Gas loss and isotopic fractionation induced by pumping during ice core gas extractions

Ice core trapped gases are crucial paleoclimate archives, yet various gas loss processes introduce fractionation that obscures climatic signals. This study quantifies pump-induced fractionation during sample evacuation. Using a horizontal ice core from coastal East Antarctica, we investigated the responses of gas ratios and isotopic compositions (δ18Oatm, δO2/N2 and δAr/N2) to varied rotary or turbo pumping durations (0.5 – 90 minutes). First, we found that the extent of gas loss is pump-dependent (turbo > rotary) but independent of pumping duration. We observed no correlation between pumping duration and gas loss magnitude. The δAr/N2 – δO2/N2 fractionation slope from pair differences between samples subjected to different pumping durations was 0.85, similar to the slope of 1 for size-dependent gas loss process. This pump-induced slope is distinct from the slopes for bubble close-off and post-coring gas loss, which are usually ≤ 0.5. Isotopic enrichment in δ18Oatm correlated with gas loss magnitude, with slopes of -0.0109‰‰-1 and -0.0119‰‰-1 against δO2/N2 and δAr/N2, respectively. Our observations imply that pumping primarily evacuates gases from ice cracks, which are mainly influenced by mass-dependent fractionation, leaving bubble-resident gases fractionated by lattice diffusion (size-dependent). In order to reduce gas losses and associated uncertainties, we propose optimized protocols that minimize ice sample pumping duration after achieving a critical vacuum, particularly for fractured ice such as Antarctica blue ice samples.