Gammaceranoic acids (GAs) have rarely been reported in geological samples, and little is known about their origin and geochemical significance. In this study, their occurrence and evolution were investigated in genetically related crude oils spanning a range of thermal maturities. Two compounds (C31–32 GAs) were tentatively identified based on chromatographic behavior and mass spectra, including 3-carboxymethyl-gammacerane (C32 GA), which is reported here for the first time. Comparisons of structural characteristics and homolog distributions between GAs and 3-carboxy(alkyl) steranes indicate that they may be formed by addition at the C-3 positions of their respective diagenetic intermediates (i.e., gammacerene and sterene). Similar trends of maturity change were observed for GAs and gammacerane by studying concentrations of these compounds. Their concentrations first increased at low maturity levels and then decreased with increasing maturity. This correlation pattern may reflect the release of kerogen-bound GAs during early maturation and their subsequent conversion to gammacerane. The findings in this study suggest that the understudied GAs occurring in geological samples may represent an important diagenetic intermediate in the formation of gammacerane.
Dibenzothiophene (DBT) isomer parameters are commonly employed as tracers for thermal maturity and hydrocarbon migration. However, thermochemical sulfate reduction (TSR) may distort these geochemical indicators, and research on this topic remains limited. In this study, a total of 13 Ordovician ultra-deep crude oil samples, collected from five fault zones [F1, F4, northern part of the F5 (F5N), F7, and F8] in the Shunbei Oil and Gas Field of the Tarim Basin, were systematically analyed for diamondoids, 2-thiadiamondoids, and dibenzothiophenes. Through quantitative analysis, the alterations induced by thermochemical sulfate reduction (TSR) on DBT isomer parameters were systematically constrained. The physical properties and the diamondoid-based parameters consistently indicate that the maturity sequence of the crude oil samples in the study area is F7 < F5N < F1 < F4 < F8. The concentrations of 2-thiadiamondoids confirm that the F4 crude oil samples have undergone TSR. In crude oil samples unaffected by TSR, DBT isomer parameters, including MDR (4-/1-methyldibenzothiophene), DBTI [(2- + 3-) /(2 × 1-) methyldibenzothiophene], and 4,6-/1,4-, 2,4-/1,4-, and (2,6-+3,6-)/1,4-dimethyldibenzothiophene, exhibit a regular increasing trend with rising thermal maturity. Meanwhile, the highly mature F8 samples show a significant decrease in DBT concentration, and only a slight reversal of these DBT isomer parameters, which can be attributed to extreme thermal cracking. In contrast, the TSR-affected F4 crude oil samples, despite their high thermal maturity and considerable thermal cracking intensity, display a significant increase in DBT concentration but a significantly and anomalously low these DBT parameter values. Integrating these findings, it is proposed that TSR, through a selective sulfur incorporation mechanism, may preferentially generates α-substituted (e.g., C-1 position) DBT isomers, thereby leading to reduced DBT isomer parameters. This study elucidates the possible mechanism by which TSR interferes with widely used DBT-based tracer indicators and provides key constraints for the proper application of these sulfur-containing aromatic parameters in deep petroleum accumulations.
Millets are receiving renewed attention due to their short growing cycles, resilience and role as fodder and summer cereals. As C4 plants adapted to arid and tropical environments, millets were domesticated in East Asia and Africa, with species such as broomcorn millet spreading into Europe during the Bronze Age. In European archaeology, millets are key indicators of crop diversification among human communities, including in northern Italy. Archaeological evidence of their use derives from charred grains, phytoliths and stable isotope and molecular biomarkers analyses. Among molecular biomarkers, miliacin (olean-18-en-3β-ol methyl ether) has been widely recognised as a biomarker for Panicum miliaceum (L.)/broomcorn millet detected in pottery, sediments and charred residues. Carbon enrichment typical of C4 plants in Middle Late Bronze Age human remains has often been linked to millet consumption, although wild panicoid grasses may also have contributed to these isotopic signals. Among wild panicoid grasses, generally more difficult to trace archaeologically, Echinochloa crus-galli (L.) P. Beauv./barnyard grass represents an exception since its charred grains have often been found. This study investigates crusgallin (taraxer-14-en-3β-ol methyl ether) as a potential biomarker for barnyard grass. Different anatomical parts of modern plants were chemically analysed to determine crusgallin distribution, and the data were used as reference for the study of sediments from the Middle Bronze Age site of Oppeano 4D, where byre-houses containing herbivore dung and plant bedding were identified. Archaeobotanical deposits yielded abundant barnyard grass seeds together with miliacin and crusgallin, suggesting that these biomarkers may derive from both plant remains and herbivore dung, additionally indicating millets as possible components of animal fodder.
Raney nickel (RN) desulfurization is widely applied to access sulfur-bound biomarkers, yet its quantitative reliability remains poorly constrained. Here we systematically evaluate RN desulfurization through parallel (reproducibility) and mass-gradient (linearity) experiments on polar fractions of a Tertiary crude oil. Results demonstrate exceptional reproducibility of biomarker ratios and strong linearity (R2 > 0.8) between product yields and reactant masses (10–20 mg), establishing a firm methodological foundation for quantitative desulfurization analysis. Importantly, carbon-number dependent yields of n-alkanes reveal molecular selectivity of natural sulfurization, validating RN desulfurization as a robust quantitative tool for tracing genuine biosignatures and calibrating paleoenvironmental reconstruction derived from sulfur-bound biomarker pools.
Intramolecular isotope analysis is a powerful tool for tracing molecular origins and formation pathways by characterizing isotope distributions among distinct molecular moieties. This study presents an on-line gas chromatography–hydropyrolysis–gas chromatography–isotope ratio mass spectrometry method for methyl- and phenyl-moiety-specific carbon isotope analysis of toluene. The key innovation of this method is the introduction of an on-line hydropyrolysis step. First, optimal hydropyrolysis conditions for toluene were determined through systematic experimentation: a pyrolysis temperature of 880 °C, an H2 pressure of 0.15 MPa, and a carrier gas flow rate of 0.8 mL/min. Under these optimized conditions, methane and benzene were identified as the main pyrolytic fragments, and no appreciable amounts of other carbon-containing compounds were detected. Subsequently, an isotope-dilution experiment showed that methane was derived predominantly from the methyl carbon of toluene. Together with the near 1:1 M ratio of methane to benzene, this result indicates that benzene carbon is mainly derived from the remaining phenyl moiety and that secondary reactions are negligible within analytical uncertainty during hydropyrolysis. Finally, the results demonstrate that isotopic fractionation during pyrolysis is stable and reproducible at a given temperature. Therefore, the δ13C value of the methyl carbon and the average δ13C value of the phenyl moiety can be calculated by applying the fractionation factors of the hydropyrolysis process to the measured δ13C values of the pyrolytic fragments. This work establishes a simple, rapid, and sensitive intramolecular carbon isotope method for determining the δ13C value of the methyl carbon and the average δ13C value of the phenyl moiety of toluene.
