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This study represents the first use of the artificial radionuclides 129I and 236U, released into the ocean mainly from Nuclear Reprocessing Plants, as a dual tracer in the vicinity of Iceland with novel estimation of ocean circulatory pathways and mixing in the region. Iceland lies at the gateway to the Arctic where warm, saline Atlantic waters interact with waters of Arctic origin in ways that have critical consequences for the strength and stability of the Atlantic Meridional Overturning Circulation. Many of these interactions are not yet fully understood, such as how Atlantic water circulates around the Arctic Ocean and Nordic Seas and the composition and fate of the major overflows of the Greenland‐Scotland Ridge. Using new and previous measurements of 129I and 236U in seawater, we present a new method of appraising water mass provenance and mixing in the form of the 129I–236U dual mixing plot. With this method, we estimate that at least half the Atlantic‐origin water entering the Arctic Ocean circulates around the Canada Basin before exiting at Fram Strait and that this outflow is increased by about 40% by mixing with Return Atlantic Water "short‐circuiting" the Arctic Ocean at Fram Strait. We present tracer‐based evidence that water carried by the East Greenland Current has an unbroken pathway to the Faroe‐Shetland Channel and that Iceland‐Scotland Overflow Water (ISOW) entrains 60% Labrador Sea Water during transit past southeast Iceland. We present an unambiguous way to differentiate ISOW from DSOW after they partially merge in the Irminger Sea. Plain Language Summary: This study is the first to use a pair of man‐made nuclear products to study ocean currents near Iceland. Understanding these currents is important because Iceland sits where warm Atlantic waters meet colder Arctic waters, affecting a key ocean circulation system and the global climate. However, many details about how these waters move and mix are still unclear. Using new and previous measurements of these tracers in seawater, we demonstrate a new method to estimate the origins of currents and how they mix. We estimate that the outflow of Atlantic‐origin water from the Arctic Ocean at Fram Strait is increased by about 40% by Atlantic water bypassing the Arctic Ocean altogether at this location. Some of this Atlantic‐origin water then flows all the way from Iceland to Shetland in an unbroken pathway and the water that spills over the Iceland‐Scotland ridge increases by 60% by mixing with water from the Labrador Sea southeast of Iceland. Finally, we present a new way to distinguish this Iceland‐Scotland Overflow Water from similar water that overflows the ridge at Denmark Strait. Key Points: Recirculating Atlantic Water contributes about 40% to Atlantic layer of the East Greenland CurrentAtlantic‐origin waters can be traced from the East Greenland Current to the Faroe‐Shetland ChannelIceland‐Scotland Overflow Water and Denmark Strait Overflow Water are traceable separately after joining Deep Western Boundary Current [ABSTRACT FROM AUTHOR]

Water mass transport and transformation processes in the North Atlantic Ocean play a crucialrole in the Atlantic meridional overturning circulation (AMOC). In this context, circulationprocesses leading to the transport of substances or properties such as heat or greenhousegases from the sea surface to the ocean interior (e.g. by dense-water formation) are of specialinterest. These processes are known to be closely linked to water mass transport in theNordic Seas and to the water exchange between the North Atlantic and the ArcticOcean through Fram Strait. In order to understand the natural variability of the watercirculation and how it will respond to anthropogenic climate forcing, chemical tracers aresuitable tools to complement hydrographic measurements and models. Amongst them,the long-lived artificial radionuclides 129I (T1∕2 ≈ 15.7 Myr) and 236U (T1∕2 ≈23.4 Myr) have been introduced into the marine environment as part of the globalatmospheric deposition of nuclear weapon test fallout (mainly 236U) and as a timelyvarying, point-like release from two nuclear fuel reprocessing plants in westernEurope (mainly 129I). Due to the quite recent introduction of these tracers (sincethe 1950s), the upper water column in the North Atlantic and the Arctic Oceancontains relatively high amounts of 129I and 236U that evolve in time due to thevarying historical releases from reprocessing plants. Thus, the combination of bothradionuclides provides a tool to investigate water mass transport pathways and to estimatetime scales of circulation. Here we combine 129I and 236U results from the FramStrait (2016 and preliminary results from 2018) and the OVIDE section (2014 and2018) to study the water circulation between the Arctic Ocean, the Nordic Seasand the subpolar North Atlantic. We find high concentrations of 129I and 236U aswell as high 129I/236U ratios in the surface waters outflowing the Arctic Ocean(concentrations about twice as high as in the inflowing Atlantic waters in the WestSpitzbergen Current) which are being transported southwards along the coast ofGreenland by the East Greenland Current (EGC). These high concentrations arefurther observed in the deep overflow waters in the Labrador and Irminger Seasdue the influence of waters entering through the Greenland-Iceland passage. Thepresence of 129I has notably increased in time in both the Fram Strait and most ofthe OVIDE section, showing the potential of this tracer for future oceanographicstudies. Finally, the combination of these results with earlier observations allows tounderstand transport pathways and transit times of Atlantic Waters into the NorthAtlantic Ocean. We find both a short loop through the Nordic Seas (8-10 yrs transittime) and a longer loop through the Arctic Eurasian Basin (16-18 yrs transit time). [ABSTRACT FROM AUTHOR]