A study of particle exchange at the sediment-water interface in the Benguela upwelling area based on 234Th/238U disequilibrium

The natural isotope Th-234 is used in a small-scale survey of particle transport and exchange processes at the sediment-water interface in the Benguela upwelling area. Results from water and suspended particulate matter (SPM) samples from the uppermost and lowermost water column as well as the underlying sediment of three stations are compared. The stations are situated in different sedimentological environments at 1200-1350m water depth at the continental slope off Namibia. Highly differing extent and particle content of the bottom nepheloid layer (BNL) are determined from transmissometer data. Three models are presented, all explaining the Th-234 depletion of the BNL and Th-234 excess of the surface sediment that were observed. While the first model is based solely on local resuspension of surface sediment particles, the second evaluates the influence of vertical particle settling from the surface waters on the Th-234 budget in the BNL. The third model explains Th-234 depletion in the BNL by sedimentation of particles that were suspended in the BNL during long-range transport. Particle inventory of the BNL is highest at a depocenter of organic matter at 25.5 degrees S, where strong deposition is presently taking place and lateral particle transport is suggested to predominate sediment accumulation. This is supported by the high settling flux of particles out of the BNL into the sediments of the depocenter, exceeding the vertical particle flux into sediment traps at intermediate depth in the same area by up to an order of magnitude. High particle residence/removal times in the BNL above the depocenter in the range of 5-9 weeks support this interpretation. Comparison with carbon mineralization rates that are known from the area reveals that, notwithstanding the large fraction of advected particles, organic carbon flux into the surface sediment is remineralized to a large extent. The deployment of a bottom water sampler served as an in situ resuspension experiment and provided the first data of Th-234 activity on in situ resuspended particles. We found a mean specific activity of 86 disintegrations per minute (dpm)g(-1) (39-339dpmg(-1)), intermediate between the high values for suspended particles (in situ pump: 580-760dpmg(-1); CTD rosette: 870-1560dpmg(-1)) and the low values measured at the sediment surface (26-37dpmg(-1)). This represents essential information for the modeling of Th-234 exchange processes at the sediment-water interface. (c) 2006 Elsevier Ltd. All rights reserved.