Organic Geochemistry最新文献

The coastal zones in Antarctica play an important role in the polar carbon cycle through the efficiency of the biological pump. In this study, Prydz Bay in East Antarctica was selected to investigate factors controlling organic matter composition in sediments using source-dependent biomarkers. The results show that fatty acids are the most abundant biomarkers in sediments, followed by sterols, fatty alcohols, and alkanes. Although microalgae are well known to be the main source of sedimentary organic matter in Prydz Bay, the distribution of low molecular weight branched alkanes and alkenes with an even–odd predominance suggest the importance of bacteria during organic matter transformation, and its contribution to sedimentary organic matter. High concentrations of highly branched isoprenoid alkenes (HBIs) with a distinctive enriched ¹³C signature indicate substantial inputs of ice algae to sediments. Principal component analysis of source-dependent biomarkers reveals that the spatial heterogeneity of organic composition in sediments of Prydz Bay is mainly controlled by the distribution of sea surface phytoplankton community in different geographical zones. Redundancy analysis demonstrates that seasonal sea-ice cover is the main driver for blooms of distinctive algae in each geographical zone as a result of spatial succession of the phytoplankton community. The result clearly exhibits that, at least in Prydz Bay, spatial succession of the sea surface phytoplankton community in austral summer can be recorded in sediments, suggesting that a high-resolution sediment record of source-dependent biomarkers can be used to reconstruct the evolution of sea surface phytoplankton community structure during geological history in Antarctic coastal zones.

The application of routine geochemical analyses for identification of dry natural gas sources and post generation processes poses a challenge as its hydrocarbon composition is typically dominated by CH₄ (∼≥99 %) while the presence of non-hydrocarbons may be limited. In particular, the possibility of dry gas interaction with H₂S at reservoir conditions (low temperatures, high pressures, millions of years residence time) is not established as previous studies of this reaction focused on conditions typical for industrial reactors (high temperature, ambient pressure, residence time of seconds). To address these issues, we studied the molecular and isotopic (δ³⁴S) composition of volatile organic sulfur compounds (VOSC) formed during pyrolysis experiments between CH₄ and H₂S at 360 °C (4–96 h), reaching %Ro equivalent of 0.80–1.25 which represents thermally mature oil and gas reservoirs. The results demonstrated that methanethiol (MeSH) is the main product of the reaction, while its δ³⁴S value suggest equilibrium isotopic effect with its parent H₂S. Subsequent analysis of the molecular and isotopic (δ¹³C, δ²H, δ³⁴S) composition of thermogenic, H₂S containing, dry natural gases from the Jiannan gas field in China revealed the presence of short thiols and sulfides dominated by MeSH. The δ³⁴S of the VOSC identified suggests they all formed by gas-phase reaction of alkanes (mainly CH₄) with the associated H₂S.

This study demonstrates the applicability of VOSC as a proxy for identification of interaction between H₂S and dry gas and identification of H₂S sources within a gas reservoir or a basin.

Deep-sea hypersaline anoxic basins (DHABs) are extreme environments harbouring unique microbial assemblages. They have, however, rarely been identified in the geological record. Here, we investigate the potential use of organic biomarkers to detect former DHAB presence in the eastern Mediterranean during the Early Pliocene. Our findings suggest challenges in identifying DHABs in the geological record using biomarker records; this is due to pelagic organic matter overprinting the DHAB signals.

Steroid acids are unique molecular fossils composed of a steroid carbon skeleton linked to a carboxyl group. These biomarkers contain special geological and biological clues in their structures. However, the origin of steroid acids remains a subject of debate, and there is a lack of direct evidence in previous studies. Here, we present experimental evidence for the origin of steroid acids. The origin of steroid acids during biodegradation in crude oil was observed for the first time. Molecular structures of these steroid acids reveal that they arose from steranes via bacterial oxidation of sidechain terminal methyl groups. Our results suggest that microbial activities indeed contribute to the formation of carboxyl groups on the eukaryotic steroid skeletons, and we believe steroid acids are a promising biomarker for both geomicrobial processes and ancient eukaryotic life in the rock record.

