Tackling Voids in Observations: An Approach to Reconstruct Rainfall ³⁵S Time Series from Proxy Parameters

Due to its short half-life (87.4 days) and omnipresence, cosmogenic radio-sulfur (35S) is an attractive tracer for investigating subyearly groundwater residence times. 35S is transported to the lower atmosphere by large-scale air mass circulation and transferred to groundwater by precipitation. For groundwater dating, the variability of 35S concentration in precipitation requires a 35S input function. However, the required 35S data are often not available. To fill this gap, we present an approach to reconstructing 35S concentrations in precipitation based on proxy parameters of better availability. The tested parameters include natural 7Be and 3H, parameters that allow quantifying cosmogenic 35S production, and parameters that are correlated to the intensity of 35S washout from the atmosphere. In comparison with an unrivaled 4 year time series of 35S in precipitation, we discuss the correlations of all parameters with 35S, assess their individual applicability as 35S proxies, and evaluate the predictive power of joint data sets in varying combination. As a result, we present a modeling approach that allows reconstruction of a 35S input function with a monthly temporal resolution based on proxy parameters. This novel modeling approach provides a valuable tool for groundwater dating using 35S as a tracer in studies that lack directly measured 35S input data.