Thermochemical sulfate reduction (TSR) is an important organic–inorganic reaction that occurs within sedimentary basins and alters the original chemical compositions and isotopic structures of hydrocarbons in natural gases. We used the GC-Py-GC-IRMS method to study TSR and obtained a novel finding related to intramolecular carbon isotope fractionation in natural propane. The results show that the ΔC-T (δ13Ccentral-δ13Cterminal) and δ13Ccentral values significantly increased to 44.7 ‰ and 11.9 ‰, respectively, with increasing TSR alteration. In contrast, the δ13Cterminal values of propane remained largely unaltered by the TSR reaction. This difference in position-specific isotope fractionation can be attributed to the central carbon’s reactivity being higher than that of terminal carbon during TSR. In sum, the results indicate that the δ13Cterminal values of propane can serve as robust indicators for source rock identification of natural gas altered by post-generation reactions such as TSR and anaerobic microbial oxidation.

The isotopic composition of ocean water is crucial in studying water masses and mixing in deep oceans, isotope mass balance in ocean water regulated by high-temperature and low-temperature hydrothermal alterations, and the exchange of water among crust-ocean-mantle reservoirs. We collected 40 water samples from Challenger Deep and the water column above at the Mariana Trench (down to 10,923 m) and 12 from the Yap Trench (down to 6,300 m) in the western Pacific Ocean in three hadal cruises from 2016 to 2018. The delta 2H values at the Mariana and Yap Trenches average 0.1 +/- 0.2 parts per thousand (1 sigma error). The delta 2H records from this study, together with existing databases, manifest that deep waters have delta 2H values varying between -2 and +2 parts per thousand (except for the Weddell Sea, the Greenland, Iceland, and Norwegian Seas, and the Mediterranean Sea), with increasing values from the Southern Ocean to the Pacific and Indian Oceans, and to the Atlantic Ocean. The average delta 18O value of water samples from both trenches is -0.04 +/- 0.03 parts per thousand (1 sigma error). The correlation between delta 18O and salinity distinguishes abyssal water masses at the study region, UCDW (Upper Circumpolar Deep Water) and LCDW (Lower Circumpolar Deep Water). These water samples from the Mariana and Yap Trenches gave an average 17Oexcess value of -6 +/- 1 ppm (1 sigma error). Our 52 data records of 17Oexcess expand the 38 existing records for the deep ocean. Both delta 2H and 17Oexcess of modern ocean have rolled as anchor points to reconstruct compositions of Earth's early ocean.

The late Paleoproterozoic was a key period in the evolution of Earth's surface environments and life. It is generally accepted that during the 2.43-2.06 Ga Great Oxidation Episode, the atmospheric oxygen level rose to an intermediate state (0.1% to 10% of the present atmospheric level) and the chemical composition of the oceans changed significantly. The paleontological record indicates that eukaryotes appeared during the late Paleoproterozoic. Nitrogen, an essential nutrient for life, is thought to have been one of the key factors in eukaryote evolution. However, nitrogen bioavailability and spatial heterogeneity in nitrogen cycle in the late Paleoproterozoic oceans remain controversial. Here, we examine carbon and nitrogen cycling in the late Paleoproterozoic ocean, via analysis of redox-sensitive element contents and organic carbon and nitrogen isotope compositions of black shales of the ca. 1.9 Ga Nuvilic Formation of the Povungnituk Group, Cape Smith Belt, Canada. Two diamond drill-holes (DDHs) 718-3333 (-50 m-long) and 4G8069 (-90 m-long) investigated in this study contain turbiditic greywackes and black shales deposited on the northern margin of the Archean Superior craton below the storm wave-base. Sedimentary rocks of DDH 4G8069 might have been deposited closer to the continental source area, as indicated by sandstone abundance, than those of DDH 718-3333. Redox-sensitive element (V, U, and Mo) contents in black shales of DDH 4G8069 are low and show positive correlation with Al contents, whereas those of DDH 718-3333 are irregularly elevated. Fe/Al ratios of DDH 718-3333 are low, which likely reflect Fe loss from sediments under anoxic water-column conditions. Depositional environment is inferred to be above and below the redoxcline for DDHs 4G8069 and 718-3333, respectively. Organic carbon isotope values of DDH 718-3333 are approximately 1.7%o lighter than those of DDH 4G8069, which might reflect methane cycling at the redoxcline. Nitrogen isotope values are positive (> +3%o) for all the black shale samples, corresponding to aerobic nitrogen cycling and presence of bioavailable nitrate on the northern margin of the Superior craton during the late Paleoproterozoic. Furthermore, nitrogen isotope values for DDH 718-3333 are approximately 2%o heavier than those for DDH 4G8069. The observed isotopic heterogeneity is similar to that in modern oxygen-minimum zones and could represent intense denitrification at the redoxcline. Although no ca. 1.9 Ga microfossils reported to date are considered unambiguously eukaryotic, the aerobic nitrogen cycling inferred for the Nuvilic Formation, as well as for the previously studied Rove, Virginia, and Menihek formations, suggests that nitrogen limitation was unlikely to have inhibited biological evolution during the late Paleoproterozoic, a finding that is consistent with molecular clock studies suggesting that the emergence of eukaryotes occurred during the Paleoproterozoic.

