Lewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.), 1995 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 141 23. CARBON ISOTOPE COMPOSITIONS OF ORGANIC MATTER FROM THE CHILE CONTINENTAL MARGIN 1 Amane Waseda, 2 Borys M. Didyk, 3 and Yoshiteru Kajiwara 2 ABSTRACT Stable carbon isotope compositions as well as organic carbon contents and pyrolysis results of cored material are reported for the Pliocene to Pleistocene sediments from the Ocean Drilling Program (ODP) Sites 859, 860, 861, and 863. The δ 13 C values of kerogen show large variation with no downhole trend, ranging between 34.2%e and 22.6‰ with average values around 25‰, whereas the organic carbon contents in most sediment samples are consistently low (<0.5%) except for enrichment in the surface layer at Sites 859, 860, and 861. The δ 13 C values of kerogen indicate the kerogen in sediments of the Chile Margin has a predominantly terrestrial origin, and the fine grained turbidite material is associated with terrestrial organic matter. The average δ 13 C values become lighter with increasing distance from land among three sites drilled along an east west transect, suggesting fine turbidites with terrestrial organic matter are transported further offshore in this trench slope setting. INTRODUCTION During Ocean Drilling Program Leg 141 in the vicinity of the Chile Triple Junction, the Pliocene to Pleistocene sedimentary se quences were recovered at four sites on the Chile continental slope (Fig, 1). This paper reports the results of stable carbon isotope mea surements, organic carbon contents, and pyrolysis results for samples collected at Sites 859, 860, 861, and 863 from Leg 141. The carbon isotope compositions of organic matter in sediments are used to determine the variation of relative proportions of terrig enous and marine organic carbon inputs (Sackett, 1989). In many marine environments, relative contributions of terrigenous and ma rine inputs have been documented through the use of stable carbon isotopes and paleoclimate has been inferred. The carbon isotope com positions of the kerogens have indicated the sediments of the Nankai accretionary prism, Japan contain a higher portion of terrestrial or ganic matter (Berner and Koch, 1993). In the Gulf of Mexico, down core variations have been correlated with glacial and interglacial epi sodes which are related to sea level lowering and the changing influ ence of the Mississippi River (Parker et al., 1972; Newman et al., 1973). The influence of glacial episodes on marine sequences has been also inferred from the isotopic compositions of organic matter in sediments of the Oman Margin (Muzka et al., 1991), the Labrador Sea, and Baffin Bay (Macko, 1989). The use of δ 13 C values as a parameter of relative proportions of terrigenous and marine organic carbon inputs are based on the prem ise that the δ' 3 C values of organic material in sediments are reflective of the isotope composition of the source biota and do not change through diagenesis. The factors of diagenesis that may be of impor tance in affecting the carbon isotope composition include: (1) isotope effects during bacterial degradation of organic matter; (2) preferential elimination of compound groups and preferential preservation of others which differ significantly in δ 13 C from the average plant mate rial; and (3) decarboxylation reactions, which would remove i3 C enriched groups from the organic material leading to 13 C depletion in the residual (Deines, 1980). However, consistent diagenetic trends in sediments are difficult to document (Deines, 1980; Galimov, 1980; Lewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.), 1995. Proc. ODP, Sci. Results, 141: College Station, TX (Ocean Drilling Program). 2 J APEX Research Center, 121 Hamada, Mihama ku, Chiba 261, Japan. 3 Empresa Nacional del Petróleo, Refineria de Petróleos Concern S.A., Casilla 28-D, Vina del Mai, Chile. Sackett, 1989). Thus these diagenetic factors above probably can account for only minor changes of 1 or 2‰ in the δ 13 C of organic carbon, but do not produce a significant carbon isotope shift in Mio cene to Holocene sediments (Arthur et al., 1985; Dean et al., 1986). The carbon isotope composition of organic matter in sediments has been also used as a paleotemperature tool for studying deep sea sediments, which is based on temperature dependent fractionation of carbon isotopes by marine phytoplankton (Sackett, 1989). The basic premise in the use of δ 13 C values of organic matter as a paleotempera ture parameter