High-resolution mass spectrometry can be utilized to select specific proxies for the quantitative assessment of crude oil biodegradation degree, offering higher accuracy and convenience compared to conventional GC-MS methods. However, the current evaluation proxies are invalid for severely biodegraded crude oil. In this study, freshwater and saltwater lacustrine crude oils from the Liaohe Western Depression (Bohai Bay Basin) with varying degrees of degradation, were characterized using negative ion electrospray ionization [ESI (−)] Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results show that seven heteroatom classes were identified including N₁, N₁O₁, N₁O₂, O₁, O₂, O₃ and O₄. Certain differences exist in the abundance of heteroatom compounds in the nondegraded crude oils from the two origins, but both are dominated by N₁. The relative abundance of O₂ class species significantly increases, while the relative abundance of O₁ and N₁ class species decreases with an increase in the degree of biodegradation, reflecting the increase in the content of acid compounds as biodegradation products. O₂ class species become the predominant compound in the severe degradation stage. Biodegradation results in the enrichment of compounds with greater condensation, while the abundance of highly alkyl-substituted compounds decreases. The nitrogen-containing compound pairing proxies (DBE_12,13,15/DBE_9∼11-N₁) can be employed to assess the degree of biodegradation in crude oil under conditions of similar maturity. As the degree of biodegradation increases, the content of 2 ∼ 5-cyclic naphthenic acids increase, while the content of acyclic acids with weaker resistance to degradation decreases. The ratio of acyclic acids to 2 ∼ 5-cyclic naphthenic acids (Modified A/C Ratio 2) can effectively assess the biodegradation level of crude oils ranging from nondegraded to severe degradation. The O₂/(N₁ + O₁) Ratio reflects the formation of acids during the biodegradation process and exhibits a robust positive correlation with crude oil density and degradation degree. The new proxies provide higher precision and broader applicability compared to conventional methods, enabling quantitative evaluation of biodegradation levels. The application of ESI FT-ICR MS technology holds significant importance in the assessment of heavy oil and the exploration of its genetic mechanisms.

Abnormal stable carbon isotopic (δ¹³C) compositions, deviating from the conventional order of δ¹³C₁ < δ¹³C₂ < δ¹³C₃, are frequently observed in natural gas reservoirs. For thermogenic gas, these anomalies, such as δ¹³C₁ < δ¹³C₃ < δ¹³C₂ or δ¹³C₁ > δ¹³C₂ > δ¹³C₃, have multiple formation mechanisms including gas mixing in conventional systems and desorption processes of gaseous hydrocarbons in unconventional shale gas systems due to their inconsistency with Rayleigh fractionation processes. Considering distinct reaction pathways (e.g., C₁ polymerized to C₂), these aberrant δ¹³C signatures are often construed as intrinsic hallmarks of extensively evolved natural gas. However, on the basis of gas generation simulation, not all findings exhibit abnormal δ¹³C values, hinting at multifaceted and intricate mechanisms governing the isotopic fractionation of alkane gas components. This study conducted gold tube pyrolysis of an Australian torbanite, revealing four distinct types of δ¹³C anomalies in hydrocarbon classes. Polycyclic aromatic hydrocarbons (PAHs) exhibited δ¹³C values more negative than co-occurring n-alkanes. δ¹³C₃ displayed a negative trend shift from EasyRo = 3.5 %, resulting in a partially δ¹³C-reversed gas (δ¹³C₁ < δ¹³C₃ < δ₁₃C₂) formed at EasyRo ≈ 4.1 %. Moreover, intramolecular δ¹³C₃ (both terminal and central carbons, termed δ¹³Ca and δ¹³Cb, respectively) reversed alongside the overall δ¹³C₃ trend. Additionally, the evolution of site preference in δ¹³C₃ (termed ‰SP = δ¹³Ca – δ¹³Cb) transitioned from progressively negative to positive. The results of this study demonstrate that the potential conversion between saturated hydrocarbons, aromatic hydrocarbons, and alkane gases is at least partially responsible for δ¹³C anomalies, but also that gaseous hydrocarbons formed from other fractions at high maturity cannot be ruled out, such as kerogen and pyrobitumen.

