A rapid hydroclimate change and its influence on the Songliao Basin during the Santonian-Campanian transition as recorded by compound-specific isotopes

Reconstruction of the paleoclimate and paleoenvironment of the Songliao Basin has important theoretical and practical significance to reveal the regional continental climate characteristics and continental organic matter accumulation under the Cretaceous greenhouse climate conditions. Here, we report the composition of biomarkers and the stable carbon and hydrogen isotope compositions of n-alkanes from Nenjiang Formation sediments in the Songliao Basin, which provide information on the source of organisms, paleoenvironmental conditions, and paleoclimate changes during the Late Cretaceous. Our hydrogen isotope records of n-alkanes displayed a significantly negative shift, indicating that a dramatically wet climate occurred during the Santonian-Campanian transition. A wet climate, such as that during a prolonged rainfall event, had a significant impact on the lake and resulted in the lake level increasing and the salinity decreasing in the surface water during this period. However, the freshwater input increased the salinity difference between the upper and lower water layers, which caused stable water stratification to occur in the lake. In addition, the high total organic carbon (TOC) content indicates high primary productivity because of increased precipitation carrying more terrigenous nutrients into the lake, which resulted in eutrophication during this period. Notably, the increased aryl isoprenoid ratios and the occurrence of isorenieratane indicates that photic zone euxinia (PZE) was enhanced during the Santonian-Campanian transition, which was possibly due to the dramatic change in the hydroclimate. We speculate that the large freshwater input promoted the formation of PZE in the lake. Then, PZE restricted the habitat available for aerobic organisms, which had fatal effects on the lake ecosystem

4-Methyl-24-Ethylcholestane in Upper Permian Fossil Conifer Wood from Paleo-Midlatitude of NE Pangea, Wutonggou Low-Order Cycle, Southern Bogda Mountains, NW China

The use of C30 4-methylsteranes as an indicator of source organic matter relies on accurate identification of the compounds from their isomers and distribution of their precursors (i.e. sterols) in contributing organisms. C30 4-methylsteranes have been reported in many Mesozoic and pre-Mesozoic samples but not in Permian rocks. They may be produced from dinoflagellates or aquatic plants or methylotrophic bacteria. Significant amount of C30 4-methylsteranes was detected in preserved materials in two Upper Permian fossil conifer stems. The 4- methylsteranes have a 4-methyl-24-ethylcholestane structure, without 4α, 23, 24-trimethylcholestanes (also known as dinosteranes). The preservation of C30 4-methylsteranes with C29 steranes and absence of C27-28 steranes with rare hopanes suggest that the 4-methyl sterols produced by conifers may be an important potential precursor for 4-methylsteranes. Alternative sources include freshwater dinoflagellates or methylotrophic bacteria in the environments where the conifers lived, transported, and deposited. Significant amount of C30 4-methylsteranes in Permian fossil conifer in fluvial-lacustrine environments shows that regarding the 4-methylsteranes as an indicator of diagnostic marine input can lead to erroneous conclusions. The rarity of this biomarker in geological records makes it an effective proxy for nonmarine sequence stratigraphic correlation and oil-oil and oil-source rock correlation

Geochemical Evolution of Occluded Hydrocarbons inside Geomacromolecules: A Review

Within geomacromolecules, such as kerogen, asphaltene, and solid bitumen, other compounds can be adsorbed and even occluded as free molecules. The occluded components have been well preserved by the macromolecular structure, and retain some of the primary geochemical information. In this work we try to probe the geochemical evolution of occluded hydrocarbons inside geomacromolecules associated with the geomacromolecule evolution from kerogen -> asphaltene -> solid bitumen. The results show that occluded hydrocarbons can be transferred steadily from kerogen -> asphaltene -> solid bitumen. Later-evolved geomacromolecules not only inherit the occluded hydrocarbons from the former ones, but can also occlude some new free molecules. Occluded hydrocarbons are subject to a relatively independent thermal evolution, whereas the evolution of adsorbed molecules is constrained by other factors besides thermal stress. Elucidation of the geochemical evolution of occluded hydrocarbons inside geomacromolecules will be helpful in oil (bitumen) source correlation, identification of mixed-source reservoirs, and characterization of hydrocarbon accumulation and evolution

