III 1 1•il l l l. || I III ll II I I 1 1 | |
Figure 5. Distributions of bound (non-solvent-extractable) geolipids from three closely spaced Cenomanian samples from the Hatteras For-mation. See Figure 2 legend for details.
126-128 cmGreen
I I I I I || | . . . 1 . 1 . 1 . | | 1 l l l l l l
1200
ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES
Figure 6. Distributions of free (solvent-extractable) geolipids from three closely spaced Albian samples from the Hatteras Formation. SeeFigure 2 legend for details.
Figure 7. Distributions of bound (hon-solvent-extractable) geolipids from three closely spaced Albian samples from the Hatteras Forma-tion. See Figure 2 legend for details.
Figure 8. Distributions of free (solvent-extractable) geolipids from five closely spaced Albian samples from the Hatteras Formation. SeeFigure 2 legend for details.
C28 and C 3 0 /t-alkanols. Long-chain /2-alkanols such asthese have been interpreted to be indicators of terrige-nous geolipids in marine sediments (Brassell et al., 1982).The majority of the samples contain large quantities ofC22 w-alkanol, which has been observed in other deep-ocean organic-lean sediments (Keswani et al., 1984) andmay result from microbial processing of sediment or-ganic matter.
Blake-Bahama Formation
Concentrations of extractable and bound geolipids aregiven in Tables 2 and 3 in terms of parts per million of
dry sample weight and relative to the organic-carbon con-centrations. Both the organic-carbon-rich and organic-carbon-lean samples follow the same observations in theHatteras Formation. In comparison to the adjacent stra-ta and other deep-ocean black-shales, these Blake-Baha-ma Formation shales are geolipid-lean.
Free and bound geolipid information for the Blake-Bahama Formation samples are compared in Figures 10through 15. Differences between the distribution of n-alkanes in organic-carbon-rich and organic-carbon-leansamples are not straightforward. All of the samples, how-ever, have a strong terrigenous geolipid signature with
1202
100
50 k-
ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES
Section 603B-40-2, bound fraction
26-2JRe(
)ci
m
h I I . I . I l l . . . . i l l ll I II15 17 19 21 23 25 27 29 " 31 33 14 16 18 20 22 24 26 28 30 14 16 18 20 22 24 26 28 30
I I I H I M i 11 i LI 1 | ,.l 115 17 19 21 23 25 27 29 31 33 14 16 18 20 22 24 26 28 30 14 16 18 20 22 24 26 28 30
100
50 h-
I
51-54 cmRed
. i l l 1 1. 1 1 1 . . . • I . . . I . I .1 ., 1 i
-
15 17 19 21 23 25 27 29 31 33
n-Alkane chain length
14 16 18 20 22 24 26 28 30
π-Fatty acid chain length
14 16 18 20 22 24 26 28 30
π-Alkanol chain length
Figure 9. Distributions of bound (non-solvent-extractable) geolipids from five closely spaced Albian samples from the Hatteras Forma-tion. See Figure 2 legend for details.
abundant C2η, C29, C 3 1, and C 3 3 «-alkanes. The ratio ofthe isoprenoid hydrocarbons, pristane and phytane, isgenerally less than one when available.
Unlike the /2-alkanes, very little variability is observedin the «-fatty acid distributions between individual blackshale units and adjacent strata. As in the Hatteras For-mation, C 1 6 n-fatty acid dominates the distribution pat-tern. Significant amounts of longer chain lengths indi-cate strong land-derived w-fatty acids (Simoneit, 1978).
The distribution of both free and bound rc-alkonolsshows a terrigenous geolipid dominance. Short-chain-length algal inputs are not observed in the free fraction;
however, some of the bound fractions from the Blake-Bahama Formation contain C 1 6 and Qg «-alkanols. Thepresence of C 2 2 «-alkanol is variable. It ranges from lowlevels to the most abundant n-alkanol in Sample 603B-36-3, 135-139 cm (Figs. 10 through 15).
