A High Precision Method to Measure Neon Isotope Fractionation at the Firn-to-Ice Transition on Polar Ice Sheets
Atmospheric gases and their isotopes are crucial to understanding biogeochemistry, and trapped air in ice cores affords a unique extension of atmospheric records back into the past. However, certain gases become fractionated during the bubble close-off process due to leakage from overpressured bubbles. Therefore, a detailed understanding of the leakage mechanism during this process is crucial to making corrections to measured past atmospheric isotope ratios in glacial ice. Neon isotope measurements are proposed for making such corrections for two main reasons. First, the isotopic ratio is constant in the atmosphere, so any observed changes of the ratio represent only the effect of the bubble close-off process. This can be useful for making corrections to dioxygen and dihydrogen isotope measurements. Second, the neon atom is smaller than the critical size (3.6 Å) of the opening in the ice lattice, so quantifying neon isotopes can test and verify the widely adopted velocity-dependent hopping theory, which predicts that the light isotope should diffuse faster through the ice lattice due to its higher velocity. Although helium isotopes clearly show this effect, neon can provide insight about observed mass-dependent fractionation of other gases and its puzzling absence in argon. The newly developed neon extraction method and the first high-precision neon isotope (22Ne/20Ne) measurements have been successfully made with La Jolla air. Greenland firn air measurements will be next, which will quantify the mass-dependent gas loss at the firn-to-ice transition and can help improve the correction that is used for past atmospheric content reconstruction.