Organic matter preservation and sulfur uptake in sedimentsfrom the continental margin off Pakistan

Andreas Luckgea,*, Brian Horsfieldb, Ralf Littkec, Georg Scheedera

aBundesanstalt fur Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655 Hannover, GermanybGeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

cLehrstuhl furGeologie,GeochemieundLagerstattendesErdolsundderKohle,RWTHAachen,Lochnerstrasse4-20,52056Aachen,Germany

Received 14 September 2000; accepted 11 December 2001(returned to author for revision 10 January 2001)

Abstract

The bulk and molecular geochemical characteristics of a suite of kerogens isolated from organic matter-rich sedi-

ments from the Pakistan margin has been characterised by elemental analysis, Rock-Eval pyrolysis and pyrolysis-gaschromatography (Py-GC). Special attention was given to the geochemistry of alkylthiophenes which are thought toreflect the incorporation of sulfur into organic matter (OM) in the context of early diagenetic sulfate reduction. Orga-

nically bound sulfur comprises up to 60% of total sedimentary sulfur. The weight ratios for organic sulfur (Sorg) overorganic carbon (Corg) varies from 0.01 in some of the recently deposited sediments to 0.11 in older samples. The resultsshow that sulfur incorporation into organic matter occurs primarily within the upper few meters of the sediments. The

amount of sulfur which can be incorporated into the organic matter clearly depends on the quality of OM. Thehydrogen index values [HI; which characterise the total amount of generated hydrocarbon equivalents of the pyrolysate(in mg HC equiv./g Corg)] are related to the extent of reworking of organic matter either in the water column or at thesediment-water interface or to remineralization processes by sulfate reduction itself which is strongly controlled by the

availability of reactive iron. High quality organic matter characterised by elevated HI values is capable of sequesteringhigher amounts of sulfur [as expressed by the thiophene content, which represent the organically bound sulfur of thepyrolysate (in mg/g Corg)]. As indicated by the ratio of thiophenes versus HI, which characterises the proportion of

thiophenes in the total pyrolysate, sulfurisation of organic matter seems to be terminated at a ratio of 0.01. # 2002Elsevier Science Ltd. All rights reserved.

1. Introduction

The microbial degradation of organic matter (OM) atthe sediment–water interface exerts an important influ-

ence on the biogeochemical cycling of elements. Organicmatter is oxidized in a characteristic pathway of oxi-dants dictated by the Gibbs free energy yield (Froelich et

al., 1979). Oxidation of organic matter by dissimilatory

sulfate reduction is a ubiquitous and important processin anoxic marine sediments underlying high productivityareas (Jørgensen, 1983). Electron transfer reactions cat-

alyzed by sulfate reducing bacteria determine the abun-dance and speciation of carbon and sulfur preserved inmarine sediments. Berner (1980) and Berner and Rais-

well (1983) have shown that sulfur species which wereproduced by sulfate reducing bacteria are fixed as pyritewhich is the major sink for sulfides in the majority ofcontinental margin sediments. However, in sediments

enriched in organic carbon, such as those underlyingproductive upwelling zones, organically bound sulfur isalso quantitatively significant (Mossmann et al., 1991;

0146-6380/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0146-6380(01 )00171-1

Organic Geochemistry 33 (2002) 477–488

www.elsevier.com/locate/orggeochem

* Corresponding author. Tel.: +49-511-643-2789; fax: +49-

511-643-3663.

E-mail address: a.lueckge@bgr.de (A. Luckge).

Zaback and Pratt, 1992; Schimmelmann and Kastner,1993; Suits and Arthur, 2000). This is because sulfide isformed in quantities exceeding that portion that can bescavenged by iron (Jørgensen, 1982). Part of this sulfide

excess may re-oxidize or disproportionate to poly-sulfides, elemental sulfur, sulfate, thiosulfate etc. (Jør-gensen, 1982, 1988, 1990; Berner and Westrich, 1985).

Another portion of these reduced inorganic sulfur spe-cies such as H2S or polysulfides may be incorporatedinto organic matter by the reaction with functionalized

organic molecules during the early stages of diagenesis(Aizenshtat et al., 1983; Brassell et al., 1986; Francois,1987; Sinninghe Damste et al., 1989a,b, 1990; Kohnen et

al., 1991; Eglinton et al., 1994; Aizenshtat et al., 1995;Wakeham et al., 1995; Hartgers et al., 1997; Werne etal., 2000; Kok et al., 2000a). Laboratory experimentshave shown that under ambient temperatures phytol

and its derivatives, alkenes, aldehydes, and ketones canreact with hydrogen polysulfides and can subsequentlyform organic sulfur molecules such as isoprenoid thio-

phenes (Schouten et al., 1994a,b; Rowland et al., 1993;Fukushima et al., 1992; Grossi et al., 1998; de Graaf etal., 1992).

Several studies have proven that a variety of organicsulfur compounds can be identified in marine sediments(Sinninghe Damste and de Leeuw, 1990 and references

therein). The diagenetic incorporation of sulfur has beenproposed to be an important mechanism to preservebiolipids and to prevent biodegradation of mainly algal-derived organic matter (Sinninghe Damste et al., 1989b;

de Leeuw and Sinninghe Damste, 1990; Kohnen et al.,1991). Principially, two pathways of sulfur incorpora-tion are evident. The intramolecular incorporation of

sulfur leads to the formation of low molecular weight,cyclic sulfur compounds such as thiolanes and thio-phenes. Intermolecular addition leads to the fixation of

organic compounds to the kerogen by sulfide bridges(Sinninghe Damste et al., 1989a; Kohnen et al., 1991).However, in spite of the well known (mechanistic) reac-tion pathways of sulfurisation documented in several

studies, questions on the extent of sulfur addition toOM and possible limitations of sulfur incorporation byOM itself and iron remain. Remineralization and remo-

bilization of organic matter as well as incorporation ofsulfur from sulfate reduction exerts an important influ-ence on quality/reactivity of sedimented organic mate-

rial (Luckge et al., 1999). Intense sulfate reductiondepresses the hydrogen index value (HI), which is com-monly used to determine the petroleum generation

potential and quality of sedimentary OM (Littke et al.,1997).The samples used in this study originate from organic

matter-rich marine sediments underlying the prominent

high productivity zone at the continental margin offPakistan. We describe processes which account forvariable concentrations of organic sulfur in kerogens

and provide evidence that this depends on the quality oforganic matter deposited.

