In situ measurement of dissolved H2 and H2S in high-temperature hydrothermal vent fluids at the Main Endeavour Field, Juan de Fuca Ridge

The first in situ measurements of dissolved H2 and H2S in high-temperature vent fluids were made at the Main Endeavour Field (Juan de Fuca Ridge) using the submersible Alvin and a newly developed electrochemical sensor. The measurements were successfully conducted in chimneys at sites of venting fluid and in pools of more quiescent hydrothermal fluid that underlie flanges on chimney structures at a depth of 2200 m below the sea surface. Fluid temperatures measured simultaneously with dissolved gas concentrations were up to 370°C. At the highest temperatures, dissolved H2 and H2S concentrations were 0.72 and 17.3 mmol/kg, respectively, which are consistent with data obtained at the same sites through conventional sampling methods. The relatively high concentration of dissolved gases measured by both techniques, however, may be linked to recent tectonic and volcanic activity. The ability to measure in situ dissolved gas concentrations simultaneously with fluid temperature in real time represents a major advance in the approaches available to study the origin and temporal evolution of seafloor hydrothermal systems at mid-ocean ridges. Although the present investigation is primarily based on sensor deployment for relatively short-term measurement of vent fluids, long-term monitoring of vent fluid holds great promise for further applications.