64. CARBON AND ITS ISOTOPIC COMPOSITION IN PACIFIC OCEAN BOTTOM SEDIMENTS,LEG 56, DEEP SEA DRILLING PROJECT

E. A. Romankevich and M. P. Nesterova, P. P. Shirshov Institute of Oceanology,USSR Academy of Sciences, Moscow, U.S.S.R.

andI. P. Shadsky and J. I. Grinchenko, I. M. Gubkin Institute of Petrochemical and Gas Industry, Moscow, U.S.S.R.

INTRODUCTION

We have analyzed inorganic and organic carbonsand determined the isotopic composition of both sedi-mentary organic carbon and inorganic carbon in car-bonates contained in sediments recovered from Holes434, 434A, 434B, 435, and 435A in the landward slopeof Japan and from Hole 436 in the oceanic slope of theJapan Trench.

Both inorganic and organic carbons were assayed atthe P. P. Shirshov Institute of Oceanology, in the samesample, using the Knopp technique and measuring evolvedCO2 gravimetrically. Each sample was analyzed twicein parallel. Measurements were of a ±0.05 per cent ac-curacy and a probability level of 0.95.

Carbon isotopic analysis was carried out on a MI-1305 mass spectrometer at the I. M. Gubkin Institute ofPetrochemical and Gas Industry and the results pre-sented as δC13 values related to the PDB standard. Theprocedure for preparing samples for organic carbon iso-topic analysis involved (1) drying damp sediments at60°C; (2) treating samples, while heating, with 10 NHC1 to remove carbonate carbon; and (3) evaporatingsurplus HC1 at 60°C. The organic substance was turnedto CO2 by oxidizing it in an oxygen atmosphere. To pre-pare samples for inorganic carbon isotopic analysis wedecomposed the carbonates with orthophosphoric acidand refined the gas evolved. The δC13 measurements, in-cluding a full cycle of sample preparation, were of a±0.5 per cent accuracy and a probability level of 0.95.

RESULTS

The data on inorganic and organic carbon content ofLeg 56 sediments and on organic carbon isotopes aresummarized in Table 1. Lithology of organic carbon ispresented in Figure 1.

Carbon Distribution

Sediments from the landward slope of Japan (Holes434, 434A, 434B, 435, and 435A) contain inorganic car-bon in quantities from less than 0.01 to 6.16 per cent,averaging 0.44 per cent. Sediments from the oceanicslope of the Japan Trench contain inorganic carbon(Hole 436) in amounts from less than 0.01 to 0.12 percent, averaging 0.06 per cent. Quantities of organic car-bon in the same holes in the landward slope vary from0.14 to 1.29 per cent, averaging 0.85 per cent. Although

these sediments clearly show certain maxima and mini-ma in organic carbon content, there is no general pat-tern of distribution of organic carbon. The oceanic slopesediments (Hole 436) contain organic carbon in quanti-ties from 0.08 to 0.73 per cent. There is an average of0.12 per cent organic carbon in the middle Miocene sedi-ments, which is low in comparison with younger strata.For example, the average value for Pleistocene sedi-ments is 0.46 per cent. Sediments from the oceanic slopeof the Japan Trench show low organic carbon content(0.29 per cent on the average) by contrast with the land-ward slope sediment.

Isotopic Composition of Organic Carbon

The isotopic composition (Figure 2) of organic car-bon varies from -21.5 to -24.2 per mill in Holes 435and 435A; from - 18.6 to -25.2 per mill in Holes 434,434A, and 434B; and from -22.1 to -25.5 per mill inHole 436.

In general, the sediment layer shows a predominanceof heavy organic carbon isotopes typical of organicsubstance of pelagic origin (Galimov, 1973). The gen-eral pattern of distribution throughout core samples re-veals that the amount of isotope C1 2 increases withdepth.

Isotopic Composition of Inorganic Carbon inCarbonates

The isotopic composition of carbon in two analyzedcarbonate samples, as well as other characteristics ofthese samples and their matrices, are presented in Table2 (see Whelan, this volume; Okada, this volume; andSite Summary Chart for Sites 434 and 435, back pocket,Part 1). One of the two samples, which is a carbonateconcretion, was sampled from Hole 435 at a depth of75.3 meters sub-bottom from a core of carbonate-freediatomaceous, clayey, methane-containing silt. The in-organic carbon of this sample is characterized byrelatively light isotopic composition (-11.2 per mill).The other sample, a marl fragment, was sampled fromHole 434 at a depth of 244.5 meters sub-bottom from acore of diatomaceous clay containing 8.3 per centCaCO3. This core was free of methane and other gases,and the sample we tested consisted of heavier carbonisotopes (-5.9 per mill). The composition of carbon inthis sample is closer to that of sedimentary pelagic car-bonates, whose average istopic composition approacheszero (Galimov, 1968).

