On the link between Total Air Content (TAC) changes and local surface climate conditions in Greenland and Antarctica

While air bubbles in polar ice cores preserve past atmospheric composition, the quantity of trapped air, known as Total Air Content (TAC), also carries significant paleoclimatic information. First applied to reconstruct past ice sheet elevation, TAC later became an orbital dating tool due to its correlation with local summer insolation. To address knowledge gaps and better understand TAC as an environmental proxy and as an orbital dating tool, we investigate the relationships between surface parameters, pore volume at close-off depth, and TAC changes at spatial and temporal scales.

We present and analyze a new dataset extending the EDC TAC record from 440 to 800 ka, as well as new TAC records from TALDICE and EDML covering the last glacial-interglacial cycle. We combine these new datasets with a compilation of published TAC data from deep and shallow ice cores across Antarctica and Greenland to explore the influence of surface climate parameters controlling the changes in TAC. Our spatial-scale analysis demonstrates that present-day TAC values relate primarily to atmospheric pressure and elevation. When examining pore volume at close-off (i.e. TAC values corrected for ideal gas law effects), we evidence a correlation with local half-year summer insolation for sites located in East Antarctica, suggesting a direct control of local insolation on firn densification in this region. Temporal-scale analyses on TAC records covering at least 45 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 linear regression analyses incorporating both local insolation and reconstructed surface temperatures or accumulation better predict the observed TAC temporal changes, particularly during large glacial terminations. Our new EDC high-resolution record also reveals significant millennial-scale TAC changes during these glacial-interglacial transitions, highlighting that in addition to orbital-scale impacts of local summer insolation, millennial-to-multi-millennial-scale changes in surface climate parameters also influence temporal TAC changes. Our findings have implications for the use of TAC as an orbital dating tool as they suggest that performing an orbital tuning between TAC and local insolation without accounting for additional surface climate controls could introduce dating uncertainties of 1–4 ka. Building on these results, we present a new TAC profile measured on the Beyond EPICA Oldest Ice Core (BEOIC) core between 2438 and 2485 m depth and evaluate its potential for providing orbital age constraints on ice older than 800 ka and up to 1.2 million years.