NSFGEO-NERC: Understanding the Drivers of Inert Gas Saturation to Better Constrain Ice Core-Derived Records of Past Mean Ocean Temperature

The integrated heat content of the global ocean (OHC) is a fundamental climate variable for understanding Earth's energy balance. The OHC is intimately tied to high-latitude processes, which regulate air-sea fluxes of heat and radiative gases and control rates of deep-water formation. To quantitatively resolve past changes in OHC, a new ice core proxy for global mean ocean temperature (MOT) has recently been developed. This MOT proxy employs high-precision measurements of globally well-mixed atmospheric noble gases trapped in polar ice, which are highly sensitive to global ocean warming or cooling (Baggenstos et al., 2019; Bereiter, Shackleton, et al., 2018; Shackleton et al., 2019, 2020). Noble gases are powerful tracers of physical interaction between the atmosphere, ocean, and cryosphere due to their chemical and biological inertness, lack of long-term sinks and sources, and spatially uniform distribution in the atmosphere. Changes in krypton (Kr) and xenon (Xe) mixing ratios in the troposphere are quantitatively linked to the MOT due to the strong control of temperature on the solubility of these gases in seawater. That is, as the whole ocean warms, Kr and Xe solubilities decrease, which leads to net degassing of these dissolved gases from the global ocean and thereby increases their atmospheric concentrations. Because the heavy noble gases - Kr and Xe - have stronger solubility temperature dependences than nitrogen (N2), the ratios Xe/N2 and Kr/N2 measured in past atmospheric air bubbles trapped in ice cores can be used to constrain past MOT. Using measurements from multiple polar ice core archives of ancient atmospheric air, past changes in MOT (and therefore in OHC) over the past 25 thousand years have been quantitatively reconstructed in several recent studies.

The quantitative translation of past atmospheric Xe/N2 and Kr/N2 to MOT relies not only on knowledge of the solubility functions of these gases in water, but also on past changes in global ocean volume, salinity, sea-level pressure and the saturation states of Xe, Kr, and N2 in the global ocean. In the modern ocean, Kr and Xe are systematically undersaturated at depth by several percent throughout the global deep ocean, whereas N2 is closer to solubility equilibrium (Hamme et al., 2017; Loose et al., 2016; Loose & Jenkins, 2014; Nicholson et al., 2016; Seltzer et al., 2019). The well documented undersaturation of heavy noble gases in the modern ocean is thought to result from a complex function of global ocean circulation and high- latitude processes, such as changes in the wintertime cooling rates of high-latitude surface waters, sea-ice extent, glacial meltwater input, and wintertime storm intensities driving variable degrees of diffusive versus bubble-mediated air-sea gas exchange. The degree to which Kr and Xe may have been undersaturated during the last glacial maximum (LGM) presently remains an entirely open question, yet one that is essential for reconstructing past MOT.

To quantify the importance of past changes in undersaturation of inert gases in the deep ocean for ice core MOT reconstruction, there is a need for simulation of these gases in the global ocean under past climate states. We propose to use a suite of numerical model experiments, both equilibrium and single- forcing (e.g., isolating the effects of sea ice, ocean circulation, air-sea gas exchange dynamics), to estimate the Kr, Xe, and N2 saturation states of the past ocean, with particular emphasis on the LGM and periods of abrupt warming during the last deglaciation. This will not only allow us to refine existing polar ice core noble gas records of MOT by producing the first estimates of a presently unconstrained but important variable (Deq), but it will also enable better understanding of the physical drivers of undersaturation and their relationship to high-latitude ice-ocean-atmosphere interaction in preindustrial, glacial, and future climates.