is that all of the organic matter in the sediments was synthesized by algae growing in an open marine environment. The sediments from Leg 141 were predominantly deposited in a trench slope environment and consist of continent derived debris or turbid ity flow deposits. This tool therefore can not be applied to the sedi ment in this study, and the variations in isotope signature possibly represent the variation of relative proportions of terrigenous and marine organic carbon inputs. This paper gives an overview of the origin of organic matter in relation to the trench slope sedimentary environment near the Chile Triple Junction. GEOLOGIC SETTING At the Chile Triple Junction (Fig. 1) an active spreading ridge and adjacent young oceanic crust are being subducted beneath the conti nent of South America. During ODP Leg 141, Pliocene to Pleistocene sedimentary sequences were recovered at four sites. Three sites (859, 860, and 861) were drilled along an east west dip transect about 35 km north of the Chile Triple Junction at water depths ranging from 1652 to 2741 m. Site 863 (2564 m water depth) was drilled at the base of a trench slope basin directly above the subducted spreading axis as a strike transect with Site 859. METHODS Wet sediment samples were dried for 16 hours at 60°C and then crushed and acidified with 6N HC1 to remove carbonate. The carbon ate free residue was then washed, dried, and sonically extracted with a benzene methanol mixture (7:3). A portion of dried material was then weighed and combusted in a quartz glass tube for 2 hr at 900°C in presence of purified cupric oxide wire and silver granule. The CO 2 gases obtained were cryogenically isolated from other combustion products and analyzed on a VG Isotech Sira Series II mass spectrome ter. The carbon isotope ratios are reported in the usual δ notation relative to the PDB (Pee Dee Belemnite) standard:
Transcript
Lewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 141
23. CARBON ISOTOPE COMPOSITIONS OF ORGANIC MATTERFROM THE CHILE CONTINENTAL MARGIN1
Amane Waseda,2 Borys M. Didyk,3 and Yoshiteru Kajiwara2
ABSTRACT
Stable carbon isotope compositions as well as organic carbon contents and pyrolysis results of cored material are reported forthe Pliocene to Pleistocene sediments from the Ocean Drilling Program (ODP) Sites 859, 860, 861, and 863. The δ 1 3C values ofkerogen show large variation with no downhole trend, ranging between -34.2%e and -22.6‰ with average values around-25‰, whereas the organic carbon contents in most sediment samples are consistently low (<0.5%) except for enrichment in thesurface layer at Sites 859, 860, and 861. The δ 1 3C values of kerogen indicate the kerogen in sediments of the Chile Margin has apredominantly terrestrial origin, and the fine-grained turbidite material is associated with terrestrial organic matter. The averageδ 1 3C values become lighter with increasing distance from land among three sites drilled along an east-west transect, suggestingfine turbidites with terrestrial organic matter are transported further offshore in this trench-slope setting.
INTRODUCTION
During Ocean Drilling Program Leg 141 in the vicinity of theChile Triple Junction, the Pliocene to Pleistocene sedimentary se-quences were recovered at four sites on the Chile continental slope(Fig, 1). This paper reports the results of stable carbon isotope mea-surements, organic carbon contents, and pyrolysis results for samplescollected at Sites 859, 860, 861, and 863 from Leg 141.
The carbon isotope compositions of organic matter in sedimentsare used to determine the variation of relative proportions of terrig-enous and marine organic carbon inputs (Sackett, 1989). In manymarine environments, relative contributions of terrigenous and ma-rine inputs have been documented through the use of stable carbonisotopes and paleoclimate has been inferred. The carbon isotope com-positions of the kerogens have indicated the sediments of the Nankaiaccretionary prism, Japan contain a higher portion of terrestrial or-ganic matter (Berner and Koch, 1993). In the Gulf of Mexico, down-core variations have been correlated with glacial and interglacial epi-sodes which are related to sea-level lowering and the changing influ-ence of the Mississippi River (Parker et al., 1972; Newman et al.,1973). The influence of glacial episodes on marine sequences hasbeen also inferred from the isotopic compositions of organic matterin sediments of the Oman Margin (Muzka et al., 1991), the LabradorSea, and Baffin Bay (Macko, 1989).