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E. A. ROMANKEVICH, M. P. NESTEROVA, I. P. SHADSKY, J. I. GRINCHENKO

TABLE 1Carbon and Isotopic Composition of Organic Carbon

in Sediments from Leg 560.5 1.0

Organic Carbon (%)

0.5 1.0 0.0 0.5

Section

Hole 435

4-17-5

12-215-1

Hole 435A

3-45-56-27-1

10-111-3

Hole 434

1-44-15-17-19-1

12-115-219-120-122-123-525-131-1

Hole 434A

1-4

Hole 434B

11-215-316-224-125-3

Hole 436

1-23-18-49-2

12-313-516-325-127-228-132-233-335-236-137-640-6

Sub-bottomDepth

(m)

28.061.9

105.0132.3

163.7184.5189.6197.4225.8238.6

4.126.036.154.873.6

102.8132.8169.2178.8196.8209.5226.3282.5

1.0

383.1422.3429.9505.2517.1

2.618.570.676.8

106.4116.2144.2227.1247.6255.6294.9305.6324.4332.3348.9376.2

OrganicCarbon

(%)

0.960.690.861.10

1.290.710.760.990.980.92

1.170.630.820.630.820.750.461.090.830.830.760.730.95

-

1.020.840.800.940.60

0.730.580.430.120.370.400.400.260.300.220.160.120.180.080.200.09

InorganicCarbon

(%)

1.160.121.350.25

0.170.010.030.080.160.32

0.050.100.410.210.060.120.176.160.09

—0.220.870.03

-

0.070.110.050.350.31

0.020.050.060.010.02

_0.040.060.040.040.080.050.110.120.040.08

OrganicCarbonδCl3

(per mill)

-22.2-21.5-23.1-21.5

-22.5-22.5-21.7-22.7-24.2-22.4

-21.8-23.2-25.2-23.1-23.0-23.3-18.6-22.8-22.9-23.1-22.8-23.4-22.9

-23.0

-23.2-23.8-23.9-24.2-25.1

-22.6-22.1-22.3-23.6-23.0-22.5-22.1-23.3-23.1-24.2-24.2-25.5-24.2-24.2-23.1

-

ε 2001-

Ho!es435, 435A Holes 434/434A/434B

Figure 1. Organic carbon in sediments of DSDP Leg56. 1A: Clayey diatomaceous ooze; IB: diatoma-ceous clays tone; 2: radiolarian-diatom clay stone; 2A:vitric diatomaceous clay stone; 2B: tuffite; 3 A: pe-lagic clay.

,,-21 - 2 2 - 2 3 -18 -19 -20 -21 - 2 2 -23 - 2 4 -25 - 2 2 - 2 3 - 2 4 - 2 5

1 200 r-

Holes 435, 435A Holes 434/434A/434B

DISCUSSION

The organic carbon of sediments cored from Hole436 lightens isotopically as it decreases in quantity (seeFigure 3). This tendency can be traced in samples from

Figure 2. Isotopic composition of organic carbon, DSDPLeg 56.

Holes 434A and 434B but is not evident those fromHoles 435 and 435A. Because organic matter in theJapan Trench sediments is mostly planktonic in origin,additional amounts of allochthonous, isotopically light

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CARBON AND ITS ISOTOPIC COMPOSITION

TABLE 2Characteristics of Sea Carbonates, Leg 56

OrganicSection/Sub-bottom CaCθ3 δ C l j Carbon CaCOß

Depth (m) (%) (per mill) Lithology (%) (%) Methane

434-27-1244.5Marl

435-9-175.3Carbonateconcretion

22.36 -11.2

Diatomaceoussilty claystone

Diatomaceoussilty clay

1.0 8.3 Present

0.6 0.2 Absent

-26

Figure 3. Plat of organic carbon content relative to itsisotopic composition for DSDP Leg 56 sediments (+,samples from Holes 434, 434A, 434B; , samplesfrom Hole 435, 435A; O, samples from Hole 436).

organic carbon should produce increased quantities oforganic carbon as well as isotopic lightening. On theother hand, organic matter transformation during sedi-mentation and during the initial stages of diagenesis,which leads to a loss of unstable and isotopically heavycomponents of organic matter, will result in decreasedamounts of organic carbon and in isotopic lightening(Shadsky et al., 1978). Thus these processes seem to beat odds with one another.

