The triple argon isotope composition of groundwater on ten-thousand-year timescales
Understanding the age and movement of groundwater is important for predicting the vulnerability of wells to contamination, constraining flow models that inform sustainable groundwater management, and interpreting geochemical signals that reflect past climate. Due to both the ubiquity of groundwater with order ten-thousand-year residence times and its importance for climate reconstruction of the last glacial period, there is a strong need for improving geochemical dating tools on this timescale. Whereas 14 C of dissolved inorganic carbon and dissolved 4 He are common age tracers for Late Pleistocene groundwater, each is limited by systematic uncertainties related to aquifer composition and lithology, and the extent of water-rock interaction. In principle, radiogenic 40 Ar in groundwater acquired from decay of 40 K in aquifer minerals should be insensitive to some processes that impact 14 C and 4 He and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic 40 Ar signals in groundwater has been limited to a small number of studies of extremely old groundwater (>100 ka). Here we present the first high-precision (<1‰) measurements of triple Ar isotopes ( 40 Ar, 38 Ar, 36 Ar) in groundwater. We introduce a model that distinguishes radiogenic 40 Ar from atmospheric 40 Ar by using the non-radiogenic Ar isotopes ( 36 Ar, 38 Ar) to correct for mass-dependent fractionation. Using this model, we investigate variability in radiogenic 40 Ar excess (Δ 40 Ar) across 58 groundwater samples collected from 36 wells throughout California (USA). We find that Δ 40 Ar ranges from ~0‰ (the expected minimum value) to +4.2‰ across three study areas near Fresno, San Diego, and the western Mojave Desert. Based on measurements from a network of 23 scientific monitoring wells in San Diego, we find evidence for a strong dependence of Δ 40 Ar on aquifer lithology. We suggest that Δ 40 Ar is fundamentally controlled by the weathering of old K-bearing minerals and thus reflects both the degree of groundwater-rock interaction, which is related to groundwater age, and the integrated flow through different geological formations. Future studies of Late Pleistocene groundwater may benefit from high-precision triple Ar isotope measurements as a new tool to better interpret 14 C- and 4 He-based constraints on groundwater age and flow.