Abstract :
Deep-sea hydrothermal systems are aimed as an on-site field analysis on the behavior and diffusion of CO2 in deep ocean. Through ocean ridge volcanism, a large amount of elements including carbon as a form of CO2 are supplied to deep ocean. Hydrothermal vent fluids at highly enriched in CO2 and show low pH ( pH 3) relative to seawater. Total carbonate, total CO2 in seawater, and pH were determined in samples at hydrothermal active area in S-EPR. The concentration of total carbonate and pH in the hydrothermal fluid samples ranged from 16 to 5 mM and from 3.1 to 7.6, respectively. The hydrothermal fluids discharged from the vents were rapidly diluted with ambient seawater, therefore total carbonate concentration and pH value in the plume waters become close to that of ambient seawater near the vents. The positive anomaly of total carbonate and negative anomaly of pH associated with hydrothermal plumes were observed on the seafloor along S-EPR axis. The diffusion of total carbonate plumes both westward and eastward in the bottom water along 15°S across the S-EPR were also detected, but pH anomalies were not obtained in the plume. These suggest the possibility of discharging of CO2 through hydrothermal systems to the ocean. Recent estimation of CO2 fluxes to the ocean through MOR was calculated at 0.7 15×1012mol C year−1. These values are 3–4 orders of magnitude smaller than the annual CO2 fluxes through terrestrial and marine respiration, therefore the importance of CO2 input from MOR on oceanic carbon cycle is thus minimal on shorter-term time scale. However, the CO2 input from MOR is significant at 106 107 years scales, and CO2 concentration in hydrothermal fluids at hotspot and back-arc basin is 10 100 times higher than that of MOR. The flux of CO2 from deep-sea hydrothermal systems to the ocean may be significant.
