A molecular and stable carbon isotopic study of lipids in
late Quaternary sediments from the Arabian Sea
Stefan Schouten
a,*, Marcel J.L. Hoefs
b, Jaap S. Sinninghe DamsteÂ
a,b aNetherlands Institute for Sea Research, Department of Marine Biogeochemistry and Toxicology, PO Box 59,1790 AB Den Burg, Texel, The Netherlands
bUtrecht University, Institute of Earth Sciences, PO Box 80021, 3508 TA Utrecht, The Netherlands
Received 20 July 1999; accepted 9 March 2000 (returned to author for revision 2 December 1999)
Abstract
The distribution of apolar and polar lipids and their stable carbon isotopic compositions were determined for a number of sediment samples from dierent sites in the Arabian Sea. Lipids are mainly derived from planktonic Archaea and a range of algae, including Haptophytes, Eustigmatophytes and diatoms. The stable carbon isotopic compositions of compounds from diatoms, highly branched isoprenoids, fall into distinct groups suggesting other sources beside the diatom speciesRhizosolenia setigerafor these compounds. High amounts of sterol ethers, which may be derived from diatoms, were also detected. A recently identi®ed triterpenoid, malabaricatriene, was also present in high abundance in these samples. The terrestrial input of lipids consists ofn-alkanes and their carbon isotopic com-positions show that they are derived from aeolian dust input from the Arabian Peninsula. The isotopic comcom-positions of C37 alkenones from two cores during the last 15 ka is relatively constant and suggests that growth conditions for
Haptophyte algae (averaged over 500 years) did not change signi®cantly over this period.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Algae; Diatoms;13C of lipids; Alkenones; Malabaricatriene; Highly branched isoprenoids
1. Introduction
Sediments from the Arabian Sea have become a sub-ject of great interest since the area is sensitive to changes in climate, especially with respect to glacial cycles, which eect monsoon intensity and, in turn, upwelling and primary productivity. The Arabian Sea is uniquely situ-ated since it is very sensitive to these atmospheric forces, which leads to great seasonal variability in this part of the ocean (Smith et al., 1998). In addition, the combi-nation of high productivity and moderate ventilation of the thermocline leads to an intense oxygen minimum zone (OMZ) between 150 and 1250 m, in turn leading to
deposition of relatively organic matter-rich sediments enabling organic geochemical analyses.
A number of organic geochemical studies have reported on sediments from the Arabian Sea. Ten Haven et al. (1992) investigated a number of Pleistocene sediments of upwelling regions, including those taken at oshore Oman. A wide variety of extractable lipids were found, including alkanediols, long-chain alkenones and sterols in dierent relative concentrations. Eglinton et al. (1997) analysed the13C and14C-content of a number of
biomarkers in a sediment from the Arabian Sea at o-shore Oman. They found a widespread variability in their13C- and14C-contents indicating multiple sources,
even for compounds with the same carbon skeleton, and indicating dierent positions of organisms in the water column. Schubert et al. (1998) investigated the distribu-tion of several algal biomarkers in a core taken at o-shore Pakistan, which spanned the last 200 ka. Using
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 0 3 1 - 0
Organic Geochemistry 31 (2000) 509±521
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standard biomarkers and TOC data, they inferred from the data that the phytoplanktonic community was stable during deposition of the sediments.
Although a number of organic geochemical studies have been done in this area, studies on apolar hydro-carbons or stable carbon isotopic compositions of lipids to determine their sources in recent sediments have been limited. Thus, a detailed molecular and isotopic organic geochemical study was undertaken of several sediment cores from the Arabian Sea taken during the Nether-lands Indian Ocean Program 1992±1993 (Van Weering et al., 1997; Table 1, Fig. 1). Three sites (921, 451 and 453) were examined in detail for the distribution and stable carbon isotopic composition of apolar hydro-carbons. Since these components are present in very low amounts, large composite samples were taken from trip cores. Three cores from dierent sites (455, 464 and 921) were subsampled in a time range covering c. 15 ka and total lipid fractions were analysed. Finally, long-chain ketones were isolated from all samples and analysed for their stable carbon isotopic compositions.
