20. GEOCHEMISTRY OF CARBON: DEEP SEA DRILLING PROJECT LEG 61,NAURU BASIN, NORTH PACIFIC1

Karl S. Schorno, Phillips Petroleum Company, Bartlesville, Oklahoma

INTRODUCTION

Seven quartered sections of Pliocene to Mesozoic(Cenomanian) cores from the Nauru Basin contain pri-marily marine organic matter admixed with detectableamounts of terrigenous organic matter. The mixture isimmature with respect to organic genesis. Chemicalproperties of this organic matter are compared withproperties of other deep-ocean cores from DSDP sites inthe central Pacific.

RESULTS AND DISCUSSION

The sampling and analysis procedures used in thisstudy were reported previously (Schorno, in press). Theorganic and inorganic compositional data obtainedfrom the homogenized core, along with sub-bottomdepths and chronostratigraphies, are given in Table 1.Table 2 contains the results of the organic analyses ofthe bitumen fractions. The results of capillary gas chro-matographic analyses of the saturate fraction arereported in Table 2 as the ratios of marine to terrigenousrt-alkanes, S (S = («-C21 + A2-C22)/(«-C28 + n-C29)),the ratios of pristane to phytane, pristane to π-C17, andphytane to n-C18, and OEP (the ratio of odd to evenn-alkanes between n-C23 and «-C35). The followingreport is divided into a discussion of the origin of theorganic matter in these sediments, its state of matura-tion, and the oxidizing/reducing state of the deposi-tional environment. A brief comparison of these sedi-ments with similar sediments from DSDP sites in theNorth Pacific also is made.

ORGANIC SOURCE

The organic matter within the cores taken from UnitsI, II, and III appears to be derived primarily from ma-rine rather than terrestrial organisms. This assignmentof source is based on the occurrence of primarily marinecarbonates within these units, a ratio of marine to ter-rigenous n-alkanes indicative of a marine source, a nor-mal distribution of n-alkanes (neither an odd nor aneven preference) in the «-C23 to «-C35 carbon-numberrange, and the large amount of organic nitrogen in thesesediments.

Marine Carbonates

The carbonate-carbon of the first five cores listed inTable 1, in the interval from the Pliocene at 24 meters tothe upper Eocene at 336 meters, appears to have been

derived from marine organisms. With the exception ofCore 462-3, Section 4, the marine carbonate content inthese two units amounts to over 9% of the total weightof the sediment, that is, more than 80% as calcium car-bonate. Core 462-3, Section 4, was probably depositedbelow the CCD (Site Summary, this volume). The car-bonate content is still relatively high in the Lower Maes-trichtian and Campanian cores from Unit III, averaging44%, and then drops off sharply in the Cenomanian to1.4%.

Marine versus Terrigenous n-Alkanes

The ratio of marine to terrigenous n-alkanes, as givenby S = (n-C21 + «-C22)/(«-C28 + n-C29) (Philippi,1974) ranges from 1.8 near the top of Unit I to 0.5 forCore 462-59, Section 1, in the Cenomanian at 650meters. Philippi has demonstrated that S is typically lessthan 0.8 for terrigenous crude oils, and greater than 1.5for marine crude oils. Two of the seven cores studiedshow S values greater than 1. The S values for four ofthese cores fall between these two ranges. If oneassumes that S values derived from crude oils can beused for bitumens, one can speculate that the bitumensfor which S is 1 to 1.5 contain a 50/50 mixture of marineand terrigenous organic matter. The deepest samplestudied has an 5 value of 0.5. In this case, the terrige-nous input appears to have been high. However, as willbe noted later, the values for the pristane-to-phytaneratio and OEP are not in agreement with this interpre-tation. The assumption that S is a quantitative indicatorof the amount of either terrestrial or marine input intothe environment is probably erroneous. As noted byCornford (1978), a minor input of plant cuticle can pro-duce a major change in the amount of C28 and C2 9

n-alkanes, whereas a major algal input is necessary togive only a slight change in the C2i and C22 n-alkanecontent. Therefore, an S value near 1 is probably still in-dicative of a major marine input into the total organicpool. In addition to the ratio of marine-to-terrigenousn-alkanes, other parameters support the mostly marineassignment, at least for Units I and II and most of UnitIII. These are the odd/even ratio of n-alkanes, as givenby OEP < 1.3 (OEP is a mathematical expression of thechanges in the odd-to-even distribution of «-alkanesbetween /z-C23 and n-C35, as given by Scalan and Smith,1976) and the high ratio of organic nitrogen to organiccarbon in these sediments.