2. Samples and background

The samples were collected during cruise ‘‘SONNE

So-90’’ (von Rad et al., 1995) and consist of recent,organic matter-rich sediments deposited underneath thehighly productive surface waters in the northeastern

Arabian Sea off Pakistan at a water depth of about 600m. The main oceanographic feature of this margin is theexpanded oxygen-minimum zone (OMZ) between 200

and 1200 m water depth with suboxic or anoxic condi-tions at the sea floor (von Rad et al., 1995, Schulz et al.,1996). Basic data on the elemental composition, bulkgeochemical and petrologic composition of sedimentary

OM have been published elsewhere (Littke et al., 1997;Luckge, 1997; Luckge et al., 1999). Briefly, all kerogensstudied were isolated from sediments characterised by

high organic carbon (Corg) contents which range from0.7 to 4.5 wt.%. The total sedimentary sulfur con-centrations are highly variable and range from 0.1 to 1.1

wt.%. Organic petrographical studies as well as trans-mission electron microscopy (TEM) have revealed anamorphous habitus of the OM (Luckge, 1997). More

than 80% of sedimentary OM is unstructured and ofmarine origin. The OM is mainly composed of amor-phous brown to orange flakes without any apparenttexture and shape. TEM observations indicate even at

very high magnification that the OM is amorphouswithout any obvious biological structures. This kind ofamorphous OM occurs also underneath other recent

marine high productivity areas (e.g. Peru and Omanupwelling zones; Luckge et al., 1996). Curie-point-pyr-olysis experiments in kerogens from the Kimmeridge

Clay Formation (UK) by Boussafir et al. (1995) haveshown that this type of nanoscopically amorphous OMmay originate from the addition of sulfur to organicstructures. Van Kaam-Peters et al. (1998) have proposed

that this amorphous OM is strongly associated withsulfur-bound carbohydrates in the kerogens.

3. Experimental

Kerogens were obtained by treating pulverized sedi-ment in a first step with hydrochloric acid (25%) for 2 h.After washing and neutralization samples were treated

with hydrochloric acid (25%) and hydrofluoric acid(48%) and stirred for 16 h at 50 �C. Thereafter, thekerogen concentrates were washed and neutralizedagain. Subsequently they were dried in an oven at 30–

40 �C. Pyrite was not removed.Organic carbon (Corg) and sulfur (S) contents were

measured by combustion under an oxygen flow using a

478 A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488

LECO IR-112 carbon-sulfur analyzer. Average repro-ducibility based on replicate measurements for Corg andS are about 0.02 and 0.03%, respectively.Rock-Eval pyrolysis was performed using a Rock-

Eval-II analyzer (GEOCOM). Fundamentals anddetails of Rock-Eval pyrolysis are described in Espitalieet al. (1977) and Tissot and Welte (1984). This technique

has been used as a rapid method to determine thehydrocarbon source rock potential of sedimentary rocks(Peters, 1986). Rock-Eval pyrolysis can also be used to

determine the hydrogen richness of protokerogens inmodern sediments (Liebezeit and Wiesner, 1990; Arthuret al., 1994, 1998; Dean and Gardner, 1998).

Pyrolysis-gas chromatography (Py-GC) was carriedout on selected kerogen samples. The Py-GC systemused was similar to that described by Horsfield (1989).Kerogen samples (up to 5 mg) were heated in a flow of

helium. Products released at temperatures below 300 �C(300 �C held for 10 min) were vented. Pyrolysis productsgenerated between 300 and 600 �C (50 �C/min) were

collected in a cryogenic trap (liquid nitrogen cooling)from which they were then liberated by heating to300 �C. Gas chromatographic analysis of the pyrolysis

products was performed using a Hewlett Packard 5731Agas chromatograph equipped with a fused silica column(25 m � 0.32 mm i.d.) containing an apolar phase (BP-

1.1 mm thickness). The oven temperature was pro-grammed from 40 to 320 �C (5 �C/min). Prominent com-pounds, namely n-alkanes and n-alkenes, aromatichydrocarbons, alkylphenols, alkylpyrroles and alkylthio-

phenes were identified (based on their GC retention times)and quantified by external standardisation and publishedreports (Sinninghe Damste et al., 1988, 1992; Horsfield,

1989; di Primio, 1995). The precision was better than5% for all compounds except the alkylpyrroles andalkylthiophenes (better than 10%). Peak identification

on selected samples was confirmed by Py-GC–MS using

a Fisons GC 8000 coupled to a Fisons MD 800 massspectrometer. GC conditions were identical to thoseused for Py-GC, except that a 50 m column wasemployed to compensate the vacuum of the ionisation

chamber of the mass spectrometer and allow optimizedflow. Analyses were performed in full scan mode. Fullscan mass spectra were recorded over the mass range m/z

10–420.

4. Results and discussion

4.1. Elemental analysis

The elemental composition, calculated Sorg con-centrations, as well as Sorg/Corg ratios of the kerogensare presented in Table 1. The organic sulfur content was

calculated as the difference between total (measured)and pyritic sulfur (using the stoichiometry of FeS2)assuming that sulfur is exclusively fixed in pyrite or as

organic sulfur. Based on this approach we calculatedthat organic sulfur forms up to 60% of total sulfur. Thisis similar to results of Mossmann et al. (1991) and

Eglinton et al. (1994) for kerogens from the Peruviancontinental margin. Most of the kerogens analysed canbe classified as Type II-S with Sorg/Corg ratios greater

than 0.04 (Orr, 1986). Samples deposited very recently(< 0.2 kyear) show ratios lower than 0.04. According toFrancois (1987), elevated Sorg/Corg ratios are an indica-tion for the incorporation of sulfur into the organic

matter via early diagenetic processes. Sulfur incorpora-tion by assimilatory biosynthesis in marine phyto-plankton and bacteria leads to Sorg/Corg ratios in the

range from 0.01 to 0.03. The sulfur found in these organ-isms is mainly bound to proteins and amino acids, whichare considered amongst the most reactive biochemicals.

