Organic matter preservation on the Pakistan continental
margin as revealed by biomarker geochemistry
Sonja Schulte *, Kai Mangelsdorf, JuÈrgen RullkoÈtter
Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, PO Box 2503, D-26111 Oldenburg, Germany
Received 18 January 1999; accepted 17 July 2000 (returned to author for revision 15 June 2000)
Abstract
In order to assess whether the oxygen-minimum zone (OMZ) in the Arabian sea has an eect on the preservation and composition of organic matter in surface sediments we investigated samples from three dierent transects on the Pakistan continental margin across the OMZ. In addition to determining the total amount of organic carbon (TOC), we analyzed the extractable lipids by gas chromatography, combined gas chromatography/mass spectrometry, and compound-speci®c stable carbon isotope measurements. The extractable lipids are dominated by marine organic matter as indicated by the abundance of lipids typical of marine biota and by the bulk and molecular isotopic composition. Sediments from within the OMZ are enriched in organic carbon and in several extractable lipids (i.e. phytol,n-alcohols, total sterols,n-C35alkane) relative to stations above and below this zone. Other lipid concentrations, such as those of
totaln-fatty acids and totaln-alkanes fail to show any relation to the OMZ. Only a weak correlation of TOC with mineral surface area was found in sediments deposited within the OMZ. In contrast, sediments from outside the OMZ do not show any relationship between TOC and surface area. Among the extractable lipids, only then-alkane con-centration is highly correlated with surface area in sediments from the Hab and Makran transects. In sediments from outside the OMZ, the phytol and sterol concentrations are also weakly correlated with mineral surface area. The depositional environment of the Indus Fan oers the best conditions for an enhanced preservation of organic matter. The OMZ, together with the undisturbed sedimentation at moderate rates, seems to be mainly responsible for the high TOC values in this area. Overall, the type of organic matter and its lability toward oxic degradation, the mineral sur-face area, the mineral composition, and possibly the secondary productivity by (sedimentary) bacteria also appear to have an in¯uence on organic matter accumulation and composition.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Organic carbon; Lipids; Preservation; Surface sediments; Oxygen-minimum zone; Biomarkers; Arabian sea
1. Introduction
The possible causes of organic matter enrichment in marine sediments have already been a central issue of numerous organic geochemical investigations. Many of these studies revealed concentration maxima of organic
carbon at intermediate water depths on continental slopes (e.g. Premuzic et al., 1982) where an oxygen-minimum zone (OMZ) in the water column impinges on the sea¯oor. This contributed to the widely accepted view that preservation of organic matter in marine sediments is chie¯y controlled by low bottom-water oxygen levels (e.g. Schlanger and Jenkyns, 1976; Thiede and van Andel, 1977; Demaison and Moore, 1980; Par-opkari et al., 1992, 1993; van der Weijden et al., 1999).
In contrast to this, other investigators collected evi-dence for organic matter enrichment by other factors such as upwelling-induced high primary productivity,
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* Corresponding author at present address: Geosciences, University of Bremen, Postfach 330 440, D-28334 Bremen, Germany. Tel.: +49-421-218-7108; fax: +49-421-218-3116.
winnowing, sediment texture, grain size, and mineral surface area (Calvert, 1987; Pedersen and Calvert, 1990; Calvert and Pedersen, 1992; Calvert et al., 1995; Berga-maschi et al., 1997; Ganeshram et al., 1999). The role of adsorption of organic matter on mineral surfaces was emphasized in a number of studies (Keil et al., 1994 a, b; Mayer, 1994). Ransom et al. (1998) pointed out that mineralogy and surface area of California margin sedi-ments play a more important role in preservation than the oxygen level of bottom waters and the organic mat-ter origin (marine or mat-terrestrial), whereas Hulthe et al. (1998) suggested that oxygen does have an in¯uence on organic matter degradation in continental margin sedi-ments. Hartnett et al. (1998) pointed out that ``the heart of the preservation controversy lies in the fact that no single factor seems to control preservation under all conditions''. The present organic geochemical investi-gation of three series of surface sediments from dierent transects across the OMZ on the Pakistan continental margin in the northeastern Arabian sea, collected during the SONNE-90 cruise in 1993, is a contribution to the ongoing discussion on the parameters aecting organic matter preservation. Besides determining bulk para-meters like organic carbon content, stable carbon iso-tope composition of whole organic matter and mineral surface area, we have placed particular emphasis on possible eects of the OMZ on the composition and concentration of selected extractable lipids in the surface sediments.
2. Study area
The northern Arabian sea is characterized by strong seasonal variability of monsoonal upwelling (Slater and Kroopnick, 1984) and high primary productivity (Kabanova, 1968; Quasim, 1977, 1982) which, together with the supply of oxygen-depleted intermediate-depth water masses from the south (Swallow, 1984; Olson et al., 1993), favor the development of an exceptionally broad and stable mid-water OMZ. This OMZ (200± 1100 m; down to 0.5 mg O2/l) for the purpose of this
study was de®ned by laminated sur®cial sediments and the absence of benthic organisms (Fig. 1; von Rad et al., 1995). Recently, Schulz et al. (1996) demonstrated that the OMZ was persistently present during the last 6 kyears.
