Abstract
Sediment profiles of 228Ra and 232Th and water column profiles of 228Ra are presented for San Nicolas and San Pedro Basins, located in the Southern California Borderland. These data are used to estimate fluxes of 228Ra from the sediments, the vertical eddy diffusivity (KZ) in the water column, and the upwelling velocity (wZ). The fluxes of 228Ra from sediments, estimated by integrating the downcore deficiency of 228Ra with respect to its parent nuclide 232Th, are 335±115 atoms/m2s for San Nicolas Basin and 166±50 atoms/m2s for San Pedro Basin. San Nicolas Basin sediments have an excess of 228Ra in the upper 4 cm, but irrigation and diffusion create a 228Ra deficiency from 4 to 30 cm. San Pedro Basin sediments are anoxic and laminated, have a 228Ra deficiency above 6–12 cm depth, and have no surface excess of 228Ra. Basin waters appear to be well‐mixed horizontally below sill depth and in steady‐state with respect to 228Ra. Concentrations of 228Ra increase from sill depth to the bottom, ranging from approximately 1.0 dpm/100 kg to 2.4 dpm/100 kg in San Nicolas Basin and from 1.6 dpm/100 kg to 3.1 dpm/100 kg in San Pedro Basin. Estimates of KZ and wZ at several horizons in these basins are made by evaluating two steady state mass balance equations, one for 228Ra and one for heat. This approach is hindered by an inability to accurately define the temperature in waters flowing over the sills and the apparent failure of topography to completely restrict horizontal exchange in the upper part of the water column below sill depth in each basin. Upper limits for KZ are 23 cm2/s at 1700 m in San Nicolas and 2.8 cm2/s at 850 m in San Pedro Basin. Our best estimates for KZ and wZ for the deeper part of the basins are: KZ = 5–17 cm2/s and wZ = 50–150 m/y in San Nicolas Basin, and KZ = 1–3 cm2/s and wZ = 20–50 m/y in San Pedro Basin. The differences in KZ between basins reflect differences in the buoyancy gradients. Upwelling velocities increase from the bottom to the sill of each basin.
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