Assessing methane cycling in the seep sediments of the mid-Okinawa Trough: Insights from pore-water geochemistry and numerical modeling

Abstract

The methane-enriched fluids in cold seeps are likely to crystallize as gas hydrates and serve as crucial sources of carbon to seawater. In this research, we analyzed the pore-water composition in terms of CH4, dissolved inorganic carbon (DIC), Cl−, Br−, SO42−, Na+, Mg2+, Ca2+, Sr2+, and NH4+, and the δ13CDIC, δ13CCH4, and δDCH4 values of two gravity cores and six remotely operated vehicle (ROV) video-guided push cores retrieved from fault scarps and dome-like structures (DSs) on the western slope of the mid-Okinawa Trough. In addition, a reaction–transport model was applied to quantify the methane fluxes and related biogeochemical processes. Active seepage of biogenic (δ13CCH4 ~ –70‰ V-PDB) and thermogenic (δ13CCH4 = −40‰ to −56‰ V-PDB) methane was identified on fault scarps and dome structures, respectively. Methane seepage was controlled by the transport and dissolution of the ascending gas rather than by clay dehydration or gas hydrate dissociation-induced fluid advection. The high methane concentrations and shallow sulfate–methane transition zones (SMTZs; between 0.1 and 0.4 mbsf) at sites R3-C2, R4-C4, and R6-C1 suggest strong methane seepage at three of the four studied DS (the highest gas dissolution rates are 6450, 1475, and 515 mmol m−2yr−1). Site GC08, located along a fault scarp, exhibits a moderate methane seepage; the SMTZ is located at ~2.5mbsf, and the rate of anaerobic oxidation of methane (AOM) is 130 mmol m−2yr−1. The methane migrating from depth is mainly consumed by AOM. However, ~12%–66% of the methane released from the two most intensive seep sites escapes to the water column. The precipitation of high–Mg calcite (at all sites) and aragonite (only at site R3-C2) has fixed 27%–50% (average = 39%) of the DIC. Therefore, the carbon outputs to the water column have been reduced. In the study area, the area-weighted seafloor CH4 and DIC fluxes are ~30 and 20 mmol m−2yr−1, respectively. Together, they correspond to ~7%–14% of the organic carbon burial rate, indicating that sediments could not be simply regarded as a stable carbon sink because they provide methane and a certain amount of DIC to the water column. Our findings contribute to the results of the ongoing efforts in understanding carbon cycling in submarine cold seep systems.