Detailed ana / lysis of acient ice cores can be found in the paper: «Microbial Ana.lyses of
Ancient Ice Core Sections from Greenland and Antarctica» — Caitlin Knowlton, Ram Veerapaneni, Tom D'Elia, and Scott O. Roger
Perhaps Professor Salby should have acquainted himself with glaciology research before making such comments, because CO2 from
ancient air trapped in the
ice cores is precisely what is measured, albeit with some uncertainty in dating some
sections.