2. Methods
2.1. Samples
The samples investigated in this study are listed in Table 1. Sites 451, 453, 455 and 464 were sampled in the Arabian Sea during the 1992±1993 expedition of the R.V. Tyro in the Indian Ocean (Netherlands Indian Ocean Program; Fig. 1). From site 451 and 453 only a trip core sample was studied. Sediment samples from
NIOP 455 and 464 were obtained by subsampling piston cores from this site at speci®c intervals (Table 1). The samples represent an average of approximately 500 years. Site 921 is oshore Oman and was sampled during a dif-ferent leg of the same Tyro-expedition. One large sample (depth of 5±8 cm) from the core and several smaller sub-samples were analysed from this site (Table 1).
2.2. Extraction and fractionation of soluble organic matter
Sediment samples were freeze-dried and ground in a rotary disc mill and subsequently Soxhlet extracted with dichloromethane:methanol (7.5:1, v/v) for 24 h. The extracts were concentrated with a rotary evaporator at 30C. Part of the extracts (total lipid fraction) were
methylated by diazomethane and silylated using BSTFA/ pyridine and analysed by gas chromatography (GC), and GC±mass spectrometry (GC±MS). A selected number of extracts from composite samples of sites 451, 453 and 921 were separated using a column (252 cm; column volume 35 ml) packed with alumina (activated for 2.5 h at 120C). The apolar and polar fractions were eluted with
hexane:dichloromethane (9:1, v/v; 150 ml) and metha-nol:dichloromethane (1:1, v/v, 150 ml), respectively. The apolar fractions were further separated by argentatious thin layer chromatography (TLC) using hexane as developer. The AgNO3-impregnated silica plates (2020
cm; thickness 0.25 mm) used for this purpose were pre-pared by dipping them in a solution of 1% AgNO3 in
methanol/ bidistillated water (4:1, v/v) for 45 s and subsequent activation at 120C for 1.5 h. Four fractions
(A1, Rf=0.9±1.00; A2, Rf=0.3±0.9; A3, Rf=0.1±0.3;
A4,Rf=0.00±0.1) were scraped o the TLC plate and
Table 1
Samples analysed in this study
Site Location Water depth (m) Sediment interval (cm) Approximate age (ka)a
451 23 4005300N 542 0±100 ±
66 0209700E
453 23 1503000N 1556 0±100 ±
65 4405000E
455 23 3304000N 995 34±36 7.7
65 5703000E 74±75 10
89±91 12
139±141 13
464 22 1504000N 1470 25±28 1.4
63 3501000E 48±50 5.1
64±67 7.6
84±88 10.8
115±118 14.4
921 16 0402300N 455 5±8 ±
52 3602800E 0±0.9 ±
0.9±1.8 ±
1.8±2.7 ±
2.7±3.6 ±
3.6±4.5 ±
ultrasonically extracted using ethyl acetate (3). Separation of the fractions from silica/Ag+ proceeded
using a small pipette (volume 2 ml) half ®lled with alu-mina (not activated), by elution with hexane:dichloro-methane (9:1, v/v).
Ketone fractions were isolated from polar fractions or total extracts by separation using a small column/pipette (volume 2 ml) half ®lled with alumina (not activated) and elution with hexane:dichloromethane (1:1, v/v).
All total lipid and TLC fractions were analysed by GC, GC±MS and isotope-ratio-monitoring gas chromato-graphy±mass spectrometry (irm-GC±MS). Since the frac-tions sometimes contained polyunsaturated compounds they were hydrogenated in ethyl acetate containing a few drops of acetic acid with H2/PtO2for 1 h at room
tem-perature. The hydrogenated fractions were analysed by GC, GC±MS and irm-GC±MS.
Some of the total lipid extracts or polar fractions were treated with HI/LiAlH4 to release ether bound
com-pounds as described previously (Hoefs et al., 1997). Brie¯y, the fractions were re¯uxed in a solution of 56 wt% HI in water for 1 h and the released alkyl iodides were treated with LiAlH4in 1,4-dioxane for 1h to convert them
to hydrocarbons.
