Seafloor activity and deep-subsurface geology of gas hydrate areas revealed from δ13C of methane-derived authigenic carbonates along the eastern margin of the Sea of Japan

Abstract

The collection of microbial CH4 hydrates and methane-derived authigenic carbonates (MDACs) around Sado Island in the Sea of Japan has recently increased. In this study, the carbon isotopic composition of CaCO3 in MDAC (δ13CCaCO3) within these microbial CH4 hydrate areas was compared to that observed at the thermogenic CH4 hydrate mounds of the Umitaka Spur offshore of Joetsu, Sea of Japan. δ13CCaCO3 from thermogenic CH4 hydrate mounds (‐39.1 to -6.1‰ Vienna Pee Dee Belemnite [VPDB]) was more enriched in 13C than those of microbial CH4 areas (‐57.6 to -41.0‰ VPDB). The δ13CCaCO3 variations represented by 2σ standard deviations were also greater for the thermogenic CH4 hydrate mounds (8–11‰) compared to those of the microbial CH4 areas (6–7‰). Because the precipitation of MDACs with various δ13CCaCO3 compositions can occur over areas undergoing dynamic geochemical changes, larger variations suggest that the current thermogenic CH4 hydrate mounds with active gas seepage must have been geochemically dynamic during past MDAC precipitation. Oil-containing gas hydrates in the thermogenic CH4 hydrate mounds suggest a link with deep oil and gas reservoirs. CH4 leakage from deep-subsurface gas reservoirs may drive the higher activity observed at shallow sediment depths in thermogenic CH4 hydrate mounds. δ13CCaCO3 variations revealed differences in paleogeochemical dynamics at shallow sediment depths, and the geochemical characteristics may provide insights into deep-subsurface geological situations for CH4 supply.