The use of δ13C values as a parameter of relative proportions ofterrigenous and marine organic carbon inputs are based on the prem-ise that the δ'3C values of organic material in sediments are reflectiveof the isotope composition of the source biota and do not changethrough diagenesis. The factors of diagenesis that may be of impor-tance in affecting the carbon isotope composition include: (1) isotopeeffects during bacterial degradation of organic matter; (2) preferentialelimination of compound groups and preferential preservation ofothers which differ significantly in δ13C from the average plant mate-rial; and (3) decarboxylation reactions, which would remove i 3C-enriched groups from the organic material leading to 13C depletion inthe residual (Deines, 1980). However, consistent diagenetic trends insediments are difficult to document (Deines, 1980; Galimov, 1980;
2 J APEX Research Center, 1-2-1 Hamada, Mihama-ku, Chiba 261, Japan.3 Empresa Nacional del Petróleo, Refineria de Petróleos Concern S.A., Casilla 28-D,
Vina del Mai, Chile.
Sackett, 1989). Thus these diagenetic factors above probably canaccount for only minor changes of 1 or 2‰ in the δ13C of organiccarbon, but do not produce a significant carbon isotope shift in Mio-cene to Holocene sediments (Arthur et al., 1985; Dean et al., 1986).
The carbon isotope composition of organic matter in sedimentshas been also used as a paleotemperature tool for studying deep-seasediments, which is based on temperature-dependent fractionation ofcarbon isotopes by marine phytoplankton (Sackett, 1989). The basicpremise in the use of δ13C values of organic matter as a paleotempera-ture parameter is that all of the organic matter in the sediments wassynthesized by algae growing in an open marine environment. Thesediments from Leg 141 were predominantly deposited in a trench-slope environment and consist of continent-derived debris or turbid-ity flow deposits. This tool therefore can not be applied to the sedi-ment in this study, and the variations in isotope signature possiblyrepresent the variation of relative proportions of terrigenous andmarine organic carbon inputs. This paper gives an overview of theorigin of organic matter in relation to the trench-slope sedimentaryenvironment near the Chile Triple Junction.
GEOLOGIC SETTING
At the Chile Triple Junction (Fig. 1) an active spreading ridge andadjacent young oceanic crust are being subducted beneath the conti-nent of South America. During ODP Leg 141, Pliocene to Pleistocenesedimentary sequences were recovered at four sites. Three sites (859,860, and 861) were drilled along an east-west dip transect about 35km north of the Chile Triple Junction at water depths ranging from1652 to 2741 m. Site 863 (2564 m water depth) was drilled at the baseof a trench-slope basin directly above the subducted spreading axis asa strike transect with Site 859.
METHODS
Wet sediment samples were dried for 16 hours at 60°C and thencrushed and acidified with 6N HC1 to remove carbonate. The carbon-ate-free residue was then washed, dried, and sonically extracted witha benzene-methanol mixture (7:3). A portion of dried material wasthen weighed and combusted in a quartz glass tube for 2 hr at 900°Cin presence of purified cupric oxide wire and silver granule. The CO2
gases obtained were cryogenically isolated from other combustionproducts and analyzed on a VG Isotech Sira Series II mass spectrome-ter. The carbon isotope ratios are reported in the usual δ-notationrelative to the PDB (Pee Dee Belemnite) standard:
A. WASEDA, B.M. DIDYK, Y. KAJIWARA
Figure 1. Tectonic sketch map of the Chile Triple Junction and its surroundingsshowing location of Sites 859, 860, 861, and 863. Solid bold lines indicatespreading segments of the Chile Ridge, separating Nazca Plate (north) andAntarctic Plate (south), barbed line is the frontal thrust of the South Ameri-can forearc.
δ13C (‰) = {[(13C/12C)sample/( !3C/12C)standard] - 1} x 1000.
The reproducibility of δ13C (‰) in combustion and measurementis within +0.2‰.
The total organic carbon (TOC) contents of carbonate-free residuewere determined using Yanaco MT-3 CHN Corder. The TOC of thewhole rock were calculated by correcting for carbonate removal.
Rock-Eval pyrolyses of whole rock samples was carried out usinga Rock-Eval II as described by Espitalié et al. (1977). Programmedpyrolysis of samples from 300°C to 550°C gives the amount of pre-formed hydrocarbons (Sj), the amount of hydrocarbons released dur-ing heating (S2), and the amount of CO2 released during pyrolysis to390°C (S3). These values provide the basis for calculation of thehydrogen index (HI), where HI = 100 × S2/TOC, the oxygen index(01), where OI = 100 × S3/TOC, the production index (PI), where PI= Sj/tSj + S2]. The temperature of maximum hydrocarbon releaseduring pyrolysis (Tmax) is also obtained and can be interpreted as ameasure of organic matter thermal maturity.
