Radionuclides as tracers of Arctic outflows: pathways, water mass mixing, and freshwater input to Davis Strait and the Labrador Sea

Abstract. In Davis Strait, one of two primary Arctic gateways between the Arctic and Atlantic Oceans, the exchange between northward-flowing Atlantic waters and southward-flowing Arctic waters possibly influences deep-water formation in the subpolar North Atlantic. Yet, these circulation pathways and mixing processes, that may be changing, remain uncertain, underscoring the need for new tracers to resolve their structure and variability. This study employs observations from 2022 and 2024 of two artificial radionuclides, 129I and 236U, as new tracers of water masses in Baffin Bay, Davis Strait and the Labrador Sea. Samples were collected during three expeditions: the AZOMP occupation of the AR7W Line in May 2022, Davis Strait Observation Program in October 2022, and the Amundsen Expedition as part of the Refuge Arctic and Transforming Climate Action programme in September–October 2024. By defining the characteristic 129I and 236U concentrations of the main inflowing waters, we examined the distribution, origin and formation of key water masses in Baffin Bay. Our results reveal a substantial contribution (75 %) of West Greenland Shelf Water to Arctic Water on the surface of central Baffin Bay. 236U-rich Arctic-Atlantic-derived water entering Baffin Bay via Lancaster Sound contributes 40 %–60 % to the formation of the Transition Water observed in Baffin Bay. In contrast, cold Arctic Water appears to originate mainly from Nares Strait, and is derived primarily (up to 70 %) from Arctic-Atlantic Water. Notably, the contribution of the Baffin Bay Transition Water to the formation of LSW was significant, exceeding 20 %. However, the binary mixing model showed limitations in quantifying the origin of North East Atlantic Deep Water due to low tracer concentrations and the likely influence of multiple water mass sources. This study offers novel insights into the origin and transformation of water in Baffin Bay and the Labrador Sea and enhances our understanding of the complex interactions between the Arctic Ocean and the subpolar North Atlantic.