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Because the Cretaceous was an unusually warm period in Earth's history, with high evaporation rates, PDB has an unusually high¹⁸O/¹⁶O ratio. As a result, most Pliocene–Pleistocene samples have a negative δ¹⁸O, with small negative numbers recording maximum glacial ice volumes and larger negative numbers recording minimum glacial ice volumes. Because different organisms selectively fractionate¹⁸O and¹⁶O, a range of organisms must be analyzed and the results averaged when determining global changes in¹⁸O/¹⁶O. Nothing is ever as easy as it first seems.

It should be noted that many δ¹⁸O analyses have used a different standard. This standard is the average¹⁸O/¹⁶ ratio in ocean water known as standard mean ocean water (SMOW). Of course, that has been complicated by global warming, that generally increases evaporation rates, changing the ratios in natural waters.

Because the original SMOW and PDB standards have been used up in comparative analyses, yet another standard, Vienna standard mean ocean water (VSMOW), is also used. This name is misleading as the Vienna standard is actually a pure water sample with no dissolved solids. There is currently much discussion concerning the notion of which standards are most appropriate and how δ¹⁸O and other isotope values should be reported.