The Arctic, a region highly sensitive to global climate change, has experienced a temperature rise more than four times the global average since the industrial era. The Arctic warming has led to significant sea ice loss and shifts in the thermohaline structure of the seawater, with potential impacts on Arctic ocean currents. Understanding how these currents respond to climate change is critical for addressing its broader implications. Large amounts of iodine-129 (¹²⁹I) have been released into the atmosphere and North Atlantic by two European nuclear fuel reprocessing plants (NFRPs). Atlantic waters, enriched with NFRP-derived ¹²⁹I, enter the Arctic through the Fram Strait and Barents Sea, while airborne ¹²⁹I, carried by westerlies, gradually deposits across different regions, leaving distinct signatures. The annual release of ¹²⁹I from NFRPs into the atmosphere and ocean is well documented, making ¹²⁹I a reliable tracer for studying the temporal and spatial movement of ocean currents. Despite extensive research using ¹²⁹I to explore ocean currents in the Eurasian Basin of the Arctic Ocean, studies in the Canada Basin and surrounding seas are limited. The water mass composition in the Canada Basin is complex, influenced by Pacific water, Atlantic water, and river-transported freshwater, with freshening over recent decades holding significant climatic and ecological relevance. This study aims to explore the composition and origin of water masses in the Canada Basin and adjacent seas using ¹²⁹I as a tracer to investigate their variability and interaction with the subarctic Pacific.

We present a decade-long analysis of temporal changes in ¹²⁹I concentrations in the southern Canada Basin, Chukchi Sea, and Bering Sea from 2013 to 2023. Our findings show a decreasing influence of Atlantic water and an increasing influence of Pacific water on the surface of the Canada Basin over time. A remarkably high ¹²⁹I signal was detected at the bottom of the cold halocline, confirming that the Arctic Shelf Break Branch extended into the southern Canada Basin. Additionally, the ¹²⁹I time series suggests that the transit time of Atlantic water from the Norwegian coast to the intermediate layer of the southern Canada Basin was approximately 25 years. Our study reaffirms that ¹²⁹I is an effective and reliable tracer for studying ocean currents and offers new insights into the variability and dynamics of water masses in the southern Canada Basin and adjacent seas.

iodine-129, Canada Basin, water masses