Estimates of particle flux and reworking at the deep-sea floor using 234Th/ 238U disequilibrium

Abstract

Because of high specific activities of excess 234Th ( t 1/2 = 24.1 days) on suspended particles in the deep sea, this nuclide is potentially an extremely sensitive indicator of particle inputs and dynamics at the seafloor. Measurements were made at two deep-sea sites in order to examine this potential. Inventories of excess 234Th at a low-current hemipelagic mud site (3990 m) in the Panama Basin were ∼ 1.5 (September, ′81) and ∼ 2.5 (June, ′82) dpm/cm 2. The steady state fluxes to the seafloor calculated from these inventories are in rough agreement with radionuclide fluxes measured in sediment traps. Small-scale ( ∼ 100m) spatial variability in inventories implies biologically significant heterogeneity in particle inputs. Sediment from the continental rise site in the northwest Atlantic (2800 m), a site with higher current velocities than the Panama Basin, had an inventory of ∼ 1.9dpm/cm 2 . This inventory is also in rough agreement with predictions made on the basis of nearby sediment trap data. Particle mixing coefficients of ∼ 30cm 2/yr calculated at the Pacific and Atlantic sites are similar to those in shallow water deposits but could reflect disturbance during handling. Based on 210Pb data from the Panama Basin, sediment from below ∼ 6cm is mixed at a rate ∼ 10 × slower than the near-surface sediment to a depth of at least 20 cm. Agreement between 234Th predicted mixing rates at the Panama Basin site with 210Pb profiles and in-situ experiments with glass bead tracers implies that these rates are real. Although the diffusion of dissolved 234Th into deep-sea sediments complicates interpretations, 234Th xs distributions in bottom sediments offer a useful adjunct to sediment traps for investigation of particle dynamics near the deep-sea floor.