3. Samples and analytical methods
3.1. Samples
During the SONNE-90 cruise in 1993, surface sedi-ments in, and adjacent to, the OMZ on the Pakistan continental margin were collected, using box cores, on
three dierent transects across the slope and at a distal reference site (Fig. 2). A preliminary description of sediment characteristics including the enrichment of organic carbon within and just below the OMZ was published by von Rad et al. (1995).
3.1.1. The Makran transect (Area A)
The area o the Makran coast south-west of Omara (Area A; Fig. 2) is an active continental margin slope with sedimentation rates between 2.5 and 3 mm/year over the last 90 kyear and a tendency of decreasing values with increasing water depth (Schulz and von Rad, 1997). The typical sediment in Area A is a hemi-pelagic carbonate- and quartz-rich silty clay with abun-dant chlorite. The annual surface productivity in this area (64.5E/24.5N) is estimated to be around 480 g C
m2/year, with highest values in July during the SW
monsoon, from a digitized productivity atlas (developed by Denis Dollfus, CEREGE Aix-en-Provence, France) based on satellite chlorophyll observations and a model of photosynthesis (Antoine et al., 1996). The mineral surface area of Area A sediments varies between 10.8 and 17 m2/g sediment and slightly increases with water
depth (Keil and Cowie, 1999; Fig. 3).
3.1.2. The Hab transect (Area B)
The Hab transect south-west of Karachi (Area B; Fig. 2) has an extremely complicated slope morphology (Schulz et al., 1996). On the basis of varve counting and
210Pb measurements, a sedimentation rate of 1.6 mm/
year was determined (core 39 KG) and considered repre-sentative for the whole transect (von Rad et al., 1999; U. von Rad and H. Schulz, personal communication). The mass accumulation rates (MAR) for sediments from this transect range from 333 to 1000 g m2yearÿ1with higher
values occurring within the OMZ (Sutho et al., 2000). As in the Makran transect, the sediments are also char-acterized by hemipelagic carbonate-rich, silty clays with chlorite as an abundant clay mineral. The estimated primary productivity (about 435 g C m2/year at 65.5E/
24.5N) is only slightly lower than in Area A. The
sur-face area of the sediments increase with water depth from 5.4 to 17.6 m2/g (Keil and Cowie, 1999; Fig. 3).
3.1.3. Murray Ridge (Area C)
The Murray Ridge (Area C; Fig. 2) was selected as a pelagic reference site unaected by downslope sediment transport, which is still located below the extended OMZ (Schulz et al., 1996). The estimated annual pri-mary productivity in this area (64.5E/23.5N) is 375 g
C m2/year. The reference sediment has a mineral surface
area of 16.9 m2/g (Keil and Cowie, 1999).
3.1.4. The Indus transect (Area D)
less aected by sediment redeposition than Areas A and B (Schulz et al., 1996). Based on 210Pb measurements
and varve counting, the sedimentation rate ranges from 0.6 to 1.1 mm/year (U. von Rad and H. Schulz, unpub-lished results; Crusius et al. 1996; Cowie et al., 1999). The hemipelagic carbonate-rich silty clays contain det-rital carbonate fragments and quartz. The estimated primary productivity in this area (66.5E/23N) is about
375 gCm2/year, i.e. closer to that on the Murray Ridge
than in Areas A and B. The mineral surface area of sediments from the Indus transect (13.8±20.7 m2/g) are
higher than in the other areas and do not show a sys-tematic trend with water depth (Keil and Cowie, 1999; Fig. 3).
3.2. Methods
We analyzed 32 surface samples (upper 2 cm) from above, within, and below the OMZ for bulk parameters. Based on these results, 22 samples where selected for
detailed studies of the molecular composition of the extractable organic matter.
After freeze-drying and grinding, the sediments were analyzed in duplicate or triplicate (when relative dier-ences were larger than 5% after duplicate measure-ments) for total carbon content by combustion in a LECO-SC-444 instrument. Carbonate contents were determined in duplicate by acidi®cation using a UIC-Coulometrics CM 5012 device. A detailed description of the system as well as a discussion of the accuracy and reproducibility of the method were provided by Engle-man et al. (1985) and Jackson and Roof (1992). Total organic carbon (TOC) contents were calculated as the dierence between total and inorganic carbon data.
Solvent extractions were performed for 48h in a Soxhlet apparatus with dichloromethane containing 1% methanol. After addition of internal standards (squa-lane, erucic acid, 5a-androstan-17-one) and removal of the hexane-insoluble fraction by the addition of a large excess ofn-hexane (`asphaltene' precipitation), the
hexane-soluble portion of the extract was separated by medium-pressure liquid chromatography (MPLC; Radke et al., 1980) into fractions of aliphatic hydro-carbons, aromatic hydrohydro-carbons, and polar hetero-compounds (NSO fraction). Subsequently, carboxylic acids were separated from the NSO fraction on a col-umn ®lled with KOH-impregnated silica gel (63±200
mm; a solution of 0.5 g KOH in 10 mliso-propanol was added to 4 g silica gel). The nonacidic compounds were eluted with 120 ml dichloromethane. Following this, 120 ml of a solution of formic acid (2% in dichloromethane) transformed the potassium salts of the acids back to the original acids and eluted them from the column. For molecular analysis, the acids were methylated with diazo-methane, and the nonacidic polar compounds were silylated with N-methyl-N -trimethylsilyltri¯uoroacet-amide (MSTFA).
