Organic Geochemistry 35 (2004) 157–168
www.elsevier.com/locate/orggeochem
Steroids in sediments from Zabuye Salt Lake, western Tibet:
diagenetic, ecological or climatic signals?
R.L. Wanga,*, S.C. Brassellb, S.C. Scarpittaa, M.P. Zhengc, S.C. Zhangd,
P.R. Haydea, L.M. Muenchb
a
Chemistry Department, Brookhaven National Laboratory, Bldg. 555, Upton, NY 11973, USA
b
Biogeochemistry Laboratory, Indiana University, Bloomington, IN 47405, USA
c
R&D Center of Saline Lake and Epithermal Deposits, Chinese Academy of Geological Sciences, Beijing 100037, PR China
d
Key Laboratory of Petroleum Geochemistry, China National Petroleum Co., Beijing 100083, PR China
Received 10 April 2003; accepted 2 October 2003
(returned to author for revision 6 August 2003)
Abstract
A 45 cm long core from Zabuye Salt Lake (Tibetan Plateau, S.W China) was studied to reveal the possible
interference between diagenesis and climate signals. Steroids, including sterols and sterenes, dominate the soluble
organic matter in these cores. The relative abundance of C27 sterol to the C29 sterols decreases with depth, resulting in a
predominance of C29 sterols at the bottom section of this core. This change in the relative molecular distribution could
be attributed to both environmental/ecological change and diagenetic complication of molecular signals. Sterols in the
shallow sediments are relatively enriched in 13C compared to those from lower within the core. This enrichment is
possibly associated either with environmental/climatic change (e.g., increase of salinity and global pCO2 level change
etc.) or it could be attributed to the biogeochemical change of organic matter during early diagenesis. 4,4-dimethyl
spirosterenes and their possible precursors, 4,4-dimethyl sterenes, constitute a major component of the apolar fraction
of organic matter. 13C values of the 4,4-dimethyl sterenes indicate that they are derived from phytoplanktonic algae
rather than from bacteria. The 13C values of the regular and spiro steroids differ by >2% suggesting either backbone
arrangement of steroids might have involved isotopic fractionation or that these steroids are derived separately from
different biological sources.
Published by Elsevier Ltd.
1. Introduction
Steroids occur ubiquitously in eukaryotic organisms
from microorganisms to macro algae and vascular
plants and are present in most sedimentary organic
matter. Thus, steroids are an important group of fossil
compounds providing valuable information about sources of organic matter in recent sediments and ancient
sedimentary rocks, as well as crude oils. Diagenesis of
the organic matter occurring in waters and sediments
* Corresponding author.
E-mail address: [email protected] (R.L. Wang).
0146-6380/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.orggeochem.2003.10.003
modifies the structures of precursor steroids in complex
ways. Examples of these processes include: oxidation of
sterols to sterones, dehydration of the hydroxyl group
(of sterols) leading to the formation of steradienes,
isomerisation of double bonds of sterenes via tertiary
carbon atoms to different positions in the carbon skeletons (e.g., Mackenzie et al., 1982; Brassell et al., 1984;
de Leeuw et al., 1989; Sinnighe Damsté et al., 1999),
backbone rearrangement and the formation of spiro
steroids (Peakman and Maxwell, 1988), and aromatization on the A,B,C-rings to form aromatic steroids
(Brassell et al., 1984). Incorporation and cleavage of
sulfur atoms from steroids also play an important role
in the early diagenesis of organic matter. Sterols and
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their diagenetic products contain functionalities such as
double bonds, carbonyl and hydroxy groups, that under
anoxic conditions are susceptible to incorporation of
inorganic sulfur species by natural sulfurization during
early diagenesis (e.g., Schouten et al., 1993). Steroids
also are valuable tools for determining the complex
diagenetic pathways that transform biological
precursors into steroid hydrocarbons in all crude oils,
their source rocks, and even in the migration pathways
of crude oils (e.g., Huang and Meinschein, 1976, 1979;
Brassell and Eglinton, 1981; Summons and Capon,
1988, 1991; Summons et al., 1987; Meyers, 1997; Kok et
al., 2000; Curiale, 2002). Detailed knowledge about the
diagenetic pathways of sterols is important since their
diagenetic products (steranes) in crude oils are very
useful markers in assessing the geological age of their
source rocks, the degree of thermal maturity, and types
of organisms present during the deposition of source
rocks (e.g., Peters and Moldowan, 1993).
