Triple Oxygen and Hydrogen Isotopes of Soil Water Across Climates, Seasons, and Depths

Soil waters are a central component of the terrestrial water cycle. Their δ²H, δ¹⁸O, and d-excess values contain rich information about hydroclimate, including the source of moisture and its evaporative history. Paleoclimate workers seek to investigate ancient hydroclimates via triple oxygen isotope geochemistry, termed Δ′¹⁷O, which is analogous to d-excess but can be measured in oxygen-bearing minerals in paleosols. Observations of Δ′¹⁷O in soil water across climates, seasons, and depths are needed to establish a framework for applying Δ′¹⁷O to paleohydrology. We measured the δ¹⁸O, δ²H, and Δ′¹⁷O values in soil waters (0–1 m) and precipitation in the Mojave, Great Basin, and Chihuahuan Deserts and in a temperate forest in Michigan. Evaporation drives covariation in Δ′¹⁷O and d-excess in soil waters. The range of Δ′¹⁷O values in soil waters (−148 to 44 per meg) spans the heretofore observed range of Δ′¹⁷O in the water cycle. The uppermost 20 cm of the soil profiles experience the greatest isotopic variation as soil water is subject to infiltration and evaporation. Each environment displays a distinct pattern in δ¹⁸O, d-excess, and Δ′¹⁷O of soil water. Soil water at the forested site has isotope values that closely adhere to meteoric waters. In contrast, at the dryland sites, evaporation causes soil waters to deviate from meteoric waters along site-specific trajectories. The Δ′¹⁷O patterns in soil waters adhere to expectations based on d-excess, strongly affirming the utility of Δ′¹⁷O in fossilized soil sediments and in modern soil waters to investigate the water cycle.