Initial Reports of the Deep Sea Drilling Project, Volume 61.

Odd-Even Predominance (OEP)For five of the eight sediments given in Table 2 OEP

calculated for n-alkanes between n-C23 and n-C35 is less

621

K. S. SCHORNO

Table 1. Geochemical data on homogenized cores from Leg 61, Hole 462.

Sample(interval in cm)

Unit I

462-3^, 100-1257-5, 100-125

12-5, 100-125

Unit II

22-5, 100-12536-3, 100-125

Unit III

49-4, 100-13054-2, 120-14059-1, 120-140

Chronostratigraphy

PlioceneLate Miocene-

PlioceneMiddle Miocene

Late OligoceneLate Eocene

Lower MaestrichtianCampanianCenomanian

Sub-bottomDepth

(m)

24.363.6

112.0

207.0336.8

461.0511.7550.5

Carbonate

Carbon(wt. %)

0.211.1

9.7

9.69.7

4.16.41.4

asCaCθ3(wt. %)

293

80

8081

345111

OrganicCarbon(wt. %)

0.160.07

0.08

0.060.07

0.190.070.04

Kjeldahl N (ppm)

AmmoniumNitrogen

(half)

6637

40

3433

413334

OrganicNitrogen

(full-half)

42396

98

9592

12585

103

AtomicRatio

OC/ON

49

10

79

18105

Note: Calcium carbonate determined by dividing carbonate carbon by 0.12.

Table 2. Geochemical data on the bitumen fraction of cores from Leg 61, Hole 462.

Core

Unit I

37

12

Unit II

2236

Unit III

495459

Section

455

53

421

Bitumen C a

(ppm)

136

13

66

621

Bitumen C

TOC(wt. %)

0.80.91.6

1.00.9

0.30.30.3

Saturate(wt. %)

6b13

bb

1111b

Aromatic(wt. %)

19b14

bb

3023b

Asphaltic(wt. %)

74b73

bb

5966b

OEP

1-4,1.0d

1.2

l . l d

1.21-61.4d

S

1.31.8

c

1.01.7

0.91.10.5

Pristane

Phytane

0.90.1

c

0.30.4

0.60.60.4

Pristane

n-C, 7

0.21.1_ c

0.61.0

1.10.61.8

Phytane

«-Cl8

0.34.9

c

2.43.0

1.40.64.7

a The bitumen-carbon content is determined by dividing the weight of bitumen by weight of rock times 10° (ppm), divided by afactor of 1.25.

° Insufficient sample for analysis.c Data lost." Because of the sparseness of bitumen in these cores, OEP was determined on the bitumen and not on the saturates fraction.

than 1.3. Generally, OEP is either maturity-dependent,source-dependent, or both. OEP is less than 1.3 foreither organic matter in the catagenetic or metageneticstage of evolution, or for organic matter derived pri-marily from marine organisms. In the first case, it hasbeen well established that immature sedimentary or-ganic matter containing only a trace of terrigenous or-ganic matter has OEP values greater than 1.3. As matu-ration proceeds, an evolution of w-alkanes with neitheran odd nor even preference in the C23 to C25 range oc-curs, which consequently wipes out the original highOEP value. In the second case, Hunt and others (Hunt,1979) have shown that marine organisms synthesize/z-alkanes with neither an odd nor even preference in theC23 to C35 range. Moreover, Koons et al. (1965) andPowell and McKirdy (1973) have shown that for purelycarbonate sediments OEP is less than 1.3 at shallowdepths (low temperatures), even in Tertiary sediments.Thus, if the input of terrigenous organic matter is muchless than that of marine organic matter, then OEP willbe near unity early in its evolutionary history as well aslate; thus OEP in this case is source-dependent, and notmaturity-dependent, suggesting a marine source for theNauru Basin sediments.

Organic Nitrogen from Marine Organisms

The amount of organic nitrogen in these cores isrelatively high, as indicated by an atomic ratio oforganic carbon (OC) to organic nitrogen (ON) less than20. In general, marine organisms contain a higher per-centage of organic nitrogen than continental plants. Wehave set a tentative cut-off value for OC/ON of 20 as anapproximate upper limit for purely marine organic mat-ter. This value is empirically derived from a compilationof values obtained from at least 500 DSDP samplesanalyzed in this laboratory in the past 10 years. It ismerely an estimate of marine content, and should onlybe used to support other source parameters. Theaverage value for this ratio for the cores under study is9, with a range from 4 to 18, well below the tentativecut-off value of 20.

