25. GEOCHEMISTRY OF CARBON: DEEP SEA DRILLING PROJECT LEG 41

J.G. Erdman and K.S. Schorno, Phillips Petroleum Company, Bartlesville, Oklahoma

INTRODUCTION

Eleven core sections taken at Site 368 on DSDP Leg41 from the Cape Verde Rise (Figure 1) were analyzedfor organic content. The samples represent the full suiteof cores taken for organic geochemical study for thissite. The data provide information on depth control ofthe organic composition.

SAMPLING AND STUDY PROCEDURESThe core samples were collected in their plastic liners,

frozen onboard ship, and remained frozen thereafter. Achannel was cut down two sides of the plastic liner ofeach frozen core section with a milling machine to splitthe liner, and an aspirator was used to remove chips ofplastic. All parts of the milling machine which couldcome in contact with the sample were washed withmethylene chloride to eliminate any possible oilcontamination. The channels were cut so that only amillimeter or so of plastic remained. The final cutsevering the liner and cutting the core longitudinallywas made with a band saw, the blade of which had alsobeen cleaned with methylene chloride.

The flat surfaces of the core section were smoothedusing the milling machine for description andphotographing while the cores were still frozen.Photographs of the cores, diagrams of textural or colorpatterns, and brief lithological descriptions arecontained in Figures 2 through 12. The sections areoriented in the photographs with the top of the corelocated near the zero point of the scale.

Chip samples, 10 to 20 mg in size, of each texturaltype and color were removed for determination ofcarbonate carbon and organic carbon content. Thesevalues are listed in Table 1. The procedures forseparation and analysis of these samples have beendescribed previously (Erdman and Schorno, in press).

RESULTS AND DISCUSSIONNumerical data for the homogenized samples are

provided in Tables 2 through 4 and Figure 13. Table 2gives depth and ages of each core, the averagecarbonate and organic carbon content of the coresections, the proportions of the organic fractionconsisting of lipids and kerogen, and the carbonisotopic ratios of the lipid and kerogen fractions. Table3 contains the nitrogen composition of the dried core asdetermined by the full- and half-Kjeldahl methods.Table 4 provides the normalized composition ofsaturates, aromatics and asphaltics fractions, theelemental composition of carbon, hydrogen, nitrogen,

sulfur, and oxygen in the lipid fractions, and the moleratios of hydrogen to carbon.

The lithologies of these sediments range from clayand claystone in the Upper Cretaceous to calcareousooze in the upper Miocene and Plio-Pleistocene.

The organic content is relatively uniform averaging1.1 weight percent with a range of 0.3 to 2.7. Only thePaleocene Core 368-51-2 contained insufficientamounts of organic matter to support petroleumgenesis.

Lipid makes up 1% to 3% of the total organic matterfor 9 of the 11 cores which falls within the range of mostDSDP samples studied in this laboratory. The UpperCretaceous Section 368-63-3 contains three to five timesas much lipids as the remaining cores. The increase inlipids reflects the increased rate of petroleum genesisdue to the injection of a diabase sill at 957 to 970.5

40c

35'

30'

25C

20c

15C

10c

30 25" 20° 15" 10° 5° 0°

Figure 1. Location of site from which samples for geochem-ical study were obtained.

849

J. G. ERDMAN, K. S. SCHORNO

FREEZING

CRACKS

Figure 2. Site 368, Core 3, Section 5. Foraminiferal ooze, lightolive-gray (5Y 6/1), highly calcareous. Foraminifers abundant,up to 1 mm diameter.

Figure 3. Site 368, Core 8, Section 3. Calcareous ooze,yellowish-gray (5Y 8/1). No fossils discernable. Clay clasts,olive-black (5Y 2/1).

Figure 5. Site 368, Core 18, Section 5. Gay,grayish-blue-green (5BG 5/2). Contains siliceousspicules and abundant disseminated pyrite.

Figure 4. Site 368, Core 15, Section 3.Gay, greenish-gray (5G 6/1).

Figure 6. Clay, greenish-gray (5GY 6/1). Con-tains scattered pyrite.

850

GEOCHEMISTRY OF CARBON

Figure 7. Site 368, Core 32, Section 2. Clay, greenish-gray (5GY 6/1)with subangular to rounded clasts of white clay to 2 cm pyrite presentin one 5 mm nodule.

Figure 8. Site 368, Core 37, Section 3. Clay, dustyyellow-green (5GY5/2) streaked with olive-black (5Y2/1)along continuous bedding surfaces. Well indurated, fissile.