Spirosterenes are a group of sedimentary sterenes
with a unique molecular backbone rearrangement
between C- and D- rings (e.g. Peakman et al., 1984;
Peakman and Maxwell, 1988; Summons and Capon,
1988, 1991; Schüpfer and Gülaçar, 2000; Kok et al.,
2000; Rushdi et al., 2003). This family of steroids has
dominant mass fragment ions at m/z 206+14n (n=0–2)
and a major ion at m/z 121. Spirosterenes are observed
in various environments including shales and evaporite
sediments (Brassell et al., 1984; Peakman et al., 1984;
Peakman and Maxwell, 1988). Beside the natural
occurrence of spirosterenes, a mixture of 20R and 20S
isomers of 12,14(a’-cyclo-12,13-seco-5a(H)-cholest-b
(H)ene (spirosterene) also were synthesized from 5a(H)cholest-7-ene and the corresponding C28 homologues
from
(20S)-24-methyl-5a(H)-cholest-7-ene
(5a(H)ergost-7-ene) (Peakman et al., 1984). Both natural and
synthesized spirosterenes gave major ions at 206 or 220
(from M.+ of C27 or C28 homologues respectively by
cleavage through C11–C12 and C8–C14) and m/z 121
(from subsequent cleavage through C20–C22) (Peakman
et al., 1984). The mass-spectral features make this group
of steroids rather unique and different from regular
steroids. The configuration at C20 was assumed by
analogy with the acid catalyzed formation of diasteranes
(Peakman and Maxwell, 1988) and by the assumption
that the presumed 20R isomer was formed first during
the rearrangement (Peakman and Maxwell, 1988).
Identifying molecular indicators that are controlled
primarily by climate and the depositional environment
and least impacted by diagenetic processes is an
extremely important issue in paleoclimatic study. Our
current research involves understanding the Pleistocene/
Holocene climate change on the Tibetan Plateau using
isotopic and molecular geochemical techniques on sediments from a core taken from the Zabuye Salt Lake,
located in western Tibet Plateau (ZSL, 31.35.N, 84.07.E,
Fig. 1). The Zabuye Salt Lake is at 4421 m above sea
level (a.s.l.), about 1000 m above the global tree line and
is surrounded by high mountains ca 4600–6000 m a.s.l.
In such an extreme environment, the influence/pollution
from vascular higher plant organic matter could be
virtually negligible. The remote ZSL is also far from
industrial and anthropogenic perturbation and the
impact from allochthonous organic sources to the organic
matter is effectively minimized by nature itself. The
pristine environment and special geographic location of
ZSL made this lake an ideal site for geochemical study
of autochthonous biological marker constituents, their
Fig. 1. Maps of the location of Qinghai-Tibet Plateau (A), distribution of saline, brackish and fresh water lakes in the western part of
the plateau (B) and the coring site of ZK3 in Zabuye Salt Lake (C) (Based on Zheng et al., 1989 and modified after Wang et al., 2002).
R.L. Wang et al. / Organic Geochemistry 35 (2004) 157–168
variation with changing environment/climate, and
diagenetic/geochemical change with time.
In this study, we report on the distributions of sterols,
sterenes and their carbon isotope composition from the
ZSL and their relationship to climate, source input and
early diagenetic process. In particular, we report the
presence of two isomers of the unusual spirosterenes
with two methyl groups attached on the A-ring, and
discuss the possible biological and diagenetic precursors
and pathways of the formation for these steroids
according to their carbon isotope compositions. These
types of spirosteroid structures are found normally with
sediments or crude oils associated with saline and
hypersaline environments. It was anticipated that this
study would help clarify the molecular and isotopic
characteristics of this group of unusual steroids, and
that a better understanding of steroid diagenetic processes will yield molecular fossils that more accurately
reflect climate changes.
2. Geological setting
Samples were taken from the northern basin of ZSL
(Core ZK3) located in western Tibetan Plateau, S.W.
China. ZSL is the terminal basin of the Taro-Zabuye
lake chain and has a current area of 243 km2. Due to the
enhanced evaporation and extremely arid climate
condition, the lake size has been decreasing every year.
