Cryoconites from Alpine glaciers: Radionuclide accumulation and age estimation with Pu and Cs isotopes and ²¹⁰Pb

Cryoconites ("cold dust", derived from the Greek) are aeolian sediments accumulated on glacier surfaces. In cryoconites from the surface of the Stubacher Sonnblickkees, a temperate Austrian glacier, extremely high activity concentrations of artificial and natural radionuclides were found. Artificial radionuclides stem from two clearly distinguishable sources, global fallout from the nuclear weapons testing era deposited over a period of years until roughly 1966 and the fallout from Chernobyl in 1986, which was essentially deposited as a single input during one week. Anthropogenic radionuclides identified were ¹³⁷Cs, ¹³⁴Cs, ²³⁸Pu, ^239+240Pu, ⁹⁰Sr, ²⁴¹Am, ⁶⁰Co, ¹²⁵Sb, ¹⁵⁴Eu, and ²⁰⁷Bi. The naturally occurring radionuclides detected were the long-lived radon decay product ²¹⁰Pb, the primordial radionuclide ⁴ K and the cosmogenic ⁷Be. Isotopic ratios of ¹³⁴Cs/¹³⁷Cs and ^239+240Pu/²³⁸Pu were used to separate the nuclide inventory into the contributions of the two aforementioned sources, which show varying degrees of mixing and provide information on the mixing age of the cryoconites. Since isotopic ratios of Pu often have high uncertainties due to low absolute concentrations, age estimation based on this method can be quite inaccurate. Additional information about the age of cryoconites was obtained through analysis of ²¹⁰Pb, which is constantly deposited over time.