Do bottom mixed layers influence 234Th dynamics in the abyssal near-bottom water column?

Dynamics of the natural radioactive particle tracer 234Th (half-life: 24.1 days) within the abyssal water column up to 1000 m above bottom and within surface sediments of the northeast Atlantic (Porcupine Abyssal Plain; depth: ≈4845 m) were investigated. Distributions of transmissometer voltages and potential temperature indicated a subdivision of the near-bottom water column into a benthic mixed layer (BML; thickness: ≈10–65 m) and the layer above the BML up to the upper boundary of the bottom nepheloid layer (BNL; thickness: ≈1000 m). Comparison of 234Th fluxes (dpm m−2d−1) in sediment traps, vertical fluxes derived from 234Th/238U-disequilibrium in the near-bottom water column and excess 234Th inventories in the surface sediment provided evidence for lateral advection of 234Th-depleted water and a ‘missing sink’ for 234Th. A simple one-dimensional steady-state box-model approach was applied to investigate 234Th dynamics. Estimated residence times suggest the BML and the resuspension zone of the surface sediment to be highly dynamic systems with respect to particle cycling and sorptive reactions on a time scale of weeks. Model results indicate that, through the chemical forcing of changing particle concentration, a thickening BML results in (1) increasing residence times of particulate 234Th in the BML with respect to the net fluxes across the upper boundary of the BML and into the surface sediment; (2) declining adsorption rate constants in the BML; (3) increasing desorptive fluxes in the BML resulting in (4) enhanced 234Th decay in the BML; (5) decreasing net fluxes of particulate 234Th from the BML to the upper BNL above the BML and to the sediment. Potential consequences for carbon cycling in the water column of the deep ocean are discussed.