2.3. Gas chromatography
GC was performed using a Hewlett Packard 5890 series II chromatograph equipped with an on-column
injector and ®tted with a 250.32 mm fused silica capillary column coated with CP-Sil 5 (®lm thickness 0.12mm). Helium was used as the carrier gas and the oven was programmed from 70 to 130C at 20C/min,
followed by an increase of 4C/min to 320C (10 min
hold time). Detection was performed using a ¯ame ionization (FID).
2.4. Gas chromatography±mass spectrometry
GC±MS analyses were performed using the same chromatograph and conditions as described for GC. The column was directly inserted into the electron impact ion source of a VG-Autospec Ultima mass spec-trometer, operated with a mass range ofm/z40±800, a cycle time of 1.8 s and ionization energy of 70 eV.
2.5. Isotope-ratio-monitoring gas chromatography±mass spectrometry
Isotope-ratio monitoring was performed using a DELTA-C irm-GC±MS system (Schouten et al., 1998a), equipped with an on-column injector and ®tted with a 250.32 mm fused silica capillary column coated with CP-Sil 5 (®lm thickness 0.12mm). Helium was used as carrier gas and the oven was programmed from 70 to 130C at 20C/min, followed by an increase of 4C/min
to 320C (20 min hold time). Isotopic values were
calcu-lated by integrating the mass 44, 45 and 46 ion currents
Fig. 1. Map showing the location of the samples investigated in this study.
of the peaks produced by combustion of the chromato-graphically separated compounds and those of CO2-spikes
produced by admitting CO2with a known13C-content at
regular intervals into the mass spectrometer. Two analyses were carried out for each sample and the results were averaged to obtain a mean value and to calculate the standard deviation.
3. Results and discussion
Five sediment cores from dierent sites of the Ara-bian Sea were sampled and analysed for total lipids. From three cores (NIOP 451, 453 and 921) samples were taken and analyzed for the distribution and stable car-bon isotopic compositions of apolar hydrocarcar-bons as well. The fractions of samples 451 and 453 were hydro-genated prior to irm-GC±MS analysis to simplify the mixture of compounds. Two other cores (NIOP 455 and 464) were subsampled at several intervals and analyzed for total lipids. In addition, long-chain ketones were isolated from the total lipid extracts or polar fractions and analyzed for their13C-contents.
It is interesting to note that no sulfur compounds were detected either in the apolar and polar fractions of the sediments. Indeed, ¯ash pyrolyses of some of the kerogens also yielded very low amounts of sulfur com-pounds (Hoefs et al., 1995a and unpublished results). This suggests that no sulfur was incorporated into the organic matter, consistent with the observation that no sulfate reduction was observed in the pore waters at these sites (Passier et al., 1997). Hence, no free hydrogen sul®de or polysul®des were available to react with the organic matter (Sinninghe Damste et al., 1989).
3.1. C37alkenones
All sediments contain relatively high amounts of C37
and C38 diunsaturated methyl and ethylketones (I, see
Appendix; e.g. Fig. 2). These lipids are biosynthesized by Prymnesiophyte algae (e.g. Volkman et al., 1980, 1995; Marlowe et al., 1984) and have been reported in numerous sediments, including in several parts of the Indian Ocean (e.g. Sonzogni et al., 1997). Both Gephyr-ocapsa oceanicaandEmiliania huxleyioccur abundantly in the northern part of the temporary Indian Ocean (e.g. Kleijne et al., 1988) and both could be the source for the alkenones. Based on the dominance of the C37ketones
over the C38ketones and the Uk'37correlation with
tem-perature, Sonzogni et al. (1997) suggested that E. hux-leyiis the main source of alkenones in sediments from the Indian Ocean. The distribution of the di- vs the tri-unsaturated C37ketone is used, via the Uk'37ratio
(Bras-sell, 1986), as an indication for paleotemperatures. As noted by Sonzogni et al. (1997), the diunsaturated ketone is dominating, yielding Uk'37always higher than
0.9. This is due to the high water temperatures (26± 28C) of the Indian Ocean.
Since the total lipid fraction consisted of a complex mixture of coeluting compounds, analysis of their stable carbon isotopic compositions was impossible. The analy-sis of the C37diunsaturated methyl ketone was, however,
feasible after isolation using column chromatography (see experimental). Bidigare et al. (1997) and Popp et al. (1998) have shown with cultures ofE. huxleyi that the stable carbon isotopic composition of this compound depends on the isotopic composition and concentration of [CO2]aq and the growth rate of its parent organism.
