Hydrothermal venting along axes of seafloor spreading in the eastern Pacific gives rise to bouyant plumes of warm water which rise and mix with ocean bottom waters. These plumes are an important measure of hydrothermal output and dispersal of both heat and chemicals, and an exploration guide for new vent fields, yet little is known about their physical structure or the chemical processes which occur within them. Drs. Mottl and Von Herzen have developed a method for measuring and characterizing the hydrothermal flux of heat and chemicals on the scale of an individual vent field, using a 50 m high moored instrument array suspended above the submersible ALVIN via flotation. With this device, which was tested successfully on the East Pacific Rise in 1984, they can measure the three-dimensional temperature field of a plume and collect two-dimensional profiles of flow velocity, conductivity, pressure, and particle concentration via light transmittance, plus samples of water containing suspended particles of black sulfide "smoke" from various heights above the vents. They propose to apply this technique to a series of contiguous vent fields along a 10 km segment of the Juan de Fuca Ridge, as part of an integrated program of physical, chemical, and geologic observations their objectives are to characterize the physical and chemical structure of the vent field plumes, to compare these with plume theory, to estimate the chemical and heat fluxes from the vent fields and along the total segment due to both localized hot spring and diffuse warm spring venting, to identify chemical processes occurring within the plumes, and to determine the importance of these processes for the fate of the hydrothermal input. They will also compile a geologic map and description of the vent field and associated massive sulfide deposits.
