Distribution and speciation of arsenic around methane-seep on Joetsu Knoll, Japan Sea

Abstract

We analyzed arsenic in surface sediments and porewaters from Joetsu Knoll, which collected during the environmental survey for shallow-type methane hydrate. Arsenic concentration of sediments increased in the sulfate reduction zone at the reference site, but did not increase around the bacterial mats, despite its active sulfate reduction-anaerobic methane oxidation. Arsenic concentrations of porewaters generally varied 5-100 µg/L, and the depth of concentration peak increased with distance from the bacteria mat. The arsenic concentration of porewater was exceptionally high at the red snow crab aggregation site (105-187 µg/L), which may be attributed to the decomposition of organic matter in the deep sediments. Furthermore, speciation analysis of arsenic in the porewater detected unknown arsenic compounds, suggesting that leached arsenic may have become thioarsenic compounds in the presence of hydrogen sulfide. This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).