Organic Geochemistry最新文献

Hydroxylated Isoprenoidal Glycerol Dialkyl Glycerol Tetraethers (OH-isoGDGTs) have recently been utilized as paleothermometers in the marine environment. However, their ability to reconstruct temperature in the Mediterranean and Red Sea has not been adequately investigated. Previous research has shown that archaeal communities inhabiting different water depths in these basins exert a substantial influence on the regular isoGDGT distributions and associated proxies such as the ${TEX}_{86}$. However, the impact of these archaea on OH-isoGDGTs and their corresponding proxies remains unclear. In this study, we examined the distribution of OH-isoGDGTs and their associated proxies ($\%OH$, RI-OH, RI-OH′ and ${TEX}_{86}^{OH}$) in surface sediments of the Mediterranean and the Red Sea. We observe strong correlations between the fractional abundances of OH-isoGDGTs, relative to all isoGDGTs and OH-isoGDGTs, and water depth which suggests that deep-water archaeal communities have a lower OH-isoGDGT abundance compared to the shallow communities. As a result, $\%OH$ and ${TEX}_{86}^{OH}$ are strongly correlated with water depth, particularly at depths <500 m in the Mediterranean Sea. Interestingly, RI-OH and RI-OH′ show no correlation with water depth in the Mediterranean Sea. Instead, they correlate more strongly with satellite-derived sea surface temperature compared to other isoGDGT-based proxies, indicating their potential as paleothermometers. Finally, unlike ${TEX}_{86}$ and ${TEX}_{86}^{OH}$, the $\%OH$, RI-OH and RI-OH′ do not exhibit distinct 'Red Sea cluster' and display comparable values to sediments from other tropical oceans. Further research on sedimentary OH-isoGDGT distributions with broader geographical coverage within these basins and enrichment cultures of deep-water archaea are needed to confirm these observations.

The Mesoproterozoic Jixian System in the Yanliao Rift of the North China Craton contains organic-rich shales within the Hongshuizhuang, Tieling, and Xiamaling formations. The biomarker composition of these formations is characterized by the dominance of 13α(n-alkyl)-tricyclic terpanes. The distinctive structure and main occurrence of 13α(n-alkyl)-tricyclic terpanes in the Mesoproterozoic Jixian System in the Yanliao Rift have intrigued researchers, raising questions about their origin and formation mechanisms. This study found that δ¹³C values of 13α(n-alkyl)-tricyclic terpanes ranged from −32.7‰ to −26.2‰, similar to those of kerogen and n-alkanes, but significantly more depleted in ¹³C compared to 13β(H),14α(H)-tricyclic terpanes, which ranged from −28.5‰ to −25.7‰. The difference in δ¹³C values suggested that 13α(n-alkyl)-tricyclic terpanes were not formed by the same mechanism or precursors as 13β(H),14α(H)-tricyclic terpanes. No sterane biomarkers indicative of eukaryotic source input were detected. Instead, the comparable concentrations of 13α(n-alkyl)-tricyclic terpanes and n-alkanes in the samples implied a possible origin from primary organic matter producers, notably cyanobacteria. Moreover, the biomarkers obtained from distinct formations within the Mesoproterozoic Jixian System exhibited an impressive degree of simplicity, similarity and mutual correlation. These findings collectively suggest the prevalence of a cyanobacteria-dominated primitive ecosystem during the Mesoproterozoic Era within the confines of the Yanliao Rift.

Mangrove sediments are valuable archives of relative sea-level change if they can be distinguished in the stratigraphic record from other organic-rich depositional environments (e.g., freshwater swamps). Proxies for establishing environment of deposition can be poorly preserved (e.g., foraminifera) in mangrove sediment. Consequently, differentiating mangrove and freshwater sediment in the stratigraphic record is often subjective. We explore if biomarkers can objectively identify mangrove sediment with emphasis on their utility for reconstructing relative sea level. Our approach is specific to identifying in situ sediment, which has received less attention than identifying allochthonous mangrove organic matter. To characterize mangrove and non-mangrove (freshwater) environments, we measured n-alkane, sterol, and triterpenoid abundances in surface sediments at three sites in the Federated States of Micronesia. Elevated taraxerol abundance is diagnostic of sediment accumulating in mangroves and taraxerol is particularly abundant beneath monospecific stands of Rhizophora spp. Taraxerol was undetectable in freshwater sediment. Other triterpenoids are more abundant in mangrove sediment than in freshwater sediment. Using cores from Micronesian mangroves, we examine if biomarkers in sediments are indicative of in situ deposition in a mangrove, and have utility as a relative sea-level proxy. Taraxerol concentrations in cores are comparable to surface mangrove sediments, which indicates deposition in a mangrove. This interpretation is supported by pollen assemblages. Downcore taraxerol variability may reflect changing inputs from Rhizophora spp. rather than diagenesis. We propose that taraxerol is a proxy that differentiates between organic sediment that accumulated in mangrove vs. freshwater environments, lending it utility for reconstructing relative sea level.

