Twenty-one isotopically determined “interglacial” and an equal number of isotopically determined “glacial” or near-“glacial” stages are recognized in the isotope record of Globigerinoides sacculifer, a planktonic foraminiferal species, from a well-dated equatorial Atlantic core representing the past 2.3 m.y. Many of the glacial stages (especially before 1 m.y. B.P.) are less pronounced than the most recent glacial maximum.
The observed maximum change in oxygen-isotope values is 1.1‰. At least 90% of the changes in the isotopic composition is attributable to variation in the isotopic composition of ocean water, which is due to the waxing and waning of large continental glaciers. Thus, the isotope record could more appropriately be called an ice-volume record rather than a temperature record, although the two records are closely related. A long period of relatively decreased ice volume occurred from 1.2 to 1.0 m.y. B.P. Low sea-level stands (that is, periods of maximum glaciation) comparable to the Wisconsin maximum occurred at 145,000, 240,000, 530,000, and 750,000 B.P.
Correlation between this record and isotope records from the Caribbean Sea and equatorial Pacific Ocean is good. Because the oxygen-isotope record primarily represents an ice-volume record, it is an extremely useful correlative tool. Ice-volume changes should be recorded in the oceans almost synchronously, because complete oceanic mixing occurs in less than 1,500 yr.