Organic Geochemistry最新文献

This research outlines a fast and efficient technique for separating the pyrrolic, phenolic-ketonic, and carboxylic acid fractions from oils in a small-scale setting. This approach enables the separation and analysis of neutral nitrogen fractions that contain indoles, carbazoles, and benzocarbazoles, as well as ketonic-phenolic fractions that contain phenols and ketones. The study showcases the potential for accurately measuring petroleum composition. The reliability of the fractions’ concentration results is determined by the mass of the separated fraction, with completely reliable results achievable for masses exceeding 1.4 mg. The fraction’s mass can also be used to evaluate the relative expanded uncertainty of a single separation procedure. As the mass and concentration of the fraction increase, the relative expanded uncertainty of the concentration decreases, and this can be modelled using a power function.

Dissolved organic carbon (DOC) is a key component of coastal biogeochemical cycles, but its composition and reactivity depend on the relative contribution of autochthonous aquatic versus allochthonous terrigenous DOC (tDOC). In complex coastal waters, tDOC is commonly quantified using the bulk DOC stable carbon isotope ratio (δ¹³C_DOC). However, several limitations can hamper the use of δ¹³C_DOC in marine ecosystems, such as (1) the narrow and often overlapping separation of the autochthonous and allochthonous endmembers, and (2) mineralization of tDOC to dissolved inorganic carbon creates a reservoir effect such that autochthonous DOC can acquire a terrigenous-like δ¹³C_DOC. The stable isotope ratio of non-exchangeable hydrogen in the DOC (δ²H_n) has emerged as a new tool that can potentially overcome these limitations: (1) δ²H_n has a large separation between aquatic and terrigenous endmembers (>50‰) and (2) it is not subject to reservoir effects caused by tDOC mineralization. Here, we evaluate the potential of δ²H_n obtained from solid phase-extracted dissolved organic matter (SPE-DOM), by comparing it to δ¹³C_DOC and chromophoric DOM (CDOM) optical properties. We collected samples at a site in Southeast Asia’s Sunda Shelf that experiences substantial seasonal variation in tDOC input, driven primarily by the monsoon-induced physical advection of peatland-derived tDOC. Over a 1-year monthly time series, the terrigenous fraction of DOC (f_terr) determined using δ²H_n of SPE-DOM and δ¹³C_DOC of bulk DOC was well correlated (r² = 0.42), and there was no significant difference in f_terr between the two isotope systems. In fact, δ²H_n displayed slightly stronger correlations with salinity and CDOM optical properties compared to δ¹³C_DOC. Our results indicate that δ²H_n of SPE-DOM is effective for quantifying tDOC across coastal gradients, potentially offering greater sensitivity than δ¹³C_DOC, and is a viable alternative in settings where δ¹³C_DOC is inadequate.

Ionisation suppression is a persistent issue in electrospray ionisation mass spectrometry, which decreases the signal of co-eluting analytes. In non-targeted analysis, where analyte and organic matrix identity is unknown, this poses a very challenging analytical problem when it comes to quantitatively assessing differences between samples, including in a compositional sense. In this study, I demonstrate the problems that arise due to ionisation suppression using a very simple sample mixing scheme between a fresh, metabolite rich sample (a leaf leachate) and a forest pond water. Samples were analysed after solid phase extraction on Agilent PPL and using high performance liquid chromatography coupled to electrospray ionisation – Orbitrap mass spectrometry, charged aerosol detector and diode array detector, the latter two allowing quantification of eluting material. I found that more than half of the well-resolved analytes expected to be present (at equal concentration) were completely lost from detection after mixing with pond water DOM. The average recovery of analytical signal (i.e., the signal weighted average), was about 50%, and was highly variable between analytes. Ionisation suppression also affected the signal obtained from the geochemical background DOM, and material recovery decreased slightly when mixing samples and extracting at a higher volume on PPL. Overall, the results showed that ionisation suppression is an extremely important problem for comparison of biogeochemical samples, even when only considering presence and absence of detected features. A multi detector approach and liquid chromatographic separation adds great value in comparison to use of only high resolution mass spectrometry (in direct infusion mode).

