Porewater geochemistry indicates methane seepage in the Okinawa Trough and its implications for the carbon cycle of the subtropical West Pacific

The Kuroshio Current, originating from the West Pacific Warm Pool and flowing through the Okinawa Trough, connects the equator and the subtropical West Pacific and plays an important role in the carbon cycle in the West Pacific Ocean. Previous studies have focused on the effects of surface export productivity and bottom redox conditions on the carbon cycle in the late Quaternary. Recent studies have demonstrated that methane seepages are very common in the Okinawa Trough, however, their significance for the carbon cycle remains not well understood. Here, methane seepages and sulfate-driven anaerobic oxidation of methane (SD-AOM) are constrained by the geochemical characteristics of porewater at site CSHC-4 (Length: 1555 cm) derived from the water depth of 934 m on the western continental slope of the Okinawa Trough. Using the △(DIC+ Ca2++ Mg2+)/△SO42− ratio, it is suggested that the sulfate reduction in the upper part of site CSHC-4 is dominated by organoclastic sulfate reduction (OSR), while that between 655 and 1055 cm is gradually dominated by SD-AOM with deepening depth. However, with the depletion of sulfate, the methanogenesis process predominates at the bottom of site CSHC-4. The depth of the sulfate-methane interface (SMI) and the methane diffusive flux of site CSHC-4 is calculated to be ∼1156 cm and ∼ 63 mmol m−2 yr−1, respectively. Near the SMI, the changes in concentrations of Ca2+, Mg2+, Sr2+ and ratios of Mg/Ca and Sr/Ca in the porewater indicate the possibility of the precipitation of authigenic high-Mg calcite in the sediments. In contrast, the increase in the concentration of Ba2+ in porewater below the SMI may imply the dissolution of barite (BaSO4). In addition, redox-sensitive elements in porewater exhibit various geochemical behaviours due to changes in redox conditions caused by SD-AOM. The above SD-AOM processes will affect the reconstruction of surface productivity and redox conditions of bottom water in the Okinawa Trough since the late Quaternary based on biogenic elements (e.g., Ca and Ba) and redox-sensitive elements (e.g., U and Cr), which requires careful evaluation. Therefore, our new findings illustrate the modern methane seepage in the Okinawa Trough and provide a reference for calculating the ancient carbon cycle in the subtropical West Pacific.