Carbon monoxide (CO) is one of the most important components in the troposphere, which plays a significant role in the oxidation and reduction chemistry of the atmosphere. Surface rupture zones are one of the main regions of gas degassing. The Wenchuan MS 8.0 earthquake produced the ~240-km-long Yingxiu-Shuiguan rupture zone and the ~72-km-long Tongji-Hanwang rupture zone. In this study, soil CO flux in the surface rupture zones produced by the Wenchuan MS 8.0 earthquake (WSRZ) was measured using the closed-chamber method. The results showed that soil CO flux in the WSRZ ranged from 0.2 to 25.4 mg m−2 d−1 with background value of 5.2 mg m−2 d−1, which indicated gas was still degassing after the Wenchuan MS 8.0 earthquake. Abiogenic CO in the WSRZ might origin from the low crust flow and chemical reactions of the C-H-O fluids in the active fault zone. The spatial variations of CO fluxes along the WSRZ show significant segmentation characteristics. This research implicates the geochemical characteristics and origin of CO from natural fault zones as well as provides basic data for geochemical characteristics of soil CO degassing in co-seismic surface rupture zones.
Crude oil obtained from the Southwest Depression of the Tarim Basin contains high concentrations of rearranged hopanes. However, the geochemical factors controlling this phenomenon, and the correlation between the rearranged hopanes and other biomarkers remain unclear. In this study, nine crude oil samples were collected from this region and analyzed using the gas chromatography-mass spectrometry in terms of hopane compositions, by combining the Pearson correlation analysis (PCA) to understand the cause of the unusually high abundance of rearranged hopanes. Three series of rearranged hopanes were detected in the samples: 17α(H)-diahopanes (C29 and C30 rearranged hopanes), 18α(H)-neohopane (Ts and C29Ts), and C30 early eluting rearranged hopanes (30E). The high content of these rearranged hopanes relative to hopanes correlated well with each other, indicating that they were derived from the same source. The related ratios of the rearranged hopanes and other saturated biomarkers suggest that the unusually high abundance of rearranged hopanes in the crude oil samples are closely related to redox conditions, whereas other biomarkers, which represent water salinity condition, thermal maturity, and biological sources are weakly correlated with the rearranged hopane series relative to hopanes. The PCA confirmed that rearranged hopanes are readily present in crude oils and are developed in an anoxic and fresh lacustrine source rock depositional environment.
The extinct, relatively short-lived nuclide 182Hf produced 182W as a decay product. Fractionation of Hf-W in the very early Earth led to variations in the 182W/184W ratios of terrestrial rocks; however, because these variations are very small, quantifying 182W/184W ratios requires an extremely precise method. Here, we propose an improved method for highly precise and accurate method for measuring the 182W/184W ratios of terrestrial rocks. Samples were extracted with 4-methyl-2-pentanone and purified by cation and anion exchange chromatography prior to determination of the W isotope ratio by multiple collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) system coupled with a desolvating nebulizer. Sample preparation removed matrix elements (e.g., Hf, Ta, Os, and dimers of Nb and Mo) with masses similar to those of W isotopes, resulting in these elements having a negligible influence on the measured 182W/184W ratios. A W standard solution processed by ion exchange chromatography and/or solvent extraction showed a 183W deficiency, even after mass fractionation correction of the measured isotope data. As reported previously, mass-independent fractionation increases the 182W/184W ratio if the 183W/184W ratio is used to correct for mass fractionation to for better precision in natural samples. However, accurate 182W/184W ratios for a basalt reference material (JB-2) were obtained, even if 183W was used for mass fractionation correction. Our results show that it is also possible to correct for the effects of mass-independent fractionation on the 183W/184W ratio by sample-standard bracketing using a W standard solution subjected to the same preparation procedure used for the samples. A major advantage of the newly developed method is that it requires a smaller amount of sample (0.2–0.3 g; 50–80 ng W for JB-2) compared with that needed for other reported methods (typically 0.7–15 g; 500–1000 ng W). This decrease in sample amount was possible by removing matrix elements from the sample solutions, and cleaning the membrane of the desolvating nebulizer between analyses also contribute to enhancing the W ion beam intensity and to high precision. Analysis of different basalts from the Loihi, Kilauea islands and Ontong Java Plateau with various W isotopic compositions consistent with the previous studies demonstrated the reliability of the method.
Here we quantify radiocarbon (14C) content in CO2 and CH4 from hydrothermal fluids collected at 5 vent sites in the Okinawa Trough. These data provide preliminary insights on how different carbon sources and circulation processes contribute to vent fluids and take into consideration all potential contaminations during sample processing and analysis with accelerator mass spectrometer. For CO2, 14C-dead values observed in fluids of most sites (Hatoma, Hitoshi, Noho) demonstrate the complete removal of the inorganic carbon in the recharged seawater. Conversely, the significantly positive 14CO2 are detected in fluids from the Daiyon-Yonaguni site, which implies the persistence of the recharged seawater-derived inorganic carbon and/or a contribution from thermal degradation of sedimentary 14C-containing carbon during fluid upwelling. The CH4 in these fluids were predominantly 14C-dead or nearly 14C-dead, suggesting a source of deep-buried, 14C-dead sedimentary carbon. The methods outlined in our study exemplify a useful way of elucidating radiocarbon contents of geofluids, and provide future studies with a means to improve our understanding of carbon circulation between hydrosphere and lithosphere.
A set of oils from Ancon oilfield (Neogene Progreso Basin, Ecuador), were studied by GC-MS in terms of the occurrence and distribution of cyclohexylbenzenes, biphenyls, and diphenylmethanes, and their use as indicators of thermal maturity and biodegradation. In the studied oils, biphenyls are more abundant than the other related bicyclics; 3-methylbiphenyl is the most predominant over remnant homologs. A weak correlation between different biphenyl ratios and sensitive parameters for thermal maturity (MPI-1 and %Rc) is observed; probably, the input of terrestrial organic matter could be responsible for the low correlation. The linear correlation between biphenyls and cyclohexylbenzenes is explained through dehydrogenation reactions during oil formation. However, no correlation is observed with diphenylmethanes nor cyclohexylbenzenes. The biodegradation achieved in the studied oils is equal to or less than 3 in the PM scale, according to the negligible effects over the distribution of these compounds, confirmed by aromatic biodegradation indices derived from alkylnaphthalenes.