Compound-specific radiocarbon analysis (CSRA) is a promising tool for dating sediment sequences where traditional dating methods are impractical. However, the applicability of CSRA of short-chain fatty acids as a dating tool remains poorly understood, especially in lacustrine settings. Accordingly, we determined the radiocarbon content (Δ¹⁴C) of individual fatty acids in sediments of Lake Yamanaka (central Japan), as well as their stable carbon and hydrogen isotope ratios, to evaluate the potential of CSRA as a dating tool in volcanic lake environments. We found that the Δ¹⁴C values of plant-derived (C₂₄, C₂₆, and C₂₈) n-fatty acids (–99‰ to –149‰) were considerably lower than the Δ¹⁴C of charred plants (139‰) within the sediments and those of living aquatic plants (–52‰ to –58‰) in Lake Yamanaka, suggesting that contributions from pre-aged terrestrial and aquatic plant materials likely affect these acids. Similarly, the Δ¹⁴C of C₁₆ n-fatty acid (–95‰) in surface sediments was much lower than the Δ¹⁴C of modern aquatic plants (–52‰ to –58‰), as well as the Δ¹⁴C of dissolved organic carbon (DIC) in surface water (–48‰). Together with the stable isotope results, we conclude that in addition to autochthonous aquatic sources, contributions from pre-aged terrestrial carbon sources significantly affect the Δ¹⁴C of C₁₆ n-fatty acids. Comparing fatty acid Δ¹⁴C and concentration data across lakes within the Mt. Fuji region suggests that CSRA of the C₁₆ acid provides valid chronological information only when the C₁₆ originates exclusively from autochthonous aquatic sources, with minor allochthonous terrestrial input.

Understanding the formation of long-term persistent soil organic matter (SOM) is key to optimizing soil management and predicting the response of the terrestrial organic carbon (OC) pool to climate change, yet our knowledge of the soil-type dependent weight of different stabilization pathways (e.g., recalcitrance and mineral binding) is fragmentary. Owing to their stratigraphy, the exceptionally SOM-rich (up to 2 m of mineral soil with >5% OC) colluvial slope deposits of Atlantic Europe (Haplic Umbrisol [colluvic/hyperhumic]) are archives of palaeo-environmental conditions including SOM formation pathways. The objective of this study was to determine how the different drivers of persistent SOM formation influenced the formation of these organic-rich soils. For this purpose, we use Holocene (∼9000 yrs) molecular composition records obtained from pyrolysis-GC–MS (Py-GC–MS) and thermally assisted hydrolysis and methylation (THM-GC–MS). The results emphasize three pathways to stability (i.e., persistence on millennial timescales): 1) palaeofires that generated recalcitrant pyrogenic SOM, 2) release of root-derived aliphatic macromolecules (suberin-like SOM), and 3) formation of microbial necromass. Pathways 1 and 2 are controlled by land use: Pathway 1 was relatively important under intense anthropogenic fire regimes and pyrophytic shrubland expansion; Pathway 2 was stimulated during early forest phases and under pasture conditions, when past societies focused vegetation management on grazing instead of fire; Pathway 3 was controlled by binding with aluminium-dominated mineral phases. However, we found indications that Pathway 2 (suberin input and preservation) relied partially on sorptive preservation as well. Aided by structured equation modeling (SEM), we show that the formation of persistent SOM pools was driven by balanced weights of i) microbial vs. plant-derived SOM and ii) intrinsic chemical properties of SOM (recalcitrance continuum) vs. mineral binding/occlusion, which varied in keeping with interactions between past land use, topography and vegetation. These findings are inconsistent with the prevalent paradigm of persistent SOM formation by sorptive/occlusive preservation of microbial necromass alone.