DISCUSSION
Hatteras Formation
Organic matter in the black shales is a mixture of ma-rine and varying proportions of continentally derived or-ganic matter. The high C/N values may record selective
1203
K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM
Free fraction
100
50
603B-49-2,98—101 cm
. I I • h l l • l •15 17 19 21 23 25 27 29 31 33 14 16 18 20 22 24 26 28 30 14 16 18 20 22 24 26 28 30
Figure 11. Distributions of bound (non-solvent-extractable) geolipids from three Blake-Bahama Formation black shales. See Figure 2 leg-end for details.
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ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES
Figure 12. Distributions of free (solvent-extractable) geolipids from three closely spaced Hauterivian samples from the Blake-Bahama For-mation. See Figure 2 legend for details.
Figure 13. Distributions of bound (non-solvent-extractable) geolipids from three closely spaced Hauterivian samples from the Blake-Baha-ma Formation. See Figure 2 legend for details.
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K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM
100
5 0
Free fraction
603B-53-4,140—144 cm
. I I I l l I , . , I I i l Mil15 17 19 21 23 25 27 29 31 33 14 16 18 20 22 24 26 28 30 14 16 18 20 22 24 26 28 30
Figure 14. Distributions of free (solvent-extractable) geolipids from two limestones and one sandstone from the Blake-Bahama Formation.See Figure 2 legend for details.
Figure 15. Distributions of bound (non-solvent-extractable) geolipids from two limestones and one sandstone from the Blake-Bahama For-mation. See Figure 2 legend for details.
603B-76-3,135-139 cm
| | | . . . . . I I I . . ,11 1 . . . • 1 |
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ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES
preservation of carbonaceous components relative to ma-rine organic nitrogenous components. Organic matterin the organic-carbon-lean green and red claystones ofthe Hatteras Formation appears to be made up of detri-tal, highly oxidized materials similar to those found inmost deep-sea sediments (cf. Degens and Mopper, 1976).Such sediments commonly have low C/N ratios in olderstrata.
Comparison of carbon isotope compositions of blackshales with their adjacent strata shows a positive rela-tionship between higher organic-carbon concentrationsand a stronger marine isotope character in black shalesamples (Fig. 16A). The isotope values of rocks surround-ing the black shales display a wide range that suggestsvarying origins of the organic matter in these layers ofturbiditic, organic-carbon-lean rock. This relationshipimplies that the organic-matter concentration of Hat-teras Formation black shales is directly related to burialof greater proportions of marine organic matter super-imposed upon a background of poorly preserved organ-ic matter from variable sources. C/N and δ13C valuesare both indicators of the source of organic matter. HighC/N values from vascular land plants should theoreti-cally relate to light δ13C values and lower C/N valuesfrom marine organic matter with heavier δ13C values.There is not such a relationship between C/N values andδ13C values in these Hatteras Formation samples (Table1). This further indicates the susceptibility of C/N val-ues, in particular, to diagenetic changes.
Geolipid distribution and concentrations in black shalesand the adjacent strata are difficult to interpret. Thereappears to be no emergent geolipid pattern in relation-ship to other organic geochemical parameters. For ex-ample, three black shale samples—Samples 603B-33,CC,1-18 cm; 603B-34-2, 85-87 cm; and 603B-42-3, 42-43
cm—have dissimilar n-alkane geolipid distributions (Fig.2). Sample 603B-34-2, 85-87 cm has an «-alkane geoli-pid pattern typical of terrigenous organic matter. Theδ13C value for this sample, however, is the most marinein character compared to the other two black shales.Part of the explanation for this characteristic may arisefrom enhanced preservation of nonlipid components ofsediment organic matter in black shales. Such dilutionof geolipid concentrations is postulated for black shalesrelatively rich in organic carbon (Meyers et al., 1984). Inblack shales with relatively low amounts of organic car-bon, such as Sample 603B-42-2, 42-40 cm, it appearsthat even the lipid components have been microbially re-worked and nearly all of the remaining organic matter isdetrital inert matter.