MATURITY

The sedimentary organic matter within these cores isin an early stage of maturation (the diagenetic stage asdefined by Tissot and Welte, 1978a). This assignment isbased on a presumably low-temperature history forthese sediments, low ratios of both bitumen to TOC and

622

GEOCHEMISTRY OF CARBON

saturates plus aromatics to total bitumen, and the pre-dominance of phytane over «-C18.

The percentage of the bitumen-carbon portion of theTOC for marine organic matter should be greater than5°7o (Philippi, 1965) for sediments in either the cata-genetic or early metagenetic stages of organic matura-tion. The average value for the weight per cent bitumencarbon for these cores is 0.8%, with a range from 0.3 to1.6%. These values are well below that expected for thestages of evolution of marine organic matter notedabove.

The amount of saturates plus aromatics fraction ofthe total bitumen (HC) increases with a correspondingdecrease in the asphaltic fraction (non-HC) with depthin these units. The ratio of non-HC/HC never reachesthat typical of source rocks of petroleum, i.e., the cata-genetic stage of organic evolution. Based on publishedHC and non-HC values (Tissot and Welte, 1978b), theratio of HC/non-HC for sources of petroleum is 0.91for an average of rock types, 0.87 for shales and silts,0.96 for calcareous shales, and 1.4 for carbonates. Thisratio for Cores 462-3, Section 4, and 462-12, Section 5,are highly indicative of immature sediments (2.96 and2.70, respectively). The ratio decreases with depth to1.44 and 1.94 for Cores 462-49, Section 4, and 462-54,Section 2, in Unit III. Both values are above the valueexpected for source rocks of petroleum, as noted above.

The third maturity parameter used in this report isthe ratio of phytane to A/-C18. The ratio of phytane to«-C18 is high (greater than 1), suggesting that matura-tion has not progressed beyond the diagenetic stage. Therate of conversion of phytol to phytane is relatively fastcompared to the rate of formation of «-alkanes. Thus,phytane (in reducing environments) or pristane (in ox-idizing environments) forms faster than the n-alkane«-C18 («-C17 for oxidizing environments); thus, theisoprenoid predominates over its associated n-alkane inthe earliest stages of genesis (the diagenetic stage).

The hydrogen and oxygen indices (HI and OI) (Espi-talié et al., 1977) generated aboard ship contradict thematurity assignment just mentioned for at least thedepth interval 400 to 800 meters. In this case, thehydrogen and oxygen indices obtained from the pyroly-sis of the whole-rock kerogen suggest that the organicmatter in these sediments is either in a late stage ofmaturation (catagenesis stage), is derived from mostlyterrigenous sources, or is highly oxidized, as indicatedby low values for HI (less than 100) and high values ofOI (greater than 50). The averages of these two valuesfor 29 cores from Site 462 are 83 mg HC/ and 887 mgCO2/g TOC.2 The equivalent kerogen H/C and O/Cvalues obtained from linear-regression analyses of Es-pitahVs (1977) original data (H/C = 0.00137 HI +0.589 and O/C = 0.00395 OI + 0.04) are approxi-mately 0.71 and 3.4, respectively. In the latter case, an

The shipboard data were found to be invalid and will not be published elsewhere in thisvolume.

O/C ratio of 3.4 is considered improbable. Assumingthat at least the HI data are correct, the three possiblecases just mentioned directly contradict the results thatwe present herein. A reason for this contradiction maybe that assignments of organic source and maturity andof the reducing state of the depositional environmentare based herein solely on data obtained from thebitumen fraction, whereas the hydrogen index is deter-mined solely from the whole-rock kerogen, in this casepossibly reworked organic matter. Because only a smallmarine portion of the total kerogen is necessary to pro-duce a marine bitumen fraction, the kerogen could beprimarily terrigenous, while the bitumen is marine.

