Early melt-season nutrient and inorganic carbon sediment-water fluxes in the Bering and Chukchi Seas

The Bering and Chukchi Seas are important oceanic regions of carbon sequestration, owing to enhanced gas solubility in cold surface waters and the rapid uptake of carbon dioxide (CO₂) during intense spring blooms. The biogeochemical impacts of decreasing sea ice extent and earlier onset of spring ice melt in this region are yet uncertain. As these marginal seas of the western Arctic Ocean are quite shallow, mostly <60m depth, there is extensive interaction across air-sea-sediment boundaries, but the transformations and fluxes of inorganic carbon in Bering and Chukchi Sea sediments have not been directly quantified. In May-June 2021, we collected water column samples at 14 stations and sediment cores at 5 stations spanning the eastern Bering Sea and southern and eastern Chukchi Sea. Duplicate cores were incubated for several days at in situ temperature, and core-top water was sampled to estimate inorganic carbon and nutrient fluxes. The stations spanned a range of surface ice coverage history, from greater than one month to less than one day of ice-free conditions. In the Chukchi Sea, salinity-normalized bottom water nutrient and dissolved inorganic carbon (DIC) concentrations increased northward, indicating a net input of remineralization products, although effluxes of these parameters from the sediments decreased northward. Moving northward in the Chukchi Sea, the surface water had greater sea ice concentrations, inhibiting surface productivity and air-sea exchange. This may have reduced the rain of labile carbon to the seafloor, resulting in the decreased benthic remineralization. The combination of increasing northward ice coverage and the northward flow of nutrient and IC-rich Pacific-sourced waters influences the bottom-water concentration of remineralization products and sediment-water fluxes. We expect our northeastern Chukchi Sea flux observations are representative of baseline low wintertime sediment-water flux conditions, while the more southerly stations represent at least one month post-ice melt benthic fluxes when surface water productivity is high and the air-water-sediment system openly interacts. We note that some duplicate core measurements were highly heterogeneous, especially in the Bering Sea, illustrating the dynamic nature of this macrofauna-dominated benthic environment and the range of possible fluxes under different rates of bioturbation. While these observations may serve as a seasonal reference, they may also demonstrate how sedimentary fluxes will evolve under future conditions that are expected when sea ice retreats earlier in the season. Here we present our sediment-water flux and water column DIC and nutrient measurements and place them in context with previous work in the region.