Figure 10. Site 368, Core 51, Section 2. Unit A-Clay, greenish-gray (5G Y 4/1). Dark flecks ap-pear to be plant macerals. Unit B-Clay, same asUnit A but dark yellowish-brown (10YR 4/2).

Figure 9. Site 368, Core 43, Section 2. Clay,olive-gray (5 Y 4/1). Well indurated, fissile.

Figure 11. Site 368, Core 58, Section 4.Clay, dark greenish-gray (5GY 4/1)with light gray clasts (2). Containsscattered pyrite in 1-2 mm blebs.

851

J. G. ERDMAN, K. S. SCHORNO

TABLE 1Carbonate and Organic Carbon Valuesfor Inclusions Noted in Figures 2-12

Figure

2

3

4

5

6

7

8

9

10

11

12

Site-Core-Section

368-3-5

368-8-3

368-15-3

368-18-5

368-27-2

368-32-2

368-37-3

368-43-2

368-51-2

368-58-4

368-63-3

Inclusion

1234

12

1

12

12

1

1

123

12

12

123

Carbonate

Carbon(wt %)

3.45a

5.729.36

8.71a

4.80

0.17a

<O.l2.03

0.11

2.37

0.200.240.22

0.200.24

<O.l0.26

<O.l<O.l<O.l

CaC03(wt %)

28.75a

47.6778.00

72.58a

40.00

1.42a

<0.8316.92

0.92

19.75

1.672.001.83

1.672.00

<0.832.17

<0.83<0.83<0.83

OrganicCarbon(wt %)

0.800.500.290.33

0.270.56

0.92

1.562.00

0.991.50

0.55

0.60

0.190.470.47

0.190.47

0.270.14

1.500.531.30

insufficient sample size for determination.

meters. The Plio-Pleistocene Section 368-3-5 containsvery little lipid which probably is an indication of a veryearly stage of petroleum genesis.

All 11 cores contain organic matter derived frommarine sources as determined by the carbon isotopicratios of the lipid fraction ranging from -28.6 to -23.7

Figure 12. Site 368, Core 63, Section 3. Unit A-Clay,olive-gray (5Y 3/2), plastic. Thin laminae of light grayprobably due to drilling disturbance. Unit B-Clay, darkgreenish-gray (5G Y 4/1).

δC;P

3

DB, Admixture of a considerable contribution ofterrestrial organic matter into all but the two carbonatesediments is indicated by the very light carbon isotopicvalues for the kerogen.

Both the ammonia and total nitrogen contents arerelatively constant from the Upper Cretaceous to theOligo-Miocene Section 368-18-5 and then decrease inthe younger sediments. The highest values for theammonia and the organic nitrogen, i.e., the differencebetween the total nitrogen as determined by the full-Kjeldahl and the ammonia nitrogen as determined bythe half-Kjeldahl, are associated with the sedimentscontaining the highest organic content.

The percentage of saturated hydrocarbons isrelatively constant, averaging 2.7 weight percent with arange of 1.3 to 6.2, in the Plio-Pleistocene to the lower

TABLE 2Carbonate Content and Concentration of Organic Matter (in wt %) and Carbon Isotopic Ratio Values of the

Lipid and Kerogen Fraction (in δC Units) From Geochemical Samples From DSDP Leg 41rDt>

Site-Core-Section

368-3-5368-8-3368-15-3368-18-5368-27-2368-32-2368-37-3368-43-2368-51-2368-58-4368-63-3

Geologic 4ge

Plio PleistU. MioceneM. MioceneU. Oligo/1 MioM. EoceneM. EoceneM. EoceneL. EocenePaleoceneU. CretaceousU. Cretaceous

SubbottomDepth (m)

50.1-50.2181.4-181.5250.1-250.2271.3-271.4391.3-391.4451.3-451.4541.4-541.5611.4-611.5721.3-721.4931.4-931.5981.3-981.4

Percent of Rock

Carbonate

Carbon(wt %)

4.379.58

<O.IO<O.IO<O.IO<O.IO<O.IO

0.18<O.IO<O.IO

0.25

CaCθ3(wt %)

36.4279.83<0.83<0.83<0.83<0.83<0.83

1.50<0.83<0.83

2.08

Organic Carbonate

Total(wt %)

0.650.640.990.682.121.372.701.780.350.790.97

Lipid(wt %)

0.0020.0170.0120.0090.0340.0130.0220.0300.0070.0150.096

Kerogen(wt %)

0.650.620.980.672.091.362.681.750.340.780.87

%ofTotal

OrganicCarbon

Lipid

Carbon(wt %)

0.362.601.221.311.600.950.811.702.121.869.89

Lipid

a-26.5-28.6-26.2-27.8-26.0-27.2

a-26.2-23.7-28.1

Carbon IsotopicComposition

Kerogen

-21.3-20.1

aa

-29.5a

-29.2-33.4-27.7-31.3-29.6

Diff.

