Molecular and isotopic characterization of organic matter in
recent and sub-recent sediments from the Dead Sea
$
Thomas B.P. Oldenburg
a,1, JuÈrgen RullkoÈtter
a,*, Michael E. BoÈttcher
b,
Arie Nissenbaum
caInstitute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, PO Box 2503,
D-26111 Oldenburg, Germany
bMax Planck Institute of Marine Microbiology, Celsiusstraûe 1, D-28350 Bremen, Germany cDepartment of Environmental Sciences, The Weizmann Institute of Science, 76100 Rehovot, Israel
Received 9 November 1998; accepted 12 January 2000 (returned to author for revision 2 July 1999)
Abstract
Near-surface sediments from two sections in the Nahal Zeelim delta of the Dead Sea (Israel) with low total organic carbon contents of 0.4±0.8% were studied by molecular and isotopic organic geochemical techniques to determine the origin of the extractable lipid components. The molecular investigation showed most of the material in this extremely hypersaline environment to be of terrestrial origin. This was indicated by a dominance of 24-ethylcholest-5-en-3b-ol and 24-ethylcholesta-5,22-dien-3b-ol in the sterol distribution as well as an abundance of angiosperm triterpenoids like
b-amyrin,a-amyrin, lupeol and their oxidized derivatives. The n-alkane distribution patterns are very similar in all samples studied and typical of an origin from epicuticular waxes of higher land plants. This is corroborated byd13C
values of then-alkanes between ÿ28.1 andÿ33.6%. The even-over-odd carbon number predominance of the
long-chain fatty acids (C20±C30) and their range ofd13C values (ÿ27.3 toÿ31.3%) are also in accordance with an origin
from C3terrestrial plants. The pronounced13C depletion of the short-chain fatty acids (C14±C18) further substantiates
the dominance of terrestrial plant material in the sediments and highlights the limited importance of autochthonous biomass in the Dead Sea water. Then-alcohol distribution patterns show a strong even-over-odd carbon number pre-ference and, compared to then-alkanes, are enriched in13C by 2±5%, which suggests a small contribution of aquatic
organic matter particularly to the short-chain homologues. Indications for a supply from autochthonous organisms to the sedimentary organic matter were derived from the high amounts of phytol withd13C values between
ÿ22.8 and ÿ19.7%, the isotopic composition of cholesterol (ÿ23.9 toÿ21.9%) and low concentrations of 24-methyl-5a -cholest-7-en-3b-ol and 24-ethyl-5a-cholest-7-en-3b-ol. They are attributed to the only primary producer in the Dead Sea descri-bed so far, i.e. the unicellular green algaDunaliella parva. In addition, the archaean cell walls of halophilic bacterial communities likeHalorubrum sodomenseare represented by signi®cant amounts of bis-O-phytanylglycerol (ÿ22.3 to ÿ23.0%).#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Carbon isotopes; Dead Sea;Dunaliella parva; Fatty acids; Halophilic bacteria; Sterols
1. Introduction
The Dead Sea basin is located in the deepest part of the Jordan Rift Valley. The Dead Sea itself is a terminal lake fed by the Jordan river, surface run-o during the winter wet season, and a few perennial fresh and saline springs and streams (Fig. 1). The Dead Sea, a 320 m
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 1 5 - 2
www.elsevier.nl/locate/orggeochem
$ Presented as a poster at the International Meeting on
Organic Geochemistry, Maastricht, The Netherlands, Septem-ber 1997.
* Corresponding author.
E-mail address:[email protected] (J. RullkoÈtter).
1 Present address: Institute of Petroleum and Organic
deep water body (which makes it the deepest hypersaline lake in the world), is characterized by being the lowest exposed surface on the face of the earth, with a present lake level of aroundÿ408 m below mean sea level, and
by its extreme hypersalinity. The total dissolved salt content of the lake is about 340 g/l. Its chemistry is also unusual by having Mg2+as the dominant cation (44 g/l)
followed by Na+(40 g/l), Ca2+(16 g/l) and K+(6 g/l).
The dominant anion is, by far, Clÿ(220 g/l), distantly
followed by Brÿ(5.4 g/l). Sulfate and carbonate are very
minor (Gavrieli, 1997).
The lake level of the Dead Sea has been continuously rising and falling since its formation. Since 1971, the increased utilization of fresh water in the drainage area and in particular of upstream damming the Jordan river and its tributaries caused the lake level to drop con-siderably, at an annual rate of about 0.5 m, until it
reached the present value of ÿ408 m below mean sea
level. Along the present-day coast, the receding water has exposed some of the areas which previously were part of the lake bottom at water depths reaching 10 m or so only 25±30 years ago. The samples were taken in the fan complex of Nahal Zeelim (Fig. 1). Winter run-o and ¯ash ¯oods have cut several deep and narrow `canyons' of about 7 m depth in the sediments which allowed sampling of the laminated sample sequence (C). The age of the section is not known, but it is estimated that the bottom of the section is a few hundred years old. In addition, two near-surface samples, of unknown age but possibly younger than 100 years, composed of sticky, deep black sediments were collected by coring to a depth of 80 cm, about 1 km north of the `canyon'. Those samples are `black muds' (BM) which are com-mercially used for pharmaceutical and cosmetic purposes.