Five series of benzohopanes were identified in a set of immature to early-oil-window early Cretaceous sediments from the ZK1006 well, Chaoyang Basin, NE China. Series a (C₃₁-C₃₅), b (C₃₃-C₃₅), and c (C₃₁-C₃₅) benzohopanes form by different pathways, while the newly-found series d (C₃₁-C₃₄, maybe to C₃₅?) and e (C₃₁-C₃₂, maybe to C₃₅?) benzohopanes have similar formation mechanisms. Series a is usually the predominant benzohopane series, but occasionally samples are dominated by series c benzohopanes. Series b benzohopanes may be associated with terrigenous organic matter. Series a and b benzohopanes are possibly influenced by redox properties, while paleosalinity more likely controls the formation and enrichment of series c benzohopanes. Two novel parameters (a benzohopane index, and the C₃₅/C₃₄ benzohopane ratio) are proposed to be useful paleoenvironmental proxies in the study area. A high benzohopane index and C₃₅/C₃₄ benzohopane ratio may indicate an anoxic environment.

The Dongpu Depression is a rift lake in the Bohai Bay Basin of eastern China, where abundant oil and gas resources were discovered. Previous geochemical investigations of Dongpu Depression oils have revealed a notable lack of data regarding the isotopic compositions of individual polycyclic aromatic hydrocarbons (PAHs) from the discovered Cenozoic crude oils. Using gas chromatography-mass spectrometry (GC–MS) and gas chromatography-isotope ratio mass spectrometry (GC-IRMS), a novel approach combining molecular parameters and stable carbon isotope compositions of individual PAH was applied to study the depositional environment conditions, the different source inputs, the maturation stage and to identify the potential family of five crude oils selected from Dongpu Depression.

A decrease of Ga/C₃₀H together with low Pr/Ph ratios and changes in isotopic compositions indicate that the five Dongpu Depression oils were derived from OM deposited in an anoxic saline environment with a stratified water column where mixing of saline water and freshwater occurred. The low isoprenoid, tricyclic and tetracyclic terpane ratios in combination with the δ¹³C values of 1,6-DMN, 1,2,5-TMN, 1,3,6,7-TeMN, 9-MP and 1-MP attest that their sources have a mixed origin with high aquatic organism (planktonic) input and a lower land plant (C₃ plant) contribution. The δ¹³C values of phenanthrene between −30.0 ‰ and −20.0 ‰ indicate that the studied Dongpu Depression oils belong to a single family generated from the peak to the late oil generation stage (Rc-TNR-2 between 0.85 and 1.15 %). A correlation with various basins indicates that oil samples featured by δ¹³C values of Phen between −30.0 ‰ and −20.0 ‰ correspond to crude oils derived from mixing of aquatic and high plant contributions with different degrees of mixtures, which have not yet reached their cracking stage (Rc < 2.0 %). Crude oils from the Carboniferous and Mesozoic-Cenozoic Tarim rocks, Australian petroleum systems, Termit Basin, Bongor Basin, Fushan Depression and the studied Dongpu Depression oils belong to that group of oils. Crude oils characterized by lighter phenanthrene isotopic compositions (δ¹³C values of Phen < −30.0 ‰) are mainly derived from marine/aquatic input and have already entered the cracking window (Rc > 2.0 %). This group of oils is represented in this study by the Cambrian-Ordovician Tarim Basin oil samples. The research shows the importance of the aromatic isotopic compositions in petroleum system characterization and could be used as a reference for a practical exploration campaign of petroleum.

Cholestane is produced by diagenesis of cholesterol in the geosphere being known to have four main stereoisomers. The abundance ratios of these stereoisomers are important indicators of the geospheric thermal history. However, it has been unclear whether the isomeric ratios are caused by kinetic or thermodynamic dominance. In this study, we analyzed the isomeric ratios of cholestane under thermodynamic dominance using the density functional theory. The calculated isomeric ratios are in good agreement with the limit values in the geosphere, indicating that the isomeric ratios in the geosphere can be obtained under thermodynamic control. We also discussed the factors contributing to their stabilities.