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

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Inthorn, M. , Rutgers v. d. Loeff, M. and Zabel, M. (2006): A study of particle exchange at the sediment-water interface in the Benguela upwelling area based on 234Th/238U disequilibrium , Deep-sea research i, 53 (11), pp. 1742-1761 . doi: 10.1016/j.dsr.2006.08.004
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Abstract:

The natural isotope 234Th is used in a small scale survey on particle transport and exchange processes at the sediment-water interface in the Benguela upwelling area. Results from water andsuspended 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 differentsedimentological environments at 1200 to 1350 m water depth at the continental slope off Namibia. Highly differing extension and particle content of the bottom nepheloid layer (BNL) are determined from transmissometer data. Three models are presented, all explaining the 234Th depletion of the BNL and 234Th excess of the surface sediment that were observed. While the first model is based solely on localresuspension of surface sediment particles, the second evaluates the influence of vertical particle settlingfrom the surface waters on the 234Th budget in the BNL. The third model explains 234Th depletion in the BNL with 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°S, where strong deposition is presently taking place and lateral particle transport is suggested to have predominant significance onsediment 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 water 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 to 9 weeks strengthen this interpretation. Comparison with carbonmineralization 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 ofa bottom water sampler served as an in-situ resuspension experiment, and provided first data of 234Th activity on in-situ resuspended particles. We found a mean specific activity of 86 dpm g-1 (39-339 dpm g-1), intermediate between the high values for suspended particles (in-situ-pump: 580-760 dpm g-1; CTD-rosette: 870-1560 dpm g-1), and the low values measured at the sediment surface (26-37 dpm g-1). This representsessential information for the modeling of 234Th exchange processes at the sediment-water interface.

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