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Orbital dating using O2/N2 records is a powerful tool for constructing ice core chronologies in deep ice cores due a widely observed anti-correlation with summer solstice insolation (SSI). While understood to be linked to near-surface snow metamorphism, the physical mechanisms driving this process remain poorly constrained and the role of local accumulation rate and temperature have been scarcely considered. We primarily present the results of our new study which uses a compilation of records from 14 ice cores to show a significant dependence of mean O2/N2 on local accumulation rate and temperature. Using EPICA Dome C as a case study, we then show that during rapid climatic changes, an accumulation/temperature signal may be superimposed on top of the SSI signal and therefore should be accounted forwhen using peak-matching techniques for future dating of deep ice cores, such as the EPICA or Beyond EPICA cores.Further to our study, we include new O2/N2 data measured in shallow, bubbly ice just below close-off from two newly drilled firn cores at sites with distinct close-off conditions; D-47 and Little Dome C (the Beyond EPICA site), which support our findings. Moreover, thanks to parallel firn air pumping campaigns, overlapping data from open and closed porosity at these two sites promise greater insight into the mechanisms driving close-off fractionation.
Stuart et al. (Fri,) studied this question.