Since the pioneering work of Elazari-Volcani (1936, 1940, 1943a,b), a number of halophilic and halotolerant micro-organisms have been isolated from the lake. The dominant organism and only primary producer found in the oxic upper water layer of the Dead Sea was the halotolerant unicellular green alga Dunaliella parva
(Chlorophyceae) while a few types of red halophilic archaea were detected as the main heterotrophs (Kaplan and Friedmann, 1970; Nissenbaum, 1975; Oren, 1983, 1988, 1992, 1994). Simulation experiments in the laboratory and in outdoor tanks (Oren and Shilo, 1985) showed that growth ofDunaliellarequired a brine con-centration below 1.21 g/ml (compared to about 1.235 g/ ml of undiluted Dead Sea water) and that phosphate was the limiting nutrient. Rain ¯oods are expected to supply large amounts of phosphate to the Dead Sea, although quantitative estimates are not available (Oren, 1995).
The unusual membrane permeability properties of
Dunaliellaspecies created an interest in their lipid com-position. The algae were found to have unusually high contents of total lipids, particularly polyunsaturated C16
and C18 acids, and carotenoids (Ackman et al., 1968;
Chuecas and Riley, 1969; Ben-Amotz and Avron, 1980; Tornabene et al., 1980; Ben-Amotz et al., 1982; Evans et al., 1982; Fried et al., 1982; Evans and Kates, 1984). Isopranylglycerol diethers or tetraethers from their membranes are typical biomarkers of halophilic archaea (Kates, 1978; Langworthy et al., 1982) and may be expected to occur in Dead Sea sediments. Due to the low volatility of these compounds they previously escaped direct analysis by gas chromatography (GC) or
gas chromatography±mass spectrometry (GC±MS).
Archaeen hydrocarbons were the only biomarkers of this group of organisms in shallow sediments (Brassell et al., 1981) while diagenetic or catagenetic transformation of isopranylglycerol ethers was already known to be common in sediments with more mature organic matter and in crude oils (Moldowan and Seifert, 1979; Albai-ges, 1980; Albaiges et al., 1985). Anderson et al. (1977), Chappe and Albrecht (1982), Ward et al. (1985), Pauly and van Vleet (1986), and Vella and Holzer (1990) developed methods to generate molecules amenable to GC and GC±MS analysis by cleavage of the iso-pranylglycerol ether bonds and subsequent derivatiza-tion. Later, Teixidor et al. (1993) detected entire isopranylglycerol diethers in sediments and cultures of halobacteria by using high-temperature capillary col-umns in GC and GC±MS analysis.
The ®rst organic geochemical investigations of anoxic Dead Sea surface sediments from a water depth of 300 m were carried out by Kaplan and Baedecker (1970). They suggested that the compounds extracted from the sediments, i.e. the diphytanyl glycerol ether analogue of phosphatidyl glycerophosphate as well as phytanic acid, originated from the Halobacteria population of the
Dead Sea. Nissenbaum et al. (1972) performed a detailed analysis of four surface sediment samples, two from the shallower parts of the lake which were depos-ited under oxidizing conditions and two from the deeper parts of the lake where the overlying waters were (at that time) reducing. The total organic carbon (TOC) content of the sediments was between 0.23 and 0.40%. The compound classes investigated included lipids (fatty acids and hydrocarbons), humic and fulvic substances, amino acids and chlorins. The d13C signature of the
bulk organic matter was highly uniform (ÿ24.0 to ÿ24.3%) and similar both to soil organic matter from
the land surrounding the Dead Sea (ÿ24.3%) and
par-ticulate organic matter from the lake water (ÿ24.8%).
The authors related the presence of phytane, pristane (in high concentrations relative to other lacustrine sedi-ments), phytol and dihydrophytol to an origin from the phosphatidyl glycerophosphate esters typical of halo-philic bacteria. High concentrations of the C18
unsatu-ratedn-fatty acid, presence of unaltered chlorophylla, but absence of chlorophyll b, were also noted by the authors. In addition, the concentrations of humic and fulvic substances were lower in the oxic sediments than in those from the reducing environment. Later, Ander-son et al. (1977) analyzed ®ve sediment samples depos-ited under reducing conditions for their isoprenoid content. They detected only the 3R,7R,11R isomer of phytanic acid, suggesting that the phytanyl units in Dead Sea sediments were most likely derived from extreme halophiles rather than from phytol of the chlorophyllasidechain. No other organic geochemical studies on Dead Sea sediments have been published since.
This paper reports the results of an extensive mole-cular study of extractable lipids in six selected sediments comprising compound-speci®c carbon isotope analysis (GC-irm-MS) in addition to GC and GC±MS analysis of extractable lipids. X-ray ¯uorescence spectroscopy was used to determine the elemental composition of the mineral matter. The objectives included the origin of the organic matter, a search for speci®c biomarkers of autochthonous organisms, a comparison of lipid and mineral matter composition of the `canyon' sediments and the `black mud' samples never analysed before and the unknown nature of the dark color of the `black muds'. This, altogether, was intended to contribute to a better geochemical characterization of an extreme, `end-member' environment and, thus, to provide a basis for comparison with other, mainly saline or hypersaline environments.
2. Analytical methods
combustion in a LECO CS-444 instrument. The TOC contents of the surface sediments were determined by a coulometric method using a StroÈhlein Model 7012 apparatus.