Stable isotope (delta C-13(ker), delta C-13(carb), delta O-18(carb)) distribution along a Cambrian outcrop section in the eastern Tarim Basin, NW China and its geochemical significance

This study investigated the geochemical features of the lower Paleozoic strata of Yaerdang Mountain outcrop along with the core samples from well TD2∈ in the eastern Tarim Basin, NW China. The total organic carbon abundance, hydrocarbon-generating precursor biospecies, and stable isotope ratios of organics and carbonate (δ13Cker, δ13Ccarb and δ18Ocarb) were comprehensively studied for their possible correlative constraints during sedimentary evolution. The results revealed that the δ13Cker (VPDB) of Cambrian kerogens along the outcrop section varied from −34.6‰ to −28.4‰, indicating an increasing tendency from the lower Cambrian to the upper Cambrian. This was on the whole accompanied by the variation in the δ13Ccarb and δ18Ocarb along the profile, which might be associated with the changes in the sea level and also in the compositional variation of benthic and planktonic biomass. The large variation in the stable carbon isotope ratios up to 6‰ along the outcrop section reflected the heterogeneity of the Cambrian source rocks from the eastern Tarim Basin. Hence, the 13C-enriched crude oils from well TD2∈ might have been derived from a localized stratum of Cambrian source rocks. The results from this study showed the possibility of multiple source kitchens in the Cambrian–lower Ordovician portion of Tarim Basin

Determination and geochemical implication of multiple series of long-chain oxygen-bearing compounds trapped in kerogen in the Lucaogou Formation, Santanghu Basin, NW China

Studies of occluded oxygen-bearing compounds inside kerogen are scarce, although such studies are important in understanding early-stage transformation of organic matter. A relatively low-maturity kerogen from the late Permian mudstone of the Lucaogou Formation of the Santanghu Basin, northwestern China, was successively extracted with n-hexane, acetone and dichloromethane to release the adsorbed components, and the extracted kerogen was treated with a mild oxidation reagent H2O2 to release the occluded components. Gas chromatography-mass spectrometry analysis of the adsorbed and occluded components revealed a suite of oxygen-bearing compounds, including n-alkan-2-ones, n-aldehydes and fatty acid methyl esters. The n-alkan-2-ones were mainly detected in the adsorbed fraction and n-aldehydes in the occluded component. Comparison of the long-chain oxygen-bearing compound distributions and their individual stable carbon isotope characteristics both suggest that the oxygen-bearing compounds released by H2O2 treatment were occluded in the kerogen. The shorter-chain (C-15 to C-19) occluded n-aldehydes reflected similar isotopic distribution trends to the adsorbed n-alkan-2-ones, and the delta C-13 values of the mid-length (C-21 to C-25) occluded n-aldehydes were much closer to the adsorbed n-alkanes. Bacterially mediated methylation and decarbonylation of the n-aldehydes during kerogen formation may be important source(s) of n-alkan-2-ones and n-alkanes, respectively. The mid-chain ketones mainly originated from hydrothermal alteration of n-alkanes after kerogen formation. Uplift of the Lucaogou Formation in the Yuejingou section allowed surface-enhanced aerobic reactions. (C) 2018 Elsevier Ltd. All rights reserved

Separation of crocetane and phytane and measurement of their compound-specific carbon isotopic compositions

Crocetane and phytane are two isoprenoids isomers with similar molecular structures and often present together in methane-seep sediments and some Palaeozoic crude oils. Their commonly co-elution on gas chromatography is challenging for quality and quantity analysis, making it impossible to determine their compound-specific isotopic composition, and thus, insight their geological and geochemical significance. A new gas chromatography method is reported here using a DB-17MS column (50%-phenyl-methyl polysiloxane as the stationary phase) that successfully achieved baseline separation of crocetane and phytane and can be used to accurately identify and quantify them on gas chromatography and gas chromatography-mass spectrometry. Routine steroids and terpenoids biomarkers can also be analysed simultaneously. Additionally, their compound-specific carbon isotopic compositions were also measured without matrix influence using this method. This is the first time that a simple chromatographic method for direct determination of compound-specific carbon isotopic composition of crocetane has been reported publicly. (C) 2019 Elsevier B.V. All rights reserved