Gas chromatography was performed on a Hewlett Packard 5890 Series II instrument equipped with a Gerstel KAS 3 temperature-programmed cold injection system and a fused silica capillary column (30 m length, inner diameter=0.25 mm, coated with DB 5 (J&W), ®lm thickness=0.25 mm). Helium was used as the car-rier gas, and the oven temperature was programmed from 60C (1 min) to 305C (50 min) at 3C/min. The
injector temperature was programmed from 60C (5 s
hold time) to 300C (60 s hold time) at 8C/s. GC/MS
studies were performed under the temperature condi-tions given above with the same type of gas chromato-graph interfaced directly to a Finnigan SSQ 710 B mass spectrometer operated at 70 eV.
Molecular isotope analyses were performed using the same type of gas chromatograph (fused silica capillary column; 30 m length, inner diameter=0.25 mm, coated with Ultra 2 (HP), ®lm thickness=0.17mm; temperature conditions as given above) attached to a Finnigan MAT 252 isotope ratio mass spectrometer via a combustion interface. This instrument was calibrated with a CO2
standard at the beginning and the end of each analysis and with compounds of known isotopic composition which were co-injected with each sample and served as internal isotopic standards. Isotopic ratios are expressed asd13C
values in per mil relative to the V-PDB standard.
4. Results
4.1. Composition of lipids
4.1.1. Gross molecular composition
The aliphatic hydrocarbon fractions of the surface samples are dominated by C17 and C43n-alkanes in a
carbon number range between with components con-taining more than 40 carbon atoms being present only in small amounts (Fig. 4; Schulte, 1997). Then-alkane distribution maximizes at n-C31in eighteen of the
ana-lyzed samples, and at n-C35 in the other four samples
which were all deposited within the OMZ. The long-chain homologs (C25±C35) show an odd-over-even
car-bon number predominance with a carcar-bon preference index (CPI) between 1.3 and 4 (Schulte, 1997). Besides the n-fatty acids, branched-chain (isoandanteiso) iso-mers of the C15 and C17 acids were detected.
Mono-unsaturated acids with 16 and 18 carbon atoms as well as a diunsaturated C18acid were also present. The fatty
acid distributions have a pronounced maximum in the short-chain compound range (at C16:1, C16:0, or C18:0)
and a second, lower maximum at C24:0or C26:0(Schulte,
1997). The even-carbon-numbered fatty acids are domi-nant in all samples.
Sterols were detected between C27 and C30 among
which the C28 and C30 pseudohomologs dominate
(Schulte, 1997). The most prominent sterol is dinosterol: 24-ethylcholest-5-en-3b-ol, its saturated counterpart, and cholesterol also occur in high abundance (Schulte, 1997; Fig. 5).
In then-alcohol series ranging from 14 to 28,n -hexa-decanol is the most abundant homolog. Furthermore,
phytol, alkandiols, keto-ols, hopanoic acids and alco-hols, and as well as C37 and C38 alkenones are
sig-ni®cant constituents of the NSO fraction (Fig. 5; Schulte, 1997).
4.1.2. Isotopic composition of single compounds
Two samples, one from above and one from within the OMZ were investigated for the isotopic composition of n-alkanes. Most importantly, the isotopic composi-tion of then-C35alkane signi®cantly diers from those
of the other long-chainn-alkanes (Schulte, 1997; Schulte et al., 1999). In the sample from above the OMZ, then -C35alkane has ad13C value ofÿ26.2%compared with a
mean d13C value of
ÿ28.1% (n=8, S.D.=1.21%) for the other long-chainn-alkanes (C27±C39). In the sample
from within the OMZ, the n-C35 alkane (ÿ24.2%) is
isotopically even 4%heavier than, on average, the other
long-chain n-alkanes (ÿ28.06%; n=9, S.D.=1.95%) (Schulte et al., 1999). Chromatographic coelution of an unknown component with then-C35alkane can be
exclu-ded on the basis of GC/MS and GC-irm-MS analyses (Schulte, 1997).
For one sample from within the OMZ (12 KG, Table 1), the isotopic compositions of several fatty acids were determined. Thed13C values vary between
ÿ24.6%and
ÿ28.4%with a mean value ofÿ26.3%(n=9, standard deviation=1.17%). Then-C24andn-C26acids have an
isotopical composition that is 2% lighter than that of
the othern-fatty acids (mean=ÿ25.8%,n=7, standard deviation=0.73%; Schulte, 1997).
4.2. Organic matter abundance in relation to the OMZ
4.2.1. Total organic carbon (TOC)
In a plot versus water depth, the TOC data exhibit variations between 0.4 and 4 wt.% (Fig. 6). As was
noted in previous studies (e.g. Paropkari et al., 1992; Calvert et al., 1995), there is a broad mid-slope organic carbon maximum, i.e. organic carbon values in surface sediments are higher on the continental slope within the present OMZ than above and well below (Table 1 and Fig. 6). On the Pakistan continental margin, this max-imum is most pronounced for the Indus transect and less so in Areas A and B. These regional dierences are in general agreement with the measurements of Par-opkari et al. (1992). The shallow-water rise of the organic carbon content at the top of the OMZ is fairly sharp, whereas the carbon values decrease gradually with increasing water depth below the OMZ.