Pyrite and elemental sulfur were released separately from freeze-dried samples using a cold (2 h) or hot (1 h) acidic Cr(II) chloride solution, respectively (Allen and Parkes, 1995). The produced H2S was trapped
quanti-tatively as Ag2S in a 1 M AgNO3solution and
quanti-®ed gravimetrically (precision 2s: 10%). It is assumed that all iron monosul®de originally present in the sedi-ment was oxidized to elesedi-mental sulfur upon
freeze-dry-ing. The measured elemental sulfur, therefore,
represents the original S and a secondary sulfur
frac-tion. The amount of total reduced inorganic sulfur (TIRS; sum of iron monosul®des, pyrite and elemental sulfur) equals the sum of the measured pyrite and ele-mental sulfur fractions. Total sulfur (TS; sum of pyrite, iron monosul®des, elemental and organic sulfur, pore water sulfate) was measured on freeze-dried samples using a LECO CS-444 analyzer (precision 2s: 2.3%).
For alkaline hydrolysis, the sediments were heated in a solution of 5% potassium hydroxide in 4:1 metha-nol:water (Farrington and Quinn, 1971) for 4 h. After decanting, the sediment residue was ultrasonically extracted ®ve times with dichloromethane. The com-bined total extracts were evaporated to a small volume, and after addition of internal standards (squalane, anthracene-d10, erucic acid [n-C
22:1], 5a
-androstan-17-one, 5a-androstan-3b-ol) the n-hexane-insoluble por-tions (`asphaltenes') were precipitated by addition of a large excess ofn-hexane.
The n-hexane-soluble fractions were separated into fractions of aliphatic hydrocarbons, aromatic hydro-carbons, and polar compounds (NSO fraction) by
medium-pressure liquid chromatography (MPLC;
Radke et al., 1980). Prior to separation, elemental sulfur was removed from the n-hexane-soluble fractions by treatment with copper ®lings.
The carboxylic acids were separated from the NSO
fractions using a liquid chromatography column
(12010 mm) ®lled with KOH-impregnated silica gel
100 (63±200mm; a solution of 500 mg KOH in 10 mliso -propanol was added to 4 g silica gel). After elution of the nonacidic compounds with 120 ml dichloromethane, the potassium salts of the acids were converted to the free acids with 50 ml of a solution of formic acid (2% in dichloromethane) and eluted with 80 ml dichloro-methane. Subsequently, the acid-free NSO fractions were separated into sterols and ketones plusn-alcohols by ¯ash chromatography (Still et al., 1978) under a moderate overpressure of nitrogen. A 20010 mm
col-umn ®lled with 5 g silica gel 60 (40±63mm, deactivated with 5% by weight of water) was washed with 30 ml of dichloromethane. Then the ketone fraction was eluted with 60 ml dichloromethane at a rate of two drops per
second, the steroid alcohols were recovered from the column with 50 ml of a mixture of dichloromethane and methanol (10% by volume). For molecular analysis, the acid fractions were methylated with diazomethane, ketone and alcohol fractions were silylated withN -tri-methylsilyltri¯uoroacetamide (MSTFA).
Gas chromatography was performed on a Hewlett± Packard 5890 series II instrument equipped with a Ger-stel temperature-programmed cold injection system and a fused silica capillary column (length=30 m, inner
diameter=0.25 mm, ®lm thickness=0.25 mm, DB-5
[J&W]). Helium was used as the carrier gas, and the temperature was programmed from 60C (1 min) to
305C (50 min) at a rate of 3C/min.
GC±MS and GC-irm-MS studies were conducted with the same type of gas chromatograph using the capillary column and temperature conditions described before. For GC±MS analysis, the GC was connected to a Finnigan SSQ 710 B mass spectrometer operated at 70 eV. The compound-speci®c carbon isotope analysis of individual fractions of two samples was carried out with a GC coupled to a Finnigan GCC-II-combustion and triple collector gas mass spectrometer 252. Carbon iso-tope ratios are expressed relative to the V-PDB stan-dard. Calibration was performed by injecting several pulses of CO2of known13C value at the beginning and
the end of each GC run and by adding n-alkanes of knownd13C value close to the retention times of
com-pounds of interest where the complexity of the gas chromatogram allowed this without interference of the standard with sedimentary lipids.
Elemental composition of the mineral matter was determined using a Philips X-ray ¯uorescence spectro-meter PW 2400 with a rhodium tube. The water extracts of the sediment samples were analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES; Perkin±Elmer Optima 3000XL) to determine the portion of soluble salt cations in the total element composition.
3. Results and discussion
3.1. Bulk characterization of organic and inorganic matter
All sediments, including the `black muds', have low TOC contents of only 0.45±0.81% (Table 1). These values are in accordance with previous investigations of Dead Sea sediments by Nissenbaum et al. (1972; 0.3± 0.5%) and Anderson et al. (1977; 0.38±0.87%). The sulfur content (0.3±0.85%) is of the same order of mag-nitude as the TOC content with the lowest values occurring in the black mud sediments. In the absence of major amounts of TOC, most or all of the sulfur is
inorganic carbon (TIC) between 3.84 and 5.21% (cor-responding to 32±43.4% CaCO3) are due to detrital
calcite and authigenic aragonite. This is con®rmed by the correlation between TIC and calcium (after sub-traction of the content of trace amounts of soluble salt) with a correlation coecient ofR2=0.99.
