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.