Upwarding gas source and postgenetic processes in the shallow sediments from the ARAON Mounds, Chukchi Sea

Abstract

The methane (CH4) emission from the Arctic Ocean is crucial to understand global carbon cycle. Here, we investigated sulfate (SO42−) in pore water and compositional and isotopic gas signatures at ARAON Mounds (hydrate/nonhydrate-bearing sites) and background site in the Chukchi Sea. Sulfate-methane transition (SMT) did not reach at the background site but occurred at shallow depths (≤3.3 m below the seafloor) at all ARAON Mounds sites. The SO42− profiles at ARAON Mounds also clearly indicate the unsteady state due to upward gas migration by high flux at the hydrate-bearing sites compared to the nonhydrate-bearing sites. The isotopic signatures of gas samples at the hydrate-bearing sites and below the SMT at the nonhydrate-bearing sites reflect thermogenic source transported across at least 1 km through faults/fractures in the Chukchi Sea. The headspace (HS) gas samples above/near the SMT at the nonhydrate-bearing sites are affected by the biogenic CH4 with enriched 12C; they indicate biogenic or thermogenic/biogenic mixed sources. The thermogenic gases below the SMT at ARAON Mounds have high C1/C2+ ratios (>300), much higher than those of normal thermogenic gases in offshore shallow sediments (<100), due to postgenetic processes during migration.The carbon isotopic fractionation (εc = δ13CCO2 - δ13CCH4) in HS samples of the background site and ARAON Mounds above the SMT are consistent with the biogenic gas range generated via microbial CO2 reduction. However, εc below the SMT is anomalously low (13–42‰) and is higher at the hydrate-bearing sites than at the nonhydrate-bearing sites. We postulate that this low εc is explained by the two-phase fluid transport model of Kim et al. (2012) and that gas hydrates highly influence this value. We suggest that εc can be used as a powerful geochemical proxy for the upward gas migration and gas hydrate occurrence in shallow marine sediment systems.