The Arctic experiences climate changes that are among the fastest in the world and affect all Earth system components. Despite expected increase in terrigenous inputs to the Arctic Ocean, their impacts on biogeochemical cycles are currently largely neglected in IPCC-like models. We used a state-of-the-art high-resolution ocean biogeochemistry model, that includes carbon and nutrient inputs from rivers and coastal erosion, to produce twenty-first-century pan-Arctic projections. Surprisingly, even with an anticipated rise in primary production across a wide range of emission scenarios, our findings indicate that climate change leads to a counterintuitive 40% reduction in the efficiency of the Arctic's biological carbon pump by 2100, to which terrigenous inputs contribute 10%. Terrigenous inputs will also drive intense coastal CO2 outgassing, reducing the Arctic Ocean's carbon sink by at least 10% (33 TgC yr-1). These unexpected positive feedbacks, mostly due to accelerated remineralization rates, lower the Arctic Ocean’s capacity for sequestering carbon.