Source, level and migration behavior of plutonium isotopes in cryoconite: A review.

Cryoconite, a dark sediment present on glacier surfaces, plays a significant role in the accumulation of radioactive contaminant plutonium (Pu). As global temperatures rise and glaciers rapidly melt, the Pu isotopes stored in cryoconite could be remobilized, posing environmental risks. The present review collected worldwide data regarding Pu isotopes in cryoconite samples (N = 221) to analyze their level and source term as well as investigate their accumulation and migration behavior. Specifically, 203 samples were analyzed for Pu activity concentrations and 161 for Pu/Pu atom ratios. The Pu/Pu atom ratios ranged from 0.064 to 0.205 with an average of 0.157, and the ratio was below 0.1541 in 58 % of the samples, indicating that they originated from global fallout and weapons-grade sources. On average, glaciers located above 60° latitude showed a weapons-grade Pu contribution of approximately 21.7 %, while Alpine glaciers exhibited a lower value of around 15.1 %. The activity concentrations of Pu in cryoconite samples ranged from 0.025 to 215.44 Bq/kg with an average of 26.05 Bq/kg, reaching two or even three orders of magnitude higher than that in soil, lake sediments and moss. This supports the assertion that cryoconite functions as an effective sink for Pu. The high activity concentration and accumulation of Pu are influenced by the physicochemical properties of cryoconite such as organic matter content, and external environmental controls including elevation, topography, precipitation, andglacial hydrology. Among them, organic matter content (r²=0.72) and precipitation (r²=0.58) exhibit strong positive correlations. Finally, methods for tracing the migration behavior of Pu adsorbed by cryoconite in the cryosphere, and future research gaps regarding Pu isotopes in cryoconite are discussed.