Lipid-Kerogen

-6.4

13..51.7

2.04.31.57.61.5

lSample insufficient or unsuitable for this determination.

852

TABLE 3Full and Half Kjeldahl Deter-

minations From Sediments FromDSDP Leg 41

Site-Core-Section

368-3-5368-8-3368-15-3368-18-5368-27-2368-32-2368-37-3368-43-2368-51-2368-58-4368-63-3

Nitrogen (ppm)

Half

47136

1574

150162262218213237221

Full

300200200400700200700600500600400

10 15 20 25Carbon number

30 35 40

Figure 13. Normalized n-Alkanes versus carbon numberplots from the Upper Cretaceous Sections 368-58-4 and368-63-3.

GEOCHEMISTRY OF CARBON

Eocene sediments, then increases sharply to 16.4% and18.6% above and below the sill in the UpperCretaceous. The sharp increase in saturates togetherwith a sharp decrease in the ratio of methane to ethane,as reported in the Initial Shipboard Report, givesfurther confirmation that the rate of catageneous of thesedimentary organic matter is influenced by proximityto the diabase sill.

The π-alkane distributions were obtained for the twoUpper Cretaceous samples. These distributions aresimilar as shown in Figure 13. The differences can beattributed to either a more advanced state of petroleumgenesis in the deeper core in that the odd to evenpreference is reduced, a different source of organicmatter as indicated by the large difference in δC'|DBpfthe lipid fractions, or oil migrating into these sections.

The percentage of carbon in the lipid fractionincreases with depth from 74.3% in the middle Eoceneto 81.8% in the Upper Cretaceous, but the percentageof hydrogen, nitrogen, and sulfur remain constant. Theratios of hydrogen to carbon, however, decrease from1.64 (cyclohexane = 2.0) to 1.57 (toulene = 1.2) overthis interval indicating that the lipid fraction isbecoming slightly more aromatic with depth.

CONCLUSION

The organic content in these unconsolidatedsediments is relatively high and in an early stage oforganic diagenesis for all except the Upper CretaceousSection 368-63-3 lying 10 meters below the diabase sill.Because the temperature effects of the sill on thesesediments is localized to only 2 or 3 meters from the sill-sediment interface, the changes that are observed in thelipid fraction of the Upper Cretaceous Section 368-63-3are probably due to nonindigenous oil that hadmigrated from sediments lying closer to the sill.

REFERENCEErdman, J.G. and Schorno, K.S., in press. Geochemistry of

carbon: Deep Sea Drilling Project Leg 38. In Talwani, M.,Udintsev, G.B., et al., Initial Reports of the Deep SeaDrilling Project, Volume 38: Washington (U.S.Government Printing Office).

TABLE 4Normalized Percent Composition of the Carbon, Hydrogen, Nitrogen, Sulfur, Oxygen, and Saturate,

Aromatic, and Asphaltic Fractions in Core Samples From DSDP Leg 41, Site 368

DSDPDesig-nation

3-58-3

15-318-527-232-237-343-251-258-463-3

Extract Fractions (wt %)

Saturate

c1.32.26.21.81.81.94.08.7

16.418.6

Aromatic

c83.477.380.056.086.067.843.078.844.354.2

Asphaltic

c15.320.513.842.012.230.353.012.539.327.2

Elemental Composition of Total Extraci

Carbon

cccc

74.3c

74.776.7

c79.181.8

Hydrogen

cccc

10.2c

10.311.3

c10.410.7

Nitrogen

cccc

0.4c

0.40.8

c0.30.5

Sulfur

cccc

3.7c

1.04.7

c2.32.5

(wt %)

Oxygena

cccc

11.4c

13.66.5c7.94.5

Hydrogen"

Carbon

cccc

1.64c

1.631.76

c1.581.57

aOxygen determined by difference.t>Molar ratio of hydrogen to carbon,insufficient sample for analysis.

853