Investigating the relationship between Total Air Content (TAC) variations in polar ice cores and local surface climate conditions

Abstract. While air bubbles in polar ice cores are well known for archiving past atmospheric composition, the quantity of air trapped in these bubbles also carries significant paleoclimatic information. This parameter, known as Total Air Content (TAC), was first used to reconstruct past ice sheet elevation and later became an orbital dating tool after its temporal variations revealed a strong correlation with local summer insolation. However, limited understanding of relationships between TAC, pore volume at close-off depth, and surface parameters has restricted its widespread use. In this study, we investigate the link between surface parameters (e.g. temperature and accumulation) and changes in TAC at both spatial and temporal scales in order to better understand TAC as an environmental proxy and as an orbital dating tool. To do so, we first present a new dataset extending the TAC record from the EPICA Dome C (EDC) ice core back to 800 ka, as well as new TAC records from the TALDICE and EDML ice cores covering the last glacial-interglacial cycle. Second, we combine these new datasets with a compilation of published TAC data from ice cores across Antarctica and Greenland to explore the influence of surface climate parameters controlling the changes in TAC both at spatial and temporal scales. Our spatial-scale analysis first examines how recent TAC values relate to atmospheric pressure and elevation. We then investigate pore volume at close-off (i.e. TAC values corrected for ideal gas law effects) to assess the influence of mean annual temperature, accumulation rate, and local summer insolation. We evidence a strong correlation between the pore volume at close-off and local half year summer insolation for East Antarctic sites, suggesting a direct control of local insolation on firn densification in this region. Temporal-scale analyses on TAC records covering at least 40 ka confirm that TAC records contain an orbital-scale signature of local insolation but also show that local summer insolation alone cannot capture the full TAC variability. Multiple regression analysis incorporating local insolation and reconstructed surface temperatures or accumulation better predicts TAC temporal changes, particularly during glacial terminations. Our new EDC high-resolution record also revealed significant millennial-scale TAC changes during these glacial terminations. Hence, our results highlight that in addition to the orbital-scale impact of local summer insolation, millennial-to-multi-millennial-scale changes in surface climate parameters also influence the temporal-scale TAC changes. Our findings suggest that orbital tuning between TAC and local insolation that neglects surface climate controls could introduce age uncertainties of 1–5 ka, calling for surface climate-related corrections prior to TAC-based orbital dating.