The carbonate chemistry of Arctic Ocean seafloor and its vulnerability to ocean acidification remains poorly explored. This limits our ability to quantify how biogeochemical processes and bottom water conditions shape sedimentary carbonate chemistry, and to predict how climate change may affect such biogeochemical processes at the Arctic Ocean seafloor. Here, we employ an integrated model assessment that explicitly resolves benthic pH and carbonate chemistry along a S—N transect in the Barents Sea. We identify the main drivers of observed carbonate dynamics and estimate benthic fluxes of dissolved inorganic carbon and alkalinity to the Arctic Ocean. We explore how bottom water conditions and in-situ organic matter degradation shape these processes and show that organic matter transformation strongly impacts pH and carbonate saturation (Ω) variations. Aerobic organic matter degradation drives a negative pH shift (pH < 7.6) in the upper 2—5 cm, producing Ω < 1. This causes shallow carbonate dissolution, buffering porewater pH to around 8.0. Organic matter degradation via metal oxide (Mn/Fe) reduction pathways further increases pH and carbonate saturation state. At the northern stations, where Ω > 5 at around 10–25 cm, model simulations result in authigenic carbonate precipitation. Furthermore, benthic fluxes of dissolved inorganic carbon (12.5—59.5 µmol cm−2 yr−1) and alkalinity (11.3—63.2 µmol cm−2 yr−1) are 2—3-fold greater in the northern sites due to greater carbonate dissolution. Our assessment is of significant relevance to predict how changes in the Arctic Ocean may shift carbon burial and pH buffering into the next century.

Iron is a key limiting nutrient for phytoplankton. Continental shelf and slope sediments are important sources of dissolved iron (DFe). Stable iron isotopes (d56Fe) are a particularly useful tool to quantify the DFe sources and sinks in the ocean. The isotopic signature of the sedimentary DFe source is controlled by environmental factors such as bottom water redox conditions, carbon oxidation and bioturbation by burrowing fauna, but the exact relation on a global scale is poorly understood. We developed a reaction-transport model capable of tracing dissolved iron isotope fractionation in marine sediments to quantify the isotopic signature of benthic DFe fluxes under a wide range of environmental conditions. We derived fractionation factors for iron reduction (-1.3 permille), iron oxidation (+0.4 permille), iron sulphide precipitation (+0.5 permille and dissolution (-0.5 permille and pyrite precipitation (-0.7 permille) that were in line with existing literature. At bottom-water oxygen concentrations >50 µM, bioturbation increased the benthic DFe flux and increased the d56Fe signature. In contrast, at bottom-water oxygen concentrations <50 µM, a reduction in bioturbation led to a decrease in the benthic DFe flux and its d56Fe value. On a global scale, a model simulation without bioturbation decreased the sedimentary DFe release from ~158 Gmol DFe yr-1 to ~70 Gmol DFe yr-1, and decreased the variability in the d56Fe signature of the DFe flux. Finally, we find that a decrease in ocean oxygen content by 40 µM can increase global sedimentary DFe release by up to 103 Gmol DFe yr-1.