Since the isotopic composition of [CO2]aq remained
fairly constant in the time frame that our sediments were deposited (thed13C values of the inorganic
carbo-nate in the cores are relatively constant, i.e for 464 varying between +0.75 to +1.15%; Reichart et al.,
1997), pCO2remained constant within a factor of 1.2 in
the last 15 ka (IndermuÈhle et al., 1999) and the tem-peratures were relatively constant between 25 and 27C
based on the Uk'37 values, the13C-value of this
com-pound should provide information on the average growth rate of E. huxleyi during times of deposition. Since our samples are an average of c. 0.5 Ka short term patterns will be averaged out and only long term trends will be visible. Fig. 3 shows the13C-contents of the C
37:2
methylketone in the Arabian Sea sediments which shows that at both at sites 464 and 455 no signi®cant variations are observed and in fact remains rather constant at
ÿ23.50.3%. This suggests that growth rates did not signi®cantly vary at the sampled resolution during the last 15 Ka. The average value in itself is relatively high compared to data reported from the Equatorial Paci®c, Santa Monica Basin and Bermuda Atlantic but lower than that for the Peru Upwelling area (Bidigare et al., 1997), suggesting excellent growth conditions for hap-tophyte algae due to a high input of nutrients in this upwelling area.
3.2. Highly branched isoprenoids
All samples contained C25 and C30 highly branched
isoprenoid (HBI) polyenes. The A3 and A4-fractions contained C25(II) and C30HBI's (III) with 2±4 and 4±6
double bonds, respectively (Fig. 4). In addition, a C35
HBI (IV) with 7 double bonds was present as well, as reported previously (Hoefs et al., 1995b). These com-pounds are encountered in numerous sediments (Row-land and Robson, 1990) and the C25 and C30 carbon
skeletons are known to be biosynthesised by two diatom species Haslea ostrearia and Rhizosolenia setigera
and Madhupratap, 1996). One of the more common species occurring in the diatom community are dierent strains ofRhizosolenia spp. among which isR. setigera. Thus R. setigeracould be the main source for the C25
and C30 HBI's though it is likely that there are more
diatom sources as well (see below).
Most13C-contents of the diatom-derived biomarkers
are generally between ÿ22 toÿ24% (Fig. 4; Table 2). However, two speci®c C30HBI isomers are very depleted
in13C with values of
ÿ37%. A similar phenomenon was noted in an Arabian Sea sediment by Eglinton et al. (1997). They found HBI-isomers with values betweenÿ19.9 and
ÿ23.2%and one C30HBI isomer with ad13C value of ÿ37.1%. Kohnen et al. (1992) and Schouten et al. (1997)
also detected dierent HBI-isomers (albeit in sulphurized form) with widely dierent isotopic compositions in sedi-ments from the Vena del Gesso basin and the Monterey Formation, respectively. The isotopic dierence between
Fig. 2. GC-traces of total lipid fractions of samples (a) site 455, sample 139±141 cm and (b) site 464, sample 84±88 cm.
the C25and C30HBI isomers in the Arabian Sea
sedi-ments indicate thatR. setigeracannot be the only source for the HBI's and that alternative (diatom) sources have to be considered.
