Benthic cycling of silicon in Arctic and subarctic shelf sediments: insights from silicon isotopes

Spring diatom blooms in the Atlantic Arctic & Subarctic Provinces (AASP) account for significant annual net primary production and pCO₂ drawdown; recent work has shown that the supply and distribution of dissolved silicon (Si) is a key factor regulating diatom growth during this season. However, the sources and internal cycling of Si in the AASP require further investigation and quantification, particularly in coastal and shelf settings. Here, we use a combination of Si concentrations and stable isotopic compositions to investigate Si cycling in shelf sediments at two locations within the AASP: coastal southwest Greenland and the Barents Sea. A combination of silicon isotopes in porewaters and core incubation experiments show that these sediments are an important source of biologically available Si to overlying waters due to a high content of distinctive reactive silica phases. Reaction transport modelling allows us also to constrain the dominant chemical processes occurring during early burial, and to estimate total diffusive and advective Si fluxes out of the sediments, which rival those from Arctic rivers. A comparison between our Si efflux estimates and water-column diatom silica production indicates that pelagic and benthic processes are tightly coupled in our study locations. Our findings improve the understanding of the Si cycling between sediments, pore water and overlying seawater, which represents an important but understudied component of internal cycling as well as a key input into the oceanic Si budget.