228Ra and 228Th concentrations in GEOSECS Atlantic surface waters

Abstract

An average 228Ra flux of 0.6±0.1 dpm cm −2 yr −1 from continental shelf sediments can maintain the estimated total 228Ra inventory of about 5 × 10 17 dpm in Atlantic surface waters. By fitting 228Ra and potential temperature data to Munk's vertical advection-diffusion model, upwelling rates of 17±10 m yr −1 and vertical eddy diffusion coefficients of 0.4±0.2 cm 2 s −1 were obtained in the upper water column of the equatorial Atlantic. The downward fluxes of particulate 228Th across the 350-m water depth are about 0.01 to 0.04 dpm cm −2 yr −1 in the Sargasso Sea and in the regions south of 12°N and about 0.06 to 0.11 dpm cm −2 yr −1 in the northern temperate region (excluding the Sargasso Sea). The shorter half-removal time of 228Th in the equatorial (10°S to 20°N) and the northern region (35° to 65°N) are related to higher biological productivity there.