FY 1999

Sources and fluxes of hydrothermal heat, chemicals and biology within a segment of the Mid-Atlantic Ridge

Murton, B.J., L.J. Redbourn, C.R. German, and E.T. Baker

Earth Planet. Sci. Lett., 171(2), 301–317, doi: 10.1016/S0012-821X(99)00157-0 (1999)

We have conducted a segment-wide study of the chemical, biological and thermal effects of hydrothermal activity, in a bathymetrically enclosed, 50-km-long segment of the Mid-Atlantic Ridge, at 29°N. Our study involved long-term monitoring of bottom current speeds and temperatures coupled with "snap-shot" surveys of the concentrations of hydrothermal shrimp, manganese and particulate plumes occupying the water column enclosed by the axial valley. These data are combined with the calculated volume transport rates for the segment and the results compared with the heat and manganese flux from the Broken Spur vent field, located in the center of the segment. Our current meter data show that the basin-like bathymetry acts to restrict and control flow of water. Warm and shallow water, exported from the segment, is replaced by colder and deeper water. Water column profiles showed an increase in temperature of the water within the "Broken Spur" segment, compared with background stations located outside the ridge axis. The average refresh rate for the water column enclosed by the segment was found to be about 262 days. From these data, we calculate that the segment-wide flux of juvenile shrimp is ~20 × 10⁶ individuals per year. We show that the flux of manganese (exported at a rate of 5-6 mol/s) is matched by input from high-temperature effluent discharge at the Broken Spur vent field. We also calculate that the minimum average heat flux exported from the segment is about 275 MW. Of this, more than 90% is pervasively dissipated throughout the segment floor by conductively heated sea water, or simple thermal conduction, that carries little or no manganese to the water column. The remaining 10% or less of the heat flux results from localized, manganese-rich, high-temperature hydrothermal venting.