The relationship between isotopic composition oforganic carbon and its presence in sediments revealed byHole 436 samples proves that fractionation of carbonisotopes in the course of destruction of organic matterplays an important role in the formation of isotopiccomposition of organic carbon buried in oceanic slopesediments of the Japan Trench. The absence of such arelationship for the sediments of the landward slopemay indicate the relatively substantial contribution ofallochthonous isotopically light organic matter to thecreation of those sediments. This allochthonous ingre-dient obviously possesses higher resistance to destruc-tion than an autochthonous one.

The sedimentary section we examined is composed ofsimilar diatomaceous clayey sediments interspersed withvolcanic tuff. Carbonate concretions occur in the depo-sitions of the landward slope (Holes 434, 434A, 434B,435, and 435A). These sediments contain methane and

other hydrocarbon gases, whereas oceanic slope sedi-ments (Hole 436), although similar in other respects tothose of the landward slope, contain no hydrocarbongases (Whelan, this volume). Comparison of changes ofisotopic composition of organic carbon and changes ofmethane content in sediments, exposed by Holes 434,434A, 434B, 435, and 435A (Figures 4 and 5) shows thatthe extremes of these values coincide. Enrichment in C13

corresponds to extremely high magnitudes of methanein sediments during a gas-hydrate phase and showslimited migration.

It is known that the microbiological decompositionof organic matter, in which methane-producing bacteriatake part, is accompanied by generation of isotopicallylight methane (δC13, up to - 80 per mill) and of carbondioxide whose isotopic composition is heavier than thatof the original organic matter by 3 to 15 per mill (Nakai,1961; Oana and Deevay, 1960). Calculation of the bal-ance of stable carbon isotopes in such a decompositionsystem shows that the remaining organic matter shouldbe enriched with heavy carbon isotopes. Hence thehigher the generation of methane, the greater the enrich-ment of organic matter with heavy isotopes. Thus sam-ple analysis may indirectly indicate the degree of activemicrobiological decomposition of organic matter takingplace in the sediments under the influence of anaerobicmethane-producing bacteria.

The difference between the average isotopic composi-tion of carbon in organic matter of the sediments coredfrom Hole 435 (δC1 3-22.4 per mill) and the isotopic

CH

(LU)

om D

epth

Bo

ttS

ub

100

200

inn

) 201 I

v

•

i i

40I I

P

4

\

IT

1 1

1

>

—

1

60

-

I

801

-

a—

<

1

-18 -20 -22 -24 -26

δC1 3(°/oo)

Figure 4. Isotopic composition of organic carbon andmethane gas content for Holes 435 and 435A sedi-ments, DSDP Leg 56.

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E. A. ROMANKEVICH, M. P. NESTEROVA, I. P. SHADSKY, J. I. GRINCHENKO

100 -

-P 200 -

300 -

400 -

500 -

-18 -20

δC

-22

13 ,o

-24 -26

/oo)

Figure 5. Isotopic composition of organic carbon andmethane gas content for Holes 434, 434A, and 434Bsediments, DSDP Leg 56.

composition of inorganic carbon from carbonate con-cretions in the same sediments (δC13 -11.2 per mill) is11.2 per mill. This value falls within the range of magni-tudes characteristic of differences between isotopic com-position of organic carbon and CO2 produced duringmethane fermentation. Thus a carbonate concretionmay in fact be a diagenetic carbonate which has emergedfrom metabolic CO2. The essential irregularities that oc-cur in the isotopic composition of organic carbon in thesediments from Holes 434, 434A, 434B, 435, and 435Acan be attributed to the activity of methane-producingbacteria. Similar isotopic excursions occur in sedimentsfrom Hole 436 (Figure 2). Thus one may conclude thatthe methane which had been generated in oceanic slope

sediments of the Japan Trench (Hole 436) subsequent-ly escaped.