REDUCING ENVIRONMENT

The data presented below indicate that the organicmatter in these sediments was deposited in a reducingenvironment. The reducing state of the environment ofdeposition is indicated by a low OEP value, near unity,and a ratio of pristane to phytane of less than 1. Tissotand Welte (1978a), Hunt (1979), and this author (DSDPLegs 58, 59, and 60) have reported cases where OEP isless than 1.3 for open marine reducing environments. InTissot and Welte's case, OEP values are generally lessthan 0.8, from which they infer that H-alkanes aregenerated by the reduction of fatty acids from plantwaxes. In a highly reducing open marine environment,the predominance of odd-carbon-numbered acids andalcohols in the C25 to C35 range found in terrigenoussource materials would be reduced to even-carbon-num-bered rt-alkanes, whereas if the organic matter weredeposited in a highly oxidizing environment decarbox-ylation would be favored. Thus the one-carbon-less odd«-alkane would be generated. This hypothesis suggests arelatively high even-carbon predominance for immaturesediments (OEP < 0.8). In our case, OEP is neither high(>1.6) nor low (<0.8), suggesting a source of thesen-alkanes different from immature even-carbon-num-bered acids and alcohols. As noted by Tissot and Weltein this same discussion, if the environment of depositionis reducing, then the ratio of pristane to phytane shouldbe less than 1. Basically, the same type of argument usedabove for the transformation of fatty acids and alcoholsto Az-alkanes holds for these isoprenoids. Because inmost instances phytol is the precursor of pristane andphytane, in a reducing environment phytol is reducedprimarily to phytane. In an oxidizing environment,phytol is oxidized to phytanic acid, then decarboxylatedto pristene, and finally reduced to pristane. Low valuesfor both OEP and the ratio of pristane to phytane in-dicate that marine organic matter was deposited in areducing environment. Some marine organisms do syn-thesize pristane. However, because phytane predomi-nates, this contribution must be small. Now, if a con-siderable portion of the kerogen was terrigenous andwas deposited in a reducing environment, one would ex-pect to find OEP <0.8, as noted above. Because it isnot, we favor the marine source for these kerogens.

623

K. S. SCHORNO

COMPARISON OF LEG 61 ORGANICGEOCHEMICAL DATA WITH THOSE

OBTAINED FROM OTHER NORTHPACIFIC "OPEN" MARINE SEDIMENTS

I have compiled the averaged values of 11 geochem-ical properties taken from DSDP cores from sites in theNorth Pacific and listed these values in Table 3; averagevalues for each of the eleven sites listed in this table arealso given. The Nauru Basin values compare favorablywith averages for the Mariana Trough and fore-arcregions (Leg 60). The organic matter within the sedi-ments taken during Leg 60 fits the criteria for marineorganic matter deposited in a reducing environment,that is, OEP <1.3, pristane/phytane < l , OC/ON< 20, and S > 1.0. The Nauru Basin, compared to theLeg 60 sediments, appears to contain more terrigenousorganic matter, as indicated by slightly lower S values.

CONCLUSIONSBecause of a low supply of nutrients in this isolated

basin, a loss of organic matter during settling to the bot-tom, and/or the low input of transported organic mat-ter from other sources, the amount of organic matterthat finally accumulated in the bottom sediments waslow, near the average for open marine sediments (0.1%organic carbon).

Most of the sediments deposited at this site aremarine carbonates and are either autochthonous ortransported to the site. These sediments could have beendeposited on adjacent structural highs, as noted in theSite Summary (this volume). Our data indicate that pri-marily marine organic matter was deposited in either areducing environment or an oxygen-minimum zone andhas undergone some diagenetic changes (to generatepristane and phytane from phytol). The sediments thencould have been redeposited by slumping or turbiditycurrents into the Nauru Basin. Additional alteration ofthe organic matter within these sediments because of thelatter movement are not determinable by our measure-ments. The organic matter in these sediments appears tobe derived primarily from marine organisms with thepossible exception of the lowermost core, 462-59, Sec-tion 1. This core still contains a noticeable terrigenousimprint. The assignment of the source of the organicmatter in these sediments is based heavily on the twoparameters OEP (<1.3) and 5 (>l) . There is a con-

siderable amount of uncertainty in the assignment ofsource using these parameters. Also, the HI valuesreported in the Site Summary are in direct contradictionwith the marine assignments given above. Additionalresearch is needed to clarify these ambiguities, so thatthe mechanism of organic accumulation within this typeof environment can be better understood.

The average values of geochemical properties for thepelagic sediments of the Nauru Basin are compared withsimilar properties for sites in the central Pacific. Thereare some similarities between the Nauru Basin sedimentsand similar sediments of the North Pacific. As moredata are accumulated, one may be able to statisticallyestablish an averaged parameter for the deposition ofmarine organic matter in deep oceanic environments.

ACKNOWLEDGMENT

I would like to thank J. Whelan (Woods Hole Oceanographic In-stitute), J. Louda (Florida Atlantic University), and J. Natland (DeepSea Drilling Project) for their review of this paper.

REFERENCES

Cornford, C , 1978. Organic geochemistry of DSDP Leg 47A, Site397 eastern North Atlantic: organic petrography and extractablehydrocarbons. In von Rad, U., Ryan, W. B. F., et al., Init. Repts.DSDP, 47, Pt. 1: Washington (U.S. Govt. Printing Office), 511-522.

