Age and depositional environment of authigenic carbonate in the gas hydrate field offshore southwestern Taiwan

Abstract:

Formation of carbonate concretions has been suggested to be enhanced with gas hydrate decomposition and considered to be a chronometer of past methane seep events. Here we present time series of authigenic carbonates paired with ²³⁰Th dating techniques and multi-tracers including radiogenic isotopes (Sr and Pb) and rare earth elements (REE) to reconstruct past seep evolution and environment from gas hydrate sites offshore southwestern Taiwan. Using ²³⁰Th dating techniques, we determined formation ages of authigenic carbonates collected from gas hydrate sites offshore southwestern Taiwan, including Yuan-An Ridge, Good Weather Ridge, and 96 Mud Volcano Group. These carbonates deposited at three time intervals of 50-80, 150-200, and 350-400 thousand years ago (ka). Carbonate intensively formed during 150-200 ka, equivalent to Marine Isotope Stage (MIS) 6, the penultimate glacial time with sea level about 80-100 m lower than present. Most marine methane emanation generated at glacial period or the decline of global sea level, indicating that higher water pressures provide the best conditions for methane hydrate formation. Sr and Pb isotopic signatures and REE diagrams show diverse formation environments and different sources.