Organic Geochemistry最新文献

Glycerol dialkyl diethers (GDDs) are membrane lipids and a variation of the more commonly known glycerol dialkyl glycerol tetraethers (GDGTs). GDGTs include both archaeal and bacterial membrane lipids that are both frequently used for paleoclimate reconstruction in a range of terrestrial and aquatic environments. In contrast to GDGTs, GDDs lack one of the terminal glycerol moieties. Although both isoprenoidal (iso) and branched (br) GDDs have been found, this study focuses on isoGDDs. These lipids have been found in a few terrestrial and aquatic environments. However, the origin of isoGDDs is debated and the extent of their distribution across the surface of the Earth is poorly constrained. Based on a few single site studies, previous authors hypothesised that isoGDDs are degradation products of isoGDGTs, but more recent studies that isolated isoGDDs from cultured nitrososphaerota (formerly thaumarchaeota) proposed a biological source through direct archaeal biosynthesis. Here we compiled a global dataset of isoGDD and isoGDGT abundance in environmental samples to thoroughly investigate the distribution of isoGDDs and the correlation with isoGDGTs on a global scale and across a variety of environments (peat, mineral soils, lake sediments, and marine sediments). We find that isoGDDs are present in most samples that we analysed. Their abundance is strongly proportional to isoGDGT abundance (r² = 0.85), dominated by the GDGT-crenarchaeol/GDD-crenarchaeol ratio (r² = 0.94) and supported by individual compound isoGDGT/isoGDD ratios (r² = 0.56–0.94). In addition, the degree of cyclisation of isoGDDs, reflected in the ring index, is positively correlated (r² = 0.84) with that of isoGDGTs across all environments. We conclude that isoGDDs are abundant on the surface of the Earth and predominantly originate from the degradation of isoGDGTs.

In this study, the geochemical significance of the δ¹³C and δ²H values of selected C₇ light hydrocarbons (nC₇ = n-heptane; MCH = methylcyclohexane; Tol = toluene) is investigated in 58 light oil/condensate (LOC) samples from the Kuqa Depression of the Tarim Basin, NW China. For most of the LOCs in the western region, the nC₇, MCH, and Tol exhibit systematic differences where δ¹³C_nC7 < δ¹³C_MCH < δ¹³C_Tol and δ²H_Tol < δ²H_nC7 < δ²H_MCH. LOCs from the eastern area have higher δ¹³C_oil, δ¹³C_Tol, and δ¹³C_MCH values than those from the western area and their δ¹³C values for C₇ hydrocarbons are similar and δ²H_nC7 < δ²H_MCH < δ²H_Tol. These organic geochemical differences indicate that the LOCs from the western and eastern areas of the Kuqa Depression were mainly derived from lacustrine and coaly parent materials, respectively. The LOC reservoirs generally received a late hydrocarbon charge of the high-maturity natural gas, and the LOCs from the western Kela–Keshen areas subsequently experienced gas leakage. Hierarchical cluster analysis indicates that the most important influencing factor on the LOCs of the Kuqa Depression is the sedimentary facies (coaly and lacustrine), followed by gas leakage and then parent organic type (lacustrine sapropel and humic organic matter). A cross-plot of the Δδ²H_Tol–MCH versus Tol/MCH can distinguish the origins of continental facies parent materials and gas leakage of the LOCs.

The Lower Cretaceous Jiufotang Formation, situated in western Liaoning Province, China, has attracted attention for abundant Jehol Biota fossils and the potential to improve understanding of the relationships among climate change, evolution of terrigenous organisms, and organic matter (OM) enrichment in lake systems during the Early Cretaceous. Based on organic petrology and biomarker analyses, this study investigated the depositional environment, OM sources, and enrichment mechanism of Jiufotang mudstone in the Kazuo Basin. Maceral composition shows that the OM source of Jiufotang mudstone shifted from mainly aquatic to terrigenous, and biomarkers such as abundant diterpenoids and retene indicate that terrigenous higher plants were mainly conifers during deposition of the Jiufotang Formation. Pr/Ph ratios (0.07–0.65) show that the studied section was deposited in anoxic bottom water that may be necessary for OM enrichment in lake systems. Gravity flows were accompanied by input of many clastic minerals and oxygenation of bottom waters, which is unfavorable for OM enrichment. Therefore, the best OM enrichment model for lake systems includes anoxic bottom water, stable water column stratification, low sedimentation rates, and high phytoplankton contributions. The climate changed from relatively dry to humid, which drove changes in the OM enrichment model in the lake. More importantly, the climate and terrigenous plant information obtained by biomarkers is useful to reconstruct the ecological environment of the Jehol Biota.

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.