Distribution and Biogeochemical Properties of Hydrothermal Plumes in the Rodriguez Triple Junction

In 2010, we conducted seven surveys for the deep-sea hydrothermal plume through conductivity-temperature-depth profiler (CTD) “tow-yo” cast in the area of the Kairei field. We observed a turbidity anomaly with a maximum thickness of 120 m, the upper limit of which was at 2,150 m water depth, approximately 300 m above the Kairei field hydrothermal vents (~2,440 m). The depth of upper limit of turbidity anomaly around Kairei field was the same height as in previous reports. Because the maximum height of hydrothermal plumes are regulated by the density (temperature and salinity) of the end-member hydrothermal fluid and dilution by the ambient seawater, the height of the plume suggested that the hydrothermal activity of the Kairei field was also the same as 17 years ago. Deep sequencing of microbial 16S rRNA genes showed that the SUP05 phylotypes and Epsilonproteobacteria, which are known as the potential sulfur oxidizer and/or possibly hydrogen oxidizer, were propagated in the early stage of the hydrothermal plume and in the hydrothermal fluid–seawater mixing zone near the Kairei hydrothermal vents. Our exploration found a hydrothermal plume at 14 km north of the Kairei field, which had different H2/CH4 ratio expected from the end-member hydrothermal fluid of Kairei field and the ambient seawater mixing. The north plume had a lower H2, higher CH4 concentration, and higher microbial cell density than those in the hydrothermal plume around Kairei field. The north hydrothermal plume represented too oxic condition to harbor methane production by anaerobic methanogens. In addition, our microbial community structure analysis based on deep sequencing of 16S rRNA genes more than 10,000-amplicon reads per one sample showed no signal of methanogenic archaea. This suggests little in situ methanogenesis from H2 in the plume. It seems likely that high concentration of methane in the north plume is derived from another hydrothermal plume source rather than the Kairei hydrothermal fluids. Further studies will be needed to understand the cause of high methane concentration in the north plume.