Total Air Content measurements from the RECAP core

Total air content (TAC) from ice cores mainly reflects air pressure when the air is occluded and is therefore a proxy for elevation. However, there are several complications, such as melt, changes in firn structure and air pressure variability.  

We measured TAC in the RECAP ice core drilled in 2015 on the Renland Icecap in East Greenland, currently at an elevation of 2340 m. The upper 529 m of the 584 m core cover the Holocene. There is extensive melting in this part of the core, which is reflected in low air content values. Assuming constant altitude and air pressure, lower TACs at the beginning of the Holocene indicate more melting and therefore higher summer temperatures. Simulations with the regional climate model HIRHAM5 allow us to translate the observed melt fractions into summer temperatures. We conclude that summer temperatures in the early Holocene were ~2 to 3°C warmer than today, in agreement with previous findings in Greenland. The core extends into the Eemian. The air content is very low in this section, indicating excessive melting. Using the same metric as for the Holocene, we conclude that the temperature in the Eemian was at least 5°C higher than today.

The ~22m of glacial ice in RECAP from 11.7 to 119 kyr BP appear to be unaffected by melting. However, we observe large variations in total air content during periods of rapid climate change. In Greenland, similar effects have been found at NGRIP (Eicher et al., 2016). These effects cannot be due to changes in elevation. All the evidence points to a dynamic effect in the firn column that changes the pore volume. If we do not understand these effects, the interpretation of TAC in terms of elevation changes is questioned.