A large area of playa was exposed around the lake, with
mirabilite and halite currently being deposited in the
lake. In the northern basin of the lake, where core ZK3
was taken, waters are recharged mainly from melted ice/
snow (Zheng et al., 1989). Due to its exceptional high
altitude ( 1000 m above tree-line), the predominant
organic source can be attributed to the autochthonous
production of the lake system dominated by halophilic
algae (Dunaliella salina, Zheng et al., 1985). In addition
to algae, other halophilic organisms contain halophilic
bacteria as well as halophilic archaea at zones with
higher salinity (salt playa) were found. These halophilic
organisms and their decomposition products color this
salt lake reddish during the blooming season of summer.
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homogenized samples using a LECO C/S Analyzer
using the previously published methodologies (Wang
and Zheng, 1998; Wang et al., 2002).
3.2. GC and GC–MS analyses of biomarkers
Freeze-dried sediment was extracted by CH3OH/
CH2Cl2 (1:1) in a Soxhlet extractor. The total extract
was separated using silica gel column chromatography
by eluting with hexane, toluene and dichloromethane.
The polar fraction was treated with BF3/CH3OH
(Aldrich, esterification of carboxylic acids) and BSTFA
(bis(trimethylsilyl)trifluoroacetamide, Aldrich) and then
purified using the same column chromatography procedure. The collected hexane-dissolved fractions (apolar)
were analyzed by gas chromatography and gas chromatography–mass spectrometry (GC–MS). The gas
chromatography column was an Ultra 1 (Crosslinked
Methyl Silicone Gum, 60 m0.32 mm i.d.0.52 mm film
thickness). Oven temperatures were programmed from
60 to 320 C at 4 C/min and held isothermal at 320 C
for 30 min. GC–MS analyses were carried out initially
on a Finnigan MAT TSQ-700 system using electron
impact ionization (70 eV). Then the structure confirmation was repeated on a HP 5970 mass selective detector
at Brookhaven National Laboratory using the same
GC–MS conditions. Helium was the carrier gas for all
analyses.
3.3. Isotope measurements of individual biomarkers by
irm-GC–MS
An isotope-ratio-monitoring GC–MS system (irmGC–MS) (Matthews and Hayes, 1978; Merritt et al.,
1995) was employed to obtain the 13C values of
the steroid compounds. Carbon isotope values are
expressed in the traditional notation of values:
sample ð%Þ ¼ Rsample Rstandard =ðRstandard Þ 1000;
3. Experimental
where R is the abundance ratio of 13C /12C in the
samples or in the standard. Isotope ratios are reported
relative to the PDB (Pee Dee Belemnite) standard via a
secondary standard (VPDB) for carbon isotope
measurements on the individual molecular CO2 (irmGC–MS) and the bulk CO2 (algae sample).
3.1. Measurement of elemental carbon and sulfur
concentrations
4. Results and discussions
The core, 45 cm long, was separated into 10 sections
and frozen in dry ice before shipment to the laboratory.
Measurements of total carbon (TC), total organic
carbon (TOC), total inorganic carbon (TIC) and total
sulfur (TS) were performed at the Biogeochemical
Lab of Indiana University on dried, ground, and
4.1. Elemental carbon and sulfur distributions of core
ZK3
Carbon and sulfur elemental concentrations are
plotted against the depth of the core in Fig. 2. From the
bottom section of the core (i) to top section (v) (Fig. 2),
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Fig. 2. Distribution of total organic and inorganic carbon (TOC, TIC), total organic and inorganic sulfur (TOS, TIS) and total sulfur
(TS) in the core ZK3, Zabuye Salt Lake, western China, plotted versus burial depth (all in % of dry sediment weight; i–v: core sections
used in the text).
TOC and TIC decreased from about 0.6 to 0.3% and
7.5 to 6.5%, respectively. The gradual decrease of
organic carbon from section ‘i’ to ‘v’ (Fig. 2) is, perhaps,
associated with the increase of salinity (evaporation/
precipitation ratio increase, Zheng et al., 1989) and the
decline of productivity (as TOC) in the lake. The
gradual decrease of carbonate carbon (TIC) and
increase of sulfate as indicated by the increased total
sulfur (TS) in the core also are indications of increasing
salinity of the lake. Fluctuations of salinity occurred as
indicated by the variability of total sulfur contents of
the core. The rapid drop of sulfur in the topmost section
(section v, Fig. 2) could be associated with the further
salinization of the lake, such that the sulfate was largely
replaced by halite, marking a hypersaline salt lake
environment during interval ‘v’. Total organic sulfur
content remained rather low in the top sections above 30
cm, but increases significantly with depth below 30 cm,
indicating that active sulfate reduction and formation of
organic sulfur species could be taking place in the
sediment below this depth level ( 30 cm).
