Settling, dissolution and burial of biogenic silica in the sediments off Somalia (northwestern Indian Ocean)

Particle fluxes of biogenic silica through the water column, silica burial fluxes into the sediments, and the flux of dissolved silica across the sediment-water interface estimated from pore water profiles are used to assess the behaviour of biogenic silica at two stations 80 and 270 km offshore along a transect off the Somali coast in the northwestern Indian Ocean. Particulate biogenic silica fluxes varied from 0.3 mmol m−2 day−1 in the non-upwelling season to 6 mmol m−2 day−1 during upwelling on the Somali slope. Fluxes were lower in the Somali Basin, from 0.2 to 2.3 mmol m−2 day−1. Evaluation of the dissolution curves derived by wet chemical leaching in sediment trap and sediment samples shows that the Km values, the apparent reactivity rates in alkaline medium, are higher for the shallow sediment traps than for deep trap and boxcore sediments. Modelling of pore water profiles shows that in the sediment most dissolution occurs in the top halfcentimetre, and pore water effluxes are in close agreement with those from in situ benthic incubations. Our results show that less than 10% of the biogenic silica arriving on the Somali Margin is buried in the sediments, giving a burial efficiency lower than the approximately 20% reported from the open Arabian Sea.