Distribution and controls on the accumulation of fallout radionuclides in cryoconite across the global cryosphere

Fallout radionuclides (FRNs) are a product of nuclear accidents and weapons testing, and are known environmental contaminants. There has been extensive research into the risks and consequences of FRN deposition for human and ecosystem health, however this has rarely extended to the cryosphere. The results of our international collaboration reveal widespread accumulation of FRNs in cryoconite spanning 30 glacier sites and 477 samples across the Arctic, the Alps, the Caucasus, North America, the Andes, the Himalaya and Antarctica. The activity levels of FRNs found in many samples are orders of magnitude higher than those found in other environmental matrices such as mosses and lichens, and include some of the highest ever recorded outside of nuclear exclusion zones. This raises important questions around the role of glaciers, and specifically cryoconite and its interaction with meltwater, in concentrating - and eventually releasing - FRNs to levels above those historically deposited in the surrounding environment. We compare FRNs in cryoconite with a range of geographical and environmental factors, and find no significant correlation between ¹³⁷Cs and distance from Chernobyl, and moderate correlations for ²⁴¹Am and ²¹⁰Pb which deteriorate to no significant correlation when only Northern Hemisphere sites are considered. We also find no correlation with distance from the sea, or with mean elevation, but a moderate correlation between precipitation and both ¹³⁷Cs and ²⁴¹Am, highlighting the importance of scavenging of atmospheric contaminants by snow. Notably, we find a strong correlation between organic matter and activities of both ¹³⁷Cs and ²¹⁰Pb, reflecting the capacity of cryoconite to bind FRNs due the presence of extracellular polymeric substances. This research has, for the first time, shed light on the widespread occurrence of FRNs in glacier catchments across the global cryosphere. The potential risks of FRN exposure for water and environmental quality, including uptake of FRNs by flora and fauna, should be a focus of future interdisciplinary research as glaciers continue to retreat and release legacy contaminants into proglacial environments.