Particle mixing rates in deep-sea sediments determined from excess 210Pb and 32Si profiles

Abstract

Particle mixing rates have been determined for 5 South Atlantic/Antarctic and 3 equatorial Pacific deep-sea cores using excess 210 Pb and 32 Si measurements. Radionuclide profiles from these siliceous, calcareous, and clay-rich sediments have been evaluated using a steady state vertical advection diffusion model. In Antarctic siliceous sediments 210 Pb mixing coefficients (0.04–0.16 cm 2 /y) are in reasonable agreement with the 32 Si mixing coefficient (0.2 or 0.4 cm 2 /y, depending on 32 Si half-life). In an equatorial Pacific sediment core, however, the 210 Pb mixing coefficient (0.22 cm 2 /y) is 3–7 times greater than the 32 Si mixing coefficient (0.03 or 0.07 cm 2 /y). The difference in 210 Pb and 32 Si mixing rates in the Pacific sediments results from: (1) non-steady state mixing and differences in characteristic time and depth scales of the two radionuclides, (2) preferential mixing of fine-grained clay particles containing most of the 210 Pb activity relative to coarser particles (large radiolaria) containing the 32 Si activity, or (3) the supply of 222 Rn from the bottom of manganese nodules which increases the measured excess 210 Pb activity (relative to 226 Ra) at depth and artificially increases the 210 Pb mixing coefficient. Based on 32 Si data and pore water silica profiles, dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32 Si profile in the mixed layer. Deep-sea particle mixing rates reported in this study and the literature do not correlate with sediment type, sediment accumulation rate, or surface productivity. Based on differences in mixing rate among three Antarctic cores collected within 50 km of each other, local variability in the intensity of deep-sea mixing appears to be as important as regional differences in sediment properties.