We used the materials collected during the 46thvoyage of the Soviet research ship Vityaz to study theisotopic composition of organic carbon present in Holo-cene sediments in the region of Kurilo-Kamchatck andthe Japan Trench. Our analysis revealed, first, that inmore pelagic environments the organic carbon is moreenriched in the C12 isotope because of selective retentionof lipids and their polymeric forms (isotopically lightand stable fractions of organic matter) and, second, thatirregularities of isotopic composition are due to dif-ferences in the thermal conditions in which organic mat-ter was produced during photosynthesis in the warmKurosio current on the one hand and the cold water ofOiyasio on the other (Shadsky et al., 1978). The dataconfirm that the tendency established for Holocenedeposits (i.e., that C12 content increases in more-pelagicenvironments) is valid for Pleistocene sediments also(Table 3). Consequently, we conclude that on the wholethere had been no great change in conditions through-out the Quaternary. In Pliocene sediments, far awayfrom shore (Hole 436), the isotopic composition oforganic carbon is the same as in the Quaternary deposi-tions (δC13 = -22.7 per mill).

In comparison to Pleistocene deposits, the Pliocenesediments of the landward slope of Japan (Holes 434,435, and 43 5 A) are enriched by isotopically light organiccarbon. One of the causes for isotopic lightening oforganic carbon may be that during the Pliocene an iso-topically light organic carbon was brought in by a coldcurrent from colder waters of high latitudes where thatcarbon had been photosynthesized.

In the Miocene the organic carbon is isotopicallylighter still by approximately 1.5 per mill, probably as aresult of the following. In the Miocene the climate of theNorthern Hemisphere was warmer and more humidthan during subsequent periods. This enhanced thedelivery to the ocean of allochthonous isotopically lightorganic matter, which is more resistant to biological de-struction. Furthermore, methane fermentation, whichproduces an increase of heavy carbon isotopes, proceed-ed less actively in those sediments.

In Hole 436 sediments the settling of organicsubstances during the Miocene took place at a decreasedrate. This resulted in precipitation of oxidized deep-seared clays (see Site Summary Chart, back pocket, Part1). These sediments, in contrast to Pliocene deposits,settled in pelagic environments far from land. Suchlateral displacement of sediments bears out the hypoth-eses of plate tectonics.

ACKNOWLEDGMENTS

We wish to thank Professor A. A. Kartsev of the GubkinInstitute of Petrochemical and Gas Industry and Dr. A. A.Ivlev of the Ail-Union Scientific Research Geological Pros-pecting Oil Institute for their critical review of the manuscript.Thanks are also due Professor A. A. Murdmaa of the ShirshovInstitute of Oceanology for his help with problems of tax-onomy.

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CARBON AND ITS ISOTOPIC COMPOSITION

TABLE 3Average Contents of Carbon and Its Isotopic Composition in Leg 56 Sediments

Age

Pleistocene

Pliocene

LateMiocene

MiddleMiocene

Holes 435, 435A

OrganicCarbon

(%)

0.82(2)

0.81 (8)

InorganicCarbon

(%)

0.64 (2)

0.30 (8)

OrganicCarbon

(per mill)

-21.8 (2)

-22.6 (8)

Holes 434, 434A,

OrganicCarbon

(%)

1.17(1)

0.80(15)

0.77 (2)

InorganicCarbon

(%)

0.05 (1)

0.54 (15)

0.33 (2)

434B

OrganicCarbonδCl3

(per mill)

-22.4 (2)

-23.0(15)

-24.6 (2)

OrganicCarbon

0.46 (4)

0.35 (4)

0.20 (5)

0.12(3)

Hole 436

InorganicCarbon

0.03 (4)

0.03 (4)

0.06 (5)

0.08 (3)

OrganicCarbon

(per mill)

-22.6 (4)

-22.7 (4)

-24.2 (5)

-23.6 (2)

Note: Numerals in parentheses refers to total number of samples averaged.

REFERENCES

Galimov, E. M., 1968. Geochemistry of stable carbon iso-topes. Nedra, p. 49.

, 1973. Carbon isotopes in oil-gas geology. Nedra,p. 111.

Nakai, N., 1961. Geochemical studies on the formation of nat-ural gas. /. Earth Sci., Nagoya Univ., 9, 59.

Oana, S., and Deevay, E. S., 1960. Carbon-13 in lake watersand its possible bearing on paleolimnology. Am. J. Sci.,258A, 253.

Shadsky, I. P., Romankevich, E. A., Belyaeva, A. N., andGrinchenko, U. I., 1978. Isotopic composition of organiccarbon from the Pacific Ocean bottom sediments. VII Na-tional Symposium of Stable Isotopes in Geochemistry,Moscow, GEOCHI.

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