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1.
The Holocene successions of numerous shallow lakes located along the Coorong coastal plain in South Australia attest to the impact of rising sea level and changing climate on their depositional environment. Old Man Lake is one of the smallest perennial alkaline lakes in the region. Its succession comprises a basal lagoonal sand rich in humic organic matter (OM) overlain by a 3.7 m thick upward shoaling lacustrine mudstone. The latter features three discrete sapropel units deposited between 3270 and 4910 cal yr BP, a time of increasing aridity throughout southeastern Australia. A core taken from the lake’s eastern margin yielded sedimentological, mineralogical, geochronological and micropaleontological data. Coring at five other sites across the lake provided sections of the humic and sapropelic facies (n = 20) for total organic carbon and Rock–Eval analysis; isotopic characterization of their micritic carbonate (δ13Ccarb, δ18Ocarb) and co-existing OM (δ13Corg); and GC–MS and GC–irMS analysis of their free aliphatic hydrocarbons. For each ‘sapropel event’ high productivity of diatoms and green algae was the principal driver of the accumulation and preservation of OM in such high concentrations. The precursor algal blooms were likely triggered by the influx of fresh water following winter rainfall. The combination of kerogen hydrogen index and δ13Ccarb–δ13Corg, previously employed to track secular changes in algal productivity and organic preservation, proved useful in identifying synchronous geographic differences in these processes across the lake. Highly branched isoprenoids (HBI: C25:1 ≫ C20:0) are prominent components of the aliphatic hydrocarbons in the sapropels, confirming the significant contribution of diatoms to their OM. The C isotopic signatures of the principal C25:1 HBI isomer and the co-occurring C23–C31 odd carbon numbered n-alkanes further document the non-uniformity of biomass preservation within and between the three sapropel units. The evidence from this study suggests that seasonal algal blooms and meromixis, although not necessarily an anoxic hypoliminion, were required for sapropel formation in the Holocene lakes of the Coorong region. Higher resolution sampling, dating and comparative analysis (microfossil, biomarker and isotopic) of these sapropels is required to clarify their potential significance as palaeoclimate proxies. 相似文献
2.
Long-chain fatty acids (C10-C32), as well as C14-C21 isoprenoid acids (except for C18), have been identified in anhydrous and hydrous pyrolyses products of Green River kerogen (200–400°C, 2–1000 hr). These kerogen-released fatty acids are characterized by a strong even/odd predominance (CPI: 4.8-10.2) with a maximum at C16 followed by lesser amounts of C18 and C22 acids. This distribution is different from that of unbound and bound geolipids extracted from Green River shale. The unbound fatty acids show a weak even/odd predominance (CPI: 1.64) with a maximum at C14, and bound fatty acids display an even/odd predominance (CPI: 2.8) with maxima at C18 and C30. These results suggest that fatty acids were incorporated into kerogen during sedimentation and early diagenesis and were protected from microbial and chemical changes over geological periods of time. Total quantities of fatty acids produced during heating of the kerogen ranged from 0.71 to 3.2 mg/g kerogen. Highest concentrations were obtained when kerogen was heated with water for 100 hr at 300°C. Generally, their amounts did not decrease under hydrous conditions with increase in temperature or heating time, suggesting that significant decarboxylation did not occur under the pyrolysis conditions used, although hydrocarbons were extensively generated. 相似文献
3.
13C and 1H NMR spectra were obtained for humic acids isolated from marine sediments. NMR shows great promise in identifying structural components of humic acids as some new and interesting structural features are identified. Aliphatic structures were found to constitute a much larger fraction of humic acids than previously thought, and they appeared to be highly branched. Although the aromatic content of terrestrial humic acid was found to be lower than expected, the aromaticity appears to be a specific discriminator of terrestrial/aquatic source types. A humic acid isolated from an anoxic algal sapropel was found to be composed predominantly of polyuronic acids and different than other aquatic sedimentary humic substances. 相似文献
4.