As can be seen from Fig. 6, the stable carbon isotope data of the total organic matter vary slightly between
ÿ20.8%andÿ19.2%(Cowie et al., 1999), i.e. the values
are in the typical marine organic matter range of
ÿ202% (Hoefs, 1980). The d13C values of the
sedi-ments deposited within the OMZ are isotopically 0.5%
lighter (mean:ÿ20.5%;n=12; S.D.=0.3%) than those Fig. 3. Mineral surface area of surface sediments versus water
of the sediments deposited outside the OMZ (mean:-20.0%;n=12; S.D.=0.4%). No dierence between the three transects was observed. The two samples from below 2500 m water depth show d13C values that are
slightly lighter than those of the other samples from below the OMZ (Fig. 6).
4.2.2. Compound group concentrations
We analyzed a suite of biomarkers characteristic of marine and terrigenous organic matter to investigate whether or not the low bottom-water oxygen con-centrations within the OMZ have an in¯uence on the preservation of these compounds. Because these com-pounds belong to dierent chemical compound classes, they should have dierent sensitivities toward degrada-tion during early diagenesis.
The C16±C42n-alkane concentrations vary between 31
and 184 mg/g TOC, with the exception of sample 164 KG, which has ann-alkane concentration of 385 mg/g TOC (Fig. 7). The extremely high n-alkane concentra-tion in this latter sediment may be related to the sam-pling location south of the Indus Canyon. Other than the rest of the samples from Area D, the sediment represented by 164 KG may have accumulated organic
matter supplied by the Indus River and de¯ected south by the prevalent surface ocean current (Rao and Rao, 1995).n-Alkane concentrations in sediments from Areas A and B increase with water depth (r2=0.93 and
r2=0.94, respectively), whereas then-alkane
concentra-tions in Area D sediments are less signi®cantly corre-lated with the water depth (Fig. 7;r2=0.61; sample 164
KG south of the Indus Canyon not included).
The concentrations of n-fatty acids in the surface sediments from the four sampling areas vary between 460 and 2940 mg/g TOC and exhibit nearly the same average values within and outside the OMZ (991mg/g TOC within OMZ, n=12, and 884mg/g TOC outside OMZ, n=9; Fig. 7). However, there are dierences among the transects studied. In the sediments from the Indus transect, the fatty acid concentrations are almost invariable over the entire range of water depth (164 KG excluded). By contrast, the sediments from the Makran transect show a strong decrease in fatty acid concentra-tion between 200 and 1000 m water depth, and those from the Hab transect have slightly elevated concentra-tions within the depth range of the OMZ (Fig. 7).
n-Alcohols (C12±C28) were detected in concentrations
between 90 mg/g TOC for the reference sample (MR)
and 2420mg/g TOC for sample 164 KG (Fig. 8). With-out taking into account sample 164 KG, the concentra-tions of then-alcohols in sediments deposited within the OMZ on average are 1.6 times higher (430 mg/g TOC,
n=10; Fig. 8) than outside the OMZ (273mg/g TOC,
n=10; Fig. 8). We note that three samples from within the OMZ have comparatively low concentrations of n -alcohols.
The concentrations of total sterols (C27±C30) vary
between 83 and 2070 mg/g (Fig. 8). The mean con-centration in samples deposited outside the OMZ is 415
mg/g TOC (n=10), whereas the samples from within the OMZ have a mean concentration (1020 mg/g TOC,
n=11) almost 2.5 times higher with highest values being attributed to samples from Areas A and B. Sedi-ments from Area D, with the exception of sample 164 KG, fail to show elevated sterol concentrations within the OMZ.
4.2.3. Speci®c biomarkers
The major source of phytol is chlorophyllaand released from it by hydrolysis already in the water column
(Baker and Louda, 1983). Due to the highly reactive allylic hydrogen atoms, phytol is rapidly degraded by oxidation (Rontani et al., 1990). The phytol concentra-tions in the surface sediments from the Pakistan margin vary between 1.5 and 126 mg/g TOC with the highest concentrations occurring within the OMZ (up to six times higher than above and below; Fig. 9), especially in sediments from the Hab transect (B) The phytol con-centration in several sediments within the OMZ on the Makran (A) and Indus transects (D) are also sig-ni®cantly higher than the average value of samples from outside the OMZ.
Dinosterol, a speci®c marker for dino¯agellates (e.g. Boon et al., 1979; Nichols et al., 1984; Robinson et al., 1984) which were abundant organisms in plankton tows from the overlying surface waters in the study areas (von Rad et al., 1999), was detected in a concentration range from 14 to 320 mg/g TOC. Concentrations of dinosterol are two times higher in sediments from within the OMZ (mean: 165mg/g TOC, n=10) than in sedi-ments deposited outside the OMZ (mean: 85mg/g TOC,
n=10; Fig. 9). The higher abundance of dinosterol
within the OMZ is most pronounced in sediments from Areas A and B. By contrast, Area D sediments do not show a remarkable variation of dinosterol concentration as a function of water depth.