4.2. Distribution of sterols in core ZK3
Sterols are found to be the predominant components
in the total organic extract of these salt lake sediments.
The most abundant steroids are cholesterol (C27), C28
5,22-sterol, C29 5,22-24-ethyl sterol and C29 5 sterol
(Fig. 3). Other components, such as the normal-chain
moieties mainly contributed from terrestrial vascular
plant input are low in these cores (Fig. 3). The relative
abundance of C27–C29 sterols shows a steady decrease in
the cores from surface (No. 1) to the bottom (No. 10)
(Figs. 3 and 4). The relative percentage of both C29:1
sterol and C29:2 sterol in the total neutral lipid fraction
decreases slightly in shallow sediments but increases
steadily with increasing depth below 18 cm in the core.
On the other hand, C27:1 sterol abundance decreases
markedly with increasing depth at the top section (v)
(Fig. 4). The ratio of C27/C29 (see Fig. 4 for details)
decreases steadily from about 0.8 at the surface to about
0.3 at the bottom of the core (Fig. 4). These changes in
steroid distributions may be due to an ecological change
in the phytoplankton community that occurred during
the time represented by this 45 cm core. Alternatively,
the variation in the molecular distribution of these
steroids could result from a differentiation in the diagenetic transformation of different steroids. Recent
studies of early steroid sulfurization in Ace Lake
sediments showed that incorporation of sulfur is biased
toward C27 sterols (Kok et al., 2000). In the surface
sediments from Ace Lake in Antarctica, a predominance
R.L. Wang et al. / Organic Geochemistry 35 (2004) 157–168
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Fig. 3. Gas chromatograms of alcohols and sterols (as thiolated fractions) extracted from sediment core ZK3 in Zabuye Salt Lake,
Tibet (distributions of the relative abundance of compounds A,B,C are shown in Fig. 4 and 13C values of compounds A and B are
shown in Fig. 5).
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Fig. 4. Depth profiles of relative percentages of sterols and their molecular ratio in core ZK3 (chromatograms of compounds A,B and
C are shown in Fig. 3; sections i–v same as in Fig. 2).
of sulfurized C27 steroid contrasted with the distribution
of free sterols with a strong predominance of C29 sterols
(Kok et al., 2000). Therefore, diagenetic differentiation
could be responsible for the observed change of molecular distribution of sterols in these cores. In the case of
ZSL, both organic carbon and sulfur contents in these
cores are rather high (Fig. 2). In the southern basin of
ZSL, sulfur content is even higher, with TS as high as
14% (Wang et al., 2002). Under hypersaline conditions,
incorporation of sulfur into the steroid molecules can be
an important process especially at burial depths where
sulfate reduction is significant.
4.3. Carbon isotopic composition of individual sterols
The 13C values of C29 24-ethyl-5,22-sterenol and C29
ster-5-enol, the two major sterols extracted from core
ZK3, are plotted vs. burial depth in Fig. 5. Both compounds have 13C values between 25 and 28% except
for the two samples in section ‘v’. The 13C values of
sterols in this section are much lower, 31 to 32%
for both compounds. The dramatic shift of carbon isotope values (up to 4%) at the top section of the core (‘iv’
to ‘v’) is possibly due to either environmental/ecological
change (e.g., salinity change) during deposition and/or
early diagenesis. Due to the markedly increased salinity
Fig. 5. Distribution of molecular carbon isotope composition
(13C) of C29-24-ethyl-5,22-sterenol (A) and C29ster-5-enol (B)
vs. burial depth sediments in core ZK3 from northern basin of
Zabuye Salt Lake in Tibet (both peaks shown in Fig. 3; 13C
values in % VPDB).
R.L. Wang et al. / Organic Geochemistry 35 (2004) 157–168
163
Fig. 6. Partial mass chromatograms of m/z 220, 234, 412 and partial RIC showing the co-occurrence of 4,4,24-trimethyl-12,14a-cyclo12,13-seco-5a(H)-13(17)ene (black, #1 and 2) and 4,4,24-trimethyl-5a(H)-cholest-7-ene (#3 and 4). Mass spectra of compounds 1 and
2 are shown in Fig. 7.
