Organic Geochemistry最新文献

The dissolved organic matter (DOM) components in lake water have been widely studied; however, few previous studies have considered the growth of Phragmites australis in brackish lakes. It has not been well understood how salinity variations influence the DOM compositions in pore water and its bacterial mechanisms in lakes with Phragmites australis. This experiment included three salinity groups (1,200 mg/L, 3,600 mg/L, and 6,000 mg/L) to study the interactions between bacteria and DOM in pore water under a salinity gradient. The results showed that the maximum fluorescence intensity (F_max) of DOM measured by excitation-emission fluorescence spectroscopy decreased with increasing salinity. Higher salinity reduced the F_max of protein-like substances and resulted in DOM becoming more aromatic. Salinity affected DOM composition due to the responses of functional bacterial communities. Thiobacillus was salt-tolerant and dominated in the sediments, and its relative abundance was negatively correlated with the F_max of protein-like components. The relative abundance of Flavobacterium showed a positive correlation with salinity and a negative correlation with the F_max of the fulvic acid-like component. Pseudomonas, Brevundimonas, and Polaromonas were negatively correlated with salinity and the F_max of the fulvic acid-like component, while being positively correlated with the F_max of tyrosine-like and aromatic protein substances. Higher salinity inhibited the metabolism gene modules of tryptophan and tyrosine. The results of this study may offer a novel perspective on comprehending the biochemical cycling of fluorescent DOM, encompassing tryptophan-like, tyrosine-like, and fulvic acid-like components in brackish lakes with fluctuating salinity.

BrGDGTs, membrane-spanning lipids produced by bacteria, are at the basis of the MBT’_5ME, a biomarker ratio that has been used as a paleotemperature proxy. However, the response of the MBT’_5ME to temperature changes, particularly in freshwater systems, remains incompletely understood. In this study, oxic mesocosms are used to assess the temperature sensitivity of brGDGTs and their producers, sampled from a lake (Lake Rot) and a river (Sihl River) in three different seasons. Three temperature treatments are employed (10, 17.5, and 25 ℃), representing control (in-situ temperatures), cooling, and/or warming treatments, with GDGTs and the bacterial community measured at several timepoints (24 h, 1, 2, 3 and 5 weeks). The control experiments showed that this experimental approach could not replicate natural conditions exactly, with small changes in chemistry (pH, conductivity, alkalinity) and bacterial community composition. Still, our mesocosm setup yielded valuable insights into the temperature-dependent production of lacustrine brGDGTs and MBT’_5ME values, especially in warming treatments, while no response was observed in cooling treatments, potentially indicating limited sensitivity to cold temperatures. In the river mesocosms not the MBT’_5ME but the IR ratio showed a temperature dependency, potentially driven by small changes in the water pH. Coeval changes in the composition of the bacterial community and the MBT’_5ME and IR are determined to constrain potential GDGT producers. Although an increase in MBT’_5ME in response to some warming incubations is observed, the temperature-sensitivity of MBT’_5ME, as expected from GDGT studies on a global scale, is not supported by this experiment.

During the Late Paleozoic Ice Age (LPIA; particularly early Permian), organic material of the alga Tasmanites was extensively deposited in sediments of southern Gondwana, which formed the Tasmanite oil shales that have only been identified in Tasmania, Australia. It remains unclear whether other analogous geological records exist worldwide that formed at the same time, and the origin is enigmatic. This paper reports the first discovery of Tasmanite oil shales in northern Pangea, specifically in the Lucaogou Formation of the Junggar Basin, China, deposited during the Artinskian (ca. 290 Ma). The organic petrological features of Tasmanites are clearly visible, including thick-walled disks with tubes and radially arranged channels. These shales have markedly elevated C₂₈/C₂₉ steranes and tricyclic terpanes/hopanes ratios. Based on inorganic geochemical data, the salinity of the paleo-lake during deposition of the Tasmanite oil shale interval decreased abruptly from saline to brackish–freshwater. In addition, the pH changed from alkaline to nearly neutral, while the redox environment changed from anoxic to suboxic. Enhanced continental chemical weathering and the change in salinity of the paleo-lake occurred at the same time as global warming and large-scale glacier melting during the Artinskian, which promoted the habitat of the low-salinity tolerant Tasmanites. Our results provide new geological evidence for another occurrence of early Permian Tasmanite oil shales, indicating that Tasmanites flourishs and the associated oil shales may have been widely deposited at the end of the LPIA. The flourishment of Tasmanites archives biotic–environmental co-evolution.

To investigate the possibility that dissociation of subsurface methane hydrate (MH) in the eastern Nankai Trough, offshore of Japan, led to the formation of a giant colony of the bivalve Calyptogena (currently mostly dead), the carbon isotope ratios (δ¹³C) of archaeal lipids and methane were measured in near-surface core sediments at Daini-Tenryu Knoll. The irregular variation of porewater methane δ¹³C with depth (from −75 ‰ to −26 ‰) suggested that originally low δ¹³C microbial methane was degraded in different proportions by anaerobic methane oxidation. Consistent with this inference, biomarkers of anaerobic methanotrophic archaea (ANME), namely, crocetane (2,6,11,15-tetramethylhexadecane), PMI (2,6,10,15,19-pentamethylicosane), and diethers (archaeol and hydroxyarchaeols), were detected in lipid extracts. The low diether δ¹³C values (−121 ‰ to −104 ‰) were characteristic of ANME, but less variable than the methane δ¹³C values, and the relationships between diethers and methane δ¹³C values deviated from regression lines derived using worldwide data from modern methane seep sites. In contrast, δ¹³C values of the ANME source methane predicted from those regression lines and the diether δ¹³C values agreed well with methane δ¹³C values in MH samples obtained by nearby deep drilling. This result strongly suggests that most of the diethers were produced by ANME that proliferated during a past massive methane release event associated with MH dissociation. The crocetane δ¹³C value, measured in a mixture with phytane and estimated from the correlation of the δ¹³C of the mixture with the mole fraction of crocetane, was about −127 ‰. More than half of the PMI δ¹³C values were greater than −100 ‰, suggesting the background presence of fossil PMI from methanogens.

