LONG-TERM MONITORING OF HYDROTHERMAL HEAT-FLUX USING MOORED TEMPERATURE SENSORS, CLEFT SEGMENT, JUAN-DE-FUCA RIDGE

Heat flux from submarine vent fields can vary gradually on interannual scales, or nearly instantaneously in response to volcano-tectonic eve nts in the underlying crust. Neither case is well documented because m easurements of vent-field scale heat flux are scarce. We report here a new approach to hydrothermal plume monitoring sensitive to both progr essive changes and hydrothermal events. From June 1991 to May 1992 we moored 35 self-contained temperature sensors and six current meters on seven moorings located in and around the plume from a vent field on t he Cleft segment, Juan de Fuca Ridge. The hydrothermal plume was ident ified by a local temperature anomaly of 0.01-degrees to 0.03-degrees-C in the lowermost 200 m of the water column. Plume heat flux, defined as the net advection of this temperature anomaly, averaged about 250 M W during the deployment. This flux is less than previous estimates and thus supports speculation that the heat flux is declining after a sud den reinvigoration triggered by a seafloor rifting event in 1986.