Major and trace elements of the mineral matrix have a distribution similar to that in Middle-East wind-blown dust material (H.-J. Brumsack, private communication,
1997). An indication for the source of the black color of the sticky, deep-black muds is not revealed by the data of X-ray ¯uorescence spectroscopy, but treatment of the samples with hydrochloric acid causes the black color to disappear while at the same time hydrogen sul®de is evolved. This indicates that the black muds contain iron monosul®des.
In all investigated sediments, the total amount of sul-fur exceeds the separately determined total inorganic
Table 1
Total organic carbon data, carbonate contents, extraction yields, CPI values and inorganic element data for six Dead Sea sediment samplesa
Units C 020 C 030 C 060 C 160 BM 005 BM 080
TOC (%) 0.81 0.46 0.68 0.45 0.78 0.62
TIC (%) 3.84 3.96 4.43 4.92 5.09 5.21
Extraction yield (mg/gTOC) 72 127 74 53 138 114 Aliphatic hydrocarbons (%) of total extract 10 8 7 8 7 14 Aromatic hydrocarbons (%) of total extract 1 1 2 3 2 2 NSO compounds (%) of total extract 65 61 65 72 59 52 Asphaltenes (%) of total extract 23 30 26 17 32 31 CPI (C27±C32) 11.4 8.1 10.7 6.6 9.3 9.8
Data from X-ray ¯uorescence spectroscopy
SiO2 % 23.67 35.40 25.95 27.38 28.81 26.67
TiO2 % 0.491 0.664 0.511 0.465 0.536 0.516
Al2O3 % 6.41 6.38 6.21 4.95 5.96 5.64
Fe2O3 % 3.47 3.18 3.34 2.51 2.81 2.60
MgO % 6.11 4.43 5.30 5.64 4.25 5.01
CaO % 17.89 17.84 20.22 21.88 23.23 23.23
Na2O % 1.70 2.06 1.76 2.37 2.15 2.39
K2O % 1.87 1.72 1.68 1.46 1.60 1.57
P2O5 % 0.334 0.277 0.347 0.280 0.407 0.309
Ba ppm 242 302 253 299 269 259
Co ppm 9 12 8 11 11 9
Cr ppm 67 75 84 54 78 71
Mn ppm 355 590 428 1006 348 521
Ni ppm 30 25 30 23 31 26
Rb ppm 32 33 31 27 30 29
Sr ppm 455 501 516 1096 533 637
U ppm 2 2 3 2 3 3
V ppm 71 70 72 59 71 66
Y ppm 17 19 17 13 17 16
Zn ppm 63 47 79 40 69 59
Zr ppm 106 261 134 144 174 167
Main salt cations obtained by ICP-OES
Ca % 0.65 0.34 0.51 0.44 0.33 0.42
K (estim.) % 0.35 0.18 0.27 0.25 0.14 0.18
Mg % 2.01 1.05 1.56 1.42 0.79 1.03
Na (estim.) % 1.84 0.97 1.44 1.31 0.73 0.95 (%)Salt of total amount of the element in the sediment
Ca 5.08 2.67 3.53 2.81 1.99 2.53
K (estim.) 22 12 19 20 10 13
Mg 54.56 39.31 48.81 41.75 30.83 34.10
a Sample key: C=`canyon' section; BM=`black muds'; the numbers express sediment depth in [cm]. Na and K contents were
reduced sulfur (TIRS) indicating that, in the likely absence of organically bound sulfur, pore water sulfate contributes signi®cantly to the bulk sedimentary sulfur fraction. The combined TIRS and TOC data (Tables 1 and 2) fall on the trend observed by Berner (1984) for typical marine sediments with no sulfate limitation.
Except for BM080, pyrite represents the dominant inorganic reduced sulfur fraction in all sediment sam-ples. The downcore variation of the pyrite to TIRS ratios in core C (Table 2) indicates an increasing importance of pyrite sulfur with depth, likely due to the formation of FeS2from metastable precursor phases as
iron monosul®des and elemental sulfur (Morse et al., 1987). The deepest sample from the black mud core (BM080) shows the lowest relative amount of pyrite, much lower than at shallower depth. This is supposed to be a primary signal suggesting a partly depositional control on the conversion of metastable phases into
pyrite which is the most stable sedimentary iron sul®de (Morse et al., 1987).
3.2. Molecular investigations
3.2.1. Hydrocarbons
Then-alkane distribution patterns are very similar to each other in all samples studied and are typical of an origin of then-alkanes from epicuticular waxes of higher land plants (Eglinton and Hamilton, 1967). n-Alkane histograms of samples BM080 and C160 (Fig. 2) exhibit maxima at n-C31 and a strong odd-over-even carbon
number predominance. Carbon preference index (CPI) values (Bray and Evans, 1961) in the carbon number range of 27±33 vary between 11.4 and 6.6 (Table 1).