Aliphatic and aromatic degraded triterpenoids (TTs), including des-A and des-E TTs, were investigated in turbiditic and hemipelagic mudstones from the Miocene Kawabata Formation (Ishikari basin) and Abetsu and Nibutani formations (Hodaka basin) of south-central Hokkaido, Japan. des-A TTs with carbon skeletons of oleanane, ursane, and lupane are derived from angiosperms, while des-E TTs with hopane carbon skeletons are of bacterial origin. These compounds are thought to form through microbial degradation under dysoxic and anoxic conditions. We found that the concentrations and abundances of total degraded TTs, especially des-A TTs, relative to total organic carbon (TOC) were markedly higher in the Kawabata Formation despite significant levels in the Abetsu and Nibutani formations. These results clearly suggested that huge amounts of des-A TTs may have been transported to, and accumulated in, the Ishikari basin during the late Miocene. Degraded TT/TOC ratios are correlated with aquatic macrophyte n-alkane proxy (Paq) values in the Abetsu Formation. Higher Paq values are interpreted as indicating large contributions of aquatic, submerged or floating macrophytes, and are commonly observed in lake and pond environments. Thus, large amounts of degraded TTs were likely produced through the biodegradation of transported angiospermous TTs in dysoxic or anoxic environments, such as ponds and wetlands. Furthermore, we assumed that organic matter deposited in the Hidaka basin was transported from wetlands and marsh areas, especially floodplain lakes, of paleo-Hidaka Island. The class distributions of degraded TTs varied among samples from these formations. The higher relative abundances of des-A lupanes in the Ishikari basin (Kawabata Formation) suggest that TTs were supplied from mountainous forested areas, where lupenoid-producing woody plant taxa may occur. Meanwhile, less abundant des-A lupanes in the Hidaka basin (Abetsu and Nibutani formations) suggest little or no supply of TTs from mountainous forested areas.

Alkenones are biomarkers derived exclusively from species of haptophyte algae. The relative abundance of di- to tri-unsaturated C₃₇ alkenones expressed as U^K'₃₇ is widely applied as a sea surface paleotemperature proxy for Cenozoic marine sediments. However, the absence of alkatrienones prior to the Eocene has precluded application of the U^K'₃₇ proxy for assessment of Cretaceous paleoclimates. Herein, we report a C₄₀ alkatrienone (tetraconta-9E, 16E, 23E-trien-3-one; C_40:3 Et) in deep-sea sediments from southern high latitudes (International Ocean Discovery Program: IODP site U1516). This discovery extends the geologic record of alkatrienones to the late Cenomanian, ∼70 million years earlier than previous reports. The parallel occurrence of higher abundances of a C₄₀ alkadienone (tetraconta-16E, 23E-dien-3-one; C_40:2 Et) allowed calculation of the U^K’₄₀ unsaturation index, comparable to U^K’₃₇. Stratigraphic variations in the δ¹³C of C_40:2 Et revealed an elevated (∼1.5 ‰) positive carbon isotope excursion (CIE) relative to those observed in carbonate from other OAE2 sequences likely reflecting a decrease in global pCO₂. The U^K’₄₀ profile suggests a concurrent drop in sea surface temperature associated with the decline in pCO₂ during the early phase of OAE2. The timing of these environmental perturbations in the southern high latitude of the proto-Indian Ocean suggests they were triggered by volcanism associated with large igneous province (LIP) formation.

1,1,4a,6-tetramethyl-9-alkyl-1,2,3,4,4a,9b-hexahydrodibenzo[b,d]thiophene (9-alkyl HDBT) was initially found in highly desulfurized diesel oils. This and related organic sulfur compounds (OSCs) have a special shielding structure that make them extremely resistant to hydrodesulfurization. The structure of 9-alkyl HDBTs is likely directly inherited from biological precursors related to terpenoids, but their origins have not been determined. In this study, OSCs were isolated from a low sulfur crude oil from the Junggar Basin in China by the methylation/demethylation method and characterized by gas chromatography-mass spectrometry. A series of 9-alkyl HDBTs substituted with long isoprenoid alkyl chains were identified in the OSCs. Raney nickel desulfurization was conducted to assist the structural identification. The presence of 9-alkyl HDBTs with a β-carotene carbon skeleton and the predominance in specific carbon numbers of alkyl-substituted 9-alkyl HDBTs support a carotenoid origin of such OSCs. Possible geochemical pathways are proposed for the formation of these OSCs via β-carotene, involving processes such as sulfur incorporation, cyclization, aromatization, hydrogenation, and cleavage.