Reference

Lüthi D, Le Floch M, Bereiter B, Blunier T, Barnola JM, Siegenthaler U, Raynaud D, Jouzel J, Fischer H, Kawamura K & Stocker TF (2008). High-resolution carbon dioxide concentration record 650,000–800,000 years before present. Nature 453(7193):379–382.
Bereiter B, Eggleston S, Schmitt J, Nehrbass-Ahles C, Stocker TF, Fischer H, Kipfstuhl S & Chappellaz J (2015). Revision of the EPICA dome C CO2 record from 800 to 600 kyr before present. Geophysical Research Letters 42(2):542–549.