Gravitational settling fractionation of Kr and Xe isotopes as a quantitative indicator of past water-table depth:

Dissolved Kr and Xe isotopes in groundwater are inherited from air above the water table (in what’s called the unsaturated zone). Thus any fractionation of dissolved Kr and Xe isotopes relative to the atmosphere only happens in two places/stages: 1) within the unsaturated zone before dissolution into groundwater at the water table, and 2) during dissolution and the associated injection and fractionation of excess air. In the unsaturated zone, gravitational settling dominates all other fractionation mechanisms, leading to an increase in heavy-to-light Xe and Kr isotopes that scales nearly linearly with depth (see figure below, adapted from Seltzer et al., 2019). During dissolution, solubility fractionation leads to a further increase in heavy-to-light isotope ratios while injection of excess air does the opposite, but typically only by a very small amount. Because solublity fractionation of Xe and Kr isotopes is largely insensitive to recharge temperature, it is the signal of gravitational settling – a nearly linear function of water-table depth – that is responsible for nearly all the variability of Kr and Xe isotope ratios dissolved in groundwater.

Steady-state isotopic fractionation of Xe in an idealized unsaturated zone (Seltzer et al., 2019) [*Xe: mass-normalized, error-weighted mean heavy-light Xe isotope ratio, vs air]

Past changes in the 18O/16O ratio of atmospheric O2

The ratio of O-18 to O-16 in atmospheric O2 is effectively constant globally on the ~1 year mixing timescale of the troposphere. However, on glacial-interglacial timescales, it changes with the O-18/O-16 ratio of seawater and terrestrial precipitation, and with the strength, spatial distribution, and types of photosynthesis and respiration. In our 2017 CP paper, we investigated to what extent shifts in the latitude of tropical rainfall may account for observed differences between the O-18/O-16 ratio of the atmosphere and seawater (called the Dole Effect) in two merged Antarctic ice core records. Insights from the modern seasonal cycles of rainfall O-18/O-16 ratios and oxygen production are consistent with the idea that an increase in atmospheric O-18/O-16, relative to mean seawater, may reflect a southward shift of tropical rainfall.

Analysis of modern monthly-mean precipitation oxygen isotope and gross primary production data from Seltzer et al., 2017. Southward shifts of GPP (and tropical rainfall and temperature maxima) are associated with increases (note that the bottom plot's y-axis is reversed) in the global GPP-weighted mean precipitation O-18/O-16 ratio. The "TOE" refers to the "Terrestrial Oxygenesis Equator," which we defined as the median latitude of monthly oxygen production on land.

High precision measurements of Ar, Kr, and Xe isotopes in groundwater and seawater

Enabled by large strides in the noble gas isotope analytical community over the past few decades, with sufficiently large gas samples, it is now possible to measure changes in Ar, Kr, and Xe isotope ratios dissolved in groundwater - relative to the well-mixed atmosphere - at 5 per meg (ppm) precision or better. At SIO, I developed a quantitative extraction system for dissolved Ar, Kr, and Xe in groundwater and seawater. Using this system, we've been able to zoom in and see small signals in dissolved noble gas isotopes related to gravitational settling (in groundwater) and air-sea gas exchange during deep-water formation (in seawater). Check out our 2019 EPSL paper for more details.

A schematic of the dissolved noble gas extraction system at SIO that I developed during my graduate studies.