Fig. 7. Mass spectra of 4,4,24-trimethyl-12,14a-cyclo-12,13-seco-5a(H)-13(17)enes and their tentative structure assignment based on
GC–MS data (for conditions of GC–MS operation please refer to the text).
and lower productivity, a larger portion of the sterols in
the shallow sediments (top section of the core) might be
contributed by higher plant input from the allochthonous source, for example, wind blown pollen/spore and
other plant debris from aerosol precipitation into the
lake. Another possibility is that such molecular isotope
shift might be related to climate/environmental change
in a rather larger scale or even global pCO2 level change.
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However, as we discussed earlier, section ‘v’ represents
precipitation under higher salinity than that of section
‘iv’ (Fig. 2). Organic molecules synthesized in waters
with higher salinity tend to be isotopically heavier rather
than being lighter. For example, studies of the halophytes Salicornia europaea subsp. Rubra and Puccinellia
nuttalliana, show that increasing salt concentrations
result in decreased isotope discrimination in both field
samples and growth chamber samples resulting in 13C
enriched lipids (O’Leary, 1981). Therefore, it is likely that
the dramatic shift of carbon isotope values of sterols (up
to 4%) in this core section is associated mainly with
climate/environmental/ecological change, although a
‘diagenetic’ isotope fractionation between precursors and
products in the sediment cannot be excluded completely.
4.4. Occurrence of 4,4-dimethyl spirosterenes
Partial mass chromatograms of m/z 220, 234 and 412
illustrate the 20R and 20S isomers of 4,4,24-trimethyl12,14a-cyclo-12,13-seco-5a(H)-cholest-13(17)ene (4,4dimethyl spirosterene) observed in the sediment cores of
ZSL (Fig. 6). Mass spectra of 20R and 20S isomers are
virtually identical (Fig. 7a and b respectively). This
group of sterenes is a predominant component in the
hexane-dissolved fraction of the total extract of all core
samples. However, they were not observed in the extract
of algae samples. The mass spectrometric characteristics
are quite similar to those of the C28 spirosterene homologues reported previously (Peakman et al., 1984).
Spirosterenes were reported previously in a variety of
sediments including the Cretaceous black shales (Leg 50,
Fig. 8. Down-core profile of the ratio of C20R/C20S isomers of
4,4,24-trimethyl-12,14a-cyclo-12,13-seco-5a(H)-13(17)ene in the
later Holocene core ZK3 from northern basin of Zabuye Salt
Lake, western Tibet.
Moroccan Basin; Leg 71, Falkland Plateau; Leg 75,
Angola Basin) (Brassell et al., 1984), Miocene DSDP
sediments from Southern California Bight, and Cretaceous Black shale from Northern Italy (Brassell et al.,
1984; van Graas et al., 1982). These compounds show
dominant fragment ions at m/z 206+14n (n=0–2) and a
major ion at m/z 121. Peakman et al. (1984) also
synthesized a mixture of 20R and 20S isomers of 12,14acyclo-12,13-seco-5a(H)-cholest-b(H) ene from 5a(H)cholest-7-ene and the corresponding C28 homologues
from
(20S)-24-methyl-5a(H)-cholest-7-ene
(5a(H)ergost-7-ene). A C27 spirosterene with one methyl group
on the A-ring was observed as a significant component
in hydrous pyrolytic products (Abbott et al., 1995).
Both natural and synthesized spirosterenes yield major
ions at 206 or 220 (from M.+ of C27 or C28 homologues
respectively by cleavage through C11–C12 and C8–C14)
and m/z 121 (from subsequent cleavage through C20–
C22. The configuration at C20 was assumed by analogy
with the acid catalyzed formation of diasteranes
(Peakman and Maxwell, 1988) and by the fact that the
presumed 20R isomer was formed first during the
rearrangement.
The compounds extracted from ZSL sediment are
rather similar to those published previously but with
two methyl groups on the A-ring (Fig. 7). The molecular
ions (M+.) of these methyl steroids in ZSL are at m/z
412 (15%), rather than 384 as shown by Peakman et al.