The relative enrichment of these labile compounds by

Table 1

Elemental composition, HI values and pyrolysis yields of selected fractions of the kerogens

Depth(mbsf)

C(%)

S(%)

Fe(%)

Sorg(%)

Sorg/Corg HI(mg HCequiv./gCorg)

AliphaticHC(mg/g Corg)

AromaticHC(mg/g Corg)

Alkylthiophenes(mg/g Corg)

Alkylphenols(mg/g Corg)

Alkylpyrroles(mg/g Corg)

0.03 43.2 0.85 0.4 0.4 0.01 290 15.3 9.2 1.2 2.2 7.70.13 52.3 0.75 0.28 0.4 0.01 364 17.2 10.0 1.2 2.6 10.40.19 46.2 – – – – 267 15.2 9.3 1.3 2.1 5.73.9 25.9 7.93 5.99 1.1 0.04 341 17.6 11.6 3.2 2.5 6.55.21 25.4 10.3 8.19 1.0 0.04 274 13.7 9.7 2.6 2.1 4.35.67 30.6 11.6 8.82 1.5 0.05 266 12.7 9.5 2.3 2.0 5.58.13 40.2 13.1 8.99 2.9 0.07 322 16.2 11.3 3.7 2.4 5.39.75 24.2 – – – – 234 12.9 9.1 3.3 1.9 3.210.62 30.4 10.4 6.95 2.5 0.08 299 14.2 9.7 3.5 2.0 3.210.77 25.9 8.88 6.01 2.0 0.08 332 18.0 11.6 3.6 2.2 3.611.7 45.3 7.99 3.71 3.8 0.08 397 18.5 11.8 4.3 2.7 7.612.05 37.3 7.41 2.9 4.1 0.11 384 17.2 11.1 4.0 2.4 6.5

A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488 479

selective mineralization of organic carbon thereforeseems to be unlikely (Francois, 1987; Suits and Arthur,2000).

4.2. Rock-Eval pyrolysis

Obtained by Rock-Eval pyrolysis, the hydrogen index

(HI), which is the quotient of total amount of pyrolysisproducts versus organic carbon (measured as mghydrocarbon (HC) equivalents/g Corg), is a commonly

used measure for the determination of hydrogen rich-ness of sedimentary organic matter with respect to thehydrocarbon source rock potential. Because hydrogen-

rich organic matter is also the most easily metabolized,the HI is a measure of reactivity of OM (Whelan, 1977;Emerson, 1985; Henrichs and Reeburgh, 1987; Deanand Gardner, 1998). The HI values in the kerogen con-

centrates from the Pakistan margin range from about230 to about 400 mg HC equiv./g Corg (Fig. 1). A cleardepth trend can not be observed (Fig. 2). The highest

values of about 400 mg HC equiv./g Corg were measuredat the base of the sequence. Earlier microscopical stud-ies indicated that the contribution of hydrogen-poor

terrestrial OM is low (<10%) and more or less constant

(Luckge et al., 1999). This excludes the possibility thatvariable HI values are caused by variable input of OMfrom different sources. Only the sample (at a depth of9.75 mbsf) with the lowest HI value shows a slightly

enhanced portion of terrigenous OM. The low Corgcontent of 0.7 wt.% of the ‘original’ sample may alsodepress the HI value due to mineral matrix effects (Katz,

1983). Winnowing effects, which may exert importantcontrols on the organic carbon concentration anddegree of preservation as it was described for the Peru

and Oman upwelling systems (Arthur et al., 1998; Ped-ersen et al., 1992), can be excluded for the sedimentsfrom the Pakistan continental margin (van der Weijden

et al., 1999).

4.3. Molecular characterisation of kerogen

The GC-amenable pyrolysis products are dominatedby n-C6 to n-C32 alkane/alkene doublets (up to 50%), aswell as sulfur-containing thiophenic compounds (5–

20%), pyrrolic moieties (5–20%) and other heteroaro-matic compounds (20–30%; Fig. 3 and Table 1). It iswell known that hydrogen-rich kerogen generates

hydrogen-rich pyrolysates (van de Meent et al., 1980)

Fig. 1. Location map of the studied area off Pakistan (RV Sonne Cruise, So-90). The shaded area indicates the well-developed oxygen-

minimum zone between 200 and 1200 m water depth.

480 A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488

and that pyrolysate n-alkyl composition as well as thechain length distribution are closely related to the n-alkyl distribution in the precursor kerogen (Larter andHorsfield, 1993; Larter et al., 1983). The homologous

series of n-alkanes and n-alkenes drop off in abundancebetween C17 and C20, as is commonly observed for pyr-olysates derived from marine kerogens (Horsfield, 1989;Gray et al., 1991). The C1–C5 alkypyrrole distribution is

similar to that found in kerogens from the Montereyformation, an ancient upwelling system (SinningheDamste et al., 1992), and to that detected in pyrolysates

from the Peru and Oman upwelling areas (Luckge,1997). According to Sinninghe Damste et al. (1992) thealkylpyrroles seems to be derived from tetrapyrrole pig-

ments and may reflect the algal contribution and/orspecific preservation conditions typical for these envir-onments.

Organic sulfur compounds detected in the pyrolysatesconsist mainly of alkylated thiophenes. They are domi-nated by C2- and C3-alkylated thiophenes, namely 2,3-dimethylthiophene, 2-ethyl-5-methylthiophene and

2,3,4-trimethylthiophene. Many type II-S kerogens arecharacterised by high alkylthiophene contents in thepyrolysates (Sinninghe Damste et al., 1989a; Eglinton et

al., 1990, 1994; Kok et al., 2000b). Eglinton et al. (1990,1992) used the abundance of thiophenic to hydrocarbonpyrolysis products to show that thiophenic structures

represent the organically bound sulfur. A plot of thesummed concentrations of thiophenes (normalised toCorg) against the Sorg/Corg ratios shows, although mod-

ified, in analogy to the ratios used by Eglinton et al.(1990, 1992), a positive correlation (Fig. 4). This posi-tive correlation suggests that thiophenic pyrolysatecompounds are quantitatively representative for the

Fig. 3. Pyrolysis-chromatogram of a kerogen from 0.13 cm depth. Italic numbers indicate the carbon chain length of the n-alk-1-ene/

n-alkane doublets, indicated by filled circles. Pr represents prist-1-ene.

