Cross-shelf transport of Barents Sea dense water as a sink for atmospheric CO2 in the Arctic Ocean

<title>Abstract</title> Export into the deep sea can store significant amounts of atmospheric carbon (C) on millennial time scales, buffering global warming1,2. The Barents Sea is one of the most biologically productive areas of the Arctic Ocean3,4 but C retention times there were thought to be short5. Here we show that dense bottom water formation and transport over the continental slope into the deep sea6–8 result in deep injection of substantial amounts of organic C with long retention times. Observational evidence complemented by numerical model simulations revealed a deep and widespread C injection driven by Barents Sea Bottom Water transport with daily lateral fluxes of ~2.2 kt C d-1 to ~1200 m in the Nansen Basin. With increasing distance from the outflow region, the plume expanded and penetrated into even deeper waters, and towards the sediment (retention time hundreds of millennia). Numerical model and genomic data suggest a seasonally fluctuating but continuous transport of C-rich phytoplankton and resuspended material from the Barents and Kara Sea shelves. This mechanism could sequester ~1/3 of total C burial across the whole Barents Sea9. Our findings combined with those from other export regions of C-rich dense waters in the Arctic10,11 and around Antarctica12–16 highlight the importance of lateral transport of bottom water as a C sink globally. Resolving uncertainties around negative feedbacks of global warming due to sea ice decline will necessitate observation of changes in bottom water formation17 and biological productivity3 at a resolution high enough to quantify future deep C injection.