Biogeochemical processes involving acetate in sub-seafloor sediments from the Bering Sea shelf break

Abstract Biogeochemical processes involving acetate in sub-seafloor sediments from piston core PC23B from the Bering Sea shelf break were inferred by examining the stable carbon isotopic relationships between acetate and other relevant carbon compounds: total organic carbon (TOC) in the sediment solid phase, and dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in pore water. Throughout the core, the isotopic composition of acetate (δ 13 C acetate ), from −31‰ to −29‰, was 13 C-depleted by ca. 7‰ vs. DOC (δ 13 C DOC ) and its depth profile approximately paralleled that of δ 13 C DOC , suggesting that the principal process producing acetate was fermentation of dissolved organic compounds. However, the 13 C depletion in δ 13 C acetate indicates some contribution of acetogenesis to total acetate production, because acetogenesis results in 13 C depletion of the acetate produced. The relative contribution of acetogenesis via the H 2 /CO 2 reaction, calculated by using a two source isotope mixing model, increased with depth in the sulfate reduction zone from 10% to 15% and was constant at 19% in the methanogenic zone. The acetogenic contribution to acetate production in the methanogenic zone underlying the sulfate reduction zone is consistent with reported observations, whereas the occurrence of acetogenesis in the sulfate reduction zone may be related to the contribution of terrestrial organic matter (OM) to the sedimentary OM in that depth interval, because the terrestrial component likely includes precursors that favor organoautotrophic acetogenesis. The high acetate concentration (up to 81 μM) and TOC content (up to 1.4%) at the same depth (