Biogeochemical cycling of silicon in the Arctic Ocean

• Applicant: Dr. Claudia Ehlert
• Subject Area: Oceanography
• Term: from 2018 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 399076022

Silicon (Si) and its stable isotope ratios (δ30Si) provide information in the modern as well as the past ocean, e.g. about weathering and inputs from land, water mass mixing and nutrient (silicate) utilization by diatoms in surface water. The Si cycle in the Arctic Ocean is characterized by unique features and its study is of particular interest because it will provide information on biogeochemical cycles in this isolated and highly sensitive environment. The production and recycling of diatom opal are the dominant biogeochemical processes controlling Si isotope distributions in the global ocean, however, diatom primary productivity in the Arctic Ocean is generally low and restricted to a short season in the year due to low dissolved Si availability, seasonal sea-ice coverage, and light limitation. Therefore, previous studies suggested that seawater δ30Si signatures in the Arctic Ocean are dominated by physical water mass mixing, whereas biological Si cycling but also the input of terrestrial dissolved and particulate Si via rivers and subsequent dissolution of the particles might play only a minor role. However, based on preliminary analyses of dissolved Si concentrations and isotope compositions of water samples we hypothesise that diatom primary productivity in surface waters, as well as terrestrial inputs via river discharge and transport with the Transpolar Drift from the shelves to the Central Arctic, and biogenic and lithogenic particle fluxes from the surface to the deep waters lead to substantial differences in the Si isotope composition of waters and particles at all depths in the central Arctic Ocean. We want to test this hypothesis and investigate in detail the Si biogeochemical cycling in the Arctic Ocean. To understand and disentangle the different processes, we will analyse full water column profiles of dissolved and accompanying suspended particulate samples from the Central Arctic Ocean basins and the Barents Sea shelf that were collected during cruise PS94 ARK-XXIX/3 (TransArc II; PIs: U. Schauer, M. Rutgers van der Loeff, AWI) with R/V Polarstern in August-October 2015.In addition to the specific understanding of the Arctic Ocean Si biogeochemical cycling, this study will provide unique information about the transfer of the surface water Si utilization signal through the water column to the deep ocean and will therefore provide important general information on the applicability of diatom δ30Si as a paleo proxy in the global ocean. This study will further provide a baseline for future investigations on the impact of climate change in the Arctic. So far, we can assume that the general Si cycle has not changed significantly, however, rising temperatures, lower sea-ice cover, and increased terrestrial runoff as well as changes in the Siberian hinterland (e.g. melting permafrost and changes in the weathering regime) are likely to substantially affect the Arctic Si cycling in the future.

• DFG Programme: Research Grants
• Co-Investigator: Professorin Dr. Katharina Pahnke-May