Silicic acid enrichment in the deep water of the Faeroe–Shetland Channel

Abstract

To evaluate the relative contribution of continental slope sediments, resuspended particles and settling material to the refuelling of dissolved silica in a high-energy environment, a study was conducted in the Faeroe–Shetland Channel (FSC), one of the main pathways of the newly formed Atlantic Deep Water. Fluxes of silicic acid (DSi) to the water column determined by multiplying the DSi concentration gradient by the vertical eddy diffusion coefficient showed a maximum at mid-slope (600–800 m) with values reaching up to 1700 μmol m−2 d−1 near the seabed. The mere dissolution of settling biogenic silica, estimated at 30–120 μmol m−2 d−1, cannot account for this refuelling of the deep-waters of the FSC with DSi. Benthic DSi effluxes on the slope, obtained both from incubations and from modeling pore water DSi profiles, indicated higher fluxes at mid-depth which corresponds to a zone of enhanced deposition and higher biogenic silica content. Contribution of DSi from benthic fluxes range from 370 to 740 μmol m−2 d−1 and can, to a large extent, sustain the enhanced calculated flux in the water column. Furthermore, the recurrent resuspension of bottom material, as observed at mid-slope being associated with turbulent flow, contributes to the increase in DSi concentrations in the deep water of the FSC with fluxes ranging from 60 to 800 μmol m−2 d−1. Our data illustrate the importance of the geomorphological setting of the basin which, with its shallow depth, funnel shape and turbulent conditions, favors the erosion and resuspension of surface sediment and the deposition of relatively fresh sediments at mid-slope, hence focusing the refuelling of passing waters with DSi.