RESULTS AND DISCUSSION
The total organic carbon (TOC) contents of most samples arecharacterized by predominant low values, ranging from 0.2% to 0.5%and show no trend related to depth (Table 1). These values are typical
for modern open-marine environments (Mclver, 1975) and are simi-lar to the sediments of Site 808, Nankai accretionary prism (Bernerand Koch, 1993). On the other hand, in the same trench-slope system,organic carbon contents are much higher in upwelling area off Peru(up to 9%; ten Haven et al., 1990). During Leg 108 in the northwestAfrican continental margin, upwelling Site 658 displays high organiccontents of 0.5%^l%, whereas nonupwelling Sites 657 and 659 arecharacterized by low organic carbon values of less than 0.5% (Steinet al., 1989). These comparisons with results obtained from the otherareas suggest the low organic carbon contents in the Chile Marginis mainly attributed to the low productivity of organic matter in thisarea. The depth profiles of TOC from Sites 859, 860, 861, and 863 areshown in Figure 2. The shipboard data of TOC values are also plottedin Figure 2; data were determined using a Rock-Eval II with a TOCmodule at Sites 859, 860, and 861 and calculated by subtraction of theinorganic carbon content determined using a coulometer from the totalcarbon content determined using a CNS analyzer at Site 863. TheseTOC values determined through three different analytical methods arewell comparable. The surface layer (<l mbsf [meters below seafloor])show relatively higher TOC values (>1.0%) at Sites 859, 860, and 861,whereas no surface enrichment is observed at Site 863. This surfaceenrichment at Site 859, 860, and 861 suggests that intensive bacterialdegradation of organic matter occurs in near surface sediments at thesesites. This is supported by the observation that high concentrations ofbacterial methane exist at Site 859, 860, and 861, whereas methane isnot abundant at Site 863 (Waseda and Didyk, this volume).
Rock-Eval pyrolysis was only carried out for the samples whichcontain more than 0.3% TOC, since rocks containing low TOC aremost likely to be strongly affected by adsorption of pyrolysate by themineral matrix, resulting in reduced S2 and HI and increased Tmax andOI (Peters, 1986). The S2 values of many samples are too low (<0.2mg/g) to derive reliable interpretations, or S2 peaks in the pyrogramsare broad or multiple. Since the Tmax values of those samples do notrepresent true maturity of sedimentary strata, we have eliminated thosedata (Table 1). In spite of such data elimination, extensive scatter of theRock-Eval Tmax values are observed, and no downhole maturity trendexists (Fig. 3). Most of the data exceeds 430°C equivalent to thethreshold level for oil generation. However, actual level of thermalmaturity should be low since the shallow depth of burial of the sedi-ments and low bottom hole temperature. Maximum temperature at thebottom of the borehole measured by wireline during Leg 141 is 65 °Cat Site 863 (Behrmann, Lewis, Musgrave, et al., 1992). As mentionedabove, the pyrograms of the samples occasionally contain multiple S2
peaks, which suggest multiple inputs that include recycled compo-nents. Other samples have pyrograms in which the S2 peaks are broad,partially resolved, and sloping humps. This pattern suggests that theclay-induced adsorption of the pyrolysates also contributes to theerroneously high Tmax values for some of the sediment samples.
The HI-OI diagram (Fig. 4) indicates the type of organic matter isType III, terrestrial. This is consistent with the fact that the sedimentshave a turbidite and debris flow origin. However, taking into accountthe clay-induced adsorption effect, true HI values of organic mattermaybe slightly higher. Therefore the sediments probably contain con-siderable amount of marine derived organic matter.
