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The article presents a study which aims to determine the decomposition temperature of cold seep carbonate through delta carbonate method. The result shows that the carbon dioxide content yielded from all the eight samples increase gradually with time and pyrolysis temperature. Carbon isotopic composition of carbon dioxide reveals distinct isotopic-change tend of two groups with time increase and temperatures.

The accumulation of authigenic carbonate in marine sediments has been regarded as a significant contributor to the carbon cycle over Earth's history. However, accurate quantification of the extent of authigenic carbonate formation in both modern and ancient oceans has been challenging due to limited approaches. Here, we analyzed calcium, magnesium, dissolved inorganic carbon, and alkalinity pore‐water profiles of 153 sites influenced by methane diffusion from the northern South China Sea. By combining these data with published data of diffusive sulfate flux, we found a significant positive correlation between carbonate precipitation rate and sulfate flux in the subseafloor. This correlation can be used to quantify the regional extent of authigenic carbonate formation, representing an advantage over a spatial interpolation approach. We therefore propose that the observed correlation can serve as a new approach for quantifying authigenic carbonate formation in shallow sediments along continental margins for both modern and ancient oceans. Plain Language Summary: Carbonate mineral precipitation through removing seawater dissolved inorganic carbon—the marine carbonate factory—plays a critical role in shaping marine carbon cycling and global climate. The formation of authigenic carbonate from pore‐water of marine sediments represents a variable share of the marine carbonate factory throughout geological history. However, it is challenging to quantify the extent of such authigenic carbonate formation. By analyzing pore‐water parameters from 153 sites influenced by methane diffusion of the northern South China Sea, we found a significant positive correlation between the rate of carbonate precipitation and the diffusive flux of sulfate into shallow sediments. Given the existence of estimates on the sizes of the seawater sulfate pool and diffusive fluxes of sulfate into sediments over Earth's history, we propose that the observed correlation can be used for quantifying authigenic carbonate formation in shallow sediments along continental margins through time, which will greatly advance our understanding for past carbon cycling and its relation to climate. Key Points: Porewater parameters from 153 sites of the northern South China Sea were evaluated to quantify the extent of authigenic carbonate formationRate of carbonate formation strongly correlates with diffusive flux of sulfate into sediments affected by methane diffusionA correlation of carbonate formation rate with sulfate flux is used to quantify most of authigenic carbonate formation in shallow sediments [ABSTRACT FROM AUTHOR]

Marine methane hydrates are a huge and dynamic carbon reservoir found mainly at the continental margins, and their stability might be affected by climate-associated pressure and temperature changes on the seafloor. Reconstructing the growth history of cold-seep carbonates, which formed during seafloor methane leakage, could help to constrain methane hydrate stability in the geological past. However, U-Th isotope systematics of these complex carbonate cements have not yet been systematically investigated on the micro-scale, leaving uncertainties in the U-series geochronology which is commonly applied to date the seep carbonates. In this study, we have developed multiple in-situ analytical methods, including U-Th isotope analysis by laser ablation MC-ICPMS, elemental concentration mapping by laser ablation ICPMS, as well as organic distribution mapping by Raman Spectroscopy, to provide insights into the U-series geochemistry and geochronology for the different types of cold-seep carbonates. Our result demonstrates that 238U and 232Th of these carbonates are dominantly derived from seawater and detrital particles, respectively. As the [230Th/232Th] (activity ratio) of micro-domains with high [232Th/238U] is negligibly affected by U decay, we have directly determined the initial [230Th/232Th] of the cold-seep carbonates which is 0.7 ± 0.1 (2 SD, n = 12). In general, U-series isotopes show closed-system behavior within our studied analytical precision in the interior of the seep cements, and the initial [230Th/232Th] derived from the isochron approach is consistent with the in-situ direct determination. A notable exception is a calcitic pipe sample with highly enriched U in its rim which has also experienced post-depositional mobilization. Our method is then applied to a large set of seep carbonate samples from the upper continental slope of the northern South China Sea (SCS). The obtained U-series ages provide evidence that cold seep was likely continuously active since at least ∼72 ka in the northern SCS, but the timing and duration of methane leakage vary between different sites within this region. While the upper continental slope methane hydrates are highly susceptible to destabilization under changing bottom water conditions, our extensive dating on seep carbonates from the SCS further indicates that bottom water P-T conditions could exert contrasting impacts on methane hydrate stability at different depths within the same geological setting. Overall, the in-situ U-series geochronological method developed in our study has the potential to date cold-seep carbonates efficiently and reliably, offering new opportunities to probe into the history of methane leakage in the late Quaternary. [ABSTRACT FROM AUTHOR]

