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

[摘要] Benthic alkalinity production is often suggested as a major driver of net carbon sequestration in continental shelf ecosystems. However, information onand direct measurements of benthic alkalinity fluxes are limited and are especially challenging when biological and dynamic physical forcing causessurficial sediments to be vigorously irrigated. To address this shortcoming, we quantified net sediment–water exchange of alkalinity using a suiteof complementary methods, including (1)  224 Ra 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 toincorporate frequently occurring sediment classes ranging from coarse sands to muds and sediment–water interfaces ranging from biologicallyirrigated and advective to diffusive into the investigations. Sediment–water irrigation rates in the southern North Sea were approximately twice ashigh as previously estimated for the region, in part due to measured porewater 224 Ra activities higher than previously assumed. Netalkalinity fluxes in the Baltic Sea were relatively low, ranging from an uptake of  − 35 to a release of 53  µ mol m - 2 h - 1 , and inthe North Sea they were from 1 to 34  µ mol m - 2 h - 1 . Lower-than-expected apparent nitrate consumption (potential denitrification), acrossall sites, is one explanation for our small net alkalinity fluxes measured. Carbonate mineral dissolution and potentially precipitation, as well assulfide re-oxidation, also appear to play important roles in shaping net sediment–water fluxes at locations in the North Sea and Baltic Sea.

[学科分类] 大气科学