Geochemical record of methane seepage in authigenic carbonates and surrounding host sediments: A case study from the South China Sea

Sediments at marine methane seep sites provide potential archives of past fluid flow that serve to explore seepage activities over time. Three gravity cores (D-8, D-F, and D-7) were collected from seep sites on the northern slope of the South China Sea where gas hydrates were drilled in the subsurface. Various carbon and sulfur contents, delta C-13 values of total inorganic carbon (delta C-13(TIC)), delta S-34 values of chromium reducible sulfur (delta S-34(CRS)), trace element contents, grain size, and AMS C-14 dating of planktonic Foraminifera in the sediments were determined to explore the availability of related proxies at seeps and to trace past methane seepage activities. Evidence for the presence of methane seepage and consequently anaerobic oxidation of methane comes from the occurrence of C-13-depleted authigenic carbonate nodules (delta C-13 values as low as -49 parts per thousand) discovered at an interval of 150-200 cm in core D-7. This finding is supported by high S/C ratios and molybdenum enrichment in the same interval. However, low contents of CRS and negative delta S-34(CRS) values are present. It is suggested to reflect a transient methane seepage event, which continued for about 1 ka based on the C-14 ages. Cores D-8 and D-F have delta C-13(TIC) values close to zero, low S/C ratios and CRS contents, negative delta S-34(CRS) values, and no trace element enrichment, suggesting a negligible impact of methane-seepage on the sediments. The negative delta S-34(CRS) values of the studied seep impacted and background sediments suggest that the application of delta S-34(CRS) alone as a proxy to identify AOM-related process may be insufficient. Sediment carbon-sulfur-trace element systematics and 14C ages used here have the potential to be a promising tool to recognize transient methane seepages and constrain their timescales. (C) 2017 Elsevier Ltd. All rights reserved.