Because the stable carbon isotopic compositions of the C24 and C26 n-fatty acid dier from those of the
other fatty acids, they may have a distinct origin. Sediments from within the OMZ have a 1.6 times higher concentration of C24 plus C26 fatty acids
(mean: 130 mg/g TOC, n=11; Fig. 9) than sediments which were deposited outside the OMZ (mean: 80mg/g TOC,n=11; Fig 9). The maxima of C24and C26n-fatty
acid concentrations cover the water depth range of the OMZ, with no obvious dierences between the trans-ects.
Branched-chain fatty acids are more common in bac-teria than in other organisms (Volkman et al., 1980;
Parkes and Taylor, 1983), and so they are useful indi-cators of bacterial contributions to sediments. The sum of C15and C17branched fatty acids versus water depth
is shown in Fig. 10. Sediments within the OMZ are richer in branched-chain fatty acids by a factor of up to nearly 3 (maximum about 260 mg/g TOC) relative to sediments which were deposited above or below the OMZ (mean: 110mg/g TOC,n=11).
The C35n-alkane is enriched within the water depth
range of the OMZ in all transects (up to more than 15
mg/g TOC) by a factor of about 2 on average relative to the sediments deposited outside the OMZ (mean: 6mg/g TOC) (Fig. 10). The molecular carbon isotopic compo-sition of this component is signi®cantly more positive than those of the other long-chainn-alkanes pointing to a distinct origin of the C35 homolog (Schulte, 1997;
Schulte et al., 1999). In contrast to the C35n-alkane, the
C31 n-alkane which is typically derived from higher
plants (e.g. Eglinton and Hamilton, 1967; Rieley et al., 1991; Meyers and Ishiwatari, 1993) is not enriched within the OMZ. The concentration, normalized to TOC, increases with increasing water depth in Areas A and B similar to those of the C27and C29n-alkanes. In
the Indus transect, such an increase was not observed (Fig. 10).
4.3. Organic matter abundance in relation to mineral surface area
It was suggested that organic matter settling through the water column and in surface sediments may be pro-tected against oxidation and microbial degradation by sorption on mineral surfaces (Keil et al., 1994a; Mayer, 1994; Hedges and Keil, 1995, Bergamashi et al., 1997). For this reason, we compared published data on mineral surface area for the surface sediments on the Pakistan continental margin (Keil and Cowie, 1999) with organic geochemical data generated in this study.
4.3.1. Total organic carbon (TOC)
The TOC contents of the investigated samples do not show an overall signi®cant correlation with mineral surface area (Fig. 11). However, within the OMZ, the organic carbon content with a correlation coecient of
r2=0.49 may indicate an organic load on minerals of
about 2.2 mg C/m2.
4.3.2. Extractable lipids
A strong correlation betweenn-alkane concentration and mineral surface area was observed for sediments from Areas A and B (r2=0.83 and 0.84, respectively;
Fig. 11). The concentrations of the other extractable lipids analyzed do not show any relationship with the surface area, except for phytol and sterol concentrations of sediments deposited outside the OMZ (Fig. 11) exhi-biting a weak to good correlation.
Table 1
Sample number, water depth, organic carbon content and C24/ C16n-fatty acid ratio in sediments collected on the continental slope o Pakistan. (n.d.=not determined)a
Sample Area Water depth
5. Discussion
5.1. Information on the origin of organic matter based on straight-chain lipid distributions and the carbon isotopic composition of individual components
The occurrence of long-chainn-alkanes with an odd-over-even carbon number predominance in marine sediments is commonly interpreted as a contribution from higher land plants (e.g. Eglinton and Hamilton, 1967; Rieley et al., 1991; Meyers and Ishiwatari, 1993). Very long chain n-alkanes (>C35) with no carbon
number preference were detected in a surface sediment from the western Arabian Sea (Oman upwelling; Eglin-ton et al., 1997), and they do occur in a number of other marine sediments (Volkman, private communication 1998). However, the origin of these very long-chain n -alkanes is still unknown.
The stable carbon isotope ratio of higher-plant n -alkanes is lower by about 5±10% than the bulk plant
tissue (Collister et al., 1994; Lichtfouse et al., 1994). A similar dierence (7 Ð 9%) betweenn-alkanes and total biomass was reported for a cyanobacterium (Sakata et al., 1997). Nothing has been reported so far about the isotopic depletion ofn-alkanes in marine organism, but assuming the same dierence in marine organism as in other plants (about 7.5%),n-alkanes deriving from C3
and C4land plants and from marine organism should
have d13C values of
ÿ35.5%, ÿ19.5%, and ÿ27.5%,
respectively. The carbon isotope ratios measured for the
n-alkanes from the Pakistan margin sediments (C27±C39
mean:ÿ28.1%;n=9, S.T.D.=2.0%) would be closest to those of marine organism, but these values could also result from a mixed origin of then-alkanes from C3and
C4plants or a mixture from all three sources. Thus, the
origin of then-alkanes cannot be unambiguously deter-mined on the basis of their isotopic composition alone.