Fatty acids (FAs), β- and ω-hydroxy acids, α,ω-dicarboxylic acids and n-alkanes were studied in a 200 m sediment core taken from Lake Biwa, Japan. FAs showed bimodal distribution with peaks at C16 and C22-C28. Their distribution patterns clearly changed with depth from lower molecular weight (C12-C19) predominance to higher molecular weight (C20-C32) predominance in the upper 20 m interval. Analyses of related compounds (β- and ω-hydroxy acids and α, ω-dicarboxylic acids) suggest that β- and ω-oxidative degradation of C12-C19 FAs has occurred in the sediments.The ratio of bound C12-C19 to unbound FAs increases with depth in the upper 0–1 m sediments, suggesting that unbound FAs are more labile. However, the ratio varies significantly in deeper sections and may be associated with water temperature.In the sediments deeper than 20 m in depth, C12-C19 FAs gradually decrease. On the other hand, higher molecular weight FAs (HFAs: C20-C32), which were probably derived from terrestrial plants, increase in concentration from 20 m to 100 m, suddenly decrease at 100 m and show progressively lower concentration in deeper sediments. These fluctuations are interpreted in relation to paleolimnological changes of the lake and the drainage basin. ω-Hydroxy C20-C30 acids and C20-C30α, ω-dicarboxylic acids show a distribution pattern similar to HFAs. Branched chain FAs, ω-hydroxy acids and C9-C19α,ω-dicarboxylic acids show a major peak around 3–15 m in depth. This peak is probably caused by increased bacterial activity in the water column and surface sediments in the past, which may be associated with an increase in primary production of the lake. 相似文献
5.
Yongli Wang Xiaomin Fang Tongwei Zhang Yuanmao Li Yingqin Wu Daxiang He Youxiao Wang 《Applied Geochemistry》2010
This study reports the first observation of predominant even C-numbered n-alkanes from sediments in the continuous lacustrine-sedimentary section (Maogou) from the Late Miocene to the Early Pliocene (13–4.4 Ma) in the Linxia Basin, NE Tibetan Plateau. The n-alkanes showed a bimodal distribution that is characterised by a centre at n-C16–n-C20 with maximum values at n-C18 and n-C27–n-C31 as well as at n-C29. The first mode shows a strong even C-number predominance (OEP16–20 0.34–0.66). In contrast, the second mode has a strong odd C-number predominance (OEP27–31 1.20–2.45). Certain types of special autochthonous bacteria are a possible source for this distribution of even C-numbered n-alkanes in lacustrine sediments. These bacteria may have a high production rate in weak oxic–anoxic and arid depositional environments, in which a variety of geochemical parameters have recorded palaeoclimate change. 相似文献
6.
Lipid biomarkers in lacustrine sediments provide valuable information about lake history and environmental change. Here we report the occurrence of a unique set of lipid biomarkers from a freshwater crater lake, El Junco, in the Galápagos. In addition to previously reported alkadienes, botryococcenes and lycopadienes indicative of Botryococcus braunii A, B and L races, we find highly branched C25 isoprenoids (HBIs) from diatoms, monomethyl alkanes likely from insect epicuticular waxes, long chain alkenols, diols and a triol, keto-ols, hydroxy acids and keto acids. Saturated and mono-unsaturated long chain diols from C30-C36 had terminal hydroxyl groups and hydroxyl groups between the ω16 and ω20 positions. Vicinal diols with hydroxyl groups at ω9 and ω10 were likely from the floating fern Azolla. C30-C36 keto-ols, mid-chain hydroxy and keto acids had mid-chain functional groups at similar positions to the diols, suggesting common origins. The predominance of ω20-hydroxy acids and diols, together with 20,21-dihydroxy-nonacosanoic acid is indicative of an Azolla source, while ω16 and ω18 hydroxy acids and diols imply a microalgal source. 相似文献
7.
Organic matter from the Lower Paleozoic strata on the northern margin of the Tarim Basin of China contains abundant alkylcyclohexanes.The n-alkylcyclohexanes show the even(C16-C20) and odd (C17-C21) carbon number predominance and smooth distribution,and the methyl-n-alkylcycolhexanes the odd carbon number predominance(C17-C21) and smooth distribution,which may be related to their depositional environments and organic precursors.There are some differences in carbon number distribution between the two series of monocyclic alkanes and the n-alkanes marked by smooth distribution.The genetic relationship between the three series of compounds needs to be further studied. 相似文献
8.