d13C values between
ÿ28.1 andÿ33.6% (Fig. 2) for
individual homologues indicate a slight enrichment in the heavier 13C isotope compared to carbon isotope
Table 2
Amounts of dierent sulfur fractions given as per cent sulfur of freeze-dried sediment. Note: The Sfraction consists of original
ele-mental sulfur and oxidized iron monosul®des
Sample FeS2(%dwt.) S(%dwt.) TIRS (%dwt.) TS (%dwt.) FeS2/TIRS
C020 0.24 0.19 0.43 0.66 0.55
C030 0.17 0.06 0.23 0.42 0.75
C060 0.24 0.15 0.28 0.85 0.62
C160 0.24 0.01 0.25 0.37 0.95
BM005 0.17 0.07 0.24 0.32 0.70
BM080 0.06 0.09 0.15 0.30 0.39
ratios of ÿ33 toÿ39% forn-alkanes from various C3
higher plants, and dierent parts thereof, as reported in the literature (Rieley et al., 1991, 1993; Collister et al., 1994; Lichtfouse et al., 1994). Then-alkane carbon iso-tope data of this study are consistent, however, with the relatively heavy (ÿ22 toÿ25%) bulk carbon isotopic
signature of terrestrial organic matter in Recent soils around the Dead Sea (Nissenbaum et al., 1972; Magar-itz et al., 1991) which is several permil higher than in typical higher land plants in temperate regions (e.g. Fry and Sherr, 1984), and with the carbon isotope values of cellulose in recent trees from the Dead Sea area (ÿ22 to ÿ24%; Lipp et al., 1996). Carbon dioxide from fossil
fuel burning can only explain changes of about 1%
(Keeling et al., 1984) while eects of environmental conditions on isotopic fractionation during photosynth-esis can lead to dierences of 3±6%(Huang et al., 1995, 1996a,b). The enrichment in the13C isotope content of
then-alkanes in the sediments could also (partly) be due to a small contribution of autochthonous organisms (Farquhar et al., 1989), because trace amounts of n -alkanes and n-alkenes were observed when laboratory cultures of the archaeonHalorubrum sodomenseand the microalga Dunaliella parva were analysed (Oldenburg, unpublished results).
A second, but smaller n-alkane maximum at n-C17
was observed especially in the most deeply buried sam-ple C160, which may arise from diagenetic transforma-tion of functransforma-tionalized aliphatic precursors such as n -carboxylic acids (Shimyama and Johns, 1971; Tissot and Welte, 1984) which have a high relative abundance in the Dead Sea sediments. No evidence was found for a direct contribution of short-chainn-alkanes from auto-chthonous organisms (Oldenburg, unpublished results),
although they are known to occur in the biomass of extant marine organisms (Blumer, Guillard and Chase, 1971).
3.2.2. n-Alcohols
Two typical distribution patterns of n-alcohols in Dead Sea sediments shown in Fig. 3 exhibit a strong even-over-odd carbon number predominance. The most abundantn-alcohol in all samples isn-hexacosanol, fol-lowed by n-octacosanol and n-tetracosanol. This dis-tribution pattern with the long-chain n-alcohol preference is typical of an origin from epicuticular waxes of higher land plants (Eglinton and Hamilton, 1967; Kolattukudy, 1970). Compared to the terrigenous
n-alkanes, then-alcohols are enriched in13C by 2±5%
resulting ind13C values in the range
ÿ24.1 toÿ25.1%
for the C20±C24homologues and in the rangeÿ26.7 to ÿ30.3% for the longer-chainn-alcohols. For the
long-chainn-alcohols in the Dead Sea samples, the same ter-restrial origin as for the n-alkanes appears most plau-sible, the considerably stronger enrichment in the heavier carbon isotope of the shorter-chain n-alcohols indicates a contribution from autochthonous organisms. Small amounts of short-chain n-alcohols (near the detection limit) and fatty acids as their possible pre-cursors found in the autochthonous organisms (Old-enburg, unpublished results) support this assumption.
Furthermore, the n-alcohols of the `black mud' sam-ple (BM080) are depeleted in13C up to 2.2% in
com-parison to the most deeply buried `canyon' sample (C160; Fig. 3). The same trend was observed for most of the other biomarkers analyzed in this study. A pro-gressive enrichment in the 13C content of individual
compounds with increasing depth and age may possibly
be due to (i) atmospheric CO2 that was isotopically
heavier in the past than at present (1%; Keeling et al., 1984), (ii) changes in environmental conditions that aected plant isotope fractionation (Farquhar et al., 1989; 3±6% according to Huang et al., 1995), (iii) iso-topic fractionation of individual compounds that occurred during diagenetic degradation (small for recent organic matter; Hayes et al., 1990), and (iv) a variable contribution from autochthonous organisms like Duna-liella parva, the only primary producer in the lake, and the heterotrophic bacterial community, which thrives on organic matter produced by the algae.
3.2.3. n-Fatty acids
The distribution patterns ofn-fatty acids of two sam-ples are shown in Fig. 4. All investigated samsam-ples exhibit a bimodal distribution of n-fatty acids with a strong predominance of even-carbon-numbered homologs, maximizing at n-C16:0 and n-C22:0. The second
max-imum at C22:0is more distinct in the more recent
sam-ples than in samsam-ples C160 and BM080 (Fig. 4), and the concentrations of the individual compounds decrease with depth as observed for the n-alkanes. Unsaturated fatty acids have 16 or 18 carbon atoms with one of the monounsaturated n-C18:1 isomers having the highest
relative abundance (Figs. 4 and 5). In addition, trace amounts of odd-carbon-numberedisoandanteisoacids were found in the C15±C19range and only
even-carbon-numberedisofatty acids in the C14±C18range (Fig. 5).
