Millennial-scale jumps in total air content at Dome C and new total air content measurements over the MPT on the Beyond EPICA ice core

Measurements of the total air content (TAC) in ice cores have a long history and were motivated to reconstruct past changes in the altitude of the ice sheet at the drill site, as air pressure is the dominant control on TAC. To allow this, one must know the porosity at bubble closure (pore volume) and correct for temperature. Temporal changes in the porosity are difficult to constrain, limiting its use as an altitude proxy. Orbital changes in the local insolation were found to modulate the TAC signal, allowing this parameter to be used as an additional orbital dating tool without a precise process understanding of the driving mechanism. Recent measurement campaigns on the EPICA Dome C ice core covering the last 450 kyr have increased the temporal resolution and allow about 1 kyr resolution between MIS 9 and MIS 7 (around 350 to 210 kyr). Our high-resolution record corroborated the well-known orbital TAC variations; however, it also showed rapid upward jumps within 2 kyr that were not previously visible. These TAC jumps are especially pronounced in MIS 7 and 9, interglacials characterised by so-called late deglacial overshoots in CO2 and CH4, but are also visible in water isotopes and aerosol records. The characteristic sequence for these overshoot interglacials is as follows:

From a TAC maximum that is reached already before the start of the deglaciation, TAC is slowly dropping to reach a pronounced minimum right at the interglacial temperature maximum.  After this minimum, TAC values rapidly increase within 2 kyr, thus less than the age of the firn column. This suggests that the millennial-scale changes in temperature and accumulation at the start of the interglacial lead to a transient disequilibrium in firnification. I.e., during the early interglacial warming, the rise in snow accumulation, hence overburden pressure, on top of a firn column leads to a transient creep-related reduction of porosity, hence to the pronounced TAC minima. Similar millennial-scale TAC features were observed in Greenlandic ice cores (Eicher et al. 2016) during rapid DO events.

With the advent of the Beyond EPICA ice core, we can now examine the characteristics of older interglacials to answer the question which of the interglacials during the MPT exhibit these dynamic TAC features and which resemble interglacials that seem to lack them (e.g. MIS 11). First measurements on the BEOI during the MPT indicate an orbital TAC dynamic similar to those over the last 600 kyr, while pronounced TAC minima characteristic of overshoots have not yet been identified. Moreover, the TAC evolution of MIS 31 seems to resemble the overall characteristics of MIS 11, i.e., it lacks the overshoot characteristics.