Just on Science magazine website; and for 49-56 million years ago.
The earth's last major warm period probably witnessed relatively high concentrations of carbon dioxide (CO2) in the atmosphere--similar to the upper limit of concentrations we might expect in the coming century--according to an analysis of ancient mineral deposits. The clues come from the period of time between 49 and 56 million years ago, called the Eocene. Global temperatures jumped at the start of this era, judging from marine sediments as well as fossilized palm fronds and crocodiles found near the North Pole. ...
The new estimate is based on the discovery of the rare sodium carbonate mineral nahcolite in two large deposits that date to the Eocene. Crucially, nahcolite is only expected to crystallize from salty water when the atmosphere contains 1,125 ppm or more of carbon dioxide
Rare Mineral Further Implicates CO2 in Last Global Warming