Benthic alkalinity fluxes from coastal sediments of the Baltic and North seas: comparing approaches and identifying knowledge gaps

Benthic alkalinity production is often suggested as a major driver of net carbon sequestration in continental shelf ecosystems. However, information on and direct measurements of benthic alkalinity fluxes are limited and are especially challenging when biological and dynamic physical forcing causes surficial sediments to be vigorously irrigated. To address this shortcoming, we quantified net sediment-water exchange of alkalinity using a suite of complementary methods, including (1) Ra-224 budgeting, (2) incubations with( 224)Ra and Br- as tracers, and (3) numerical modeling of porewater profiles. We choose a set of sites in the shallow southern North Sea and western Baltic Sea, allowing us to incorporate frequently occurring sediment classes ranging from coarse sands to muds and sediment-water interfaces ranging from biologically irrigated and advective to diffusive into the investigations. Sediment-water irrigation rates in the southern North Sea were approximately twice as high as previously estimated for the region, in part due to measured porewater Ra-224 activities higher than previously assumed. Net alkalinity fluxes in the Baltic Sea were relatively low, ranging from an uptake of -35 to a release of 53 mu mol m(-2) h(-1), and in the North Sea they were from 1 to 34 mu mol m(-2) h(-1). Lower-than-expected apparent nitrate consumption (potential denitrification), across all sites, is one explanation for our small net alkalinity fluxes measured. Carbonate mineral dissolution and potentially precipitation, as well as sulfide re-oxidation, also appear to play important roles in shaping net sediment-water fluxes at locations in the North Sea and Baltic Sea.