Abstract

The coastal ocean is a highly dynamic and vital biogeochemical mediator between land and sea. Coastal waters frequently experience poor water quality derived from land-based anthropogenic pressure, which is often attributed to surface water sources, such as rivers. Subsurface sources (e.g., submarine groundwater discharge, benthic fluxes), however, often rival or exceed river contributions to coastal water and chemical budgets. Yet subsurface sources are understudied because they are challenging to quantify.

In this talk, I will first give a broad overview on quantifying subsurface flows using U-Th series geochemical tracers (radium, radon). Then I will present an ecosystem-scale study across the Baltic Sea, where we used 224Ra to quantify vertical mixing across a largely hypoxic deep water column. We collected radium and solute (e.g., dissolved silicate) bottom water profiles from 50 stations along a ~5000 km cruise track in the Baltic Sea. 224Ra-derived vertical mixing rates were on the order of 10-4 m2/s, well-within range of previous local-scale estimates based on modeling and sediment core incubations. Diffusive solute fluxes were also similar in magnitude to previous studies. Overall, this talk will highlight an innovative method for quantifying diffusive fluxes and contextualize findings in terms of broader biogeochemical significance.