Radionuclide tracers reveal new Arctic pathways shaping water mass mixing and formation in Baffin Bay and Labrador Sea

Abstract. The Davis Strait is one of two key Arctic gateways, where waters derived from the Atlantic flow northward and exchange with Arctic-origin waters flowing southward. This interaction may play a crucial role in shaping the formation of deep water masses in the subpolar North Atlantic. This study employs observations from 2022 and 2024 of the two artificial radionuclides ¹²⁹I and ²³⁶U measured 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, the Davis Strait Observation Programme in October 2022, and the Amundsen Expedition as part of the Transforming Climate Action programme in September–October 2024. By defining the characteristic ¹²⁹I and ²³⁶U concentrations of the main inflowing water masses (endmembers), we examined the distribution, origin and formation of key Baffin Bay water masses. This approach also allowed us to quantify the contribution of Transition Water to the formation of Labrador Sea Water (LSW) and North East Atlantic Deep Water (NEADW). Our results reveal a substantial contribution of West Greenland Shelf Water to Arctic Water on the surface of central Baffin Bay, accounting for approximately 30 %. High ²³⁶U identified a previously unknown pathway of Arctic-Atlantic-derived waters entering Baffin Bay via Lancaster Sound, contributing 40–50 % to the formation of Transition Water. In contrast, cold Arctic Water appears to originate mainly from Nares Strait, with contributions of Arctic-Atlantic Water outflowing Nares Strait reaching up to 35 %. Notably, the contribution of fresh Transition Water to the formation of LSW was significant, exceeding 30 %. However, the binary mixing model showed limitations in quantifying the origin of NEADW 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 waters in Baffin Bay and the Labrador Sea and enhances our understanding of the complex interactions between the Arctic Ocean and the subpolar North Atlantic.