Production, consumption, and migration of methane in accretionary prism of southwestern Taiwan

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.

Discharge of deeply rooted fluids from submarine mud volcanism in the Taiwan accretionary prism

“…As the capacity of AOM is limited, the efficiency of methane removal could not accommodate the extremely high supply of methane at center sites. This pattern is consistent with those obtained in onshore mud volcanoes in Taiwan 22,58 and SMVs elsewhere (e.g., the Hakon Mosby SMV 59 ).…”

“…In contrast, concentrations of total alkalinity (TA), boron, and lithium increased with depth. Between 120 and 280 cm below seafloor (cmbsf), sulfate was undetectable and , and potassium (in gray) concentration depth profiles at ten sites investigated in this and previous studies (for sites MV12-A) 22 . The solid lines represent the best fit of the modeled results.…”

“…The concentrations of headspace hydrocarbon gases were determined by a gas chromatography (GC; SRI 8610 C and Agilent 6890 N) with the precision typically better than 5% 22,58 . Ion concentrations were determined by an ion chromatography (IC, 882 Compact IC Plus) equipped with a Metrosep A Supp 5 column for anions and a Metrosep C 4 column for cations and with the detection limit around 0.1 ppm in weight 61 .…”

“…Fluid discharge from SMVs. To estimate the total discharge of brackish water from SMVs, the upward fluid velocities across the TY1 were compiled first with those obtained in the previous study 22 . The upward fluid velocities generally decreased with increasing distance from the edge of crater.…”

“…Cation abundance and total alkalinity profiles. (a) Calcium (in gray), magnesium (in black), and TA (in blue) concentration profiles at ten sites investigated in this and previous studies (for sites MV12-A)22 . (b) Lithium (gray dot) and boron (black dot) concentration profiles at eight sites.…”