The carbon isotope compositions of kerogen range between -34.2and -22.6‰ with average values around -25‰ (Table 1, Fig. 5). Inrecent sedimentary environments, the relative contributions of ter-rigenous and marine inputs have been estimated from stable carbonisotope compositions. Terrestrial plants are enriched in 12C relative tomarine plants. Tertiary kerogens of terrestrial origin have a carbonisotope range from -24‰ to-29%e and modern land plants of the C-3photosynthetic cycle range from -24%e to -32‰, whereas marineplankton varies between -18‰ and-22%e (Schoell, 1984, and litera-ture cited therein). The carbon isotope values suggest that the kerogenin sediments of the Chile Margin has a predominantly terrestrialorigin. This result is consistent with the result obtained from the
300
CARBON ISOTOPE COMPOSITIONS
Table 1. TOC, Rock-Eval, and kerogen carbon isotope data of Leg 141 sediments.
Notes: Blanks indicate not determined for low TOC (<0.3%). * indicates values were not determined for many samples for the following reasons:L = S2 values are too low (<0.2 mg/g) to derive reliable interpretations; B = S2 peaks are too broad; M = multiple S2 peaks.
301
A. WASEDA, B.M. DIDYK, Y. KAJIWARA
Site 859
TOC (%)
Site 860
TOC (%)
2 0 0.5 1
Site 861
TOC (%)
1.5 2 0
Site 863
TOC (%)
0.5 1 1.5
Figure 2. Total organic carbon (TOC) profile of sediment samples at Sites 859, 860, 861, and 863. Sample symbols: solid circles = this study; squares = ship-board analyses.
carbon isotope data of kerogen in sediments of Site 808, Nankaiaccretionary prism, Japan (Berner and Koch, 1993). In contrast, theorganic matter in organic-rich sediments underlying active upwellingarea off Peru has a predominantly marine planktonic and bacterialorigin, with minor terrigenous contribution (ten Haven et al., 1990).The relationship between the carbon isotope compositions and HIvalues show slightly positive correlation (Fig. 6). HI values of marineorganic matter are usually higher than terrestrial organic matter. Thepositive correlation is thus interpreted to indicate mixing of marineorganic matter with heavier δ13C values and higher HI, and terrestrialorganic matter with lighter δ13C values and lower HI.
The average carbon isotope values become lighter with increasingdistance from land (from Site 861 through 859) among three sitesdrilled along an east-west transect (Fig. 5), and suggest an increasingterrigenous organic carbon flux with increasing distance from land. Innormal shelf sediments, surficial sediments contain increasing amountsof the heavier isotope of carbon (13C) with increasing distance fromland and suggest a decreasing terrigenous carbon influence (Hedgesand Parker, 1976). The reverse relationship between the relative con-tribution of terrigenous organic carbon inputs and the distance fromland is observed in continental slope sediments in this study. The com-parison of the sedimentological core description (Behrmann, Lewis,Musgrave, et al, 1992) and the δ13C values of kerogen revealed thatsamples taken from the section interpreted to be turbidites have lighterδ1 3 values and samples taken from the section interpreted to be pelagicfallouts have heavier δ'3C values, indicating the fine-grained turbiditematerial is associated with terrestrial organic matter. Sediments in thisregion mainly consist of debris flow or turbidity flow deposits. At
Site 861 sediments predominantly consist of sands and conglomerateswhich represent more proximal facies of debris flow or turbidity flow,whereas more distal facies of turbidity flow are deposited at Sites 859and 860 (Fig. 7; Shipboard Scientific Party, 1992). All samples fororganic matter analyses were taken from the fine part of sediments. Theproportions of fine particles in turbidite sequences increase with in-creasing distance from land. Therefore, the parts of fine sedimentsderived from muddy turbidites with terrigenous organic matter in-crease with increasing distance from land whereas the parts of finesediments deposited by pelagic fallout containing more marine organicmatter decrease with increasing distance from land. The δ13C values ofkerogen at Site 859 and 860 show large fluctuations compared to Site861 (Fig. 5). Fine sediments sampled at Site 861 may represent pre-dominantly pelagic fallouts with heavier δ13C values. On the otherhand at Sites 859 and 860, some parts of muddy sections may repre-sent turbidites with lighter δ13C values and some parts may representpelagic fallouts.
Areal difference of marine organic productivity is another factorcontrolling the relative contributions of terrigenous and marine inputsto sediments. Since the production of marine organic matter is moreactive in coastal area than offshore area, the marine organic carbonflux decrease with increasing distance from land. Glacial environ-ments also affect the transport of terrigenous material to sediments.In glacial periods, more terrigenous organic matter is transported tooffshore due to sea-level lowering, and lateral transport of terrestrialdebris is enhanced by ice rafting (Gearing et al., 1977). Therefore, thevariation of the δ13C values of kerogen may partially reflect glacialand interglacial episodes.