Dissolved black carbon (DBC) is the condensed aromatic fraction of dissolved organic matter produced during the thermal alteration of organic material (e.g., fire). DBC concentrations are often determined using the benzenepolycarboxylic acid (BPCA) method where condensed aromatic structures are oxidized into BPCA molecular markers for quantification. However, BPCA molecules have been recently identified in fire-affected surface waters and leachates of heated soils and wildfire ash. If they survive the sample preparation and analytical procedures, the presence of these “free” BPCAs in water may result in an overestimation of DBC concentrations in aqueous samples. To assess the potential impact of free BPCAs on DBC quantification, we spiked ultrapure water, salt water, and organic matter solutions with BPCA standards and treated them as environmental samples being analyzed for DBC. Each BPCA standard was recovered in detectable amounts, with the most-substituted BPCAs having lower percent recoveries than less-substituted BPCAs. Spiked organic matter solutions had significantly higher calculated DBC concentrations than their unamended counterparts only when the conversion factor used included less substituted BPCAs. Overall, our results show that DBC quantification could be impacted by free BPCAs in aqueous samples, but the degree of impact is largely dependent upon the properties of the individual BPCA molecular marker.

Roots can add significant amounts of carbon (C) to the subsoil, which improves soil physical properties and can mitigate climate change. About 5% of croplands in Germany have been deep-ploughed (30–120 cm) at least once. This can provide better root access to the subsoil and may increase yields, but little is known on the fate of root-derived C in the subsoil (at depth greater than 30 cm) after deep-ploughing. We hypothesized that five decades after deep-ploughing, root-derived C stocks were higher than conventionally ploughed treatments due to better root development. We analysed suberin and cutin monomers as tracers for root- and shoot-derived C at three former deep-ploughed sites in Northern Germany with different soil textures and deep-ploughing depths. Concentrations of suberin monomers in the soil positively correlated with root biomass; this was most pronounced at one sandy site, but had higher variability at the other two sites due to crops with different root systems in the crop rotation, lower root development, and more favourable conditions for C decomposition. Suberin contributed more to the bulk soil organic carbon (SOC) stocks than cutin throughout the soil profile at all sites. The contribution of suberin monomers to the bulk SOC stock at silty site Banteln and the sandy site Essemühle was 38% higher in the deep-ploughed plots than at the reference plot, respectively, these differences were most visible in the subsoil of Essemühle. We conclude that as C stocks and root development increase, suberin SOC stocks also increase, especially in sandy subsoils with low pH.

Understanding the generation of secondary microbial methane (SMM) is important for the evaluation of natural gas resources and instructive for the stimulation of methane production. Coal seams are popular targets for extracting in situ preserved methane and studying microbe-stimulated methane yield. However, few studies have been done on overmature coals. Here we collected gas samples from coals varying from bituminous to meta-anthracite in the Qinshui Basin, North China, and analysed the molecular and stable isotopic compositions to systematically evaluate the influence of biodegradation in high-rank coals in geological settings. The stable isotope signatures (δ¹³C and δD) of methane are dominated by the thermal decomposition of organic matter in deep coals but inconsistent with the maturity rank of shallow burial coals. The decoupling of coal maturity with C₁/C₂₊ ratios and δ¹³C-CH₄ values, and positive δ¹³C-CO₂ values (−9.2 to +24.4 ‰) suggest biodegradation of light wet gases (C₂₊ components) and CO₂ reduction. Negative trends between δ¹³C-CO₂ and CH₄/CO₂ in shallow burial coal seams reveal the mutual conversion of CH₄ and CO₂ and carbon isotope exchange, driven by microorganisms. The calculated isotopic temperatures (33–328 °C) based on the carbon isotope fractionation factors between CH₄ and CO₂ (1.024–1.069) demonstrate that carbon isotope exchange is prevalent in high-rank coals. It also reveals that the burial depth is an imperative factor in controlling microbial environments and thus the biodegradation process. This study implicates the potential of high-rank coals as the target for microbial-enhanced methane recovery and also implies that microorganisms are widely involved in reservoir carbon cycling.