Fatty acid and alkanol concentrations in the blackshales are high compared to other deep-ocean blackshales (Meyers et al., 1984). The higher amounts of thelatter two geolipid fractions may reflect larger propor-tions of waxy land-plant material in the Hatteras For-mation samples, which is consistent with the geolipiddistributions (Figs. 2 through 9). Turbidites are commonthroughout the Hatteras and Blake-Bahama formations;therefore, transport of continental organic matter to thisdeep-sea location is feasible. It is surprising, however,that the «-alkanol distributions are so similar despiteconsiderable differences in total organic-carbon concen-trations and in «-alkane distributions. Alkanols are con-sidered to be more subject to microbial reworking thanare alkanes, with the result that the C22 fl-alkanol sus-pected to indicate microbial activity is the major alkanolfound in open-ocean sediments in which organic carbonconcentrations are low (Keswani et al., 1984). It seemsparadoxical that many of the Hatteras Formation blackshales (Figs. 2 through 10) display major contributions
1too
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anic
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Black shales
Green shales
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• Limestones
• Sandstone
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δ 1 3 C δ 1 3 C
Figure 16. Organic carbon percentages vs. carbon isotope values, Hole 6O3B. A. Hatteras Formation samples. B. Blake-Bahama Formationsamples.
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K. W. DUNHAM, P. A. MEYERS, P. L. DUNHAM
of the C22 /2-alkanol, unless these samples originally con-tained organic matter of the proper type and amount tosupport microbial activity.
Although pristane/phytane values below one have beenproposed as an indication of anoxic depositional condi-tions (Didyk et al., 1978), methanogenic bacteria gener-ate phytane during anaerobic fermentation of organicmatter (Risatti et al., 1984). Because such fermentationcan occur under anoxic conditions deeper in sedimentsas well as at an anoxic water/sediment boundary, lowpristane/phytane ratios may record bacterial fermenta-tion rather than a specific depositional condition.
Blake-Bahama Formation
Organic matter in the Blake-Bahama Formation blackshales is predominantly terrigenous with small admix-tures of marine organic matter. High C/N values for theBlake-Bahama limestones and sandstone suggest theselithologies may contain substantial amounts of coaly ma-terial eroded from continental deposits. The Rock-Evalresults (Meyers, this volume) show the relatively inertcharacter of organic matter in samples of similar lithol-ogies, even though some are fairly rich in organic car-bon.
Comparison of carbon isotope compositions for theblack shales, limestones, and sandstone does not showthe clear relationship with organic-carbon content seen inthe Hatteras Formation samples (Fig. 16B). Some rela-tionship, however, may exist in the lighter-colored, usu-ally organic-carbon-lean samples. The lean samples clusteraround -28.0‰. This cluster may indicate the normalbackground load of organic matter—highly terrigenousin nature. Sample 603B-76-3, 135-139 cm is the one ex-ception; however, it does contain a larger amount of or-ganic carbon.
All distributions of n-alkanes in the Blake-BahamaFormation black shales show a strong terrigenous char-acter (Figs. 10 through 15). Shorter chain lengths, whenpresent, do not show a major algal input. The absenceof odd-chain-length predominance may indicate consid-erable bacterial reworking (Simoneit, 1978). Fatty acidand alkanol concentrations are high compared to otherdeep-ocean black-shales, most likely for the same reasonsalready outlined for the Hatteras Formation samples.
