Uranium and thorium series isotopes as indicators of geochemical processes in recent Venezuela Basin sediments

Abstract

Several uranium and thorium series isotopes are important indicators of abyssal geochemical and physical processes. These processes, in concert, control the timing and intensity of early sedimentary diagenetic changes. Important among these processes are sedimentation, bioturbation, compaction, dissolution and precipitation. This research was designed to investigate the relative importance of each of these on early sediment diagenesis in different abyssal environments. Sediment samples were collected from three different sedimentary environments from the Venezuela Basin. Samples were designated as pelagic carbonate, hemipelagic and turbidite-rich, based on bulk sediment characteristics. Selected members of the 238U and 232Th radioisotope series (238U, 234U, 230Th, 226Ra, 210Pb and 232Th) 14C, and anthropogenic 239,240Pu were determined on selected intervals. Other sedimentary parameters, including porosity, calcium carbonate content, stable isotopes, macrofaunal abundances, organic carbon and grain size, were measured or available from the literature. Nearly constant carbonate-free 232Th activities with respect to depth and location are indicative of a constant terrigenous sediment source throughout the basin. Uranium activities at two sites correspond to activities in Amazon River particulates, while sediments from the turbidite site appear enriched in uranium. 230Th activities are low at all sites and display considerable variability with depth, suggesting a high but variable sedimentation rate. A large 226Ra peak occurs just below the redox boundaries in the turbidite and hemipelagic sediments. No redox boundary is observed for the pelagic carbonate sediments nor is a 226Ra peak observed. Excess 210Pb and 239,240Pu activities are confined to the upper 10 to 15 cm of the sediment columns and are highly variable both between sites and between subcores. A model, based on the constancy of 230Th production in sea water, was developed to determine fine-scale sedimentation rates for the last 14 ky, 6 ky and 2 ky for sediments from pelagic carbonate, hemipelagic and turbidite-rich environments, respectively. Nannofossil δ13C values correlated well with carbonate accumulation rates, indicating that variations in carbonate accumulation on the sea floor are linked to variations in carbon cycling in the surface ocean. 226Ra fluxes across the sediment/sea water interface were estimated from losses in the expected 226Ra activities...