EXCESS RN-222 ABOVE THE CLEFT SEGMENT OF THE JUAN-DE-FUCA RIDGE

The distributions of excess Rn-222 above the North Cleft segment of th e Juan de Fuca Ridge were determined from 1988 to 1991 in order to obt ain a better understanding of the temporal variability of heat flux fr om this ridge segment. Based on the inventories of excess Rn-222, 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 cro p of Rn-222, the difficulty in determining the Rn-222 to heat ratio, a nd the problem of entrainment of low-temperature waters into the risin g plume. The high-resolution sampling program carried out in 1990 perm itted the mapping of apparent plume ages for different sections of the neutrally buoyant plume. About 3 km to the east of the ridge axis the re 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 coul d indicate a current flow to the northwest during the time this transe ct was sampled. The mean Rn-222/heat ratios for the high- and low-temp erature vent fluids were 0.0017 disintegrations per minute (dpm) g-1-d egrees-C-1 and 0.0044 dpm g-1-degrees-C-1, respectively. The extrapola ted end-member concentrations of excess Rn-222 in low-temperature vent fluids are a factor of 6 greater than the high-temperature fluids, ba sed on the regressions of excess Rn-222 activity and Mg concentration in North Cleft segment vent fluid samples.