Simple Method for High-Sensitivity Determination of Cosmogenic 35S in Snow and Water Samples Collected from Remote Regions.

Cosmogenic 35S is useful in understanding a wide variety of chemical and physical processes in the atmosphere, the hydrosphere, and the cryosphere. The 87.4-day half-life and the ubiquity of sulfur in natural environments renders it an ideal tracer of many phenomena. Measurements of 35S in snow and water samples are scarce as existing analytical methods require a large volume of sample (>20 L) due to their high analytical activity background and low counting efficiency. Here, we present a new set of snow/water sample collecting and handling procedures for high-sensitivity determination of cosmogenic 35S using a low-level liquid scintillation spectrometer. Laboratory experiments using diluted 35S standards (with activities of <5 disintegrations per minute) showed a 35S recovery percentage of ∼95%, demonstrating a relatively small deviation from the true value. Using this method, we successfully measured 35S in ∼1 L of fresh snow sample collected from a glacier on the Tibetan Plateau to be 47 ± 7 mBq/L. On the basis of 35S activities in 9 natural samples measured in this study, a first proof-of-concept approximation for age determinations and source attributions was presented. This new method will provide a powerful tool in studying 35S in small volumes of snow and water samples, especially those from remote but climatically important regions such as the polar regions and the Tibetan Plateau and Himalayas. The measurements are particularly important as the radioactive sulfur provides an actual clock of glacial melting processes. With the growing rate of glacial loss, the need for measurements from remote locations becomes all the more important.