Characterizing U-series Isotope Signatures in Soils and Headwater Streams in a Complex Volcanic Terrain: Jemez River Critical Zone Observatory, Valles Caldera, NM.

Abstract

Uranium-series isotopes are an emerging tool to characterize weathering and soil forming processes in the Critical Zone, the dynamic region of earth's surface where bedrock, water, soil, and life chemically and physically interact to support ecosystems. Understanding controls on the U-series composition of soils across a range of lithologies and climate is critical to applying existing mass balance models of U-series isotopes to calculate soil formation rates in these systems. This study seeks to understand the behavior of U-series isotopes in soil profiles in the semi-arid complex volcanic terrain of the Valles Caldera, NM. (²³⁴U/²³⁸U) measured in soils ranged from 0.90 to 1.56 and (²³⁰Th/²³⁸U) values ranged from 0.48 to 1.39. Significant ²³⁰Th enrichment in upper soil profiles was interpreted as evidence of mixing with ²³⁰Th-enriched volcanic ash and significant ²³⁴U enrichment in one soil profile was interpreted as evidence of addition of U to soils from ²³⁴U-enriched soil solutions. A simple U isotope mass balance model was applied to estimate soil residence time based on U addition, which yielded a minimum residence time of ~10ka. Evidence of past episodic mixing of volcanic ash in these soils suggests modeling soil formation using a mass balance approach is problematic, and future applications of existing models in other heterogeneous volcanic soils should be applied cautiously. U-series isotopes have also shown promise as a tracer of residence time in shallow groundwater and streams. In this study, (²³⁴U/²³⁸U) in dissolved U is used to trace seasonal variation in source water contributions to streamflow in a small (3.29km²), headwater catchment in the Jemez River Basin Critical Zone Observatory within the Valles Caldera. Systematically lower (²³⁴U/²³⁸U) values in dissolved U were observed in spring and stream waters in conjunction with greater contributions of longer residence time waters during snowmelt ((²³⁴U/²³⁸U) ranged 1.7 to 2.8) vs. dry seasons ((²³⁴U/²³⁸U) ranged 1.9 to 3.1). The lower (²³⁴U/²³⁸U) values in longer residence time waters were attributed to progressive depletion of easily-weathered ²³⁴U with increasing duration of water rock interaction. Further studies with more quantitative age tracers, such as ³H, could help to establish (²³⁴U/²³⁸U) values as a powerful tracer of water sources and residence time in streamwaters at the catchment scale.