As a result, large ice sheets store water that is relatively light (has more oxygen-16), and so during a major glaciation the ocean waters become relatively heavier (contain more oxygen-18) than during interglacial times when there is less global ice.
Accordingly, the shells of marine organisms that formed during a glaciation contain more oxygen-18 than those that formed during an interglaciation.
The ratio depends on two factors, the temperature and the isotopic composition of the from which the organism secreted its shell.
Of these, only the Gelasian and Calabrian are formal intervals, whereas others await ratification by the ICS.
The Calabrian, which was previously known as the early Pleistocene, extends to the Brunhes–Matuyama paleomagnetic boundary at 780,000 years ago.
The Ionian, also known as the middle Pleistocene, extends to the end of the next to the last glaciation at about 130,000 years ago.
Pre-Pleistocene intervals of time are defined on the basis of chronostratigraphic and geochronologic principles related to a marine sequence of strata.
Following studies by a series of international working groups, correlation programs, and stratigraphic commissions, agreement was reached in 1985 to place the lower boundary of the Pleistocene series at the base of marine claystones that conformably overlie a specific in the Vrica section in Calabria.