(1984). Both 20S and 20R isomers have base fragment
ions at m/z 220 (100%, by cleavage through C11–C12
Fig. 9. Carbon isotope values (13C, %) of sterenes plotted
against the burial depth of sediments from core ZK3 [1,2: 20R
and 20S isomers of 4,4,24-trimethyl-12,14a-cyclo-12,13-seco5a(H)-13(17)enes, respectively; 3: 20R isomer of 4,4,24-trimethyl-5a(H)-cholest-7-ene (No. 4 in Fig. 6)].
R.L. Wang et al. / Organic Geochemistry 35 (2004) 157–168
165
Fig. 10. Possible pathways of the formation of spiro sterenes in the early diagenesis of sediments as observed in hypersaline ZSL
sediments; the backbone rearrangement from Nos. 3 and 4 to the spiro steroids (Nos. 1 and 2, Fig. 6) are believed to be catalyzed by
acidic minerals in peat (Peakman et al., 1984) (R=C7H15).
and C8-C14 from M+., Fig. 7) and a major ion at 121
(40%, by a secondary cleavage through C20–C22).
Other major diagnostic ions are m/z 107 (15%, m/z 121–
14), m/z 135 (10%, m/z 121+14), m/z 233/234 (15%, m/
z 220+14) and m/z 397 (M+.–CH3). The structures
(4,4,24-dimethyl-12,14a-cyclo-12,13-seco-5a(H)-cholest13(17)-ene) are assigned tentatively based primarily on
the GC–MS features compared to previously published
data on spirosterenes (e.g., Peakman et al., 1984;
Peakman and Maxwell, 1988; Abbott et al., 1995).
While these compounds are not observed in the extract
from a modern algae sampled from this lake, they may
be formed from macro-molecular lipid precursors. If so,
further study using pyrolytic degradation of this algae
sample might yield more supportive evidence. Unfortunately such data are not available at this time.
Spirosterenes detected in deep-sea sediments were
thought to be formed by an acid catalyzed rearrangement involving clay minerals (Peakman et al., 1984).
Their precursors could be 7-8(14) sterenes (van Graas
et al., 1982; Peakman et al., 1984; Peakman and Maxwell, 1988) although these alkenes are not commonly
identified in sediments. Pyrolysis studies showed that
under acidic and aqueous conditions (e.g., in hot water),
regular 5a(H)-cholestane also can undergo backbone
rearrangement yielding a C27 spirosterene (Abbott et al.,
1995). The reaction mechanism for the spirosteroids
observed in saline sediments could be much more
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complicated. The abundance of 20R and 20S isomers
shows a gradual change through the burial depth of
sediments (Fig. 8). The ratio of 20R/20S decreases
quickly in the surface sediment until it finally reaches a
steady-state ratio of 20R/20S.0.5 at about 30 cm burial
depth in the core. This indicates that 20R could be the
initial configuration inherited from its biologically
synthesized precursors, perhaps a 4,4-dimethylsterol with
an OH group on C-3 (Fig. 7). The isomerization occurring
at C-20, therefore, can be used as an indicator of early
biogeochemical transformation of organic matter under
hypersaline conditions, although such rapid isomerization
would limit the potential usage in the geological time scale.
4.5. Carbon isotope compositions of spiro steroids
13C values of 20R and 20S isomers of 4,4,24-trimethyl-12,14a-cyclo-12,13-seco-5a(H)-cholest-13(17)ene
are 24.0 26.7% (Fig. 9). The 13C values of 4,4dimethyl sterenes in these samples are in the same range,
i.e., 24 27% (Fig. 9) suggesting that the carbon
skeletons of both of these A-ring alkylated sterenes were
biosynthesized originally by the same organism. The
relatively consistent difference of 13C values between
these two groups of steroids (13C 2%) further
suggest that these compounds are related either by
biosynthesis or by diagenetic transformation. The
isotopic change (enrichment) observed in the down-core
profile could reflect biochemical response to environmental change, locally (e.g. salinity change) and even
globally (e.g., global atmospheric CO2 level change).
Alternatively, isotopic fractionation may be occurring
during early diagenesis via a multiple-step mechanism
(Fig. 10). Such a mechanism would require the formation of unidentified, isotopically lighter compounds
during the skeletal rearrangement of the 4,4-dimethysterenes. It is also possible that the gradual increase in
13C values of both spiro and regular 4,4-dimethyl
steroids is fortuitous and the 4,4-dimethylsteroids are
not the precursors of the 4,4-dimethyl spirosteroids.