Highlights • Lipid biomarker patterns of active Haima seep carbonates were studied. • UCM in seep carbonate reflects the presence of crude oil at Haima seeps. • The predominant consortia in carbonates from both sites were ANME-1/DSS. • Methane seepage was at least intermittently more intense at site ROV2 than at ROV1. Abstract Authigenic carbonates retrieved from sites ROV1 and ROV2 of the Haima hydrocarbon seeps of the South China Sea at approximately 1390 m water depth were studied using lipid biomarker analyses. Abundant molecular fossils of anaerobic methane oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB) with strong 13C-depletions (δ13C values as low as −126‰), in combination with low δ13C carbonate values (−42.7‰ to −36.8‰), provide evidence that anaerobic oxidation of methane (AOM) was the major process driving the precipitation of the studied seep carbonates. The extremely low δ13C values of archaeal biomarkers confirm that biogenic methane was the main carbon source, but the seepage of accessory crude oil is also suggested by the presence of unresolved complex mixtures in the hydrocarbon fractions. A suite of 13C-depleted biomarkers indicate the predominance of ANME-1/ DSS consortia at both sites, which indicates that the studied carbonates formed during low to medium methane flux. Somewhat higher contents of archaeal biomarkers and their stronger 13C-depletion at site ROV2 probably reflect at least temporarily higher seepage intensities than at site ROV1. Abundant bacterial dialkyl glycerol diethers (DAGEs), revealing a large offset of their δ13C values compared to SRB-derived terminally branched fatty acids, were possibly produced by distinct SRB species other than members of the DSS cluster. The encountered hopanoids are attributed to aerobic methanotrophic bacteria based on their moderate 13C-depletion. The application of molecular fossils in combination with their compound-specific isotope signatures is an efficient tool to reconstruct the composition of seepage fluids and seepage intensities. [ABSTRACT FROM AUTHOR]

The magnesium (Mg) isotopic composition of marine carbonate has been used as a proxy to constrain the geochemical composition of seawater and to trace the global Mg cycle. Sulfate-driven anaerobic oxidation of methane (SD-AOM) is a prominent process that facilitates the formation of marine authigenic carbonate. Yet the understanding of the impact of SD-AOM on the Mg isotope composition of authigenic carbonate under natural conditions is limited. Here we present the analysis of two tubular methane-seep carbonates from the South China Sea. The tubular carbonates reveal a trend of decreasing δ13C values and increasing carbonate contents from the periphery (i.e., the outer surface of the carbonate) to the inner portion (i.e., close to the inner surface of the carbonate pipes). The tubular carbonates are interpreted as former fluid conduits, indicating intense and lasting seepage that apparently increased with time. The observed variability in composition probably results from changes in the intensity of SD-AOM and correlating carbonate precipitation rates. Trends of enrichment of middle rare earth elements (MREEs), molybdenum (Mo), and uranium (U) from the periphery inward indicate more reducing formation conditions toward the inner surface of the former conduit. Along the same transects, δ26Mg values decrease from periphery to inner portion in one tubular carbonate (−2.63‰ to −3.24‰), while δ26Mg values are close to constant for the second tubular carbonate (−3.42‰ to −3.37‰); such patterns highlight the complexity of Mg isotope fractionation during seep carbonate formation. A positive correlation between δ26Mg and δ13C values and a negative correlation between δ26Mg and Mg/Ca molar ratios suggests that enhanced SD-AOM facilitates the incorporation of light Mg isotopes into the carbonate lattice. However, such assumption contradicts with the fact that higher carbonate precipitation rates reduce Mg isotope fractionation due to incomplete dehydration of Mg ions. It is hypothesized that hydrogen sulfide produced by SD-AOM may enlarge the extent of fractionation by weakening the hydration of Mg. If this interpretation is correct, the effect of sulfide on the fractionation of Mg isotopes can even offset or exceed the effect of precipitation rate. Our study indicates that SD-AOM has an effect on the Mg isotope fractionation during carbonate precipitation. Similar effects may have occurred in geological periods with more widespread carbonate authigenesis. A better understanding of the factors affecting the Mg isotopic fractionation during marine carbonate authigenesis is needed to efficiently use this archive of paleoenvironments and element cycling. [ABSTRACT FROM AUTHOR]