Sediment-water fluxes of inorganic carbon and nutrients in the Pacific Arctic during the sea ice melt season

Abstract

The Bering and Chukchi Seas are important oceanic regions of carbon dioxide (CO2) uptake owing to enhanced gas solubility in cold surface waters and highly productive spring phytoplankton blooms. Over the past several decades, sea ice extent in the Arctic Ocean has decreased and the ice-melt season has started earlier in the year, but the biogeochemical impacts of these systematic changes are unclear. As these marginal seas of the Arctic Ocean are quite shallow (mostly <60 m depth) there is extensive interaction across air-sea-sediment boundaries, and export production rates are high during the spring bloom, delivering organic carbon to the sediments. However, the subsequent transformations and fluxes of carbon in Bering and Chukchi Sea sediments have not been directly quantified. In May–June 2021, we collected sediment cores at 5 stations spanning the eastern Bering Sea and southern and eastern Chukchi Sea. These stations encompassed a range of surface water ice coverage history, from greater than one month to less than one day of ice-free conditions. In the Chukchi Sea, dissolved nutrient and inorganic carbon (DIC) effluxes from the sediments decreased northward. The highest and most variable DIC fluxes were observed in the southern Chukchi Sea, north of the Bering Strait, ranging from 2.0 to 21.5 mmol m−2 d−1, while the lowest DIC fluxes were observed at the northernmost Chukchi Sea station near Cape Lisburne, ranging from 1.1 to 2.3 mmol m−2 d−1. Moving northward, the surface water had greater sea ice concentrations, inhibiting surface productivity and air-sea exchange of CO2. The reduced export of labile carbon to the seafloor likely resulted in decreased benthic respiration and thus a lower flux of remineralization products from the sediments to the water column. 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 fluxes under different rates of infaunal activity. Core replicates with relatively higher effluxes of remineralization products also had high fluxes of radon-222, a proxy for bio-irrigation in sediments, illustrating the dominant role of benthic macrofauna in benthic-pelagic coupling in this region. While these observations serve as a seasonal reference, they may also demonstrate how sedimentary fluxes will evolve under future conditions wherein sea ice retreats earlier in the season.