Fig. 2. Depth profile of hydrogen index (HI) values of kerogen

concentrates. The shaded area represents the zone of presently

active sulfate reduction.

A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488 481

organically bound sulfur in the studied samples. A

positive intercept on the x-axis is probably caused by aslight underestimation of the organic sulfur content,which was calculated (assuming that sulfur is presentonly as pyrite and organic sulfur). Nevertheless, the

abundance of thiophenes can be used to estimate theextent of sulfur incorporation into OM during earlydiagenesis.

The recent to subrecent Pakistan kerogens show stea-dily increasing thiophene concentrations with depth(Fig. 5). The most pronounced increase from 1.2 to

about 3.1 mg/g Corg occurs between the near-surfacesamples (0.03 and 0.13 mbsf) and the sediment sampleat 3.9 mbsf which comprises about 4000 years, althoughonly a mean sedimentation rate is available (ca. 1 mm/

year). With respect to the timing of early diageneticsulfur incorporation into OM, this time interval is inaccordance with those reported by Werne et al. (2000)

and Kok et al. (2000a). These authors showed that sul-furisation reactions take place early and are completedin ca. 5000 years in the Cariaco Basin and within 1000–

3000 years in the Ace Lake, respectively. It shows alsothat sulfur incorporation has occurred in the uppermostsample from 0.03 mbsf depth as it was observed in other

recent sediments (e.g. Eglinton et al., 1994; Hartgers etal., 1997). Obviously, under the specific depositionalconditions in the OMZ environment off the Indusmouth, which is characterised by oxygen-depleted bot-

tom waters, sulfate reducing bacteria produce, near thesediment surface, excess reduced sulfur species to reactwith organic matter.

4.4. The role of iron, organic matter quality and

sulfurisation

The highly variable pyrite contents in the kerogensmake them ideal to study the relationship between

inorganic and organic sulfur (assuming that pyrite is theonly ferrous phase in the kerogen concentrates). Thetwo most recent samples (0.03 and 0.13 m sediment

depth), characterised by the lowest pyrite contents andlowest thiophene contents (Fig. 6), are located in thepresently active sulfate reduction zone (based on pore-

water data provided by Dietrich et al., 1995) wherepyrite formation as well as organic sulfur incorporationis in progress. In all other studied samples, microbialsulfate reduction and subsequent sulfide formation is

essentially complete (Luckge et al., 1999). Below thedepth of 3.9 mbsf, Fepyrite and organic sulfur con-centrations of the kerogens are negatively correlated.

This suggests that low iron concentrations favour theformation of organic sulfur. Several authors have shownthat sulfurisation of OM mainly takes place after the

bulk of pyrite precipitation (Kaplan et al., 1963;Gransch and Posthuma, 1974; Francois, 1987; Zabackand Pratt, 1992). It has been argued that the formation

of organic sulfur is limited to environments where thegeneration of sulfide is greater than the supply of reac-tive iron (Berner, 1984; Raiswell et al., 1988; SinningheDamste and de Leeuw, 1990). However, Canfield et al.

(1992) and Thamdrup et al. (1994) have shown that ironis rapidly depleted within the sediments and sufficientsulfide to react with OM is nearly always available.

Fig. 4. Relationship of the thiophenes/toluene ratio with Sorg/Corg for kerogens from the studied area. The organic sulfur content was

calculated as the difference between total (measured) and pyritic sulfur (using the stoichiometry of FeS2).

482 A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488

Nevertheless, a large pool of reactive iron makes thereaction with OM less likely. Werne et al. (2000) pointedout that the high sulfate-reducing activity required togenerate excess sulfide, which is needed to promote sul-

furisation of OM, can decompose the organic substrateitself to the point that organic matter sulfurisation is

unlikely. This probably is represented by the negativecorrelation between pyrite concentrations and HI values(Fig. 7) suggesting that samples with higher pyrite con-tents have experienced a higher degree of OM reworking

by sulfate reducers which consequently lowers HIvalues. According to Gransch and Posthuma (1974),Berner (1984) and Sinninghe Damste et al. (1989a) most

organic sulfur accumulation takes place directly afterpyrite formation is complete. If this holds true and ifsulfurisation of OM prevents further decomposition of

OM (Sinninghe Damste et al., 1989b, 1990), this meansthat the OM in samples characterised by high iron con-tents can be more extensively degraded by sulfate

reduction (associated with a lowering of HI values) thanOM in samples with low iron concentrations.In samples with low iron concentrations excess sulfide

is generated more rapidly. Sulfurisation of OM is accel-

erated and leads to the early formation of an organicresidue that has a lower susceptibility to attack by sul-fate reducers and, thus, enhances preservation of OM in

sediments (Sinninghe Damste et al., 1989b, 1990, 1998).Furthermore, Littke et al. (1997) have shown that thedegree of early diagenetic sulfate reduction has a strong

control on the HI which is a measure of the quality or‘freshness’ of OM (Dean and Gardner, 1998). High sul-fate-reducing activity in the sediments leads to the rela-

tive enrichment of more refractory organic compoundscharacterised by lower HI values, resulting in a corre-sponding reduction in reactivity of organic molecules(Littke et al., 1997).

Using the concept of SRI (sulfate reduction index)proposed by Littke et al. (1991), Lallier-Verges et al.(1993) and Veto et al. (1994), which is defined as the

OM fraction decomposed by sulfate reducers versus theOM delivered to the sulfate reduction zone, Littke et al.

Fig. 6. Thiophene concentrations (normalised to Corg ) vs. the pyrite content (FePyrite).

Fig. 5. Depth distribution of thiophenes (normalised to Corg).

The shaded area indicates the zone of presently active sulfate

reduction.

A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488 483

(1997) have demonstrated that up to 50% of OMdeposited within the OMZ sediments can be mineralizedby sulfate reducing bacteria. The calculation of a mod-ified SRI has shown that—with the exception of the

near surface samples—the OM fraction mineralized bysulfate-reducing bacteria is variable and that there is noclear depth trend (Luckge et al., 1999). Between 20 and

50% of the initially deposited OM was decomposed. Acorrelation between the calculated degree of sulfate

reduction and the thiophene concentration presented inthis study is not visible. Hence, increasing thiophenecontents with depth (below those in the shallowest sam-ples, Fig. 5) are not an indication of more intense sulfate

reduction. They also do not neccessarily mean that sul-furisation of OM is still in progress. Therefore, theamount of sulfur which can be incorporated into the OM

must be controlled by an additional parameter. As shownin Figs. 6 and 7, kerogens with high pyrite contents are

Fig. 7. Plot of HI values vs. pyrite content in the studied samples. The uppermost samples lying in the active sulfate reduction zone

are encircled.