The long-chain fatty acids (5C22) re¯ect a contribution
from terrigenous higher plants (Eglinton and Hamilton, 1967; Eglinton et al., 1974; Madsuda and Koyama, 1977) whereas the saturated and unsaturated fatty acids
in the lower carbon number range are presumably derived mainly from algae ( Cranwell, 1974; Sargent and Whittle, 1981; Shameel, 1990) with the exception of palmitic (n-C16) and stearic acid (n-C18) which are
com-mon both in microorganisms and in higher plant mate-rial. The detected iso and anteiso fatty acids are probably related to bacterial sources.
The dominance of the short-chain n-fatty acids [(C14±C20)/(C21±C30)1 for the more recent samples
and up to 2.7 for the more deeply buried samples] toge-ther with the investigations of cultures of the archaeon
Halorubrum sodomense and the microalga Dunaliella parva, which resulted in high concentrations of short-chain fatty acids (Oldenburg, unpublished results), on ®rst glance could be regarded as indicating a major organic matter supply from autochthonous organisms. However, the compound-speci®c isotope analysis yiel-ded d13C values between
ÿ26.1 and ÿ31.3% for the
fatty acids (Fig. 4) which are in the same range as those of the long-chain n-alcohols assigned to a terrestrial source, and thus also point to an origin of most of the acids from C3 terrestrial plants. The pronounced 13C
depletion particularly of the short-chain acids (C14±C20)
clearly illustrates the supply of terrestrial plant material to the sediment. The enrichment of up to 2.7%in the heavier13C isotope of the unsaturated fatty acids
rela-tive to the saturated analogs with the same carbon number is the only indication of a (small) contribution from autochthonous organisms.
3.2.4. Sterols
In Fig. 6, the gas chromatogram of the alcohol frac-tion of `canyon' sample C060 shows the elufrac-tion range of
Fig. 4. Distribution of saturated and unsaturatedn-fatty acids in two selected samples (C 160 and BM 080) with individuald13C
steroidal alcohols. Compound identi®cation, as com-piled in Table 3, is based on relative retention times and comparison with mass spectra from the literature (Wyl-lie and Djerassi, 1968; Budzikiewicz, 1972; Wardroper, 1979; Brassell, 1980; McEvoy, 1983). Quanti®cation of major compounds is based on FID response in the gas chromatograms. The number of steroid structures detected is limited because of the very restricted ecosys-tem, and their distribution shows little variation among the sediment samples investigated in this study. The sterols are dominated by 24-ethylcholest-5-en-3b-ol [p] followed by its saturated analog, 24-ethyl-5a -cholestan-3b-ol [q], and 24-ethylcholesta-5,22-dien-3b-ol [l], which reach about half the concentration of 24-ethylcholest-5-en-3b-ol [p]. All three C29 sterols and the next most
abundant sterol, 24-methylcholest-5-en-3b-ol [j], are mainly derived from terrestrial higher plants (Huang and Meinschein, 1976, 1979; Marsh et al., 1990). 24-Methylcholesta-5,22-dien-3b-ol [e] is biosynthesized by higher land plants as well as planktonic organisms (Patterson, 1970; Idler and Wiseman, 1971; Marsh et al., 1990), whereas cholesterol [c] and its saturated analo-gue, 5a-cholestan-3b-ol [d], are ubiquitous in sediments and thus are not source indicative (Mackenzie et al., 1982). Overall, the sterol distributions indicate that most of the organic matter is derived from allochtho-nous sources.
Since the degree of complexity of the sterol distribu-tion is low, giving rise to little gas chromatographic
Fig. 5. Gas chromatogram of the fatty acid fraction of sample C 030. The fatty acids are labeled with their carbon numbers. In addition, the internal standard (ER=erucic acid), the injection standard (Sq=squalane), and 3-oxo-olean-12-en-28-oic acid isomers (OL) are marked.
coelution, an attempt was made to perform compound-speci®c isotope analysis for individual sterols which is not possible without further chromatographic pre-separation in most sterol fractions from marine or lacustrine sediments. As an example, Fig. 7 shows the
d3C values of sterols in the `black mud' sample BM080.
The d13C values of the sterols assigned to terrestrial
higher land plant sources (24-ethylcholest-5-en-3b-ol [p], 24-ethylcholesta-5,22-dien-3b-ol [l], 24-methylcholest-5-en-3b-ol [j]) are very similar to each other and vary only betweenÿ25.5 andÿ26.2%. An enrichment of 1.5%in
the heavier13C isotope is common for sterols relative to
unbranched lipids in the same eukaryotic organism (Hayes, 1993). Schouten et al. (1998), in a series of laboratory algal cultures, found sterols to be around 6%
heavier than the short-chain fatty acids in the same organism. In the case of the Dead Sea sediments, this
kind of relationship is satis®ed by long-chainn-alcohols, all fatty acids and the long-chain n-alkanes which all dier from the sterols by 2 to more than 4%. On the other hand, thed13C values of cholesterol [c] of
ÿ21.9 to ÿ23.9% point toward a supply from autochthonous
aquatic organisms. This is in good agreement with investigations of laboratory cultures of the green alga
Dunaliella parva, which yielded cholesterol [c] as one of the main sterols (Oldenburg, unpublished results). Other sterols in the Dead Sea sediments, like 24-methyl-5a -cholest-7-en-3b-ol [o] and 24-ethyl-5a-cholest-7-en-3b-ol [r], described in the literature as synthesized by marine organisms (Idler and Wiseman, 1971; Volkman, 1986), are also possibly contributed by the microalga Duna-liella, as con®rmed by the analysis of the biomass cultured in the laboratory (Oldenburg, unpublished results).
