CO enrichment in hydrothermal plumes: Suiyo seamount, Izu-Bonin arc

We surveyed effluent hydrothermal plumes in and around the water column of Suiyou seamount (ca. 1,380m depth, 2km diameter), Izu-Bonin arc using R/V Kairei. Our study especially focused on the measurements of concentration and stable carbon isotopic compositions of carbon monoxide (CO) and methane (CH₄) in the plume. We also sampled venting fluids using manned submersible Shinkai 2000 to compare the chemical composition and isotopic compositions of CO and CH₄ with those in hydrothermal plume. We detect strong CH₄ anomalies not only on the water columns within the caldera, but also on those at outsides of the caldera wall at the depth of ca. 1,100-1,150m, the sill depth of the caldera wall. When we correct for the effect of ambient seawater mixing, the stable carbon isotopic compositions of CH₄ in plume coincide well with that at venting fluid, suggesting that aerobic methane oxidation is minimum within this plume. We also detect slight CO enrichment in hydrothermal plume. The stable carbon isotopic compositions of CO in the plume, however, exhibit highly 13C depleted value (-110 to -60‰ PDB) compared with those in seafloor venting fluid (around -30‰ PDB). Besides, the plume water exhibit higher CO/CH₄ ratio along with the distance from the fluid venting site. We conclude that some microbial CO production is active within this plume. The CO enrichment in deep seawater could be also good indicator for seafloor hydrothermal activity, especially for aged plume. This research is funded by Ministry of Education, Science and Technology through Special Coordination Fund ARCHEAN PARK project.