Seep dynamics as revealed by authigenic carbonates from the eastern Qiongdongnan Basin, South China Sea

Methane-related authigenic carbonates play a significant role in the marine carbon cycle and atmospheric CH4 budget. To elucidate the relationship between seeping methane and authigenic carbonate precipitation, we analyze a 72-cm long methane-related carbonate column from the W08 site in the Qiongdongnan Basin, South China Sea. Mineralogical composition, stable carbon and oxygen isotopes, major and trace element contents, and three-dimensional internal structure have been analyzed. Based on the results obtained, four types of carbonate are identified. The first type recorded decreasing content of rare earth elements plus yttrium (REEY) and increasing carbonate porosity. This is attributed to the gradual increase in methane flux intensity, raising the rate of carbonate deposition and thus reducing the time for elemental uptake. The second type of carbonate displays the same delta 18O values as the theoretical equilibrium results, the maximum REEY contents, and the densely cemented carbonate structure, indicating a reduction in gas hydrate decomposition. The third type of carbonate formation occurring at the sediment-water interface shows a synchronous and significant enrichment of Mo, V, and Cd contents, suggesting the high hydrogen sulfide flux, which is subject to intensive methane flux and sulfate reduction. The fourth type is the late infill aragonite, which exhibits lower elemental contents compared to early cement due to less exposure to host sediments, while the difference in delta 18O and delta 13C values is not significant. The compilation of (Tb/Yb)N and Y/Ho ratios of coldseep carbonates worldwide suggests that the methane flux could alter the space of redox zones (e.g. Fe reduction zone) and thus affect the sink of MREE. X-ray computed tomography (CT) reveals the internal structure of the methane-related carbonate. The continuous pore system probably suggests an intense fluid/gas pressure, and the in situ brecciations imply the carbonate precipitation did not entirely block the methane release to the water column. A better understanding of authigenic carbonates is fundamental to evaluating how carbonates modulate the carbon balance in the methane seeps.