An intramolecular isotopic study was conducted on natural gases collected from coal-derived gas reservoirs in sedimentary basins of China to determine their position-specific isotope distributions. The propane from the Turpan-Hami Basin exhibited negative ΔC-T (δ13Ccentral-δ13Cterminal) values ranging from −3.9‰ to −0.3‰, with an average of −2.1‰. Propane from the Ordos Basin, Sichuan Basin, and Tarim Basin showed positive ΔC-T values, with averages of 1.3‰, 5.4‰ and 7.6‰, respectively. Position-specific carbon isotope compositions reveal the precursors and the propane generation pathways in the petroliferous basins. Propane formed from the thermal cracking of Type III kerogen has larger δ13Ccentral and δ13Cterminal values than propane from Type I/II kerogen. The precursor for natural gases collected in this study is identified to be Type III kerogen. Comparing our data to calculated results for thermal cracking of Type III kerogen, we found that propane from the low-maturity gas reservoir in the Turpan Basin was generated via the i-propyl radical pathway, whereas propane from the Sulige tight gas reservoir in the Ordos Basin was formed via the n-propyl radical pathway. δ13Cterminal values covered a narrow range across basins, in contrast to δ13Ccentral. The terminal carbon position in propane is less impacted by microbial oxidation and more relevant to maturity levels and precursors. Thus, δ13Cterminal has a good potential to infer the origin and maturity level of natural gas. In examining post-generation processes, we proposed an improved identification strategy for microbial oxidation of natural gases, based on the position-specific carbon isotope distributions of propane. Samples from the Liaohe Depression of the Bohai Bay Basin and the Sichuan Basin were detected of post-generation microbial oxidation. Overall, position-specific carbon isotope composition of propane provides new insights into the generation mechanism and post-generation processes of natural gas in the geological period at the atomic level.

An intramolecular isotopic study was conducted on natural gases from the Santanghu Basin in China. Propane samples spanned a wide range of Delta(C-T) (delta C-13(central)-delta C-13(terminal)) values from -8.4 to 8.5 parts per thousand, delta C-13(central) and delta C-13(terminal) values varied from -37.3 to -25.8 parts per thousand and -35.7 to -28.9 parts per thousand, respectively. During thermal maturation, the position-specific carbon isotopic composition of propane shifted from delta C-13(central) < delta C-13(terminal) to delta C-13(central) > delta C-13(terminal), and the delta C-13(terminal) decreased by 6.8 parts per thousand within a R-o range of 0.46-0.50 %. The intramolecular carbon isotopic rollover and a decrease in delta C-13(terminal) with a slight increase in maturation could be explained by kinetic effects in thermal cracking of kerogen with a low proportion of branched chains. As isomeric groups were rapidly exhausted, propane generation pathway rapidly changed and primary kinetic isotope effect completely shifted from central carbon position to terminal carbon positions in propane. These changes increased carbon isotope fractionation at terminal carbon positions, thereby reducing delta C-13(terminal) values and shifting delta C-13(central) < delta C-13(terminal) to delta C-13(central) > delta C-13(terminal) within a small maturity range. This work demonstrated that position-specific isotope distribution of propane can be applied to elucidate the chemical structure of kerogen.

