The stable carbon isotope ratio of atmospheric CO2 (δ13Catm) is a key parameter in deciphering past carbon cycle changes. (science.sciencemag.org)
More recently, in the Paleocene - Eocene Thermal Maximum, about 55 million years ago, a rise of 5oC in ocean temperature (recorded in stable isotope ratios in various geological deposits) and 8oC on land occurred. (dotearth.blogs.nytimes.com)
A study by Thomas, Dennis et al 2009 [8] derived a high resolution temperature proxy record from oxygen isotope ratios from the ice core. (judithcurry.com)