James Ward, Katharine Hendry , Sandra Arndt , Johan C Faust, Felipe S Freitas, Sian F Henley, Jeffrey Krause, Christian Maerz, Hong Chin Ng, Rebecca A Pickering

Biogeochemical cycling of silicon (Si) in the Barents Sea is under considerable pressure from physical and chemical changes, including dramatic warming and sea ice retreat, together with a decline in dissolved silicic acid (DSi) concentrations of Atlantic inflow waters since 1990. Moreover, further expansion of the Atlantic realm (termed ‘Atlantification’) is expected to shift phytoplankton community compositions away from diatom-dominated spring blooms in favour of Atlantic flagellate species. The changes in pelagic primary production will alter the composition of the material comprising the depositional flux, which will subsequently influence the recycling processes at and within the seafloor. In this study we assess the predominant controls on the early diagenetic cycling of Si, a key nutrient in marine ecosystems, by combining stable isotopic analysis (δ30Si) of pore water DSi and of operationally defined reactive pools of the solid phase. We show that low biogenic silica (BSi) contents (0.26-0.52 wt% or 92-185 μmol g dry wt−1) drive correspondingly low asymptotic concentrations of pore water DSi of ~100 μM, relative to biosiliceous sediments (>20 wt% BSi) wherein DSi can reach ~900 μM. While Barents Sea surface sediments appear almost devoid of BSi, we present evidence for the rapid recycling of bloom derived BSi that generates striking transient peaks in sediment pore water [DSi] of up to 300 μM, which is a feature that is subject to future shifts in phytoplankton community compositions. Using a simple isotopic mass balance calculation we show that at two of three stations the pore water DSi pool at 0.5 cm below the seafloor (+0.96 to +1.36 ‰) is sourced from the mixing of core top waters (+1.46 to +1.69 ‰) with the dissolution of BSi (+0.82 to +1.50 ‰), supplemented with a lithogenic Si source (LSi) (-0.89 ±0.16‰). Further, our sediment pore water δ30Si profiles uncover a coupling of the Si cycle with the redox cycling of isotopically light metal oxides (-2.88 ±0.17‰). We suggest that a high LSi:BSi ratio and apparent metal oxide influence could lead to a degree of stability in the annual background benthic flux of DSi despite the pressures on pelagic phytoplankton communities. Coupled with supporting isotopic evidence for the precipitation of authigenic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget.