Carbon isotope biogeochemistry of acetate in sub-seafloor sediments in the Sea of Okhotsk near Sakhalin Island, Russia

Abstract

Biogeochemical processes involving acetate in sub-seafl oor sediments were investigated by examining the stable carbon isotopic relationships of acetate and other relevant carbon-bearing materials (i.e., total organic carbon (TOC) and ΣCO2) in fi ve piston cores retrieved from the Sea of Okhotsk off Sakhalin Island, Russia. The cores were classifi ed into two types on the basis of SO42– content: (I) those with sulfate-reducing sediments in which the depletion of pore-water SO42– with increasing depth was slight, and (II) those with methanogenic sediments in which SO42– concentrations were less than 2 mM more than 3 m below the seafl oor. Acetate was detected in all cores. The acetate content in methanogenic sediments (2.6–23.0 μM) was relatively higher than in the sulfate-reducing sediment (2.8–8.8 μM). In the sulfate-reducing sediments, the depth profi les of δ13Cacetate approximately parallel those of δ13CTOC, with the δ13Cacetate values (–39‰ to –33‰) depleted by about 12‰ relative to δ13CTOC (–24.5‰ to –22.3‰). These approximately parallel depth profi les suggest that the principal acetate production process in the sulfate-reducing sediments is fermentation of dissolved organic compounds. The fermentation products, however, tend to be similar or slightly enriched in 13C compared to their substrates. Therefore, the 13C depletion of acetate relative to TOC in the sulfate reduction zone suggests that some portion of the acetate was synthesized by acetogenesis in which the synthesized acetate is depleted in 13C compared with its precursor. Given the large contribution of land-derived organic matter in the studied sediments, organoautotrophic acetogenesis using the lignin-derived syringate monomer, which originates from land plants, is likely. In the methanogenic sediments, the δ13Cacetate values in sediments throughout the cores (–39‰ to –25‰) were depleted compared to δ13CTOC (–25.5‰ to –21.4‰). This suggests some acetogenic contribution to the total acetate production. The depth profi les of δ13Cacetate in methanogenic sediments did not parallel those of either δ13CTOC or δ13CΣCO2, probably because of the mixed isotopic effect from some production and consumption processes; namely fermentation, acetogenesis, and acetoclastic methanogenesis. * Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan ** Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan *** Institute of Low Temperature Science, Hokkaido University, N19W8 Kita-ku, Sapporo, Hokkaido 060-0819, Japan **** Present address: Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 4648601, Japan a Corresponding author. Tel: +81358414250. Fax: +81358418791. e-mail: ijiri@eps.s.u-tokyo.ac.jp Present address: Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan