The oxygen isotope variation is then almost all attributed to changes in global ice volume and sea level. Anywhere else, it is influenced by global ice and temperature. It is also more representative to work with open marine basins rather than with enclosed systems because these ones will record changes that might be only local. So, oxygene 18 ratios in open basins have a direct link to sea temperature and global ice volume. The standard used for the forams O18 composition is PDB which is a Cretaceous belemnite from the Pee Dee Formation in North Carolina.
Belemnites are extinct cephalopods with a thick calcium rostrum as part of their endosqueleton.
Isotopic analysis of these rostra revealed two successive major temperature cycles having an amplitude of several degrees and extending across 60 million years of Mesozoic time. The amount of dissolved oxygen in sea water is controlled in most part by the temperature and salinity of sea water butthe fractionation of oxygene isotopes is more affected by temperature. Some factors have to be taken in consideration to use forams for dating. Planktic forams live in upwelling areas to get enough nutrients, in tropical or subtropical regions. Emiliani used them for his isotopic studies around 1966.