UC San Diego

Observations are key to our ability to understand present climate and predict future change. However, for many climatic variables of interest, direct observations are not possible. Well-mixed atmospheric gas tracers preserved in groundwater, seawater, and ice cores provide a crucial lens into past climate and contemporary large-scale signals that cannot be resolved by modern observations. This work is predominantly concerned with dissolved noble gas isotope tracers in groundwater, which are here shown to record mean water-table depths at the time of recharge.

Noble gases are ideal tracers of geophysical processes due to their inertness and constant atmospheric concentrations on glacial-interglacial timescales. The foremost goal of this work has been to develop a quantitative tool for past water-table depth reconstruction by measuring the stable isotope ratios of Kr and Xe dissolved in groundwater at high precision. This approach relies on the dominance of depth-dependent gravitational settling fractionation in unsaturated zone (UZ) air prior to its dissolution at the water table, which is predicted by theory and consistent with observations in a deep UZ. Because this gravitational signal, in which heavy-to-light Kr and Xe isotope ratios increase nearly linearly with depth, is transferred to groundwater during dissolution, the isotopic composition of noble gases in groundwater records information about water-table depth at the time of recharge.

To fully employ these isotopic tracers of water-table depth, a complete geochemical model for fractionation between the well-mixed atmosphere and groundwater was developed, and constrained by laboratory determinations of relevant isotopic solubility and diffusivity ratios. In measurements from 58 groundwater samples across California, the expected dominance of gravitational settling was confirmed, modern water table depths were reproduced, and a mean decrease in regional San Diego water-table depth of ~20 meters during the last deglaciation was discovered, consistent with prior work that broadly indicates of the prevalence of wetter conditions during the late glacial period.

In two additional studies, noble gas isotopes in the deep north Pacific were measured to explore air-sea disequilibrium signals of rapid cooling and gas uptake during deep-water formation, and a composite record of δ18O of O2 from two ice cores was compiled to explore the sensitivity of this ratio to shifts in tropical rainfall.