Tracing Atlantic water transit time in the subarctic and Arctic Atlantic using ⁹⁹Tc-²³³U-²³⁶U

The pathway and transport time of Atlantic water passing northern Europe can be traced via anthropogenic radioisotopes released from reprocessing of spent nuclear fuels at Sellafield (SF) and La Hague (LH). These reprocessing derived radioisotopes, with extremely low natural background, are source specific and unique fingerprints for Atlantic water. This study explores a new approach using ⁹⁹Tc-²³³U-²³⁶U tracer to estimate the transit time of Atlantic water in the coast of Greenland. We isolate the reprocessing plants (RP) signal of ²³⁶U (²³⁶U_RP) by incorporating ²³³U measurements and combine this with ⁹⁹Tc which solely originates from RP, to estimate the transit time of Atlantic water circulating from Sellafield to the coast of Greenland-Iceland-Faroe Islands. Both being conservative radioisotopes, the temporal variation of ⁹⁹Tc/²³⁶U_RP ratio in Atlantic water is only influenced by their historic discharges from RP, thus ⁹⁹Tc/²³⁶U_RP can potentially be a robust tracer to track the transport of Atlantic water in the North Atlantic-Arctic region. Based on our observation data of ⁹⁹Tc-²³³U-²³⁶U in seawater and the proposed ⁹⁹Tc/²³⁶U_RP tracer approach, Atlantic water transit times were estimated to be 16-22, 25 and 25 years in the coast of Greenland, Iceland and Faroe Island, respectively. Our estimates from northeast Greenland coastal waters agree with earlier results (17-22 years). Therefore, this work provides an independent approach to estimate Atlantic water transit time with which to compare estimates from ocean modelling and other radiotracer approaches.