PDB standard . Carbon dioxide prepared from belemnites ( Belemnitella americana ) collected from the Peedee Formation (Upper Cretaceous) of South Carolina. It is used as a standard of comparison in determining the isotopic composition of carbon and of oxygen. It was first used by H. C. Urey in 1951 to determine paleotemperatures on the basis of the fractionation of oxygen isotopes. The initials stand for "Peedee belemnite".