Source, Origin, and Spatial Distribution of Shallow Sediment Methane in the Chukchi Sea

It is essential to study methane in the Arctic environment in order to understand the potential for large-scale greenhouse gas emissions that may result from melting of relict seafloor permafrost due to ocean warming. Very few data on the sources of methane in the Chukchi Sea were available prior to initiation of the Russian-American Long-term Census of the Arctic (RUSALCA) program in 2004. This article documents for the first time the spatial variation of methane concentrations in the sediment and water column in a significant region of the Pacific Arctic and the influence of methane turnover and net transport from organic-rich environments within the western Chukchi Sea. The study combines historical observations, new data obtained during the RUSALCA collaborative program, and modeling results to provide insights into the contemporary methane dynamics of the western Chukchi Sea. We compare methane evolution at two sites with distinct geological settings, depositional patterns, and methane sources: (1) the deeper, fault-bounded Herald Canyon (northern site) where methane flux is controlled by both northward CH4 transport via ocean currents and diffusive influx of thermogenic methane (formed under high-temperature conditions) from source rocks at depth in the canyon's seafloor, and (2) the shallow Chukchi shelf (southern site), where sulfate reduction and anaerobic methane oxidation play a significant role in biogenic methane production and its flux within and from the sediments into the water column. Diffusive methane fluxes at the sediment-water interface within the southern and northern sites were estimated to be 14.5 mu mol dm(-2) day(-1) and 0.7 nmol dm(-2) day(-1), respectively. In addition, we suggest that biogenic methane emanating from the organic-rich southern region is transported northward by the Anadyr Current, leading to a mix of both biogenic and thermogenic methane in Herald Canyon surface waters. Study results indicate that the South Chukchi Basin is an important source of atmospheric CH4. Further work is required to accurately quantify this flux.