The Arctic and Atlantic Oceans are connected by narrow passages, and the exchange of waters affect global climate. Using artificial radionuclides from nuclear reprocessing discharges, we traced the origin of water masses from the Arctic to the Labrador Sea. Results show that waters from Canadian Arctic origin entering via Lancaster Sound are a key freshwater source to the Labrador Sea. These flows strongly influence the formation of deep waters in the Atlantic, vital for the global circulation.