Study of an algal, sapropelic sediment from Mangrove Lake, Bermuda shows that the mass balance of carbon and stable carbon isotopes in the major organic constituents is accounted for by a relatively straightforward model of selective preservation during diagenesis. The loss of 13C-enriched carbohydrates is the principal factor controlling the intermolecular mass balance of 13C in the sapropel. Results indicate that labile components are decomposed leaving as a residual concentrate in the sediment an insoluble humic substance that may be an original biochemical component of algae and associated bacteria. An overall decrease of up to about 4‰ in the δ 13C values of the organic matter is observed as a result of early diagenesis. 相似文献
9.
《Applied Geochemistry》2004,19(1):55-72
Bulk and molecular stable C isotopic compositions and biomarker distributions provide evidence for a diverse community of algal and bacterial organisms in the sedimentary organic matter of a carbonate section throughout the Permian–Triassic (P/Tr) transition at the Idrijca Valley, Western Slovenia. The input of algae and bacteria in all the Upper Permian and Lower Scythian samples is represented by the predominance of C15–C22 n-alkanes, odd C-number alkylcyclohexanes, C27 steranes and substantial contents of C21–C30 acyclic isoprenoids. The occurrence of odd long-chain n-alkanes (C22–C30) and C29 steranes in all the samples indicate a contribution of continental material. The decrease of Corg and Ccarb contents, increase of Rock-Eval oxygen indices, and 13C-enrichment of the kerogen suggest a decrease in anoxia of the uppermost Permian bottom water. The predominance of odd C-number alkylcycloalkanes, C27 steranes, and C17 n-alkanes with δ13C values ∼−30‰, and 13C-enrichment of the kerogens in the lowermost Scythian samples are evidence of greater algal productivity. This increased productivity was probably sustained by a high nutrient availability and changes of dissolved CO2 speciation associated to the earliest Triassic transgression. A decrease of Corg content in the uppermost Scythian samples, associated to a 13C-depletetion in the carbonates (up to 4‰) and individual n-alkanes (up to 3.4‰) compared to the Upper Permian samples, indicate lowering of the primary productivity (algae, cyanobacteria) and/or higher degradation of the organic matter. 相似文献
10.
Hiromi Matsuda 《Geochimica et cosmochimica acta》1978,42(7):1027-1034
A 3-m sediment core taken from Lake Suigetsu, in which a shift from fresh to brackish water occurred about three hundred years ago, has been examined for variation with depth of organic carbon and fatty acids. From the difference in total amounts of sulphur between sediments under fresh and brackish water environments, the surface sediments above approximately 35 cm depth were deduced to be accumulated under a brackish water environment. The total contents of organic carbon and fatty acids, and percentage composition of fatty acids gave discontinuous profiles above and below the 35–40 cm sediment layer. At a depth of 12.5 cm, the distribution in chain length of the fatty acids changed from a unimodal (the predominance of C12-C18 over C20-C34) to a bimodal pattern, which was mirrored by the composition diversity index (CDI).Although the fatty acids in the surface sediments (0–40 cm) from Lake Suigetsu seemed to suffer milder degradation through microbial activity than those in a core (0–150 cm) from Lake Suwa, a freshwater eutrophic lake, both lacustrine sediments showed similar trends in the alteration of fatty acid composition with depth. 相似文献
11.
