Ice core site considerations from modeling CO₂ and O₂/N₂ ratio diffusion in interior East Antarctica

Abstract. Obtaining a continuous ice core record to 1.5 million years (Ma), which spans the Mid-Pleistocene Transition (MPT, 1.2 to 0.7 Ma) is a goal of multiple international efforts in Antarctica. Ice of such age is likely to be highly thinned and located in warm ice near the bed, conditions which promote diffusion of the stored atmospheric gases. Here, we assess the preservation of CO₂ and the O₂/N₂ ratio in the ice sheet between South Pole and Dome A where the NSF Center for Oldest Ice Exploration has surveyed with airborne radar. We employ two models: 1) a 1D, steady state ice and heat flow model to calculate the temperature and age of ice with respect to depth, and 2) a vertical gas diffusion model for clathrate ice. We analyze the preservation of CO₂ signals with a period of 40 kyr to match pre-MPT glacial cycles and the preservation of O₂/N₂ signals with a period of 20 kyr to match precession cycles. 1.5 Ma ice is lost to basal melt in much of the study area where ice thickness exceeds 3000 m. In locations that preserve 1.5 Ma ice, vertical gas diffusion is most sensitive to accumulation rate; high accumulation rate sites have more highly thinned old ice, and the steeper gas concentration gradients enhance diffusion. The most promising region for recovering 1.5 Ma ice is the foothills of Dome A, approximately 400 km from both South Pole and Dome A, due to low accumulation rates and moderate ice thickness. CO₂ signals lose on average 14 % of their amplitude, while O₂/N₂ signals lose on average 95 % for 1.5 Ma ice, suggesting precession cycles may not be identifiable. Unknown geothermal heat flow is a large uncertainty for both ice loss from basal melt and gas signal preservation.