CARBON ISOTOPE COMPOSITIONS
Tmax (°C)
(mb
sf)
Dep
th
410
u
100
200
300
400
500
600
700
800
-
4
_
-
-
430 450Λ fl~l
47
O
-
α o
DD +
D
• 859
D 860
4 861
O 863
-
-
900 -
Figure 3. Rock-Eval T m a x profile of sediment samples from Sites 859, 860,861, and 863.
SUMMARY
The δ13C values of kerogen show large variation with no down-hole trend, ranging between -34.2‰ and -22.6‰ with average val-ues around -25‰, whereas the organic carbon contents in sedimentsare consistently low except for enrichment in the surface layer at Sites859, 860, and 861. The δ 3C values of kerogen indicate the relativecontributions of terrigenous and marine inputs to sediments. Therelationship between the carbon isotope compositions and HI valuesshow slightly positive correlation, indicating mixing of marine or-ganic matter with heavier δ'3C values and higher HI and terrestrialorganic matter with lighter δ13C values and lower HI.
The comparison of the δ13C values of kerogen and sedimentologi-cal core description of samples revealed that samples taken from thesection interpreted to be turbidites have lighter δ l 3C values and sam-ples taken from the section inteΦreted to be pelagic fallouts haveheavier δ13C values, indicating the fine-grained turbidite material isassociated with terrestrial organic matter. The average carbon isotopevalues become lighter with increasing distance from land (from Site861 through 859) among three sites drilled along an east-west tran-sect, suggesting fine turbidites with terrestrial organic matter aretransported further offshore in this trench-slope setting.
ACKNOWLEDGMENTS
We are grateful to Japan Petroleum Exploration Company Ltd.and Empresa Nacional del Petróleo for allowing us to participate onLeg 141 and publish this paper. The laboratory assistance of YorikoAbe is greatly appreciated.
600 -
HI
300 -
/
-
-
i
^-ii
• D ,
^ 3 ^ ^ Φ Φ ^
• 859
D860
Φ861
O863
D
100 200
O l
300
Figure 4. Hydrogen and oxygen indices of sediment samples from Sites 859,860, 861, and 863.
REFERENCES*
Arthur, M.A., Dean, W.E., and Claypool, G.E., 1985. Anomalous 1 3C enrich-ment in modern marine organic carbon. Nature, 315:216-218.
Behrmann, J.H., Lewis, S.D., Musgrave, R.J., et al., 1992. Proc. ODP, Init.Repts., 141: College Station, TX (Ocean Drilling Program).
Berner, U., and Koch, J., 1993. Organic matter in sediments of Site 808, NankaiAccretionary Prism, Japan. In Hill, LA., Taira, A., Firth, J.V., et al., Proc.ODP, Sci. Results, 131: College Station, TX (Ocean Drilling Program),379-385.
Dean, W.E., Arthur, M.A., and Claypool, G.E., 1986. Depletion of 1 3C inCretaceous marine organic matter: source, diagenetic, or environmentalsignal? Mar. Geol, 70:119-157.
Deines, P., 1980. The isotopic composition of reduced organic carbon. In Fritz,P., and Fontes, J.C. (Eds.), Handbook of Environmental Isotope Geochem-istry (Vol. 1): The Terrestrial Environment, A: Amsterdam (Elsevier),329^106.
Espitalié, J., Madec, M., Tissot, B., Mennig, J.J., and Leplat, P., 1977. Sourcerock characterization method for petroleum exploration. Proc. 9th Annu.Offshore Technol. Conf., 3:439-448.
Galimov, E.M., 1980. C13/C12 in kerogen. In Durand, B. (Ed.), Kerogen: Paris(Technip), 271-299.
Gearing, P., Plucker, F.E., and Parker, P.L., 1977. Organic carbon stable isotoperatios of continental margin sediments. Mar. Chern., 5:251-266.
Hedges, J.I., and Parker, PL., 1976. Land derived organic matter in surfacesediments from the Gulf of Mexico. Geochim. Cosmochim. Acta, 40:1019-1029.
