No abstract

To systematically quantify the production, consumption, and migration of methane, 210 sediment cores were collected from offshore southwestern Taiwan and analyzed for their gas and aqueous geochemistry. These data, combined with published results, were used to calculate the diffusive methane fluxes across different geochemical transitions and to develop scenarios of mass balance and constrain deep microbial and thermogenic methane production rates within the accretionary prism. The results showed that methane diffusive fluxes ranged from 2.71 × 10−3 to 2.78 × 10−1 and from –1.88 × 10−1 to 3.97 mmol m−2 d−1 at the sulfate‐methane‐transition‐zone (SMTZ) and sediment‐seawater interfaces, respectively. High methane fluxes tend to be associated with structural features, suggesting a strong structural control on the methane transport. A significant portion of ascending methane (>50%) is consumed by anaerobic oxidation of methane at the SMTZ at most sites, indicating effective biological filtration. Gas compositions and isotopes revealed a transition from the predominance of microbial methane in the passive margin to thermogenic methane at the upper slope of the active margin and onshore mud volcanoes. Methane production and consumption at shallow depths were nearly offset with a small fraction of residual methane discharged into seawater. The flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes at a magnitude of 1512–43,096 Tg Myr−1 and could be likely accounted for by the sequestration of methane into hydrate forms, and clay absorption.

To explore a new discovering hydrothermal field Geolin Mounds in the Southern Okinawa Trough (SOT), a multi-scale geophysical investigation cruise including single-beam echo sounder, sub-bottom profiler (SBP) and multi-channel seismic (MCS) surveys were conducted in 2018 in a new discovering hydrothermal field we named Geolin Mounds. Taking advantage of streamer feathering caused by strong Kuroshio Current during MCS data acquisition, we performed a pseudo-3D processing and produced a pseudo-3D seismic cube. In addition to flare features in water column, "rock grove" like morphological feature above seafloor, widely-distributed amplitude anomalies including blanking zone and high-amplitude reflectors are observed around the Geolin Mounds hydrothermal field in our 2D MCS data. Pseudo-3D seismic cube, on the other hand, provide estimation of the areas of blanking zone on selected time slice; furthermore, better characterize the fault structures in the hydrothermal field. The Geolin Mounds is the first disclosed site in the SOT where a hydrothermal field is without underlying submarine volcanos. We thus suggest that the Geolin Mounds hydrothermal field is in its embryo stage of evolution and is constantly supported by vigorous subsurface hydrothermal circulation. Consequently, the Geolin Mounds hydrothermal field may grow sustainably and serves as a good observatory for development of the seafloor edifice and ore mineralization associated with hydrothermal circulation activities.