The depleted values of the C30 HBI-isomers indicate
maximum fractionation of the algae. Assuming that the HBI is 5% depleted compared to the diatom biomass
(Schouten et al., 1998a) and the isotopic composition of CO2[aq]is approximatelyÿ8%, this would mean an
esti-mated maximum Rubisco fractionation of approxi-mately ÿ24%. This extreme fractionation may be explained if they exclusively derive from diatom species which have extremely low nitrogen-limited growth rates and a relatively high cell area-to-volume ratio (Popp et al., 1998). The fact that the lipids are so depleted and require maximum Rubisco fractionation implicitly implies that they did not derive their inorganic carbon by actively using bicarbonate as suggested before from data of d13C values of diatom sterols in the Peru upwelling
region (Pancost et al., 1997). However, this may be a distinct possibility for the diatoms which biosynthesized the HBI isomers with carbon isotope values between
ÿ22 andÿ24%.
3.3. C30±C321,15-alkanediols
The total lipid fractions contain relatively high amounts of C30±C321,15- and 1,14-alkanediols (e.g.V;
Fig. 2) and a C32alka-15-keto-ol. C30±C32
1,15-alkane-diols occur in a range of marine and freshwater sediments (for a review see Versteegh et al., 1997) and are well known products from marine and freshwater Eustigmatophytes (Volkman et al., 1992, 1999). Ten Haven et al. (1992) already reported the presence of C301,15-alkanediols in
sediments at oshore Oman. The occurrences of these compounds in all sediments investigated in this study suggest that Eustigmatophytes occurred throughout the
last 15 ka in the Arabian Sea. Unfortunately, due to the complex mixture in which the alkanediols elute it was not possible to determine the stable carbon isotopic composition of these compounds directly. However, a number of extracts (921 and 451) were treated with HI to release ether-bound acyclic and cylic biphytanes (see below). During this treatment the C30 alkanediols,
together with any ester-bound counterparts, are trans-formed into a C30 n-alkane. Although other ester- or
ether-bound C30 moieties may contribute as well, the
relatively high abundance of the C30alkanediols before
HI-treatment and the C30 n-alkane after HI-treatment
suggest that the latter is predominantly derived of the former. Thed13C value of thisn-alkane varied between ÿ28.6%(site 455) andÿ30.9%(site 921). These values
are relatively depleted compared to other algal biomarkers in these sediments. Indeed the same phenomenon was observed for samples in the Black Sea (Eglinton et al., unpublished results). This ®ts well with the fact that the Eustigmatophyte algae are relatively small in size (e.g. 2±4
mm forNannochloropsis; van den Hoek et al., 1995) and therefore, may be less limited in their uptake of inorganic carbon than other algae (Popp et al., 1998).
3.4. Sterenes
The major compounds resolved in the gas chromato-graph in A2-fractions of samples of sites 451 and 921 were C27±C30 sterenes with the 2 sterenes (VI) dominating
(Fig. 4). A complex mixture of C27±C30 sterenes,
ster-adienes and steratrienes dominated the steroids present in the A3-fraction, with double bonds at positions 2, 5, 22 and/or 24(28). These components are the result of early diagenetic dehydration of the alcohol groups of C27±C30
sterols, compounds present in relatively low abundance in the total lipid fraction (Fig. 2). The sterenes are derived from numerous algal sources (Volkman, 1986).
The stable carbon isotopic composition of the cho-lestenes range from ÿ23.5% to ÿ24.8% and average aroundÿ24.70.9%(Table 2), identical to the value of ÿ25.0%reported by Eglinton et al. (1997) for cholest-2-ene in an Arabian Sea sediment sample. The13
C-con-tents of higher homologs were dicult to obtain due to the complex mixture of sterenes, even after hydrogena-tion. The C27sterenes are likely diagenetic products of
C27sterols, especially cholesterol. These compounds are
either derived from de novo biosynthesis of algae (Volkman, 1986) or are derived from higher carbon number sterols through modi®cation by zooplankton herbivory (e.g. Goad, 1981). Since the latter process has no signi®cant eect on the stable carbon isotopic com-position of the precursor sterol (Grice et al., 1998), the
13C-content of the C
27sterenes are thought to re¯ect an
average value of13C-content of sterols.
