Climatic and insolation control on the high-resolution total air content in the NGRIP ice core

Abstract

Abstract. Because the total air content (TAC) of polar ice is directly affected by the atmospheric pressure and temperature, its record in polar ice cores was initially considered as a proxy for past ice sheet elevation changes. However, the Antarctic ice core TAC record is known to also contain an insolation signature, although the underlying physical mechanisms are still a matter of debate. Here we present a highresolution TAC record over the whole North Greenland Ice Core Project ice core, covering the last 120 000 years, which independently supports an insolation signature in Greenland. Wavelet analysis reveals a clear precession and obliquity signal similar to previous findings on Antarctic TAC, with a different insolation history. In our high-resolution record we also find a decrease of 4-6 % (4-5 mL kg −1 ) in TAC as a response to Dansgaard-Oeschger events (DO events). TAC starts to decrease in parallel to increasing Greenland surface temperature and slightly before CH 4 reacts to the warming but also shows a two-step decline that lasts for several centuries into the warm interstadial. The TAC response is larger than expected considering only changes in air density by local temperature and atmospheric pressure as a driver, pointing to a transient firnification response caused by the accumulation-induced increase in the load on the firn at bubble close-off, while temperature changes deeper in the firn are still small.

Gas enclosure in polar firn follows universal law

“…An anti-correlation with local insolation was found and suggested as a new proxy (Raynaud et al, 2007). The same insolation signature was found for the V record of the NGRIP core (Greenland), but the underlying physical mechanisms are not yet resolved (Eicher et al, 2016). Based on our results, we rule out the idea of other properties influencing the porosity at close-off :::…”

“…The reduced coupling of proxies and surrounding conditions may allow for more sophisticated reinterpretations in terms of paleoclimatic changes and will foster :::::: benefit the development of new proxies, such as the air content as a marker of local insolation (Raynaud et al, 2007;Eicher et al, 2016).…”

Critical porosity of gas enclosure in polar firn independent of climate

“…An anticorrelation with local insolation was found and suggested as a new proxy . The same insolation signature was found for the V record of the NGRIP core (Greenland), but the underlying physical mechanisms are not yet resolved (Eicher et al, 2016). Based on our results, we rule out the idea of other properties influencing the porosity at gas enclosure for single layers as we do not even observe a temperature dependence.…”

“…Finally, we discuss changes in the dating and interpretation of trace-gas records that incorporation of our results in current firn air models will imply. The reduced coupling of proxies and surrounding conditions may allow for more sophisticated reinterpretations in terms of paleoclimatic changes and will benefit the development of new proxies, such as the air content as a marker of local insolation Eicher et al, 2016).…”

Using Ice Cores and Gaussian Process Emulation to Recover Changes in the Greenland Ice Sheet During the Last Interglacial

“…The relatively small overlap between air content and 18 O compatible morphologies could possibly reflect uncertainties in the air content elevation reconstruction methodology. The method relies on calculating corrections to air content due to insolation and temperature (Eicher et al, 2016;Raynaud et al, 2007) 10.1029/2019JF005237 alongside secular changes in surface pressure and winds (Krinner et al, 2000;Martinerie et al, 1994). Surface melting also affects NEEM air content measurements between 127 and 118.3 ka (NEEM community members, 2013).…”