Macko, S.A., 1989. Stable isotope organic geochemistry of sediments fromthe Labrador Sea (Sites 646 and 647) and Baffin Bay (Site 645), ODP Leg105. In Srivastava, S.P., Arthur, M.A., Clement, B., et al., Proc. ODP, Sci.Results, 105: College Station, TX, 209-231.
Mclver, R.D., 1975. Hydrocarbon occurrences from JOIDES Deep Sea Drill-ing Project. Proc. Ninth Petrol. Congr., 269-280.
Muzuka, A.N.N., Macko, S.A., and Pedersen, T.F., 1991. Stable carbon andnitrogen isotope compositions of organic matter from Sites 724 and 725,Oman Margin. In Prell, W.L., Niitsuma, N., et al., Proc. ODP, Sci. Results,117: College Station, TX (Ocean Drilling Program), 571-586.
* Abbreviations for names of organizations and publications in ODP reference lists followthe style given in Chemical Abstracts Service Source Index (published by AmericanChemical Society).
A. WASEDA, B.M. DIDYK, Y. KAJIWARA
Newman, J.W., Parker, RL., and Behrens, E.W., 1973. Organic carbon isotoperatios in Quaternary cores from the Gulf of Mexico. Geochim. Cosmochim.Acta, 37:225-238.
Parker, P.L., Behrens, E.W., Calder, J.A., and Shultz, D., 1972. Stable carbonisotope ratio variations in the organic carbon from Gulf of Mexico sedi-ments. Contrib. Mar. Sci., 16:139-147.
Peters, K.E., 1986. Guidelines for evaluating petroleum source rock usingprogrammed pyrolysis. AAPG Bull., 70:318-329.
Sackett, W.M., 1989. Stable carbon isotope studies on organic matter in themarine environment. In Fritz, P., and Fontes, J.C. (Eds.), Handbook ofEnvironmental Isotope Geochemistry (Vol. 3): The Marine Environment,A: Amsterdam (Elsevier), 139-169.
Schoell, M., 1984. Wasserstoff- und Kohlenstoffisotope in organischen Sub-stanzen, Erdölen und Erdgasen. Geol. Jahrb., Reihe D, 67.
Shipboard Scientific Party, 1992. Site 861. In Behrmann, J.H., Lewis, S.D.,Musgrave, R.J., et al., Proc. ODP, Init. Repts., 141: College Station, TX(Ocean Drilling Program), 239-299.
Stein, R., ten Haven, H.L., Littke, R., Rullkötter, J., and Welte, D.H., 1989.Accumulation of marine and terrigenous organic carbon at upwelling Site658 and nonupwelling Sites 657 and 659: implications for the reconstruc-tion of paleoenvironments in the eastern subtropical Atlantic through lateCenozoic times. In Ruddiman, W., Sarnthein, M., et al., Proc. ODP, Sci.Results, 108: College Station, TX (Ocean Drilling Program), 361-385.
ten Haven, H.L., Littke, R., Rullkötter, J., Stein, R., and Welte, D.H., 1990.Accumulation rates and composition of organic matter in late Cenozoicsediments underlying the active upwelling area off Peru. In Suess, E., vonHuene, R., et al., Proc. ODP, Sci. Results, 112: College Station, TX (OceanDrilling Program), 591-606.
Date of initial receipt: 15 July 1993Date of acceptance: 5 November 1993Ms 141SR-023
Site 859Kerogen
δ13C (‰)
Q.
Q
-35
n\J
100
200
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-
-
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Kerogen
δ13C (‰)
-30
Site 863
Kerogen
δ13C (%o)
-30 -25
Av. -27.5 Av.=-25.3 Av.= -24.3
Figure 5. Carbon isotope profile of sediment samples at Sites 859, 860, 861, and 863.
Av.= -25.0
CARBON ISOTOPE COMPOSITIONS
H I
400 r
300
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100
• 859
D860
Φ861
• 863
•
. D
DD
Φ
D
A
αΦ
α
-35 -30 -25 -20
Kerogen δ13C (‰)
Figure 6. Hydrogen indices and carbon isotope compositions of sedimentsamples from Sites 859, 860, 861, and 863.
Distance (km)
W 30 20
Figure 7. Possible depositional model for sediments recovered at Site 859, 860, and 861 (after Shipboard Scientific Party, 1992).