The stepwise oxygenation of Earth's surficial environment is thought to have shaped the evolutionary history of life. Microfossil records and molecular clocks suggest eukaryotes appeared during the Paleoproterozoic, perhaps shortly after the Great Oxidation Episode at ca. 2.43 Ga. The mildly oxygenated atmosphere and surface oceans likely contributed to the early evolution of eukaryotes. However, the principal trigger for the eukaryote appearance and a potential factor for their delayed expansion (i.e., intermediate ocean redox conditions until the Neoproterozoic) remain poorly understood, largely owing to a lack of constraints on marine and terrestrial nutrient cycling. Here, we analyzed redox-sensitive element contents and organic carbon and nitrogen isotope compositions of relatively low metamorphic-grade (greenschist facies) black shales preserved in the Flin Flon Belt of central Canada to examine open-marine redox conditions and biological activity around the ca. 1.9 Ga Flin Flon oceanic island arc. The black shale samples were collected from the Reed Lake area in the eastern part of the Flin Flon Belt, and the depositional site was likely distal from the Archean cratons. The black shales have low Al/Ti ratios and are slightly depleted in light rare-earth elements relative to the post-Archean average shale, which is consistent with a limited contribution from felsic igneous rocks in Archean upper continental crust. Redox conditions have likely varied between suboxic and euxinic at the depositional site of the studied section, as suggested by variable U/Al and Mo/Al ratios. Organic carbon and nitrogen isotope compositions of the black shales are approximately -23‰ and +13.7‰, respectively, and these values are systematically higher than those of broadly coeval continental margin deposits (approximately -30‰ for δ13Corg and +5‰ for δ15Nbulk). These elevated values are indicative of high productivity that led to enhanced denitrification (i.e., a high denitrification rate relative to nitrogen influx at the depositional site). Similar geochemical patterns have also been observed in the modern Peruvian oxygen minimum zone where dissolved nitrogen compounds are actively lost from the reservoir via denitrification and anammox, but the large nitrate reservoir of the deep ocean prevents exhaustion of the surface nitrate pool. Nitrogen must have been widely bioavailable in the ca. 1.9 Ga oceans, and its supply to upwelling zones must have supported habitable environments for eukaryotes, even in the middle of oceans around island arcs.

Drying-rewetting cycles are ubiquitous across natural and managed ecosystems. These cycles are known to mobilize carbon (C) in soils producing dramatic pulses in microbial respiration. While many factors contribute to these pulses, the drying-rewetting history of soils affecting carbon emissions remains unclear, especially in irrigated soils where soil moisture fluctuations are more repetitive and/or frequent than natural, seasonally influenced soils. To understand the controls of repeated wet-up and dry down effects on agricultural soils, we used a systems approach to examine the cross section of a furrow irrigated orchard to delineate soil C dynamics. Specifically, we compared two contrasting water regimes, (1) soils temporarily but repeatedly inundated during water delivery (i.e., furrows) and (2) soils at the base of trees (i.e., berms) that only receive water during precipitation events in a semi-arid Mediterranean climate. Overall, our findings show that the heterogeneous landscape of a furrow irrigated field results in two separate systems within the field scale in gaseous release of C as CO2, microbial selectivity of substrates, and mechanisms for C stabilization. By monitoring soil moisture as a function of depth for over two years, our results reveal that furrow soils undergo dramatic wet-dry cycles, while moisture within the berm is relatively constant. We were able to capture the distinct heterogeneity of soil moisture changes within the furrow and berm soils by continuously monitoring CO2 flux throughout water input events in both the wet and dry season. Soil CO2 efflux is suppressed upon irrigation within furrows, while carbon oxidation in berm soils exhibits pore-connectivity limitations that result in lower fluxes when dry. Solid phase soil C speciation determined by C 1s NEXAFS demonstrated C of higher aromaticity remained in furrow soil compared to berm soils. Microbial community analysis shows significantly different communities reside within berm and furrow soils, where furrow soils support more anaerobic metabolisms and spore-formers while berm soils have relatively higher abundance of aerobic microbes capable of degrading larger, more complex C compounds. Our findings show that water regime (periodic inundation vs episodic rainfall) controlling rewetting history can greatly differentiate C respiration within managed soils.