General
The proportion of continental and marine organic mat-ter present in black shale samples from the western NorthAtlantic varies considerably. Although most of the or-ganic matter appears to be terrigenous (Katz and Pheifer,1982), the marine fraction increases with distance fromNorth America (Tissot et al., 1980; Summerhayes andMasran, 1983). This pattern has been explained by Sum-merhayes and Masran (1983) to reflect the decrease inturbiditic dilution of marine sediments with continentalmaterials as distance from shore increases. An exceptionto this generality occurs in Cenomanian black shales,where large proportions of marine organic matter arefound at Site 105 (Summerhayes, 1981) located on thecontinental rise and in turbiditic environments. This ex-
ception illustrates the regional and temporal variabilitythat can exists in the mixture of organic-matter types inblack shales. On the basis of carbon isotope ratio vari-ability, the organic-carbon-lean rocks surrounding blackshale samples from Site 603 also appear to demonstrateconsiderable variation in their proportions of marine andterrigenous organic matter.
The paleoceanographic picture that emerges is thatduring the Cretaceous, the western North Atlantic ap-pears to have been a narrow sea filled with oxygenatedwater. It appears to have received an abundant supply ofterrigenous elastics from North America and to have hadat best moderate levels of marine productivity. How didblack shales form under such conditions? Whereas it ispossible that bottom-water anoxia existed from time totime in the basins comprising the proto-Atlantic, it isunlikely that enough marine organic matter would sur-vive sinking through the predominantly oxygenated wa-ter column to form black shales. It is more likely thatthe organic material reached the deep-ocean sites of blackshale deposition in the company of turbidity flows, andpreservation occurred from rapid burial in the oxygenat-ed deep basins.
This scenario of downslope transport and redepositionis essentially the same as earlier proposed by Dean, Ar-thur, et al. (1984) for Site 530 in the Angola Basin andby Robertson and Bliefnick (1983) for Site 534 in theBlake-Bahama Basin, and it helps explain the variabilityin organic character present in the sediments of the west-ern North Atlantic. This explanation is based upon theassumption that each occurrence of black shale deposi-tion is a local or regional event, loosely linked to othersuch events by global paleoceanographic conditions.Changes in climate, sea level, or oceanic circulation mightdestabilize ocean margin sediments and initiate turbidi-ty flows at numerous locations around the North Atlan-tic. Such flows need not be absolutely synchronous andneed not contain the same organic-matter content. Ex-cept for those originating where the oxygen-minimum lay-er intercepts the bottom, most would actually be poor inorganic carbon; all would have their proportions of ma-rine and terrigenous materials controlled by local condi-tions. On the seafloor, these flows would form fanlikedeposits that would overlap and interfinger, rarely creat-ing the continuous layers like those formed under shallowepicontinental seas. Although each black shale depositwould be initiated by a common set of global or ocean-wide conditions, its individual characteristics would bedetermined by regional or local factors.
CONCLUSIONS
1. The content and character of organic matter inCretaceous black shales from DSDP Site 603 in the west-ern North Atlantic are quite variable.
2. Consistent with their large fraction of terrigenousorganic matter, the black shales from Site 603 are rela-tively poor in extractable alkanes, alkanoic acids, andalkanols.
3. The lack of correspondence in isotope values be-tween the black shales and their adjacent strata, despite
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ORGANIC GEOCHEMICAL COMPARISON OF CRETACEOUS BLACK SHALES
similarities in geolipid distributions, implies differencesin sources of the bulk, nonlipid organic matter.
4. The black shales represent short episodes of en-hanced burial of organic matter within a general settingof poor preservation of carbonaceous materials. A pre-vailing oxygenated depositional setting is indicated byfaunal burrowing, oxidized minerals, and low concen-trations of organic carbon in most of the Cretaceousrocks from the western North Atlantic.
5. A scenario of downslope transport by turbidityflows from ocean margin locations within oxygen min-ima and redeposition in deep-ocean settings is invokedto explain black shale formation at Site 603 and otherwestern North Atlantic locations.
ACKNOWLEDGMENTS
We thank the Deep Sea Drilling Project for providing the samplesused in this study and for giving P. A. Meyers the opportunity to par-ticipate in Leg 93. We are especially grateful to K. C. Lohman and hisstaff for the use of the carbon isotope mass spectrometer and to J. Po-waser for laboratory assistance. This work was partially supported bythe U. S. National Science Foundation.
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