Table 3
Concentrations of steroids, triterpenoids, and isoprenoids identi®ed in six Dead Sea sediment samplesa
Concentration (mg/g TOC)
Symbol (Fig. 8) Compound C 020 C 030 C 060 C 160 BM 005 BM 080 a 5b-cholestan-3b-ol 8.2 11.5 6.2 2.4 2.8 1.5 b Unknown C27-stanol 6.2 13.3 11.3 11.9 5.4 3.4
c 5a-cholest-5-en-3b-ol 66.7 93.2 77.5 43.7 66.2 38.9 d 5a-cholestan-3b-ol 35.1 48.2 35.7 25.5 28.2 18.9 e 24-methylcholesta-5,22-dien-3b-ol 77.2 116.6 71.3 39.7 48.2 33.5 f C28steratrienol 29.2 15.8 30.0 14.6 17.2 13.5
g 5a-cholest-7-en-3b-ol 13.7 18.4 21.0 4.9 13.9 8.5 h C28steratrienol 16.9 73.7 53.3 9.9 25.7 14.6
i 24-methylcholesta-5,24(28)-dien-3b-ol 31.9 56.1 32.9 14.7 21.0 11.5 j 24-methylcholest-5-en-3b-ol 127.4 218.8 135.1 52.4 94.6 65.2 k 24-methyl-5a-cholestan-3b-ol 37.6 79.1 39.8 17.2 31.6 23.5 l 24-ethylcholesta-5,22-dien-3b-ol 108.7 227.4 144.1 75.8 104.5 62.8 m 24-ethylcholest-22-en-3b-ol 40.7 83.2 42.4 22.0 31.3 23.5
n Unknown compound 9.6 13.1 5.7 2.3
o 24-methylcholest-7-en-3b-ol 16.3 23.8 21.3 9.8 11.6 7.7 p 24-ethylcholest-5-en-3b-ol 307.0 666.7 481.7 111.6 283.4 186.9 q 24-ethyl-5a-cholestan-3b-ol 173.8 313.2 194.7 66.6 149.2 98.8 r 24-ethylcholest-7-en-3b-ol possibly coelutes
with trace amount of C30steradienol
29.2 70.5 63.0 16.6 24.1 14.6
A C27sterene 17.1 25.5 29.6 18.2 24.2 13.2
B C28sterenone 105.9 104.2 78.8 30.9 52.6 29.9
C C28sterenone 9.1 55.7 31.4 10.4 9.1 8.1
D b-amyrin 56.2 56.7 92.5 37.1 51.1 42.6 E lup-20(29)-en-3-one 47.1 46.8 43.4 31.4 6.0 10.4 F a-amyrin 27.4 28.4 59.4 19.6 26.8 21.0 G Lupeol 67.9 58.7 126.4 32.5 79.2 73.9 H 24-ethyl-5a-cholesta-3,5-dien-7-one 29.3 46.1 36.9 21.1 25.0 20.5 I unknown triterpenoid [M+*=498] 32.3 47.0 107.0 38.9 54.6 45.0
J unknown triterpenoid [M+*=498] 74.3 2.3 7.2 4.7 6.1 7.1
Phytol 55.1 96.5 89.9 140.8 140.4 106.6 6,10,14-trimethylpentadecan-2-one 8.5 36.5 20.1 31.1 19.4 22.2 bis-O-phytanylglycerol 26.8 111.8 34.9 101.7 32.7 54.1
In summary, the sterol distributions and carbon iso-tope signatures demonstrate the abundant supply of terrestrial plant material to the sediment, but also pro-vide epro-vidence for a small proportion of autochthonous organisms contributing to the sedimentary organic matter.
3.2.5. Additional terrestrial organic matter indicators
A variety of pentacyclic triterpenoids and steroid ketones was detected and quanti®ed in the ketone frac-tions of all investigated sediment samples (Table 3). The partial gas chromatogram in Fig. 8 shows the distribu-tion of these compounds in the `canyon' sample C060.
Fig. 7. Distribution of sterols in sample BM 080 with individuald13C values.
Abundant compounds of the oleanane, ursane and lupane series, like b- and a-amyrin, lupeol and their ketone derivatives, underline the dominance of alloch-thonous organic matter in the sediment (Comet and Eglinton, 1987; Laureillard and Saliot, 1993; Killops and Frewin, 1994).
High amounts (up to 200mg/g TOC) of 3-oxo-olean-12-en-28-oic acid and a possible isomer, detected as
methyl esters [OL] (Fig. 5), were only found in the `canyon' samples but not in the black muds. Fig. 9 shows the concentrations of these compounds, normal-ized to TOC, as a function of depth in the `canyon' sec-tion. The increase in concentration with decreasing depth is remarkable and may either be an indicator of a changing environment (water level of the lake), or the decrease in concentration with depth is a measure of the progress of very early diagenetic transformation.
