Geochemical responses and implications for gas hydrate accumulation: Case study from site SHC in Shenhu Area within northern South China Sea

A knowledge of gas hydrates is critical for understanding complex geological, geophysical, and biogeochemical processes and for explaining the proposed link between gas hydrate destabilization and global sea level changes. With the aim of discussing the geochemical characteristics and implications for gas hydrate accumulation, this study assesses the changes in a series of organic and inorganic geochemical gas hydrate-bearing sediments within a core section obtained at site SHC in the Shenhu Area of the northern South China Sea that range from the Late Miocene to the present. High S/C ratios (0.53-5.6) and low TOC contents ( < 0.5%) suggest that strong ancient anaerobic oxidation of methane (AOM) occurred in sediments between 96 mbsf and 182 mbsf at site SHC. Therefore, the high total inorganic carbon (TIC) content (with a peak value of 47%) in sediments close to the gas hydrates-bearing sediment zone (GHBSZ) can be partially attributed to the AOM, and this also provides a reasonable explanation for the high concentrations of captured acid-extraction CH4 (58.38-576.1 mu l/g) in the samples. The relatively lower C-13 values ( - 4.10 parts per thousand to -1.69 parts per thousand) in the carbonate indicate that the contribution of dissolve inorganic carbon is partially from methanogenesis gas within the GHBSZ. However, the relatively higher values of delta O-18, as well as the inverse correlation between values of the delta O-18 and the delta C-18 observed in the GHBSZ, likely suggest a dissociation event of gas hydrates. These features are effective for indicating the presence of AOM in ancient times and implicate the formation of authigenic carbonates in sediments. The high value of authigenic carbonates by the AOM overly the GHBSZ in sediments, which implies the authigenic carbonate may have played an important role in the accumulation and sealing of gas hydrates at the SHC site. The relatively high contents of branched glycerol dialkyl glycerol tetraethers (B-GDGTs) and the relatively high values of the BIT index in the GHBSZ at SHC site are consistent with an increase in terrestrial source inputs which are marked by fine sands and silty deposits. These features also coincide with the change trend in the TOC/TN ratio and delta C-1(3)org, value, which imply an increase in terrestrial organic mass. Such significant paleoenvironment change is consistent with the results of previous studies based on sediment cores and seismic reflection data. It is thus suggested that those proxies are related to a relative sea level fall caused by the global Messina carbon shift in the South China Sea. Such geochemical imprints not only reveal that the ancient AOM played an important role in the formation of the reservoir-cap assemblage, but also suggest that reservoir generation at SHC site is related to a relative sea level change. We propose that the formation of authigenic carbonates and their sealing effect should be considered when studying gas hydrate accumulation mechanisms.