The free, adsorbed and inclusion oils were recovered by sequential extraction from eleven oil and tar containing reservoir rocks in the Tazhong Uplift of Tarim Basin. The results of gas chromatography (GC) and GC–mass spectrometry analyses of these oil components and seven crude oils collected from this region reveal multiple oil charges derived from different source rocks for these oil reservoirs. The initially charged oils show strong predominance of even over odd n-alkanes in the range n-C12 to n-C20 and have ordinary maturities, while the later charged oils do not exhibit any predominance of n-alkanes and have high maturities. The adsorbed and inclusion oils of the reservoir rocks generally have high relative concentrations of gammacerane and C28 steranes, similar to the Cambrian-Lower Ordovician source rocks. In contrast, the free oils of these reservoir rocks generally have low relative concentrations of gammacerane and C28 steranes, similar to the Middle-Upper Ordovician source rocks. There are two interpretations of this result: (1) the initially charged oils are derived from the Cambrian-Lower Ordovician source rocks while the later charged oils are derived from the Middle-Upper Ordovician source rocks; and (2) both the initially and later charged oils are mainly derived from the Cambrian-Lower Ordovician source rocks but the later charged oils are contaminated by the oil components from the Silurian tar sandstones and the Middle-Upper Ordovician source rocks. 相似文献
12.
Lipid fraction and cell-wall materials have been separated from three types of algae (blue green, Microcystis sp.; green, Scenedesmus sp. and diatomaceous Diatoma sp.) and their KMnO4 oxidation products (aliphatic dicarboxylic acids; aliphatic normal C14–C24 monocarboxylic acids; benzoic acid and C18 isoprenoidal ketone) examined by gas chromatography and gas chromatographymass spectrometry. The results suggest that the lipid material could make a greater contribution to polymethylene chains in kerogen than the cell-wall material, when the kerogens are mainly derived from algal components. 相似文献
13.
13C NMR spectra of solid humic substances in Holocene sediments have been obtained using cross polarization with magic-angle sample spinning techniques. The results demonstrate that this technique holds great promise for structural characterizations of complex macromolecular substances such as humin and humic acids. Quantifiable distinctions can be made between structural features of aquatic and terrestrial humic substances. The aliphatic carbons of the humic substances are dominant components suggestive of input from lipid-like materials. An interesting resemblance is also noted between terrestrial humic acid and humin spectra. 相似文献
14.
A Recent unconsolidated lacustrine sediment was separated into five size fractions which were characterized by microscopy and XRD. Higher plant detritus predominated in the coarsest fractions, but decreased in proportion in the intermediate ones in which algal debris was predominant. The overall proportion of organic debris decreased from coarse to fine fractions. Quartz and feldspar were most abundant in the intermediate size fractions, while the coarsest fraction contained chlorite with some kaolinite and quartz, and the finest fraction was composed mainly of illite.Concentrations (per fraction dry weight) of solvent extract, total long chain hydrocarbons and total fatty acids decreased from coarse to fine fractions, with the exception of the finest fraction, in which they showed an increase. The change in relative abundance of higher plant to algal debris in the coarse and intermediate size fractions was paralleled by a decrease in the relative abundance of n-C29 and n-C31 alkanes and an increase in the relative abundance of -C17 alkane. There was a progressive decrease in n-alkane carbon preference index, and a progressive increase in the unresolved envelope of hydrocarbons with decreasing grain size. The unresolved hydrocarbons, which probably originate from biodegraded lubricating oil, appear to be associated with the clay fraction. Fractionation in this way and lipid analysis of the separate fractions may give more definitive information about input sources than analysis of unfractionated sediments. 相似文献
15.
The Neoproterozoic–Early Cambrian time is the cradle of multicellular, eukaryotic life and thereafter metazoan life started populating the planet. Biomarkers, which record the chronicles of biotic events on Earth, have been investigated using gas chromatography–mass spectrometry from a suite of nine oil samples extracted from oil stained sediments and a crude oil of Neoproterozoic–Early Cambrian age from Bikaner-Nagaur Basin, western India. The biomarker distribution is characterized by high concentration of both tricyclic terpanes and pentacyclic hopanes. The predominance of C29 regular sterane over C27 and C28 steranes indicates green algal contribution and may imply the prominence and diversification of the same algal group. The low pristane/phytane ratio and occurrence of substantial quantity of gammacerane, monomethyl 2-methyl-2-(2,4,8-trimethyltridecyl) chroman and 1,1,7,8-tetramethyl-1,2,3,4 tetrahydrophenanthrene are indicative of stratified water column as well as anoxic and enhanced salinity condition of the environment of deposition. This paper reports for the first time the presence of methyltrimethyltridecyl chromans (MTTCs) from Neoproterozoic–Early Cambrian time. Putative C19 norsteranes, probably indicative of sponge input, are recorded in the studied samples and also reported from other infracambrian oils and sediments. Normal alkanes (n-C17 and n-C18) and isoprenoids (pristane and phytane) are highly depleted in δ13C. The overall biomarker distribution and carbon isotope data of oils from Bikaner-Nagaur Basin show similarities with those of other infracambrian oils like Huqf oils from Oman and Baykit High oils from eastern Siberia. 相似文献
16.