Biological sources of 4,4-dimethyl steroids could
either be algae (Goodwin et al., 1988), particularly
prymnesiophyte microalgae (Volkman et al., 1990), or
methanotrophic bacterium (Bird et al., 1971; Bouvier et
al., 1976). Methylococcus capsulatus is known to
biosynthesize many hopanoids and steroids (Bird et al.,
1971; Bouvier et al., 1976), the latter being dominated
by the 4,4,14-trimethyl (lanosterol), 3-keto-4-methyl,
and 4-methyl sterols. This group of bacteria has the
ability to use simple C1 compounds such as methane,
methanol or methylamine as the sole source of carbon
for growth. Generally, methanotrophic bacteria enrich
12
C in their biosynthesis causing their lipids to be
depleted in 13C by up to 50% relative to the starting
materials due to a strong isotopic fractionation effect
(e.g. Freeman et al., 1990; Hayes, 1992). However, the
13C values of 4,4-dimethyl steroids compounds in the
salt lake sediments indicate that these compounds must
have originated from phytoplankton algae, rather than
methanotrophic bacteria. Algae, particularly the halophilic species Chlamydomnas and Dunaliella salina, are
abundant during summer blooming seasons in ZSL
(Zheng et al., 1985). The bulk algal 13C value is as high
as 18.6% (VPDB). This bulk isotope value of the algal
biomass also suggests that these algae (or generally,
phytoplankton) are likely the biological source of the
abundant steroids we observed in the sediments. Under
the hypersaline environments at ZSL, extended
residence time of the ZSL lake waters have caused
enrichment of 13C not only in the carbonate (Wang et
al., 2002), but also in the organic carbon of algae lipids,
including the regular and spirosteroids in this hypersaline lacustrine environment.
5. Conclusions
Unsaturated sterols are the predominant component
of the total extract of the sediment cores from Zabuye
Salt Lake. Abundance of C27 sterol relative to the C29
sterols decreases with depth resulting in a predominance
of C29 sterols at the bottom section of this core. The
change in the relative molecular distribution could be
attributed to environmental change (drought, salinization, etc.) of the region, and perhaps to certain degree it
could also be associated with the possible differential
sulfurization rate of C27 and C29 sterols. The obvious
enrichment of 13C in sterols was seen in samples below
6–10 cm, possibly associated with either environmental change (e.g. increase of salinity, pCO2 level
change etc.) or more or less, it might be associated with
some unknown process during early diagenesis of
organic matter. 4,4-dimethyl spirosterenes and their
possible precursors, 4,4-dimethyl sterenes were seen as a
major component of the apolar fraction of organic
matter extracted from the sediment core from ZSL. 13C
values of these compounds indicate that these steroids were
likely derived from phytoplanktonic algae rather than
from bacteria. The rearrangement of 4,4,-dimethyl 4 and
5 steroids yielding 4,4-dimethyl spirosteroids starts from
the early diagenesis of organic matter in the sediment. If
these 4,4,-dimethyl 4 and 5 steroids are really the precursors of the spirosteroids, the >2% difference of 13C
values between these two types of compounds suggest that
a multiple-step mechanism may be involved in this backbone structural rearrangement in the saline sediments.
Acknowledgements
We would like to thank our colleagues at Brookhaven
National Lab (BNL), Indiana University and Chinese
R.L. Wang et al. / Organic Geochemistry 35 (2004) 157–168
Academy of Geological Sciences (CAGS) for their
support and contribution to this research. Dr. W. Qi
(CAGS) are particularly acknowledged for assistance in
the core sampling. We thank Dr. Arndt Schimmelmann,
Mr. J. Fong and Mr. S. Studley (all at IU) for their
assistance during the irm-GC–MS analysis of steroids
and other part of the project. Finally we would like to
thank Dr. Clifford Walters (ExxonMobil), Dr. J. M.
Moldowan (Stanford) and Dr. R. Pancost (Bristol)
for many valuable comments and suggestions. Initial
research of this project was supported by a Geochemistry Fellowship from Indiana University (RLW) and
also partially supported by the National Natural
Science Foundation of China grant 49833010 (MPZ).
Associate Editor—C. Walters
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