Espitalié, J., Laporte, J. L., Madec, M., et al., 1977. Methode ra-pide de caracterisation des roches meres, de leur potential petrolieret de leur degre devolution. Rev. Inst. Fr. Pet., 32:23-42.

Koons, C. B., Jamieson, G. W., and Ciereszko, L. S., 1965. Normalalkane distributions in marine organisms: possible significance topetroleum origin. Bull. Am. Assoc. Petrol. Geol., 49:301-316.

Philippi, G. T., 1965. On the depth, time, and mechanism of petro-leum generation. Geochim. Cosmochim. Acta, 29:1021-1049.

, 1974. The influence of marine and terrestrial source mate-rial on the composition of petroleum. Geochim. Cosmochim.Acta, 39:947-966.

Powell, T. G. and McKirdy, D. M., 1973. The effects of source mate-rial, rock type and diagenesis on the n-alkane content of sedi-ments. Geochim. Cosmochim. Acta, 37:623-633.

Scalan, R. S., and Smith, J. E., 1976. An improved measure of theodd-even predominance on the normal alkanes of sediment ex-tracts and petroleum. Geochim. Cosmochim. Acta, 34:611-620.

Schorno, K. S., in press. Geochemistry of Carbon: InternationalPhase of Ocean Drilling Project Leg 60. In Hussong, D., Uyeda,S., et al., Init. Repts. DSDP, 60: Washington (U.S. Govt. PrintingOffice).

Tissot, B. P., and Welte, D. H., 1978a. Petroleum Formation and Oc-currence: New York (Springer-Verlag), pp. 69-70.

, 1978b. Petroleum Formation and Occurrence: New York(Springer-Verlag), pp. 95-96.

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GEOCHEMISTRY OF CARBON

Table 3. A compilation of average values of 11 organic chemical properties of cores from DSDP Legs 56, 58, 59, 60, and 61 in the North Pacific.

LegSites

Number ofCores SampledAge RangeCaCθ3 (wt. %)Organic Carbon(wt. %)Organic Nitrogen(PPm)Organic Carbon/Organic NitrogenBitumen (ppm)Bitumen/TotalOrganic Carbon(×IOO)OEPδ CpDB Bitumen<513Cp£)B KerogenSPr/Ph

Mariana Troughand Fore-Arc

60452, 453, 454A455, 459B, 460

11

Pleistocene-Miocene0.10.18

160

18

362.4

1.3

-26.01.90.7

Seaward SideJapan Trench

56436

6

Pleistocene-Miocene20.15

293

5

513.8

2.4-24.9-22.4

0.6

West-CentralShikoku Basin

58442A442B

10

Pleistocene-Miocene20.15

455

3

297.0 (2.0)a

2.1-27.2-24.7

0.90.8

East-CentralShikoku Basin

5844344410

Miocene40.07

314

2

2315.0

1.7-27.3-26.3

0.70.7

Basin inDiato Ridge

58445

21

Pleistocene-Eocene34

0.1

120

90

202.1

1.7-26.2-26.3

0.70.6

Diato Basin

58446

8

Miocene-Eocene1.50.16

111

217

90.8

1.5-26.2-28.6

1.50.5

Eastern SidePhilippine

Basin

59447A

2

Oligocene2.80.20

34

76

120.5

1.0-26.4-28.7

2.00.6

a Number in parenthesis is the average of those samples less than 10%.

Western EdgeKyushu-Palau

Ridge

Western SideParece-Vela

Basin

Eastern SideParece-Vela

Basin

Eastern EdgeWest Mariana

Ridge Nauru Basin

Average Valuefor All but

Nauru BasinSediments

LegSites

Number ofCores SampledAge RangeCaCθ3 (wt. <7o)Organic Carbon(wt. %)Organic Nitrogen(ppm)Organic Carbon/Organic NitrogenBitumen (ppm)Bitumen/TotalOrganic Carbon(×IOO)OEPδ CpDjj Bitumenδ CpDß KerogenSPr/Ph

59448

4

Miocene-Oligocene750.12

28

36

121.1

1.4-25 .9-27.1

1.20.7

59449

3

Miocene210.18

43

52

30.7

1.3-28 .4-27.1

1.30.4

59450

7

Pliocene-Miocene110.12

38

35

77 (14)a

4.9

1.5-27 .0-28.1

1.40.5

59451

3

Pleistocene-Miocene110.14

110

29

100.9

1.5—

-27 .61.20.5

61462

8

Pleistocene-Cenomanian540.09

140

9

81.5

1.3——1.20.5

—

—

—14.60.14

155

57

263.6

1.6-26 .6-26 .6

1.20.6

625