Kevin M. Theissen David A. Zinniker Robert B. Dunbar 《Geochimica et cosmochimica acta》2005,69(3):623-636
Our analysis of lipid molecular fossils from a Lake Titicaca (16° S, 69° W) sediment core reveals distinct changes in the ecology of the lake over an ∼25,000-yr period spanning latest Pleistocene to late Holocene time. Previous investigations have shown that over this time period Lake Titicaca was subject to large changes in lake level in response to regional climatic variability. Our results indicate that lake algal populations were greatly affected by the changing physical and chemical conditions in Lake Titicaca. Hydrocarbons are characterized by a combination of odd-numbered, mid- to long-chain (C21-C31) normal alkanes and alkenes. During periods when lake level was higher (latest Pleistocene, early Holocene, and late Holocene), the C21n-alkane, and the C25 and C27 alkenes dominate hydrocarbon distributions and indicate contribution from an algal source, potentially the freshwater alga Botryococcus braunii. The C30 4 α-methyl sterol (dinosterol) increases sharply during the mid-Holocene, suggesting a greatly increased dinoflagellate presence at that time. Long-chain alkenones (LCAs) become significant during the early Holocene and are highly abundant in mid-Holocene samples. There are relatively few published records of LCA detection in lake sediments but their occurrence is geographically widespread (Antarctica, Asia, Europe, North America). Lake Titicaca represents the first South American lake and the first low-latitude lake in which LCAs have been reported. LCA abundance and distribution may be related to the temperature-dependent response of an unidentified algal precursor. Although the LCA unsaturation indices cannot be used to determine absolute Lake Titicaca temperatures, we suspect that the published LCA U37K unsaturation calibrations can be applied to infer relative temperatures for early to mid-Holocene time when LCA concentrations are high. Using these criteria, the U37K unsaturation indices suggest relatively warmer temperatures in the mid-Holocene. In contrast to previous speculation, lipid analysis provides little evidence of a greatly increased presence of aquatic plants during the mid-Holocene. Instead, it appears that a few algal species were dominant in the lake. Based on the dramatic rise in abundances of LCAs and dinosterol during the early to mid-Holocene, we suspect that the algal producers of these compounds rose in response to a combination of physical and chemical changes in the lake. These include temperature, salinity, and alkalinity changes that occurred as lake level dropped sharply during a multi-millennial drought affecting the Central Andean Altiplano. 相似文献
17.
Three Australian brown coals have been separated into humin and humic acid fractions and studied by high resolution solid state 13C NMR spectroscopy and Fourier transform IR spectroscopy. The aromatic rings of the humic acids are highly substituted showing that extensive cross linking must have occurred during formation from wood lignin and tannin. However, the humins contain more aliphatic carbon and hydrogen than the corresponding humic acids. This shows that little cross linking has occurred with other components of the brown coal such as resins, waxes cutin and algal detritus, and cross linking has not rendered the aromatics alkali insoluble. The kinetics of extraction are complex and not simple first order. This is reflected in the chemical composition of the humic acid which is extraction temperature dependent. We also observed that there is a conversion of aromatic carbon to aliphatic carbon and gas during extraction, probably by alkaline oxidation, resulting in ring opening. A range of suitable model compounds have been studied to confirm this finding. Such a mechanism may account for the modification of lignin in oxidising environments such as those occurring in the initial stages of coalification (lignite or brown coal formation) and in soils. 相似文献
18.
