ON THE PARTITIONING OF HEAT-FLUX BETWEEN DIFFUSE AND POINT-SOURCE SEA-FLOOR VENTING

The first direct time series measurements of diffuse hydrothermal percolation velocities and tem have been obtained from a warm effluent source within a large vent field at the Endeavour Segment of the Juan de Fuca Ridge, 47-degrees-57'N, 129-degrees-06'W. A newly developed instrument using principles of electromagnetic induction to measure variations in small scale flow rates, and an integral array of thermocouples and thermistors, was emplaced directly atop a sulfide edifice from which vigorous diffuse hydrothermal percolation was observed. Samples of effluent temperatures and velocities were collected continuously for the period July 19 through September 2, 1988. Temperatures were found to be modulated by variations at semidiurnal tidal periods. The coherence between temperature and velocity signals was found to be strong at periods longer than 12 hours with a strong negative correlation between the two signals and no significant coherence at shorter periods. Effluent temperatures were found to vary from about 7-degrees-C to 13-degrees-C, and vertical effluent velocities were found to vary between 0.07 and 0.15 m s-1. Examination of transient events in the temperature and velocity records indicates that short time scale variations in effluent discharge rates appear to lead corresponding pulses in fluid temperature. A general synchronicity between effluent temperature variations and an energetic episode of local seismicity is indicated. Finally, the heat flux density from the top of the sulfide body due to this diffuse mode of effluent discharge was found to vary between about 2 and 4 MW m-2, with an arithmetic mean value of 2.91 (+/-0.23) MW m-2. Studies of point source "smoker" venting in this vent field, as well as extensive geological mapping of diffuse and point hydrothermal sources leads us to infer that heat flow out of the system due to diffuse mode flow may exceed that due to high-temperature venting by a factor of 5.