The CPI of higher plantn-alkanes ranges from 3 to 40.3 (McCarey et al., 1991 and references therein; Collister et al., 1994), whereas CPI values of bacterial and phytoplankton n-alkanes generally are closer to unity (McCarey et al., 1991). Accordingly, the CPI values between 1.3 and 4 of the Pakistan margin sam-ples point to a mainly marine origin ofn-alkanes in the sediments, which is in agreement with the molecular carbon isotope data. Zegouagh et al. (1998) interpreted
n-alkane CPI values of 1.4±2.7 in arctic surface sedi-ments as a marine signal, and a marine origin was also suggested for n-alkanes showing a CPI of 3±4 in Per-uvian surface sediments (McCarey et al., 1991). The absence of an unresolved complex mixture in the sediment extracts (Fig. 4) and the relatively low concentrations of
n-alkanes make a petrogenic origin of then-alkanes in the studied samples unlikely, except for possibly a minor
portion as indicated by traces of saturated steranes and hopanes in the extracts (Schulte, 1997).
Short-chain fatty acids (C12±C18) are commonly
pro-duced by marine micro-organisms (Schnitzer and Kahn, 1978; Sargent and Whittle, 1981), whereas long-chain fatty acids mainly derive from higher land plants (Mat-suda and Koyama, 1977). For the Pakistan margin sediments, this interpretation is supported by the iso-topic composition of the fatty acids with heavier d13C
values for the short-chain members and lighter values for the C24and C26acids (Schulte, 1997). Because lipids
in general are depleted in 13C relative to the total
bio-mass by around 4%(Monson and Hayes, 1982; Hayes,
1993) and since C3 land plants commonly have ad13C
value of ÿ28%(Bird et al., 1995), one would expect a d13C value of around
ÿ32%for a C3plant-derived lipid.
Therefore, the C24and C26acids may not be exclusively
contributed by C3land plants, but also include an
iso-topically heavier C4land plants and/or marine
compo-nent (ÿ12% and ÿ20%, respectively; Smith and
Epstein, 1971; Hoefs, 1980).
Despite the vicinity to the Indus river, the proportion of terrestrial organic matter is conspicuously small in the sediments studied. This terrigenous contribution to the sediments is most clearly re¯ected by long-chainn -alcohols (>C20) believed to derive from higher land
plants (Eglinton and Hamilton, 1967; Kolattukudy,
1976). The biomarker evidence of a predominance of the marine organic matter, however, is consistent with the d13C values of bulk organic matter (Cowie et al.,
1999) and microscopical observations indicating that in the study area OM is mainly (75±95%) amorphous with only a small fraction of terrigenous organoclasts (von Rad et al., 1995; Littke et al., 1997; LuÈckge et al., 1999).
5.2. Factors in¯uencing the accumulation of organic matter
5.2.1. Bottom water oxygen concentration
The TOC pro®le across the OMZ on the Pakistan margin shows a mid-slope maximum roughly coincident with the extension of the OMZ (Fig. 2). This result is in good agreement with those of previous studies (Par-opkari et al., 1992; Calvert et al., 1995; Cowie et al., 1999). As an exception to this general picture, the organic carbon contents of two samples directly beneath the OMZ on the Indus transect were found to be high. von Rad et al. (1995), on the basis of their Parasound data, suggested that surface sediment, originally depos-ited within the OMZ, had been displaced down-slope by slumping. This idea, however, is not compatible with the excess210Pb pro®les which, rather, indicate continuous
in-situ sedimentation below the OMZ (Cowie et al.,
1999). Thus, although the organic carbon maximum in the surface sediments appears to be related to the pre-sent position of the OMZ, not all of the organic carbon accumulation pattern can be readily explained by water-column oxygen concentrations. The more complex organic carbon patterns in the Hab and Makran trans-ects, compared to the Indus transect, may be related to the tectonic activity in those active margin areas, which is also re¯ected in common turbidities found in deeper core sections from the same areas.
Among the individual lipids and lipid groups in the sediment extracts, the C35n-alkane, the sum of the C24
and C26n-fatty acids, phytol, then-alcohols, and
dinos-terol have elevated concentrations relative to total organic carbon within the present depth range of the OMZ. This suggests that the oxygen content of the overlying water column may be an important factor controlling the accumulation of these components. For dinocysts, considered labile under normal marine con-ditions, it was recently demonstrated that anoxic condi-tions controlled their preservation in Mediterranean Sea sediments (Combrourieu-Nebout et al., 1998). Because dinosterol is a common constituent of dino¯agellates (Robinson et al., 1984), the relative extent of preserva-tion of this biomarker may well be controlled by the water-column redox conditions.
Recent studies also demonstrated slower degradation of some sterols under anaerobic conditions (Teece et al., 1994; Sun and Wakeham, 1998). This slower degrada-tion rate may be invoked for the concentradegrada-tion max-imum of the sum of sterols and of dinosterol within the OMZ of Areas A and B. On the other hand, elevated sterol concentrations do not extend over the whole depth range of the OMZ, and in Area D there is no enrichment of sterols at all within the OMZ. This also holds true for the other extractable lipids studied in some detail (phytol, branched-chain C15and C17 fatty
acids, n-alcohols) which are enriched within the OMZ, but not over the entire depth range, and which also show dierences between the three transects. Thus, although oxygen depletion in the water column may have a favorable in¯uence on biogenic lipids in marine sedi-ments, this parameter alone does not appear to be su-cient.
The concentrations of the sum ofn-alkanes, the sum ofn-fatty acids, and of the C31n-alkane fail to show any
relationship with oxygen content in the overlying water column on the Pakistan margin. It was demonstrated in incubation experiments that then-alkane concentration in sediments is not aected by dierent redox conditions (Teece et al., 1994). The chemical inertness of saturated alkanes may explain these observations, but it is less
easy to conceive that this should also hold for the fatty acids.