Fig. 8. Relationship of HI values and thiophene concentrations. The samples lying in the active sulfate reduction zone are encircled.

484 A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488

characterised by low thiophene concentrations and lowHI values and vice versa. This is probably an indicationfor a coupling of HI and thiophene yield as displayed inFig. 8. Samples characterised by higher HI values (and

thus higher yields of pyrolysis products liberated fromkerogen), seem to be able to fix more thiophenes (whichrepresent the organically bound sulfur of the pyrolysate)

than those with lower HI-values. Kerogens char-acterised by elevated HI values probably possess ahigher number of functional groups where sulfur mole-

cules can be added. Furthermore, it can be assumed thatsulfur addition is limited due to the limited availabilityof these functional groups within the kerogen. Thus, the

ratio of both pyrolysis parameters (thiophene con-centrations and HI values) should provide informationabout the status of sulfur incorporation. The highestthiophene/HI ratios of about 0.01 are established at

about 4 mbsf and remain approximately constant belowthis depth (Fig. 9). Only the sample at a depth of 9.75mbsf, characterised by a low HI value of about 230 mg

HC equiv./g Corg (and also a low Corg concentration of

0.7 wt.%), shows a higher ratio. This may be in part dueto a slightly elevated contribution of less reactive ter-restrial OM and in part due to mineral matrix effects(Katz, 1983). At the ratio of 0.01 sulfur incorporation

seems to be terminated. The organic substrate seems tobecome the limiting factor for sulfur incorporationitself. Similar thiophene/HI ratios of 0.01 were found in

kerogens from the Peru and Oman upwelling areas atthose sites where comparable environmental anddepositional conditions were observed (Luckge, unpub-

lished data).

5. Conclusions

The main results can be summarized as follows:

1. As shown by recently/subrecently depositedsediment samples off Pakistan, the addition ofsulfur to organic molecules begins at an early

stage of diagenesis (< 4 mbsf). OM sulfurisa-tion reactions mostly take place within thisuppermost part of the sedimentary column.

2. High iron contents promote degradation oforganic matter under sulfate-reducing condi-tions because it delays ‘‘vulcanization’’ of OM

which prevents the further decomposition ofOM.

3. The absolute amount of sulfur which can beincorporated into organic matter is not necessa-

rily an expression for sulfate reduction intensity.Instead, the extent of organic sulfur incorpora-tion depends on the quality of the OM which

was delivered to the sulfate reduction zone and/or on the degree of decay of the OM during theprocess of sulfate reduction itself. The quality of

organic matter can dictate the concentration oforganic sulfur in the sediments.

4. Sulfurisation of OM seems to end when a thio-phene/HI-ratio of about 0.01 (mg/g Corg versus

mg HC equiv./g Corg) is reached. This value maybe an expression for the maximum capability ofOM to store sulfur.

Acknowledgements

This study was carried out at the Institute for Petro-leum and Organic Geochemistry in Julich (ICG-4). The

technical expertise and support of Franz Leistner inperforming Py-GC analyses is gratefully acknowledged.We thank Ulrich von Rad for providing samplesfrom the northeastern Arabian Sea (RV SONNE cruise;

SO-90/PAKOMIN). M. Koopmans, J.S. SinningheDamste and an anonymous referee are thanked forsuggestions to improve the manuscript. The Deutsche

Fig. 9. Depth profile of the thiophenes/hydrogen index ratio.

The grey shaded area indicates the interval where sulfurisation

of organic matter seems to be complete.

A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488 485

Forschungsgemeinschaft provided financial support(Grant No. Li 618/1-3) in the framework of the‘‘Schwerpunktprogramm’’ Ocean Drilling Program forwhich we are grateful.

Associate Editor—J.S. Sinninghe Damste

References

Aizenshtat, Z., Krein, E.B., Vairavamurthy, M.A., Goldstein,

T.P., 1995. Role of sulfur in the transformations of sedi-

mentary organic matter: a mechanistic overview. In: Vair-

avamurthy, M.A., Schoonen, M.A.A. (Eds.), Geochemical

Transformations of Sedimentary Sulfur, ACS Symposium

Series 612. American Chemical Society, Washington DC, pp.

16–37.

Aizenshtat, Z., Stoler, A., Nielsen, H., 1983. The geochemical

sulfur enrichment of recent organic matter by polysulfides in

the solar-lake. In: Bjorøy, M. (Ed.), Advances in Organic

Geochemistry 1981. John Wiley, Chichester, pp. 279–288.

Arthur, M.A., Dean, W.E., Neff, E.D., Hay, B.J., Jones, G.,

King, J., 1994. Late Holocene (0-2000 ybp) organic carbon

accumulation in the Black SE. Global Biogeochem. Cycles 8,

195–217.

Arthur, M.A., Dean, W.E., Laarkamp, K., 1998. Organic car-

bon accumulation and preservation in surface sediments on

the Peru margin. Chemical Geology 152, 273–286.

Berner, R.A., 1980. Early Diagenesis: A Theoretical Approach.

Princeton University Press, Princeton, NJ.

Berner, R.A., Raiswell, R., 1983. Burial of organic carbon and

pyrite sulfur in sediments over Phanerozoic time: a new the-

ory. Geochimica et Cosmochimica Acta 47, 855–862.

Berner, R.A., 1984. Sedimentary pyrite formation: an update.

Geochimica et Cosmochimica Acta 48, 605–615.

Berner, R.A., Westrich, J.T., 1985. Bioturbation and the early

diagenesis of carbon and sulfur. American Journal of Science

285, 193–206.

Boussafir, M., Gelin, F., Lallier-Verges, E., Derenne, S., Ber-

trand, P., Largeau, C., 1995. Origin and preservation pro-

cesses of organic material in the short-term organic cyclicities

of the KCF (G.B.) attested by TEM, SEM and molecular

studies. Geochimica et Cosmochimica Acta 59, 3731–3748.

