"Processes at the Sediment-Water Interface of the Peru Shelf: Biogenic Fluxes and Phosphorite Formation"

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Two expeditions to the Peru Continental Margin will investigate biogeo chemical processes near the sediment surface. The P.I:'s will measure the fluxes of biogenic components across the sediment-water interface linked to bacterial mat activity, organic matter diagenesis, iron redox cycling, phosphate precipitation and sedimentary denitrification. The selected study area (11o40"S - 15oS; 50 m -600 m) exhibits high persistent upwelling, primary production and sediment organic carbon; ubiquitous microbial mats; modern phosphorites; and near-zero oxygen and "nitrate deficit" in the near-bottom water column. A smaller are (1300 sq. Km at 13o30'S), selected to study benthic processes will be box cored in a bottom-navigated array to provide a detailed map for submersible operations and bottom experiments. Fluxes will be obtained with benthic flux chambers (Landers), microelectrodes (Profilers) and mm-scale pore water profiles (box cores and Whole-Core Squeezer). The P.I:'s will also deploy near-bottom sediment traps and current meters to determine transport velocities and directions and the biogeochemical sources of particles entrained in the poleward bottom flow. Both long- and short- term in situ experiments on the seabed will test theories concerning (1) apatite nucleation and solubility; (2) sediment disturbance and sediment starvation; (3) patchy distributions of laminated sediments, mats, organic carbon and pellets; and (4) Fe- and O2-redox driven phosphorus reaction.