The distribution of geothermal fields on the Juan de Fuca Ridge

Abstract

Near bottom water temperatures were mapped along 400 km of the strike of the Juan de Fuca Ridge as part of a combined Sea MARC/Seabeam experiment to image the variability of morphology and structure along a spreading center segment. The water temperature data collected by a continuously towed thermistor chain, in addition to salinity data, indicate that there are four geothermal areas spaced at distances of 100 km from each other south of the Cobb propagator and one field just to the north of the propagator on the Endeavor Ridge segment. Each thermal region is located above a morphological dome on the spreading center. These domes are an average of 100–200 m shallower than the rest of the axis. The structure of bottom water suggests that the geothermal regions are on average 20 km long and that the heat from these fields raises the temperature in the water column by a minimum of 0.06°C up to 300 m above the bottom. Two simple models are used to estimate the heat flux associated with these features.

The influence of geothermal sources on deep ocean temperature, salinity, and flow fields

“…The source of the anomaly is geothermal heating at the East Pacific Rise. Larger-amplitude but smaller-scale anomalies have been observed at many locations in the North and South Pacific above the East Pacific Rise (Crane et al, 1985). Such anomalies however, have not previously been reported to occur near the Mid-Atlantic Ridge.…”

“…This example and other observations (e.g. Crane et al, 1985) of the structure of the background field above a ridge crest suggest that the main effect of the turbulent plume on its environment may be represented by a buoyancy driven vortex. Evidence for isolated patches of order lOkm in diameter from chemical tracer measurements is increasingly common as more complete sampling programs are conducted near ridges, and it may be speculated that nonlinear effects can produce isolated eddies from a continuous buoyancy source at the crest.…”

“…In the Pacific, isotherm deviations of several hundred meters are observed within about 10 km of heat sources, corresponding to horizontal temperature variations of about 0.1°C (Crane et al, 1985). Some of this structure is probably associated with horizontal geostrophic flow, but this possibility has not been discussed in the past.…”

“…A convenient formula based on measurements and models for the total average heat flux is 0.5/t 1 / 2 W /m 2 , where t is the age of the bottom in Ma (Morgan, 1982; age varies from 0 Ma at the crest to about 250 Ma near trenches). This formula diverges at t = 0, but the theoretical mean heat flow for age less than 1 Ma given by Sclater et al (1980) The distribution of conductive and hydrothermal heat flux along the crest is somewhat chaotic, but larger-scale patterns are beginning to appear (Crane et al, 1985). Variations in the heat flux are caused by variations in the properties of the crust and by mechanical changes such as fractures.…”

“…Variations in the heat flux are caused by variations in the properties of the crust and by mechanical changes such as fractures. Also, the nature of the underlying magmatic convection process may impress certain scales on the alongcrest distribution of heat flux (Crane et al, 1985). This latter process is suggested to be related to the occurrence of hot spots separated by about 100 km along a portion ofthe Juan de Fuca Ridge in the Pacific near 47°N, 130 0 W. Associated with explored hot spots are vent systems consisting of both high-temperature (approx.…”