Application of anthropogenic uranium radioisotopes in tracing water mass movement in the Arctic-North Atlantic Ocean

A faster decline of sea ice extent in the Arctic in recent years is caused by the rapid warming of global climate. The northward Atlantic water carries a large amount of heat from the lower latitudes of the North Atlantic Ocean, and the release of heat in the Arctic Ocean plays a key role in the sea ice melting process. Atlantic water losses the heat to be cold and dense and thus sinks to deep water layer in the Arctic and subpolar region. Colder and deeper dense water outflows back into the North Atlantic Ocean, and this circulation is defined as the Atlantic Meridional Overturning Circulation (AMOC). It is one of important part of global thermohaline circulation, which profoundly contributes to global climate. Therefore, insights on the transport pathways and timescales of Atlantic water in the Arctic and North Atlantic oceans are significant for understanding the AMOC and the influence on melting of sea ice.

Anthropogenic uranium (U) isotopes, namely ²³³U and ²³⁶U, are promising oceanic tracers owning to their long half-lives and conservative behavior in the open ocean, and their transient signals are useful to investigate water mass movement in a large spatial scale. Unique point-source signal from nuclear reprocessing plants (NRPs) at Sellafield (SF) and La Hague (LH) of European coast is herein exploited to study the transport of Atlantic water in the Arctic and North Atlantic oceans. Combining nutrients, salinity, colored dissolved organic matter (CDOM) and ⁹⁹Tc, two ²³³U-²³⁶U tracer approaches were developed to estimate Atlantic water transit time. The first approach estimated the transit time of Atlantic water through comparing the original NRPs-derived ²³⁶U (²³⁶U_NRPs) concentrations in seawater sample and the reconstructed historic concentrations in three ²³⁶U_NRPs input functions at the Barents Sea Opening (Paper I). CDOM helps distinguish different Atlantic branch waters based on the terrestrial-derived and marine-derived CDOM, and therefore constrains the corresponding input functions (Paper III). The ²³⁶U_NRPs input functions were reconstructed through some assumptions of hydrologic features, whereas the second approach chose to use the ratio of ⁹⁹Tc and ²³⁶U from the NRPs discharge instead of the reconstructed input functions, avoiding the potential uncertainties from these assumptions (Paper II).

Based on the developed new approaches, average Atlantic water transit times in surface water were obtained in the Fram Strait (14 years), East Greenland fjords (16 years) and the coasts of East Greenland (16 – 17 years), West Greenland (22 years), Iceland and Faroe Islands (25 years) (Papers I, II and III). Our observation of ²³³U and ²³⁶U suggested that the surface outflowing water from the Arctic Ocean transported to the North Atlantic Ocean along the coast of Greenland, but the intermediate dense water outflowing through the sill of the Denmark Strait had a mixed composition of Arctic Atlantic Water, Recirculating Atlantic Water from the eastern Fram Strait and the deep and bottom waters in the Fram Strait (Paper IV). The outflowing dense water moved to the North Atlantic Ocean along the seafloor and contributed to the North Atlantic Deep Water.