Organic Geochemistry最新文献

Branched glycerol dialkyl glycerol tetraether lipids (brGDGTs) are increasingly used for terrestrial paleotemperature reconstruction. However, there can be significant offsets between the estimated temperatures based on brGDGT distributions in globally-distributed individual surface soils and the corresponding instrumental temperatures suggesting that additional environmental and/or biological controls could influence these distributions. We investigated the influences of seasonality and vegetation type on the brGDGT distributions by collecting surface soils beneath sub-alpine forest and grassland in September (warm month) and January (cold month), within an identical climatic background, in southern China. The absence of apparent seasonal changes in the soil brGDGT distributions between the warm and cold months indicates an annual or longer turnover time of soil brGDGTs at our study site. However, there are differences in the surface soil brGDGT distributions expressed as MBT'_5ME (the methylation index that is related to mean annual temperature, i.e., MAT) between forest and grassland. Specifically, the forest surface soil MBT'_5ME values were generally lower than those of grassland surface soils, which is consistent with the relatively higher summer grassland surface soil temperatures. This reveals a seasonal (summer) bias in the brGDGT distributions, and the dominant influence of temperature and secondary/indirect influence of vegetation type on brGDGT distributions. Finally, for brGDGT distributions in the 288 globally-distributed surface soils with known vegetation types, the root mean square error (RMSE) between calculated and measured MAT is slightly decreased when the vegetation types are taken into account, which further indicates a possible secondary/indirect influence of vegetation type on surface soil brGDGT distributions.

Exceptionally well-preserved fossil specimens in the Fossil Basin of the Green River Formation (GRF) have made it the subject of extensive paleontological study, but the organic molecular framework that evolved during a key paleoclimatic and fossil-bearing interval during the early Eocene is poorly understood. Whereas the organic geochemistry of the larger co-eval GRF basins has been extensively characterized, our molecular understanding of the fossil-bearing layers in the Fossil Basin and the drivers of the exceptional fossilization therein remain unresolved. To bridge this gap, sediments from the famous 18″-layer — the fossiliferous horizon that is extensively quarried for exceptional soft-tissue fossils — were sampled for organic and isotopic geochemical characterisation. The results show that the Fossil Basin sedimentary archive is geochemically distinct from other GRF basins, as exemplified by the absence of the classical biomarker β-carotane and minimal evidence for the large green algal blooms that predominate in the other GRF lake basins. Photic zone euxinia (PZE), anoxia, and a freshwater cap enabled development of a productive and diverse ecosystem. Salinity and density stratification prevented vertical mixing of the water column and supported preservation of decaying carcasses. In contrast to other GRF basins, the small areal extent and ellipsoid shape of the Fossil Basin focussed terrestrial and freshwater inputs into the lake, resulting in ideal conditions for preservation of an exceptional fossil record.

Marine sediments are one of the largest organic carbon (OC) sinks on Earth. Yet, major knowledge gaps remain in our understanding of sedimentary OC cycling, particularly regarding temperature-induced alteration processes of OC from different sources. Here, we investigate OC-rich sediments of Guaymas Basin (Gulf of California) across two hydrothermal areas, one with only conductive geothermal heating and the other additionally experiencing seepage of hydrocarbon-rich fluids from deeper layers. We use Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) to investigate diagenetic OC changes and show that cold control sites in both hydrothermal areas are dominated by similar contributions of lipid-derived compounds, nitrogenous (likely protein-derived) compounds, carbohydrates, and polyaromatic hydrocarbons (PAHs). These OC compound groups are largely derived from phytoplankton detritus from overlying water. Conductively heated sediments, which reach in situ temperatures of ∼ 80 °C, have similar general OC compositions and contents to these cold sites, but show evidence of diagenetic modifications of individual carbohydrate groups in deeper layers. By contrast, strong decreases in carbohydrate and nitrogenous compound abundances at the seep sites indicate that these compound groups are not only modified but also selectively degraded at the higher temperatures (>80 °C) of these sites. Increases in pyrolysis products of PAHs, prist-1-ene, and alkanes with depth, moreover, show that import of OC by deep hydrothermal fluids contributes significantly to sedimentary OC pools mainly in deeper layers of these sites. Our study provides the first comprehensive analysis of major OC compound groups in Guaymas Basin sediment and indicates that the supply of OC by hydrothermal fluid flow only has minor impacts on particulate organic matter compositions at the seafloor, even at active seep sites. We furthermore show that temperatures up to ∼ 80 °C already result in thermochemical modifications of organic matter (OM) that are potentially linked to the onset of kerogen formation. The sequence and time scales of chemical modifications and activations in relation to temperature are an important subject for future investigations.

Compound specific stable isotope analysis of amino acids (CSIA-AA) is a practice that allows for in-depth studies of many different physiological, ecological, and environmental phenomena. The vast information obtainable through CSIA-AA has driven the exponential growth of this field since its mainstream introduction in the early 1990s. This growth, however, has been accompanied by the development of several distinct analytical approaches. Throughout this review, we outline the full CSIA-AA process and identify areas of its workflow with the highest potential to introduce measurement error. Through a meta-analysis of CSIA-AA publications, we found that rather than experimental application, the primary determinant of methodology lies in the geographic location of the analyst, likely reflective of the academic lineages of CSIA-AA practitioners as opposed to application specific method development. The relative nascency of amino acid isotope analysis gives it incredible expansion potential, but such expansion would greatly benefit from comprehensive experimentation optimizing every portion of the analytical process. While such optimization will require expertise across many areas (i.e., chemistry, mass spectrometry, and data science), uniform guidelines can ensure the highest achievable accuracy and precision for intra- and interlaboratory analyses, alike. The goal of this review is to improve data comparability and adopt standardized methodologies to uniformly generate highly accurate, precise, and reproducible data. In doing so, we make recommendations for areas that would benefit from further investigation (e.g., procedure optimization, error mitigation, and data handling methods). The creation and implementation of guidelines for optimal approaches to CSIA-AA – as has been done for applications like forensic science – can help realize the full potential of this rapidly growing field.

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.