Radium isotopes quantify vertical mixing and reveal large benthic silicate fluxes in anoxic deep waters

Benthic fluxes are important sources of nutrients and dissolved carbon to the water column. Yet, most approaches for quantifying benthic fluxes are labor intensive and cover relatively small areas of the ocean. Here, we use 224Ra to quantify ecosystem‐scale vertical mixing across a largely hypoxic deep water column in the Baltic Sea and related benthic Si fluxes. Bottom water profiles from 50 stations along a ~ 5000 km cruise track in the Baltic Sea revealed highest 224Ra activities in the deep anoxic basins near the sediment source. Dissolved Si concentrations reached 110 μM and followed a similar trend to radium. For the radium‐based vertical mixing model, 26 out of 50 stations satisfied the required assumptions. When criteria were met, the radium profiles resolved vertical mixing rates on the order of 10−4 m2 s−1 and median dissolved Si fluxes of 5.9 mmol m−2 day−1, both well within the range of previous local scale estimates based on modeling and sediment core incubations. Extrapolated benthic dissolved Si fluxes were 32 times greater than river dissolved Si flux to the Baltic Sea, highlighting the large contribution of seafloor sources to the water column.