Current Research

We are working on a number of ongoing projects - some lab-based and some model-based - addressing questions ranging from large-scale climate to molecular-scale physical chemistry. Below are a few examples of research questions that we work on. Please don't hesitate to reach out for more info!

Overview of the "noble gas paleothermometer": the temperature-dependent solubility functions of Ne, Ar, Kr, and Xe (bottom right) are used to invert measured concentrations in old groundwater into temperature signals. Because diffusion of heat in soil attenuates the amplitude of seasonal temperature fluctuations with depth (top right), the water table temperature in most regions (where it is below ~10 meters) tends to closely match the mean annual surface temperature. Because noble gases in LGM-aged groundwater were ultimately inherited at the water table during the LGM, we can use measurements in old groundwater samples to reconstruct mean annual surface temperature, after accounting for the influence of "excess" air from bubble input. (From: Seltzer et al., in review)

How Cold Was the Last Glacial Maximum (LGM) on Land?

The LGM (~20,000 years ago) is the most recent stable, long-term climate period that was substantially colder than the present. As such, it's an excellent target for evaluating climate models and estimating Earth's climate sensitivity. However, accurate and quantitative constraints on LGM surface temperature are needed, especially on land at low elevation. The "noble gas paleothermometer" enables reconstruction of mean annual surface temperature (MAST) based on measurements of neon, argon, krypton, and xenon dissolved in ancient groundwater. As shown in the figure to the left, noble gases in groundwater are inherited from the atmosphere through equilibrium dissolution of soil air at the water table (which typically maintains a temperature resembling MAST), plus bubble input. Using their well-known solubility functions and modeling the influence of "excess" air from bubble dissolution, noble gases can be used to quantitatively tell us what the surface temperature was during the LGM. We have recently completed a global synthesis several dozen groundwater records from the low-to-mid latitudes to create a composite record of MAST during the LGM.

How Efficiently Are Atmospheric Gases Transported to the Ocean's Interior?

Since the industrial revolution, the ocean has taken up ~28% of all carbon dioxide emitted by human activities (IPCC AR5). The vast majority of this uptake has occured through the "solubility pump" whereby cold high latitude waters (which can hold more gas) densify and sink into the ocean interior, bringing anthropogenic carbon with them. The solubility pump is not completely efficient, however, and its unclear how exactly its efficiency will change in the future. Because noble gases are also taken up by cooling, densifying high latitude waters but are only sensitive to physics, their concentrations throughout the ocean interior provide important clues about the physical mechanisms that regulate gas uptake efficiency during deep-water formation. Our group is working with collaborators to further develop noble gases and their isotopes as a suite of complementary tracers of the ventilation efficiency of the deep ocean. We are developing new analytical techniques, making new measurements in the deep ocean, incorporating these tracers into models, and carrying out lab experiments to better constrain their sensitivities to different processes. Ongoing work to develop Ar, Kr, and Xe isotope tracers is supported by NSF OCE Award #1923915.

Idealized model of argon undersaturation (Δ) caused by cooling of the high-latitude surface ocean prior to deep-water formation. The solid line refers to the actual concentration of argon (C), the dashed line to the equilibrium concentration at 1atm pressure (Ceq), and the red line indicates the percent undersaturation (Δ) over time. By measuring noble gases and their isotopes in the deep ocean, we can constrain more complex models to better understand the transport of gases from the atmosphere to deep ocean.
Kr and Xe stable isotope ratio anomalies, normalized by mass difference, in 58 groundwater samples from California, with respect to their ratios in the atmosphere. The observed near 1:1 relationship is similar to the expectation for the dominance of gravitational settling (red arrow, #1). For a full description of this figure, please check out the original publication here.

How Do Groundwater Levels Respond to Climate Change?

Building upon the "noble gas paleothermometer" mentioned above, we have recently developed a tool for reconstructing past water-table depths from high-precision measurements of dissolved heavy noble gas isotope ratios in groundwater. The basis for this new paleohydrological tool is rooted in gravitational settling in unsaturated zone air, a well-understood physical process that results in enrichment of heavy-to-light gas ratios with depth. As Kr and Xe in unsaturated zone air dissolve into groundwater at the water table, the gravitational signal is ‘locked in.’ Recent measurements of dissolved Kr and Xe stable isotope ratios at 5 per meg amu-1 precision (1σ) from 36 groundwater wells confirm the expected signal of gravitational settling (Seltzer et al., 2019). Reconstructions of water-table depth in modern groundwater samples agree with historical observations, and paleogroundwater measurements from San Diego (California, USA) indicate ~18-meter shallower mean water-table depths during the last glacial period, consistent with a wetter glacial climate in Southern California. We have ongoing projects to improve analytical precision and better interpret these signals in a hydrogeological framework to constrain past changes in regional water budgets. This work was supported by an NSF Graduate Research Fellowship to A. Seltzer and NSF EAR Award #1702704.

How Do Monatomic Gases Travel Through and Vibrate Within Water?

In collaboration with colleagues at Princeton, we are working to better understand what processes give rise to the small laboratory-observed differences in solubility and diffusivity among isotopes of noble gases. For example in our 2019 paper, we found that the heavy isotopes of Ar, Kr, and Xe are slightly more soluble than the light isotopes, with curious patterns of isotopic mass dependence and temperature dependence. Since 2020, we have been working on a simple set of molecular dynamics (MD) simulations using LAMMPS to simply look at how individual noble gas atoms collide with and travel around water molecules under various conditions. By examining the trajectories (and instantaneous velocities) of noble gas isotopes of varying mass at very small scales in space and time, and using a framework originally developed for vapor pressure isotope effects to understand these solubility effects, we can compare simulated isotopic solubility differences to measurements. The results have been encouraging, suggesting that classical MD simulations can resolve equilibrium fractionation factors for monatomic gases in water, and we are now beginning work on new simulations to test for solubility and diffusivity isotope effects for diatomic gases. We are in the process of preparing a manuscript about these simulations - stay tuned!

Simulation of an Ar-40 atom (central sphere; enlarged for clarity) interacting with water molecules (red/white objects: red = oxygen, white = hydrogen) in a 30x30x30 angstrom box. For a sense of scale, note that the Ar-40 atom tends to collide with a water molecule about twice per picosecond. By comparing the rattling frequencies of different isotopes, we can estimate solubility isotope effects and compare with observations.