Drying-rewetting cycles are ubiquitous across natural and managed ecosystems. These cycles are known to mobilize carbon (C) in soils producing dramatic pulses in microbial respiration. While many factors contribute to these pulses, how the drying-rewetting history of soils affects carbon emissions remains unclear, especially in irrigated soils where soil moisture fluctuations are more repetitive and/or frequent than natural, seasonally influenced soils. To understand the controls of repeated wet-up and dry down effects on agricultural soils, we used a systems-level approach to examine the cross section of a furrow irrigated orchard to delineate soil C dynamics. Specifically, we compared two contrasting water regimes: soils are temporarily but repeatedly inundated during water delivery (i.e., furrows) and soils at the base of trees (i.e., berms) that only receive water during precipitation events in a semi-arid Mediterranean climate. Overall, our findings show that the heterogeneous landscape of a furrow irrigated field results in two separate systems within the field scale in gaseous release of C as CO2, microbial selectivity of substrates, and mechanisms for C stabilization. By monitoring soil moisture as a function of depth for over two years, our results reveal that furrow soils undergo dramatic wet-dry cycles, while moisture within the berm is relatively constant. We were able to capture the contrasting response to soil moisture changes within the furrow and berm soils by continuously monitoring CO2 flux throughout water input events in both the wet and dry season. Soil CO2 efflux is suppressed upon irrigation within furrows, while carbon oxidation in berm soils exhibit pore-connectivity limitations that result in lower fluxes when dry. Solid phase soil C speciation determined by C 1s NEXAFS demonstrated C of higher aromaticity remained in furrow soil compared to berm soils. Microbial community analysis shows significantly different communities reside within berm and furrow soils, where furrow soils support more anaerobic metabolisms and spore-formers while berm soils have relatively higher abundance of aerobic microbes capable of degrading larger, more complex C compounds. Our findings show that water regime (periodic inundation vs episodic rainfall) controlling rewetting history can greatly differentiate C respiration within managed soils.

Pyrite nodules up to 20 cm in diameter are found at the top of the Marinoan (~ 635 Ma) Nantuo glacial diamictite as well as in the cap dolostones and shale/siltstones in the lower Doushantuo Formation in eastern Guizhou, southern China. Field occurrences, petrography, and stable sulfur isotopic compositions of pyrite nodules were studied from a section at Taoying, eastern Guizhou, China. Pyrite δ 34 S values from different nodules varied from 7.3 to 60.5‰ at different stratigraphic levels. No stratigraphic trend existed for the δ 34 S, supporting the scenario of pyrite formation in sediments before the precipitation of the cap dolostone. Pyrite δ 34 S values were also homogeneous within individual nodules at a 0.3 to 1 cm sampling scale, but were more heterogeneous at a 2 mm sampling scale. Homogeneity was not expected from the particular model for pyrite nodule formation in a largely closed or semi-closed environment. Thus, differential cementation and compaction of the pyrite-bearing sediments may have produced the nodular shape of the pyrite deposit.

