CHEMISTRY OF SEAFLOOR HYDROTHERMAL VENT FLUIDS: IN-SITU PERSPECTIVES FROM LAB AND FIELD INVESTIGATIONS

Redox and pH provide fundamental constraint on mass transfer processes in all geochemical systems. Accordingly, we have developed and lab tested a series of electrochemical sensors for determination of pH and dissolved H2 and H2S for real-time and time series in-situ measurements of seafloor hydrothermal vent fluids. A key element of the design of the chemical sensors is the use of yittria-stabilized zirconia (YSZ) as both an indicator electrode (pH) and reference electrode (H2, H2S). Results of experiments at temperatures, pressures and dissolved NaCl concentrations as high as 400°C, 500 bars and 0.57 mm/kg, respectively, have confirmed rapid and reversible response over a broad range of pH and redox conditions in excellent agreement with theoretical calculations.

Results of laboratory investigations have provided the requisite justification for the development of a chemical sensor system for deployment at seafloor vents. Thus, field investigations using DSRV ALVIN were recently conducted at 9°50'N, East Pacific Rise. Vent fluid temperatures ranged from 360 to 370°C (250 bars). In general, at vent conditions, steady state temperature and composition data were achieved within minutes. In-situ dissolved H2 and H2S (real-time and time series) measurements reveal concentrations of approximately 0.12 and 10.1 mm/kg, respectively, which indicate moderately oxidizing conditions in subseafloor reaction zones, in general agreement with results using more conventional sampling approaches. pH values are slightly below neutrality at in-situ conditions, although sharply lower values occur at the seawater/vent-fluid interface where mixing induced mineralization reactions contribute excess acidity. The development of in-situ chemical and physical sensors for seafloor hydrothermal applications requires an integrated approach involving lab and field investigations. The absence of either will effectively preclude the development of reliable sensor systems needed if the goals of ocean observatory research at deep-sea vents are to be fully realized.