5.2.2. Mineral surface area
A positive correlation between TOC content and mineral surface area has previously been observed in several studies of marine sediments and was invoked to emphasize that organic matter can be protected against mineralization, and thus be preserved, by sorption on mineral surfaces (Keil et al., 1994a,b; Mayer, 1994; Bergamaschi et al., 1997). In the Pakistan margin sam-ples of this study, there was no general signi®cant cor-relation between TOC and mineral surface area. Only for sediments deposited within the OMZ, a weak posi-tive correlation was observed (r2=0.49; Fig. 11), which
suggests that part of the organic matter within the OMZ is associated with mineral surfaces at a relatively high level of loading (2.2 mg C/m2). Similarly high loadings
were determined in size fractions of a Peru Margin sediment (Bergamaschi et al., 1997), but with a much higher correlation coecient between TOC and surface area. It should be noted, however, that the highest TOC contents on the Pakistan margin occur in Area D in sediments that also have the highest surface areas among the samples studied (Figs. 3 and 6). Our results are in con¯ict with those of Ransom et al. (1998) and
van der Weijden et al. (1999) who reported a positive correlation between organic matter content and mineral surface area for continental margin sediments from outside the OMZ and no correlation between TOC and surface area for sediments deposited within the OMZ.
Among the investigated biomarker concentrations, only those of then-alkanes in Area A and B samples show a signi®cant positive correlation with mineral sur-face area (Fig. 11). This is in disagreement with the results of Jeng and Chen (1995), who did not detect any grain-size eect (which is indistinguishable from the eects caused by surface area; Jeng and Chen, 1995 and references therein) for the concentration ofn-alkanes in sediments o Taiwan. By contrast, the lack of correla-tion betweenn-alkane concentrations and mineral sur-face area in Area D sediments (Fig. 11) is consistent with the results of Jeng and Chen (1995).
Phytol and the sum of sterols also are correlated with mineral surface area, but in contrast ton-alkanes this is true only for sediments from outside the OMZ. This is in accordance with the results of Jeng and Cheng (1995) who reported a pronounced grain-size eect for phytol and sterols in sediments o northeastern Taiwan. The investigation of the Pakistan margin sediments also corroborates the observation of Jeng and Cheng (1995) that n-alcohols do not correlate with mineral surface
area. Apparently, the simple presence of a hydroxyl group is not sucient to explain the association of lipids with mineral surfaces.
Mineral composition may also play a role on the extent of lipid adsorption. As was pointed out by Ransom et al. (1998), organic matter appears to be preferentially sequestered in smectite-rich sediments compared to those whose clay fractions are dominated by chlorite. The relatively high abundance of chlorite in the investigated Pakistan margin sediments (particularly Areas A and B) may explain the weak correlation between organic mat-ter content and mineral surface area.
5.2.3. Type (origin) of organic matter
Relative to marine organic matter, continentally derived organic matter commonly includes a higher proportion of biologically resistant organic compounds and a fraction already strongly degraded by soil microbes (e.g. de Leeuw and Largeau, 1993; Berga-maschi et al., 1997). Furthermore, there is evidence that terrigenous organic matter generally suers biological degradation at a lower rate than marine organic matter (Huc, 1988; Littke and Sachsenhofer 1994) which leads to a better preservation of this material.
The isotopic composition of the C24 and C26 fatty
acids suggests that they are at least partly of terrestrial origin. This may account for the pronounced enrich-ment of these compounds in sedienrich-ments deposited within
the OMZ in all investigated areas (Fig. 8). Canuel and Martens (1996) demonstrated that the terrigenous C24
acid is degraded more slowly in coastal sediments than marine-derived short-chain fatty acids (e.g. C14 and
C16:1). The C24/C16 fatty acid ratio was employed to
determine the relative contribution of fatty acids from terrestrial and marine sources (Leenheer et al., 1984; LeBlanc et al., 1989). On the Pakistan margin, this ratio in general is higher in sediments deposited within the OMZ (Table 1) possibly indicating a higher proportion of terrigenous fatty acids in sediments within the OMZ, although the non-terrigenous contribution (isotopic evi-dence) to the same acid may have varied as well. As can be seen by then-alkane concentrations, which do not relate to the OMZ, a terrestrial origin per se does not lead to a higher accumulation rate within the OMZ.
A better preservation of terrigenous organic matter within the OMZ, however, is suggested by the lighter (``more terrestrial'') d13C values of the total organic
matter in the corresponding sediments. A redox poten-tial related dierence in organic matter preservation, with heavier values above and below the OMZ, re¯ect-ing preferential remineralization of 12C compounds,
would be an alternative explanation (Cowie et al., 1999). As a third hypothesis, the supply of isotopically light organic matter by chemoautotrophic bacteria in oxygen-depleted water was suggested to explain the lighterd13C
values within the OMZ (Cowie et al., 1999). A speci®c in
situ supply of organic matter by organism living within the OMZ might also explain the enrichment of the C35
n-alkane in sediments from within the OMZ, re¯ecting a secondary productivity signal (Schulte et al., 1999). The isotopic composition of this compound is signi®cantly dierent from that of the other n-alkanes pointing to another, yet unknown source. The higher concentra-tions of the bacterial branched fatty acids within the OMZ point to signi®cant bacterial activity and support the suggestion of secondary productivity within the OMZ. This interpretation would be consistent with the results of Lee (1992) who suggested that the production of new organic matter by bacteria in sediments may be a way of preserving carbon in anoxic sediments (regard-less of O2).
