U-series isotopic signatures of soils and headwater streams in a semi-arid complex volcanic terrain

Uranium-series isotopes are emerging as a tool to characterize weathering and soil forming processes in the Critical Zone. This study seeks to understand the behavior of U-series isotopes during chemical weathering and soil formation in the semi-arid and lithologically complex volcanic terrain (rhyolitic volcaniclastics and tuff) of the Valles Caldera, New Mexico (USA). A comprehensive set of samples from the Jemez River Basin Critical Zone Observatory, including bedrock, soils, dust, soil sequential extracts, soil pore water, spring water, and stream water, was systematically investigated. (²³⁴U/²³⁸U) values measured in four soil profiles ranged from 0.90 to 1.56 and (²³⁰Th/²³⁸U) values ranged from 0.48 to 1.39. Significant ²³⁰Th enrichment in shallow soil profiles was interpreted as evidence of mixing with ²³⁰Th-enriched volcanic ash during soil formation. Evidence of past episodic mixing of volcanic ash in these soils suggests modeling soil formation using a mass balance approach without considering possible atmospheric inputs is problematic, and future applications of existing models in heterogeneous volcanic soils should be applied cautiously. Significant ²³⁴U enrichment in one soil profile was interpreted as evidence of addition of U to soils from ²³⁴U-enriched soil solutions. Soil sequential extraction confirms that most of the U is contained in organo-metal colloid and exchangeable forms in shallow soils of this profile. U-series isotopes have also shown promise as a tracer of water residence time and mixing of different water sources. In this study, (²³⁴U/²³⁸U) ratios for dissolved U are used to trace seasonal variation in source water contributions to stream flow in a small (3.29 km²), headwater catchment within the Valles Caldera. Systematically lower (²³⁴U/²³⁸U) values (ranging from 1.7 to 2.8) were observed in dissolved U in spring and stream waters during snowmelt compared to dry seasons ((²³⁴U/²³⁸U) ranging from 1.9 to 3.1) in conjunction with greater contributions of deeper groundwater sources as suggested by major element tracers Cl and Si. The lower (²³⁴U/²³⁸U) values in deeper groundwater, in contrast to previous studies, were attributed to progressive depletion of easily-weathered ²³⁴U with increasing duration of water-rock interaction and increasing chemical dissolution. Further studies with more quantitative age tracers, such as ³H, could help to establish the influence of residence time on stream source waters' (²³⁴U/²³⁸U) values. If established, (²³⁴U/²³⁸U) could be a powerful tracer of water sources and residence time in stream waters at the catchment scale.