3.2.6. Additional autochthonous organic matter indicators
Phytol, present in all investigated surface sediment samples in high concentrations (Fig. 3 and Table 3), is an ester-linked part of most chlorophylls (chlorophylls
a,band bacteriochlorophyll) and consequently an indi-cator for phototrophic organisms. Therefore, it may have been contributed to the lake sediments by plant detritus as well as by photosynthetic bacteria and algae. In this study, phytol was detected as an isotopically heavy compound with d3C values between
ÿ22.8%
(BM080) and ÿ19.7% (C160). This indicates that the
main portion of phytol is derived from autochthonous organisms, such as the microalga Dunaliella. This is supported by high concentrations of phytol in the laboratory culture of the only primary producer Duna-liella parva(Oldenburg, unpublished results). According to the study of Schouten et al. (1998), phytol should be similar or slightly lighter in carbon isotope composition relative to sterols from the same organisms. In this respect, phytol in the Dead Sea sediments more closely relates to cholesterol and the 7-sterols, for which an
aquatic origin has to be deduced, than to the con-siderably lighter sterols of terrigenous origin. The most common diagenetic degradation product of phytol,
Fig. 9. Concentration of 3-oxo-olean-12-en-28-oic acid isomers (OL) versus depth in the `canyon' section.
6,10,14-trimethylpentadecan-2-one, was also detected in high amounts in the sediment samples (Table 3).
Fig. 10 shows the mass spectrum of bis-O -phyta-nylglycerol as TMS derivative (identi®cation after Teix-idor et al., 1993), which was found in remarkably high amounts in the Dead Sea sediments (Table 3). Further-more, the compound-speci®c isotope analysis of this diether resulted ind13C values of
ÿ22.3 andÿ23.0%for
the `canyon' sample C160 and the `black mud' sample BM080, respectively. This isopranylglycerol diether has been described as a cell wall constituent of archaea (Kates, 1978; Langworthy et al., 1982; Albrecht et al., 1984; Schlegel, 1992). A culture of the archaeon Halor-ubrum sodomensewas shown to contain this ether-linked isoprenoid lipid (Oldenburg, unpublished results).
Phytol and bis-O-phytanylglycerol, based on com-pound-speci®c isotope analysis and comparison with the laboratory cultures, are the most important indicators for a contribution from autochthonous organisms to the sediments.
4. Conclusions
The Dead Sea sediments examined in this study, including the `black muds', have low TOC contents and are enriched in sulfur relative to TOC, indicating that most of the sulfur is from inorganic sul®de (and possibly sulfate) rather than bound in the organic matter. High amounts of inorganic carbon are due to detrital calcite and authigenic aragonite. Major and trace elements of the mineral matrix have a distribution similar to that of Middle-East wind-blown dust material and do not pro-vide a clue to the color of the black muds. Thus, the dark color is not due to organic matter but probably rather to poorly crystallized iron sul®des.
The molecular investigation of the extracted organic material showed that most of the lipids are of allochtho-nous terrestrial origin. There are only small dierences in the distribution patterns and the concentrations of the straight-chain lipids among the dierent sediment sam-ples. The sterol composition with components like 24-ethylcholest-5-en-3b-ol and 24-ethylcholesta-5,22-dien-3b-ol dominating the distributions as well as the large variety of pentacyclic triterpenoids from angiosperms, like
b-amyrin,a-amyrin, lupeol and their oxidized derivatives, also point to a mainly allochthonous organic matter ori-gin. Then-alkane distributions are typical for a supply from epicuticular waxes of higher land plants. This is con®rmed byd13C values between
ÿ28.1 andÿ33.6%.
The even-over-odd carbon number predominance of the long-chain fatty acids (C20±C30) and the ranges ofd13C
values (ÿ27.3 toÿ31.3%) of these compounds are also in
accordance with an origin chie¯y from C3 terrestrial
plants. The pronounced13C depletion of the short-chain
acids (C14±C18) demonstrates that even these compounds
are mainly from terrestrial plants. Then-alcohol distribu-tion patterns show a strong even-over-odd carbon number preference. The short-chainn-alcohols are enriched in13C
by up to 5%suggesting a mixture with a small proportion of autochthonous organic matter.
On the other hand, there are diagnostically distinct indications for a contribution by autochthonous organ-isms to the organic matter in the sediments. High amounts of phytol with d13C values between
ÿ22.8%
andÿ19.7%, cholesterol (ÿ23.9 toÿ21.9%) as well as
small concentrations of 24-methyl-5a-cholest-7-en-3b-ol and 24-ethyl-5a-cholest-7-en-3b-ol point toward a con-tribution from the only primary producer Dunaliella parva. In addition, an indicator for halophilic bacterial communities, possiblyHalorubrum sodomense, was pre-sent in the form of relatively high amounts of bis-O -phytanylglycerol, a component of archaean cell walls. The compound-speci®c isotope analysis of this com-pound resulted in d13C values between
ÿ22.3 and ÿ23.0%, which underlines the proposed origin.
Acknowledgements
We are indebted to Dr. Barbara M. Scholz-BoÈttcher (ICBM) for support with GC±MS analysis, Dr. Rolf Wehausen (ICBM) for performing the inorganic geo-chemical analyses, and Dr. Y. Yechieli (Geological Sur-vey of Israel) for guidance and assistance in the ®eld work. The publication bene®ts from the thorough reviews of Dr. James W. Collister (University of Utah, Salt Lake City) and an anonymous referee. We acknowledge support of the Earth Science Directorate of the Ministry of National Infrastructure, Israel.
Associate EditorÐJ.W. Collister
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