5.2.4. Reactivity toward oxic degradation
As mentioned above, phytol, then-alcohols, and the sterols all have higher organic-carbon-normalized con-centrations within the OMZ (Figs. 8 and 9). Chemically, all these compounds are alcohols, but the reactivity toward oxidation of their hydroxyl groups decreases in the order allylic>secondary>primary (March, 1992). Apparently, the relative degree of preservation under the oxygen-depleted conditions of the OMZ increases
parallel to the reactivity of the hydroxyl group under oxic conditions (phytol>sterols>n-alcohols).
The relationship between preservation and reactivity can also be demonstrated by dierences in the preserva-tion of sterols with dierent degrees of unsaturapreserva-tion. In the Pakistan margin sediments, the concentration of the sum of sterols with two double bonds is three times higher within the OMZ (mean concentration: 170mg/g TOC; n=11; Schulte, 1997) than outside the OMZ (mean concentration: 55mg/g TOC;n=9, Schulte, 1997). By comparison, the saturated and monounsaturated sterols are only enriched by factors of 2.5 and 2.3 within the OMZ (Schulte, 1997). This relationship between preservation and the number of double bonds under-lines the in¯uence of the sensitivity to oxidation of a lipid molecule on its preservation under suboxic condi-tions. This is in accordance with the conclusions of Sun and Wakeham (1998) who recently demonstrated that cholesterol is more rapidly degraded under oxic than under anoxic conditions and that under anoxic condi-tions it is degraded faster than C26±C29 dienols. All
these interpretations on speci®c compounds are con-sistent with the statement of Hulthe et al. (1998) who suggested that the eect of oxygen on the degradation of organic matter depends on its lability.
On the other hand, the lack of enrichment of sterols in sediments within the OMZ in the Indus transect (Area D) cannot be explained by reactivity alone. The mineral composition may be responsible for the higher enrichment of sterols (and alcohols in general) in Areas A and B, where the sediments are dominated by chlorite with strong adsorptive properties. In contrast, sediments of Area D are dominated by detrital carbonates.
5.2.5. Sedimentation rate and primary productivity
Sedimentation rate and primary productivity are recognized as important factors controlling the quantity of OM buried in marine sediments (cf. RullkoÈtter, 1999, for an overview). The three transects on the Pakistan margin dier in primary productivity and sedimentation rate. Highest TOC values were observed within the OMZ of the Indus transect where primary productivity and sedimentation rate are lowest, whereas lower TOC values occur in areas with higher primary productivity and sedimentation rate (Areas A and B; Fig. 6). This disagrees with the empirical relationship between TOC content of sediments and primary productivity and sedimentation rate observed by MuÈller and Suess (1979). On the basis of mean values of primary pro-ductivity and sedimentation rate (sedimentation rate may, however, vary across the margin), the accumula-tion of organic matter in the studied sediments appar-ently cannot be explained simply by dierences in primary productivity and sedimentation rate.
6. Conclusions
The composition of extractable lipids and the bulk stable carbon isotope data of organic matter in sedi-ments from the Pakistan continental margin indicate that despite the vicinity of the Indus River the organic matter is mainly derived from marine sources. This organic geochemical evidence is in good agreement with the results of organic petrographic analyses (Littke et al. 1997; LuÈckge et al., 1999).
Supported by a biological marker approach we could demonstrate that accumulation of organic matter in marine sediments underlying an extended OMZ is a complex process involving several dierent mechanisms. Apparently, the OMZ plays a signi®cant role in the accumulation and preservation of organic matter as indicated by a pronounced TOC maximum and the enrichment of single compounds within the OMZ in all investigated areas.
However, the low oxygen concentrations of the bot-tom waters underlying the OMZ are not an exclusive factor explaining all organic matter properties and their variations on the Pakistan continental margin. The type (origin) of certain organic matter constituents and their (related) lability toward oxic degradation, the mineral
surface area, the mineral composition, and possibly the secondary productivity by (sedimentary) bacteria also appear to have an in¯uence on the accumulation and composition of organic matter in this area and in some cases to explain discrepancies with published observa-tions for other continental margin settings.
Apparently, the depositional environment of the Indus Fan (Area D) oers the best conditions for enhanced preservation of organic matter. The OMZ together with the undisturbed sedimentation (slope morphology) at moderate sedimentation rates seems to be responsible for the high TOC values in this area.
Acknowledgements
We would like to thank Dr. Michael E. BoÈttcher for carrying out GC±IR±MS analyses. We are grateful to Dr. Barbara M. Scholz-BoÈttcher for her support during GC/MS analyses. We also would like to thank Greg L. Cowie for constructive comments and suggestions to improve the manuscript. This study was ®nancially supported by the Bundesministerium fuÈr Bildung, Wis-senschaft, Forschung und Technologie (BMBF), grant no. 03G0090C. The authors carry the full responsibility for the content of this publication. Data are available on http.//www.pangaea.de/home/sschulte.
Associate EditorÐS.G. Wakeham
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