Currently, certain unconventional gas is mainly extracted from high-maturity shale reservoirs, but limited attention has been given to the evolution of kerogen structure subsequent to the overmature, gas generative stage, corresponding to metagenesis and is termed the carbonization stage. In this study, we performed heating treatment on overmature kerogen samples to obtain samples in the carbonization stage and then conducted Raman, thermogravimetric-mass spectrometric analysis (TG-MS), X ray diffraction (XRD), and high resolution transmission electron microscope (HRTEM) experiments on these samples to investigate the changes in kerogen structure. Low pressure N₂ and CO₂ adsorption experiments also were performed to investigate the changes in pore structure. The results show that as the heat treatment temperature is raised to 1000 °C, the I_D/I_G ratio (D band intensity to G band intensity) experiences an increase, reaching 1.12. Additionally, the full width at half maximum (FWHM) of 002 peak consistently remains above 3.8°. This suggests that these samples were matured to an early meta-anthracite (meta-kerogen) stage, which is significantly distant from the graphite stage. During this particular stage, the most obvious changes in molecular structure are the enlargement of aromatic clusters and the decrease in hydrogen atoms, and thus, H₂ rather than methane is produced, as revealed by the results of TC-MS experiment. Following an initial increase, the pore volume and surface area of kerogen samples decrease gradually, reaching their maximum values at 700 °C. Kerogen subjected to a heating temperature of 1000 °C exhibits a greater pore volume in comparison to the initial overmature kerogen. Thus, this observation provides evidence that shale kerogen in carbonization stage, which is typically lying at significant depths, holds promise as a viable reservoir for shale gas extraction.

Investigating the mechanisms that determine the bulk and position-specific carbon isotopic distributions of propane in natural gas will help elucidate its formation and evolution. We used octadecane and squalane as model compounds that give simple precursors for gas production in gold-tube isothermal pyrolysis experiments to study the variations in intermolecular and intramolecular distributions of isotopes in the generated gaseous hydrocarbons. The δ¹³C values of the products and inverses of their carbon numbers (1/n where n = C₁–C₅) showed a negative linear relationship versus maturity, indicating that they formed by homolytic cleavage of C–C bonds and that the precursors showed homogeneous distributions of carbon isotopes. However, the significant difference in the kinetic isotopic effects (KIEs) of the gas generated by cracking of two model compounds under the same experimental conditions is not readily explained by single homolytic bond cleavage. The results indicate that besides the KIE of C–C bond cleavage, the bulk and position-specific carbon isotopic compositions of propane are related to the chemical and isotopic structures of the precursors and the propane transformation ratio. Based on the sites of C–C bond cleavage, two isotopic fractionation patterns (i.e., normal propyl and isopropyl models) may explain the bulk and position-specific carbon isotopic distributions of the generated propane. According to the propyl model, propane originates from (normal) propyl structures (CH₃CH₂CH₂*) in the precursor via C–C bond cleavage at a terminal site of a propyl group, while the isopropyl model involves propane derived from isopropyl structures (CH₃CH*CH₃) in the precursor, with C–C bond cleavage at the central site. Simulations show that these distributions for propane cracked from octadecane closely follow the propyl model, whereas propane generated from squalane showed mixed contributions from both models, and its bulk and position-specific carbon isotopic distributions depend on the proportions of propyl and isopropyl structures in the precursors. Variations of propane’s position-specific carbon isotopic distributions during the main stage of propane generation indicate that free radical reactions are the main pathway for thermogenic propane formation, resulting in similar KIEs for propane terminal and central carbons. Therefore, the position-specific carbon isotopic distribution of propane can provide evidence of its formation mechanism and shows potential for revealing the origin and evolution of natural gas.

Identification and quantification of allochthonous and autochthonous organic carbon (OC) in large and shallow lakes are crucial for knowledge of OC burial and recycling behaviors, as well as sustainable management practices. However, quantitative methods to estimate the contributions from various OC sources are still limited. In this study, we analyzed distribution patterns and δ¹³C signatures of mid- and long-chain n-alkanes from aquatic plants, riverine sediment, and surface sediments from a typically large and shallow Lake Wuliangsu in the Hetao Irrigation District, China. The results indicate that n-alkanes among submerged macrophytes, emergent plants, and riverine sediment show unique distribution patterns and δ¹³C signatures, supporting the practicability of identification and quantification of OC sources by end-member mixing models of n-alkanes’ distribution patterns and ¹³C values. Our results also suggest that introducing δ¹³C values into the end-member mixing models could effectively reduce the uncertainty, as n-alkane distribution patterns might be modified by different species, habitat environments, and potential OC degradation. In the case of Lake Wuliangsu, the model results demonstrate that the riverine sourced OC from the main channel has decreased during southward transport, indicating that Lake Wuliangsu serves as an important trap and sink for OC transported from the upper reaches of the Yellow River. The model results also show a predominant contribution of the autochthonous OC to sediments in Lake Wuliangsu, with open-water areas dominated by submerged macrophyte-sourced OC and the other areas by emergent plant-sourced OC. Higher productivities of submerged macrophytes are mainly modulated by increases in water depth, water transparency, and nutrient concentrations, while higher productivities of emergent plants are mainly associated with increasing nutrient concentrations. To improve the ecological conditions and OC burial of Lake Wuliangsu, it is necessary to manage the quality of the inflowing water from the agricultural farmland, as well as the preservation of the water body from current paludification.