Brassell, S., Lewis, C.A., de Leeuw, J.W., de Lange, F., Sin-

ninghe Damste, J.S., 1986. Isoprenoid thiophenes: novel

products of sediment diagenesis? Nature 320, 160–162.

Canfield, D.E., Raiswell, R., Bottreel, S., 1992. The reactivity

of sedimentary iron minerals toward sulfide. Am. J. Sci. 292,

659–683.

Dean, W.E., Gardner, J.V., 1998. Glacial to Holocene con-

trasts in organic matter production and preservation on the

California margin. Geol. Soc. Am. Bull. 110, 888–899.

de Graaf, W., Sinninghe Damste, J.S., de Leeuw, J.W., 1992.

Laboratory simulation of natural sulphurization I. Forma-

tion of monomeric and oligomeric isoprenoid polysulphides

by low-temperature reactions of inorganic hydrogen poly-

sulphides with phytol and phytadienes. Geochimica et Cos-

mochimica Acta 56, 4321–4328.

de Leeuw, J.W., Sinninghe Damste, J.S. 1990. Organic sulfur

compounds and other biomarkers as indicators of palaeosa-

linity. In: Orr, W.L., White, C.M. (Eds.), Geochemistry of

Sulfur in Fossil Fuels. ACS Symp. Ser. 249, pp. 417–443.

Dietrich, P.G., Volke, P., Peter, H.J., 1995. Porenwasser und

Fruhdiagenese im Bereich der Sauerstoff-Minimum-Zone vor

Pakistan, Arabisches Meer. Forschungsfahrt So90, Unpub-

lished Report, TU Bergakademie Freiberg.

Eglinton, T.I., Sinninghe Damste, J.S., Kohnen, M.E.L., de

Leeuw, J.W., 1990. Rapid estimation of the organic sulphur

content of kerogens, coals and asphaltenes by pyrolysis-gas

chromatography. Fuel 69, 1394–1404.

Eglinton, T.I., Sinninghe Damste, J.S., Pool, W., de Leeuw,

J.W., Eijkel, G., Boon, J.J., 1992. Organic sulphur in mac-

romolecular sedimentary organic matter. II. Analysis of dis-

tribution of sulphur-containing pyrolysis products using

multivariate techniques. Geochimica et Cosmochimica Acta

56, 1545–1560.

Eglinton, T.I., Irvine, J.E., Vairavamurthy, A., Zhou, W.,

Manowitz, B., 1994. Formation and diagenesis of macro-

molecular organic sulfur in Peru Margin sediments. Organic

Geochemistry 22, 781–799.

Emerson, S., 1985. Organic carbon preservation in marine

sediments. In: Sundquist, E.T., Broecker, W.S. (Eds.), The

Carbon Cycle and Atmospheric CO2: Natural Variations

from Archean to Present. AGU Monograph 32, p. 78–89.

Espitalie, J., Laporte, J.L., Madec, M., Marquis, F., Leplat, P.,

Paulet, J., Boutefeu, A., 1977. Methode rapide de caracter-

isation des roches meres, de leur potentiel petrolier et de leur

degre d’ evolution. Rev. Inst. Franc. Petr. 32, 23–42.

Francois, R., 1987. A study of sulphur enrichment in the humic

fraction of marine sediments during early diagenesis. Geo-

chimica et Cosmochimica Acta 51, 17–27.

Froelich, P.H., Klinkhammer, G.P., Bender, M.L., Luedtke,

N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D.,

Hartmann, B., 1979. Early oxidation of organic matter in

pelagic sediments of the eastern equatorial Atlantic: suboxic

diagenesis. Geochimica et Cosmochimica Acta 43, 1075–1090.

Fukushima, K., Yasukawa, M., Muto, N., Uemera, H., Ishi-

watari, R., 1992. Formation of C20 isoprenoid thiophenes in

modern sediments. Organic Geochemistry 18, 83–92.

Gransch, J.A., Posthuma, J., 1974. On the origin of sulphur in

crudes. In: Tissot, B., Bienner, F. (Eds.), Advances in

Organic Geochemistry 1973. Editions Technip, Paris, pp.

724–739.

Gray, N.R., Lancaster, C.J., Gethner, J., 1991. Chemometric

analysis of pyrolysis compositions: a model for predicting the

organic matter type of source rocks using pyrolysis-gas

chromatography. Journal of Analytical and Applied Pyr-

olysis 20, 87–106.

Grossi, V., Hirschler, A., Raphel, D., Rontani, J.-F., de Leeuw,

J.W., Bertrand, J.-C., 1998. Biotransformation pathways of

phytol in recent anoxic sediments. Organic Geochemistry 29,

845–861.

Hartgers, W.A., Lopez, J.F., Sinninghe Damste, J.S., Reiss, C.,

Maxwell, J.R., Grimalt, J.O., 1997. Sulfur-binding in recent

environments: II. Speciation of sulfur and iron and implica-

tions for the occurrence of organo-sulfur compounds. Geo-

chimica et Cosmochimica Acta 61, 4769–4788.

Henrichs, S.M., Reeburgh, W.S., 1987. Anaerobic mineraliza-

tion of marine organic matter: rates and the role of anaerobic

processes in the oceanic carbon economy. Geomicrobiology

Journal 5, 191–237.

486 A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488

Horsfield, B., 1989. Practical criteria for classifying kerogens:

some observations from pyrolysis-gas chromatography.

Geochimica et Cosmochimica Acta 53, 891–901.

Jørgensen, B.B., 1982. Mineralization of organic matter in the

sea bed—the role of sulfate reduction. Nature 296, 643–645.

Jørgensen, B.B., 1983. Processes at the sediment-water inter-

face. In: Bolin, B., Cook, R.B. (Eds.), The Major Biogeo-

chemical Cycles and their Interaction. SCOPE Rep. 21, pp.

477–515.

Jørgensen, B.B., 1988. Ecology of the sulfur cycle: oxidative

pathways in sediments. In: Cole, J.A., Ferguson, S.J. (Eds.).

Soc. Gen. Microbiol. Symposium, 42, 31–63.