Increasing soil organic carbon (SOC) stocks in agricultural soils can contribute to stabilizing or even lowering atmospheric greenhouse gas (GHG) concentrations. Cover crop rotation has been shown to increase SOC and provide productivity benefits for agriculture. Here we used a split field design to evaluate the short-term effect of cover crop on SOC distribution and chemistry using a combination of bulk, isotopic, and spectroscopic analyses of size-and density-separated soil aggregates. Macroaggregates (>250 µm) incorporated additional plant material with cover crop as evidenced by more negative δ13C values (−25.4‰ with cover crop compared to −25.1‰ without cover crop) and increased phenolic (plant-like) resonance in carbon NEXAFS spectra. Iron EXAFS data showed that the Fe pool was composed of 17–21% Fe oxide with the remainder a mix of primary and secondary minerals. Comparison of oxalate and dithionite extractions suggests that cover crop may also increase Fe oxide crystallinity, especially in the dense (>2.4 g cm−3) soil fraction. Cover crop δ13C values were more negative across density fractions of bulk soil, indicating the presence of less processed organic carbon. Although no significant difference was observed in bulk SOC on a mass per mass basis between cover and no cover crop fields after one season, isotopic and spectroscopic data reveal enhanced carbon movement between aggregates in cover crop soil.

Multiple sulphur isotope compositions of sedimentary pyrites across the Ediacaran-Cambrian (Ed-C) transition and into the early Cambrian from the Xiaotan section, Yunnan, South China, are presented to explore the evolution of the sulphur cycle. The values of delta(34)S(py)range from 13.5 parts per thousand to 35.8 parts per thousand, and the values of Delta(33)S(py)range from -0.044 parts per thousand to 0.063 parts per thousand. The first-order observation of highly positive delta(34)S(py)is consistent with sulphur isotope records from other sedimentary successions (with various degrees of enrichment in(34)S), reflecting a common feature in cycling of sulphur among ocean basins. The positive values suggest that pyrite was formed in a depositional setting with limiting availability of sulphate that suppressed the expression of microbial fractionations. The first-order observation of a 10-million-year period of negative Delta(33)S(py)beginning around the Ed-C boundary likely reflects changes in isotopic compositions of sulphur influx to the oceans. Such changes are suggested to be linked to a pulse of preferred weathering of sulphides (with negative Delta S-33) relative to sulphate, which may reflect enhanced exposure of pyrites in continental margins due to reorganization of continents at this time. Both delta(34)S(py)and Delta(33)S(py)data imply low seawater sulphate levels, and possibly heterogeneity in sulphate concentrations in the world's coastal oceans. The predictions about sulphur isotope signatures of evolved seawater (with highly positive delta S-34 and negative Delta S-33) at the Xiaotan section are testable with future measurements of carbonate-associated sulphate (CAS), a proxy of ancient oceanic sulphate that carries information about the operation of sulphur cycling on a global scale.

Oceanic nontronite deposits have been identified to be closely related to low-temperature hydrothermal activities. However, their formation mechanisms associated with microbes in diffuse hydrothermal vents still remain largely unknown. The friable deposits, collected from the low-temperature diffuse flow at the Southern Atlantic Ridge, display a layered structure. Scanning electron microscope and transmission electron microscope analyses reveal that abundant filamentous, spherical, and rod-shaped mineralized forms are preserved in the yellowish-green layer of the deposits. These mineralized forms primarily consist of Si and Fe. Selected area electron diffraction patterns of the mineralized forms indicate that they are composed of nontronite. High intensities of C-12 and (CN)-C-12-N-14 signals derived from cellular structures determined by nanosecondary ion mass spectrometry suggest the intimate relationship between nontronite and microbes. The results of 454 pyrosequencing analyses provide insights into the microbial communities involved in the biologically induced mineralization in the yellowish-green layer. We propose an evolutionary model for establishing paragenetic sequences among nontronite, Mn oxide, and Fe oxyhydroxide in the deposits. This paragenetic sequence could be widespread in modern and ancient low-temperature hydrothermal fields.