The lacustrine Lucaogou Formation was deposited during the middle Permian in the Junggar Basin, northwestern China. The depositional environment was an alkaline, brackish to saline lake. The biological diversity of the Lucaogou Formation was reconstructed using biologically informative molecules such as n-alkanes, steranes, and hopanes, and the δ¹³C composition of organic matter. C₃₀ 4α-methyl-24-ethylsterane, C₃₁ lanostane, and abundant C₃₀ − C₃₁ 3β-methylhopanes were detected in the organic matter, suggesting the presence of type Ⅰ aerobic methanotrophs including the Methylococcaceae. C₃₀ − C₃₁ 2α-methylhopanes and 7- and 8-methylheptadecanes are suggestive of the presence of cyanobacteria. Cyanobacteria and green algae are proposed to have made the dominant contribution to the organic matter. The presence of more ¹³C-enriched hopanoids in the lower unit compared to the upper unit of the Lucaogou Formation suggest a greater contribution of cyanobacteria to the organic matter in the lower unit. The high abundances of C₂₈ and C₂₉ steranes relative to C₂₇ steranes are likely due to green algae from the Chlorophyta, including the class Chlorophyceae. Dinosteranes that are diagnostic for dinoflagellates were not detected. The low concentrations of C₄₀ isorenieratane, renieratane, β-isorenieratane, renierapurpurane, and β-renierapurpurane that are indicative of photosynthetic green and purple sulfur bacteria indicate a minor contribution from Chlorobiaceae and Chromatiaceae to the organic matter. It is inferred that low to moderate amounts of ciliates grew at or below the relatively unstable chemocline in the lake. The presence of C₁₉-norisopimarane, 8β(H)-labdane, and 4β(H)-19-norisopimarane suggests the presence of coniferous plants growing around the lake, probably including Pinaceae, Araucariaceae, Cupressaceae.

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs), a relatively understudied group of bacterial membrane lipids structurally similar to branched glycerol dialkyl glycerol tetraethers (brGDGTs), appear to be strongly influenced by temperature in terrestrial settings. In surficial bottom sediments of East African lakes, the abundance of brGMGTs relative to the sum of brGMGTs and brGDGTs (%brGMGT) and brGMGT distribution are strongly related to local mean annual air temperature (MAAT), stimulating development of new paleothermometers. However, applications of these methods to lake-sediment records are currently lacking. Here we investigate brGMGT concentrations and distributions in 916 samples throughout the 250,000-year (250-kyr) sediment sequence from Lake Chala, a presently fresh and permanently stratified (meromictic) tropical crater lake. All seven previously identified brGMGTs occur abundantly, reflected in a relatively high average %brGMGT of 19%. BrGMGTs and brGDGTs concentrations throughout the sequence are strongly correlated (R = 0.83, p $<$ 0.001), suggesting that their producers and/or associated ecological niches substantially overlap. Clear distinction can be made between brGMGTs produced predominantly in the bottom sediments (H1034a and H1034c) versus the anoxic lower water column (H1020a-c and H1034b). Although a 17-month monitoring study of Lake Chala suggested brGMGTs are primarily produced in the sediments, down-core data assign greater importance to aquatic production than previously estimated. Instead of reflecting temperature, %brGMGT variations showed greatest similarity to GDGT proxies reflecting lake depth and/or mixing regime. BrGMGT-based temperature models produce ambiguous reconstructions, showing little similarity to known global temperature trends or the brGDGT-based mean summer temperature (MST) reconstruction from the same sediments.