Budgets and behaviors of uranium and thorium series isotopes in Santa Monica Basin sediments

Abstract

Nine natural decay-series isotopes were measured in six box cores collected from a transect across the Santa Monica Basin. The 210Pb-derived sedimentation rate decreases from ~80 mg/cm 2-yr at the slope to ~20 mg/cm 2-yr in the deep central basin. Sediment mixing prevails in sites underlying oxic waters, but is subdued in the anoxic deep basin below the sill depth. Uranium contents in sediments are controlled by levels of authigenic U, which are higher in the more reduced condition in the deep basin. Most of the authigenic U results from precipitation within the sediments. The 232Th- 228Th disequilibrium in sediments indicates that 228Ra is lost from the sediments from a depth of ~ 10 cm upward. Modelling the distribution of excess 228Th and 234Th in the surficial layers of the deep basin sediments results in a mean sediment mixing coefficient of 0.2 cm 2/yr and a sedimentation rate close to that based on 210Pb. There is no evidence of changing sedimentation rate in the central basin during the past century. Fluxes of excess 210Pb, 230Th and 231Pa to the central Santa Monica Basin sediments are much higher than what can be predicted from local supply. Advective input of open ocean waters coupled with enhanced scavenging of these reactive nuclides at the ocean margin is considered to be the primary cause.