Schouten et al. (1998a) showed that the isotopic composition of the C27 sterol of R. setigera, grown in
Fig. 3. Stable carbon isotopic composition of the C37:2
batch culture, is identical to that of the C25:5 HBI it
biosynthesizes. If this holds for all HBI-synthesizing diatoms, then the isotopic composition of the C27
ster-enes should be similar to that of the HBI's in case the diatoms are the main source of sterols in the Arabian
Sea. Indeed, the weighted-average isotopic composition of the HBI's in the dierent sediments is around ÿ25%,
similar to the d13C value of the C
27sterenes. However,
haptophyte algae, which also produce a range of sterols (Volkman, 1986) may also bear an imprint on the isotopic
Fig. 4. GC-traces of TLC fractions of sediment sample at site 921. (a) A2-fraction, (b) A3-fraction and (c) A4-fraction. i.s.=internal standard. Numbers in italic indicate stable carbon isotopic compositions of compounds.
patterns of the C27 sterenes. Bidigare et al. (1997) and
Schouten et al. (1998a) have shown that haptophyte algal sterols are approximately 3% depleted compared
to the long-chain alkenones inE. huxleyiandIsochrysis galbana, respectively. Since long-chain alkenones in these sediments are on average ÿ22.10.2%, sterols biosynthesized by alkenone-producing algae should have isotopic compositions of c.ÿ25%, similar to the value observed for the sedimentary sterenes. Hence, in the sediments studied here it is not possible to make a distinction between diatoms or haptophyte algae as the main source of the C27 sterenes, since their isotopic
values are similar to those expected for sterols of both species.
3.5. C27±C29sterol ethers
Some A4-fractions contain relatively high amounts of tentatively identi®ed C27±C29 5-sterol ethers with a
decyl moiety ether bound to the C-3 position (VII) (Fig. 4). Their mass spectra have similar features as those of silylated sterols with respect to the steroid fragmenta-tion pattern (Fig. 5), with the fragment of m/z 141 belonging to the fragmentation of the decyl side-chain (Boon and de Leeuw, 1979). It may be suggested that these compounds are derived from modi®cations by zooplankton of dietary algal sterols. However, the dis-tribution of the sterol ethers is dierent from that of the sterols, i.e. the sterols have a relatively high abundance of the C29 5-sterol whilst the sterol ethers are
domi-nated by the C27 5-sterol ether. Furthermore, these
compounds are rarely encountered in sediments and the alkyl-chain is restricted to the C10 homolog. This
sug-gests that sterol ethers represent a direct biological input rather than a diagenetic product. Sterol ethers have been reported before in Walvis Bay diatomaceous ooze by Boon and de Leeuw (1979) and in Miocene sediments from the Monterey Formation (Schouten et al., 2000). Both settings are known to have had high input of lipids of diatoms, as have the sediments of this study judging from the relatively high amounts of HBI's and the known abundance of diatoms in the Arabian Sea. This tentatively suggests that diatoms may be the source for these sterol ethers.
The isotopic composition of the decyl cholesterol ether (ÿ25.8%) is composed of two biosynthetically distinct
parts: a C10n-alkyl moiety and a C27sterol. Presuming
that then-alkyl part is ultimately derived from a linear fatty acid and the sterol is built from isoprenoid units, it can be expected that they will be isotopically dierent. Culture experiments of Schouten et al. (1998a) showed that the C27 sterol is enriched by approximately 1%
compared to then-C16fatty acid in two diatom strains.
If this is also the case for the diatoms producing the sterol ethers then the stable carbon isotopic composition of the sterol ethers should be only approximately 0.3%
depleted compared to sterols produced by the same algae. Indeed, the isotopic compositions of the decyl cholesterol ether is slightly depleted compared to that of the sterenes (Table 2), although this dierence is within analytical uncertainty.