Jørgensen, B.B., 1990. The sulfur cycle of freshwater sediments:

role of thiosulfate. Limnol. Oceanography 35, 1329–1342.

Kaplan, I.R., Emery, K.O., Rittenberg, S.C., 1963. The dis-

tribution and isotopic abundance of sulphur in recent marine

sediments. Geochimica et Cosmochimica Acta 27, 297–331.

Katz, B.J., 1983. Limitation of ‘Rock-Eval’ pyrolysis for typing

organic matter. Organic Geochemistry 4, 195–199.

Kohnen, M.E.L., Sinninghe Damste, J.S., ten Haven, H.L.,

Kock-van Dalen, A.C., Schouten, S., de Leeuw, J.W., 1991.

Identification and geochemical significance of cyclic di- and

trisulphides with linear and acyclic isoprenoid carbon skele-

tons in immature sediments. Geochimica et Cosmochimica

Acta 55, 3685–3695.

Kok, M.D., Rijpstra, I.C., Robertson, L., Volkman, J.K., Sin-

ninghe Damste, J.S., 2000a. Early steroid sulfurisation in

surface sediments of a permanently stratified lake (Ace Lake,

Antarctica). Geochimica et Cosmochimica Acta 64, 1425–

1436.

Kok, M.D., Schouten, S., Sinninghe Damste, J.S., 2000b. For-

mation of insoluble, nonhydrolyzable, sulfur-rich macro-

molecules via incorporation of inorganic sulfur species into

algal carboydrates. Geochimica et Cosmochimica Acta 64,

2689–2699.

Lallier-Verges, E., Bertrand, P., Desprairies, A., 1993. Organic

matter composition and sulfate reduction intensity in Oman

Margin sediments. Marine Geology 112, 57–69.

Larter, S.R., Solli, H., Douglas, A.G., 1983. Phytol containing

melanoidins and their bearing on the fate of isoprenoid

structures in sediments. In: Bjorøy, M. (Ed.), Advances in

Organic Geochemistry 1981. John Wiley, Chichester, pp.

513–521.

Larter, S.R., Horsfield, B., 1993. Determination of structural

components of kerogens by the use of analytical pyrolysis

methods. In: Engel, M.H., Macko, S.A. (Eds.), Organic

Geochemistry. Plenum Press, New York, pp. 271–287.

Liebezeit, G., Wiesner, M.G., 1990. Pyrolysis of recent marine

sediment: I. Biopolymers. In: Durand, B., Behar, F. (Eds.),

Advances in Organic Geochemistry 1989. Pergamon,

Oxford. Organic Geochemistry 16, 1179–1185.

Littke, R., Klussmann, U., Krooss, B., Leythaeuser, D., 1991.

Quantification of loss of calcite, pyrite, and organic matter

due to weathering of Toarcian black shales and effects on

kerogen and bitumen characteristics. Geochimica Cosmo-

chimica Acta 55, 3369–3378.

Littke, R., Luckge, A., Welte, D.H., 1997. Quantification of

organic matter degradation by microbial sulphate reduction

for Quaternary sediments from the northern Arabian Sea.

Naturwissenschaften 84, 312–315.

Luckge, A., Boussafir, M., Lallier-Verges, E., Littke, R., 1996.

Comparative study of organic matter preservation in imma-

ture sediments along the continental margins of Peru and

Oman. Part I: results of petrographical and bulk geochemical

data. Organic Geochemistry 24, 437–451.

Luckge, A., 1997. Ablagerung und Fruhdiagenese organischen

Materials in marinen Hochproduktivitatsgebieten. Berichte

des Forschungszentrums Julich 3413, 161.

Luckge, A., Ercegovac, M., Strauss, H., Littke, R., 1999. Early

diagenetic alteration of organic matter by sulfate reduction

in Quaternary sediments from the northeastern Arabian Sea.

Marine Geology 158, 1–13.

Mossmann, J.-R., Aplin, A.C., Curtis, C.D., Coleman, M.L.,

1991. Geochemistry of inorganic and organic sulphur in

organic-rich sediments from the Peru margin. Geochimica et

Cosmochimica Acta 55, 3581–3595.

Orr, W.L., 1986. Kerogen/asphaltene/sulfur relationships in

sulfur-rich Monterey oils. Organic Geochemistry 10, 499–

516.

Pedersen, T.F., Shimmield, G.B., Price, N.B., 1992. Lack of

enhanced preservation of organic matter in sediments under

the oxygen minimum on the Oman margin. Geochimica et

Cosmochimica Acta 56, 545–551.

Peters, K.E., 1986. Guidelines for evaluating petroleum source

rock using programmed pyrolysis. Am. Assoc. Petrol. Geol.

Bull. 70, 318–329.

di Primio, R., 1995. The generation and migration of sulphur-

rich petroleums in a low-maturity carbonate source rock

sequence from Italy. Berichte des Forschungszentrums Julich

3110, 197.

Raiswell, R., Buckley, F., Berner, R.A., Anderson, T.F., 1988.

Degree of pyritisation as a palaeoenvironmental indicator of

bottom-water oxygenation. Journal of Sedimentary Petrol-

ogy 58, 812–819.

Rowland, S., Rockey, C., Al-Lihaibi, S.S., Wolff, G.A., 1993.

Incorporation of sulfur into phytol derivatives during simu-

lated early diagenesis. Organic Geochemistry 20, 1–15.

Schimmelmann, A., Kastner, M., 1993. Evolutionary changes

over the last 1000 years of reduced sulfur phases and organic

carbon in varved sediments of the Santa Barbara Basin,

California. Geochimica et Cosmochimica Acta 57, 67–78.

Schouten, S., de Graaf, W., Sinninghe Damste, J.S., van Driel,

G.B., de Leeuw, J.W., 1994a. Laboratory simulation of nat-

ural sulphurization: II. Reaction of multi-functionalized

lipids with inorganic polysulphides at low temperatures.

Organic Geochemistry 22, 825–834.

Schouten, S., van Driel, G.B., Sinninghe Damste, J.S., de

Leeuw, J.W., 1994b. Natural sulphurization of ketones and

aldehydes: a key reaction in the formation of organic sulphur

compounds. Geochimica et Cosmochimica Acta 58, 5111–

5116.

