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: [email protected] (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
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