Thermodynamic modeling of equilibria in the system water–rock–organic acids was used to study the influence of organic acids on Ca and Mg redistribution between a solution and a solid phase in connection with the use of calcites of variable composition CaxMg1–xCO3 as indicators of paleoclimatic environments. In the thermodynamic model, high-molecular humic substances (fulvic + humic acids) were represented by a set of independent metal-binding centers. Therefore, their number was preset based on the given density of proton- or metal-binding sites. The numerical implementation of several geochemical situations involving the dissolution/deposition of calcites with different Mg contents showed that the main effect of fulvic and humic acids is the acidification of solutions and the reduction of carbonate stability. Although humic substances can play an important role in fixing Ca and Mg and removing them from solution, their actual concentrations in natural media (<<1 g/L) do not cause significant changes in the composition of CaxMg1–xCO3 phases. On the other hand, there is quantitative evidence that variations in the Mg/Ca ratio in a solution and a solid phase are significantly influenced by the evaporative concentration of Mg-oversaturated solutions, alkalization/acidification during their evolution, or CO2 content variations owing to changes in climate and lake activity. 相似文献
19.
Organic-rich mudstones with up to 10 wt% TOC from the upper portion of the Belle Fourche Formation and the lower part of the Second White Specks Formation in the Western Canada Sedimentary Basin were evaluated as source rocks. Both geochemistry and organic petrography indicate an open marine paleoenvironment with deposition of Type II kerogen based on the predominance of marine alginite and amorphous organic matter (OM), limited amounts of terrigenous vitrinite and inertinite macerals, the presence of marine fossils, and the low ratio of TOC to total sulfur (∼1.26). The prevalence of short-chain n-alkanes (n-C13 to n-C19), a predominance of C28 αββ(H)-20S steranes, and small concentrations of oleanane confirm the dominantly algal and planktonic origin of OM. Alternating oxic to anoxic paleoenvironmental sedimentary conditions are proposed based on common bioturbation, abundant inoceramid prisms, and good organic richness. Biomarker distributions are consistent with intermittent anoxia, without unequivocal evidence for water column stratification or hypersalinity. The thermal maturity measured in seven sediment cores by different methods consistently indicates a westward increase in maturity according to vitrinite reflectance, Tmax, and hopane and sterane biomarkers. Two cores are thermally immature (∼0.42 %Ro), one is early mature (∼0.65 %Ro), and four cores are within the oil window (∼0.78 to 0.89 %Ro). All thermally mature cores retain good to very good hydrocarbon potential (248 mg HC/g rock) and are dominantly oil-prone and minor gas-prone based on their maceral compositions. The upper Belle Fourche and lower Second White Specks Formations represent potential targets for unconventional light shale oil production. 相似文献
20.
Peptides were released from organic matter fractions of three Italian soils (humin, humic and fulvic acids), when the samples were hydrolyzed in Ba(ON)2-saturated solution at 105°C for 2 hr. The peptides obtained were separated using electrophoresis and paper chromatography. The presence of polypeptides in the soil organic matter was indicated by: (1) their hydrolysis by pronase; (2) the amino acids released by 6 N HCl hydrolysis; (3) The comparison of i.r. spectra of humic fractions before and after hydrolysis with 6 N HCl.Attempts at isolating the native proteinaceous compounds using electrophoresis in polyacrylamide gel failed; additionally, our attempts to hydrolyze proteinaceous components enzymatically in unfractionated soil organic matter, as well as in its fractions, before and after methylation, with pepsin, papain and pronase, were unsuccessful. Pronase demonstrated a weak proteolytic activity only at very low substrate-enzyme ratios (20 : 1) in humic and fulvic fractions and in whole phyrophosphate extract. Deproteinated substrates treated with pronase also released free amino acids, suggesting autodigestion.In humin, humic and fulvic fractions we found a total amino acid content of 40–45%, 12–24% or 1–85, respectively. Amino acid recovery from single fractions was about 70–80% of the total content in the unfractionated soil. 相似文献