Schulz, H., von Rad, U., von Stackelberg, U., 1996. Laminated

sediments from the oxygen-minimum zone of the north-

eastern Arabian Sea. In: Kemp, A.E.S. (Ed.), Paleoclimatol-

ogy and Paleoceanography from Laminated Sediments.

Geological Society, London, Special Publications 116, pp.

185–207.

Sinninghe Damste, J.S., Kock-van Dalen, A.C., de Leeuw,

J.W., Schenck, P.A., 1988. Identification of homologous ser-

ies of alkylated thiophenes, thiolanes, thianes and ben-

zothiophenes present in pyrolysates of sulfur-rich kerogens.

Journal of Chromatography 435, 435–452.

A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488 487

Sinninghe Damste, J.S., Eglinton, T.I., de Leeuw, J.W.,

Schenck, P.A., 1989a. Organic sulphur in macromolecular

sedimentary organic matter: 1. Structure and origin of sul-

phur-containing moieties in kerogen, asphaltenes and coal as

revealed by flash pyrolysis. Geochimica et Cosmochimica

Acta 53, 873–889.

Sinninghe Damste, J.S., Rijpstra, W.I.C., Kock-van Dalen,

A.C., de Leeuw, J.W., Schenck, P.A., 1989b. Quenching of

labile functionalised lipids by inorganic sulphur species: evi-

dence for the formation of sedimentary organic sulphur

compounds at the early stage of diagenesis. Geochimica et

Cosmochimica Acta 53, 1343–1355.

Sinninghe Damste, J.S., Eglinton, T.I., Rijpstra, W.I.C., de

Leeuw, J.W., 1990. Characterization of sulfur-rich high

molecular weight substances by flash pyrolysis and Raney Ni

desulfurization. In: Orr, W.L., White, C.M. (Eds.), Geo-

chemistry of Sulfur in Fossil Fuels. ACS Symposium Series

429. American Chemical Society, Washington DC, pp. 486–

528.

Sinninghe Damste, J.S., de Leeuw J.W. 1990. Analysis, struc-

ture and geochemical significance of organically-bound sul-

phur in the geosphere: state of the art and future research. In:

Durand, B. and Behar, F. (Eds.), Advances in Organic Geo-

chemistry 1989, Pergamon, Oxford. Organic Geochemistry

16, 1077–1101.

Sinninghe Damste, J.S., Eglinton, T.I., de Leeuw, J.W., 1992.

Alkylpyrroles in a kerogen pyrolysate: evidence for abundant

tetrapyrrole pigments. Geochimica et Cosmochimica Acta

56, 1743–1751.

Sinninghe Damste, J.S., Kok, M.D., Koster, J., Schouten, S.,

1998. Sulfurized carbohydrates: an important sedimentary

sink for organic carbon? Earth and Planetary Science Letters

164, 7–13.

Suits, N.S., Arthur, M.A., 2000. Sulfur diagenesis and parti-

tioning in Holocene Peru shelf and upper slope sediments.

Chemical Geology 163, 219–234.

Thamdrup, B., Fossing, H., Jørgensen, B.B., 1994. Manganese,

iron and sulfur cycling in a coastal marine sediment, Aarhus

Bay, Denmark. Geochimica et Cosmochimica Acta 58, 5115–

5129.

Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and

Occurrence. 2nd Edition. Springer, Berlin. 699 p.

van de Meent, D., Brown, S.C., Philp, R.P., Simoneit, B.R.T.,

1980. Pyrolysis-high resolution gas chromatography and

pyrolysis gas chromatography-mass spectrometry of kerogen

precursors. Geochimica et Cosmochimica Acta 44, 999–1014.

van der Weijden, C.H., Reichart, G.J., Visser, H.J., 1999.

Enhanced preservation of organic matter in sediments

deposited within the oxygen minimum zone in the north-

eastern Arabian Sea. Deep-Sea Research 46, 807–830.

van Kaam-Peters, H.M.E., Schouten, S., Koster, J., Sinninghe

Damste, J.S., 1998. Controls on the molecular and carbon

isotopic composition of organic matter deposited in a Kim-

meridgian euxinic shelf sea: evidence for preservation of car-

bohydrates through sulfurisation. Geochimica et

Cosmochimica Acta 62, 3259–3283.

Veto, I., Hetenyi, M., Demeny, A., Hertelendi, E., 1994.

Hydrogen index as reflecting intensity of sulphidic diagenesis

in non-bioturbated, shaly sediments. Organic Geochemistry

22, 299–310.

von Rad, U., Schulz, H., Ali Khan, A., Ansari, M., Berner, U.,

Cepek, P., Cowie, G., Dietrich, P., Erlenkeuser, H., Geyh,

M., Jennerjahn, T., Luckge, A., Marchig, V., Riech, V.,

Rosch, H., Schafer, P., Schulte, S., Sirocko, F., Tahir, M.,

Weiss, M., 1995. Sampling the oxygen minimum zone off

Pakistan: glacial-interglacial variations of anoxia and pro-

ductivity (preliminary results, SONNE 90 cruise). Marine

Geology 125, 7–19.

Wakeham, S., Sinninghe Damste, J.S., Kohnen, M.E.L., de

Leeuw, J.W., 1995. Organic sulfur compounds formed dur-

ing early diagenesis in the Black Sea. Geochimica et Cosmo-

chimica Acta 59, 521–533.

Werne, J.P., Hollander, D.J., Behrens, A., Schaeffer, P.,

Albrecht, P., Sinninghe Damste, J.S., 2000. Timing of early

diagenetic sulfurization of organic matter: a precursor-pro-

duct relationship in Holocene sediments of the anoxic Car-

iaco Basin, Venezuela. Geochimica et Cosmochimica Acta

64, 1741–1751.

Whelan, J.K., 1977. Amino acids in a surface sediment core

from the Atlantic abyssal plain. Geochimica et Cosmochi-

mica Acta 41, 803–810.

Zaback, D.A., Pratt, L.M., 1992. Isotopic composition and

speciation of sulfur in the Miocene Monterey Formation:

reevaluation of sulfur reactions during early diagenesis in

marine environments. Geochimica et Cosmochimica Acta 56,

763–774.

488 A. Luckge et al. / Organic Geochemistry 33 (2002) 477–488