3.6. Hopenes
Small amounts of C30±C31 hopenes (e.g. diploptene; VIII) and small amounts of C27 and C30 neohopenes
(Sinninghe Damste et al., unpublished results) were detected in A2-fractions of samples 451, 453 and 921 (e.g. Fig. 4). Similar to the sterenes, the hopenes are likely early diagenetic products of hopanols such as diplopterol, although neohopenes may represent a direct
Table 2
Stable carbon isotopic compositions of selected biomarkers in sediments from the Arabian Sea
Compound Site 921 Site 451a Site 453a
n-Alkanes Decyl C275-sterol ether
ÿ25.80.1 Decyl C275,22-sterol ether
ÿ24.80.1
Hopanoids
Hop-(22,29)-ene ÿ22.70.7
17b,21b-hopane ÿ23.10.2 ÿ24.00.1
Triterpenoids
13a-malabaricatriene ÿ21.70.4 13b-malabaricatriene ÿ21.90.2
13-malabaricane ÿ22.30.3
Miscellaneous
C30alkanediolsb ÿ30.90.3
C37alkenones ÿ21.90.1 ÿ22.20.3
C40tricyclic biphytaneb ÿ19.70.1
a Hydrogenated compounds.
biological input (Sinninghe Damste et al., unpublished results).
Hopenes may be derived from numerous bacteria, although it should be noted that major components of the picoplankton in the Indian Ocean are cyanobacteria of the genus Synechococcus (Burkill et al., 1993) and
Prochlorococcus(e.g. Liu et al., 1998). These species may possibly be major sources of the sedimentary hopanoids especially since several strains of Synechococcus are known to produce abundant hopanoid derivatives (Llo-piz et al., 1996; Summons et al., 1999). However, the isotopic compositions of the hopanoids suggest other-wise. The isotopic compositions of the normal hopenes vary areÿ23 toÿ24%(Table 2), again very similar to the data of Eglinton et al. (1997), and are thus slightly isotopically heavier than the sterenes. Popp et al. (1998) showed that the stable carbon isotopic composition of
Synechococcus is relatively independent of the CO2
concentration and growth rate. This was attributed to the relatively small cell size, which give rise to a high cell area to carbon content ratio, which in turn enhances passive diusion of dissolved CO2. Recently, however,
Keller and Morel (1999) suggested that this was due to the high amount of active bicarbonate uptake. What-ever the cause, based on the empirical observations by Popp et al. (1998) a similar degree of fractionation of
13C can be expected for the picoplanktonic
Pro-chlorococcus and thus a consistent depletion of approximately 17±18%of the biomass compared to the 13C-content of [CO
2]aq. Combined with the observation
of Sakata et al. (1997) that hopanoids of the cyano-bacteriumSynechocystisare 6±8%depleted compared to
biomass it may be tentatively expected that hopanoids derived from Synechococcus or Prochlorococcus have
13C-values between
ÿ31 toÿ34%. Since the hopanoids in the sediments studied here are considerably more
enriched it is tempting to suggest that they are not derived from picoplanktonic cyanobacteria but other bacteria.
3.7. Acyclic and cylic biphytanes
In a previous communication we showed that, after ether-bond cleavage of the polar fractions of these sedi-ments, high amounts of C40 acyclic and cyclic
biphy-tanes (IXa) are released and that they are by far the most dominant lipids in these sediments (Hoefs et al., 1997). In addition, the sediments contain relatively high amounts of C40 acyclic and cyclic biphytanediols (IXb;
Fig. 2) (Schouten et al., 1998b). The diols have not yet been identi®ed in organisms, but based on their struc-tural similarities with ether-bound C40acyclic and cyclic
biphytanes and their occurrence in sediments, Schouten et al. (1998b) suggested that they are biosynthesized by planktonic Archaea, similar to their ether-bound coun-terparts (Hoefs et al., 1997; DeLong et al., 1998). Thus a large part of the lipids in the sediments studied are derived from these organisms. Their isotopic composi-tions are enriched compared to algal sterols (Hoefs et al., 1997; Table 2). However, since it is unknown what the carbon acquisition mechanism of these Archaea is, it is dicult to interpret these values.
3.8. Malabaricatrienes
Two tricyclic triterpenoids were present in equal, rela-tively high abundance in samples 451 and 921 and in lower abundance in 453. They were identi®ed as 17(E)-13a (H)-malabarica-14(27),17,21-triene and 17(E)-13b (H)-mala-barica-14(27),17,21-triene (X) based on their identical mass spectra and retention times of the compounds reported by Behrens et al. (1999) and Werne et al.
