Tracing Atlantic water exiting the Fram Strait and its transit in the Arctic Ocean by isolating reprocessing-derived 236U and colored dissolved organic matter

The Fram Strait, located between Svalbard and Greenland is an important gateway for exchange of salt and heat between the Arctic Ocean and the North Atlantic Ocean and is also a geographically crucial region for investigating Atlantic water transport pathways and transit times, which are necessary to understand the progress of environmental changes in the Arctic. ²³⁶U from the two European nuclear reprocessing plants (RPs) at La Hague (LH) and Sellafield (SF) provides a unique signal in Atlantic water for studying its circulation pattern in the Arctic Ocean. In this study we first isolate RP-derived ²³⁶U (²³⁶U_RP) using the characteristic ²³³U/²³⁶U signature and then use colored dissolved organic matter (CDOM) to indicate transit pathways and therefore constrain the selection of appropriate ²³⁶U_RP input functions. High CDOM absorbance in the Fram Strait reflects the passage of Atlantic water transported to the Arctic by the Norwegian Coastal Current (NCC) and subsequently along the Siberian shelf where the Ob, Yenisei and Lena rivers supply terrestrial organic matter with high CDOM levels. Conversely low CDOM water represents Atlantic water that has remained off the shelf. Based on CDOM absorbance, potential temperature (θ) and water depth the path of a given body of Atlantic water could be determined and an appropriate RP input function selected so that transit times could be estimated. Waters with high CDOM levels sourced from the NCC and Barents Sea branch water (BSBW) had an average Atlantic water transit time of 12 years. Waters with low CDOM,  θ < 2 °C, and depth < 1500 m were sourced from the Norwegian Atlantic Current (NwAC), had little interaction with riverine freshwater with an advective Atlantic water transit time of 26 years.