FY 1994

Excess ²²²Rn above the Cleft Segment of the Juan de Fuca Ridge

Gendron, J.F., J.F. Todd, R.A. Feely, E.T. Baker, and D.C. Kadko

J. Geophys. Res., 99(B3), 5007–5015, doi: 10.1029/93JB03209 (1994)

The distributions of excess ²²²Rn above the North Cleft segment of the Juan de Fuca Ridge were determined from 1988 to 1991 in order to obtain a better understanding of the temporal variability of heat flux from this ridge segment. Based on the inventories of excess ²²²Rn, the hydrothermal heat flux was estimated to be 2.7 ± 1.3 GW in 1990. The large uncertainty results from errors in calculating the standing crop of ²²²Rn, the difficulty in determining the ²²²Rn to heat ratio, and the problem of entrainment of low-temperature waters into the rising plume. The high- resolution sampling program carried out in 1990 permitted the mapping of apparent plume ages for different sections of the neutrally buoyant plume. About 3 km to the east of the ridge axis there is a sharp gradient in the apparent age of the plume, while to the west the plume appears to be younger. This plume age distribution could indicate a current flow to the northwest during the time this transect was sampled. The mean ²²²Rn/heat ratios for the high- and low-temperature vent fluids were 0.0017 disintegrations per minute (dpm) g⁻¹ °C⁻¹ and 0.0044 dpm g⁻¹ °C⁻¹, respectively. The extrapolated end-member concentrations of excess ²²²Rn in low-temperature vent fluids are a factor of 6 greater than the high-temperature fluids, based on the regressions of excess ²²²Rn activity and Mg concentration in North Cleft segment vent fluid samples.