Fig. 5. Mass spectrum of decyl cholesterol ether present in the A4-fraction of sediment sample at site 921.
(2000). Furthermore, after hydrogenation four isomers were generated with mass spectra and retention times in accordance with the expected hydrogenated derivatives of the malabaricatrienes, i.e. 17R/S-13a/b (H)-malabar-icane (XI). The 13aisomer has been previously identi®ed in a lake sediment (Lake Cadagno; Behrens et al., 1999), whilst the 13bisomer was detected in a marine sediment (Cariaco Basin; Werne et al., 2000). The isotopic com-positions of the malabaricatriene isomers are the same at site 921 (ÿ21.7%andÿ21.9%; Table 2), suggesting one
source, and are relatively enriched compared to algal sterenes. Their origin remains unknown though their occurrence in these and other sediments (Behrens et al., 1999; Werne et al., 2000) suggest that they may be derived from organisms living in low oxygen environments.
3.9. n-Alkanes
The A1-fractions of the samples of sites 451, 453 and 921 contained mainly saturated hydrocarbons of which C18±
C33 n-alkanes are the most dominant compounds. The
C25±C33n-alkanes have an odd-over-even carbon-number
predominance suggesting that they are derived from a terrestrial source (Eglinton and Hamilton, 1963).
At sites 921 and 453 the stable carbon isotopic com-positions of the terrestrially derived C29 and C31 n
-alkanes could be determined. Theird13C-values are very
similar compared to each other and in each sample with an average value of ÿ28.10.3%. These values are
comparable to those reported by Eglinton et al. (1997) for C27and C29alkanes. The values indicate that
vege-tation using the C3-metabolism pathway may not be the only source for these compounds since they have typi-callyn-alkane values betweenÿ30 andÿ37%(Collister et al., 1994; Lockheart et al., 1997). It seems likely that C4- and/or CAM plants were also a source these n -alkanes. The dominant terrestrial input into these sedi-ments is likely to be transported via aeolian dust moved by hot, north-westerly winds originating from dry, arid areas in the Arabian peninsula and south-west Asia (Reichart et al., 1997). This ®ts well with our hypothesis that the terrestrial lipid input into the sediments are partly of C4/CAM origin since these type of plants especially thrive in arid climates (Mauseth, 1998). Goni
et al. (1997) also reported a signi®cant input of C4 plants in surface sediments in the Gulf of Mexico.
4. Conclusions
Lipids in Late Quaternary sediments from the Ara-bian Sea are mainly derived from planktonic Archaea and a range of algae, including Prymnesiophytes, Eustigmatophytes and diatoms. HBI's derived from diatoms have widely dispersed isotopic compositions suggesting additional sources beside the diatom species
Rhizosolenia setigera. The isotopic composition of the unsaturated C37 alkenones from two cores during the
last 15 ka is remarkably constant and suggests that 500 year-averaged growth conditions for Prymnesiophyte algae did not change signi®cantly over this period. Long-chain alkyldiols are indicative of input of Eustig-matophyte algae and their relatively depleted stable carbon isotopic compositions suggest that these algae are less limited in their inorganic carbon uptake than other algae. The terrestrial input of lipids is restricted to
n-alkanes and their carbon isotopic compositions show that they are partly derived from plants using C4/CAM-pathways.
Acknowledgements
We thank the Netherlands Indian Ocean Program and Professor Dr. C.H. van der Weijden (University of Utrecht) and Dr. W. Helder (NIOZ) for making avail-able the samples used in this study. Drs. G. Reichart and H.-J. Visser are thanked for subsampling. Ms. M. Baas, W.I.C. Rijpstra and M. Dekker are thanked for analytical assistance. Drs. P. Schaeer and X. de las Heras are thanked for their constructive reviews. This work was supported by a PIONIER-grant to J.S.S.D. by the Netherlands Organization for Scienti®c Research (NWO). Shell Internationale Petroleum Maatschappij BV is thanked for ®nancial support for the irm-GC±MS facility. This is NIOZ contribution No. 3398.
Appendix
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