Planktonic foraminifers from deep-water sediment cores in the northern Gulf of Mexico indicate alternating cold- and warm-water faunas, and are interpreted to represent glacial and interglacial stages of the Pleistocene. Globorotalia menardii (warm) and Globorotalia inflata (cold) are the most sensitive indicators for delineating severe changes in Pleistocene temperature in the Gulf of Mexico.

Based on planktonic datums the cold-warm cycles are correlated with water-depth changes in the shallower shelf environment which are interpreted to represent eustatic sea-level changes. The cold intervals represent times of lowered sea level and are correlated with the glacial stages of North America. Age relations of the glacially related events recognized in the marine Pleistocene section with the continental glacial stages are well documented to about 35,000 years ago based on carbon 14 dates. Age relations between the older part of the marine Pleistocene and the continental glacial stages are not as well documented but can be inferred from correlation between the glacial-interglacial cycles and paleomagnetic reversal events which are well dated by potassium-argon.

Onset of severe climatic deterioration and beginning of the Quaternary (Nebraskan Stage) in the Gulf of Mexico corresponds with the extinction horizon of Globoquadrina altispira. Correlation of this horizon with the paleomagnetic reversal events indicates an age of about 2.8 m.y. ago for the Pliocene-Pleistocene boundary.

Placement of the Pliocene-Pleistocene boundary at 2.8 m.y. ago allows recognition of five major glacially-related events in the marine Pleistocene based on climatic and eustatic criteria. Faunal data indicate that the Nebraskan, Illinoian, and Wisconsin were severely cold periods, whereas, the Aftonian, Kansan, and Yarmouth were warmer, the Aftonian being the warmest. Carbon 13/12 isotope data support these conclusions.

Foraminifera and discoasters from the marine Calabrian Stage of southern Italy at Santa Maria di Catanzaro indicate a correlation with the marine Nebraskan Stage, as here defined, in the Gulf of Mexico. Paleontologic evidence indicates that the Pliocene-Pleistocene boundary falls near the base of the Kansan. The cold and warm interval shown below the Pliocene-Pleistocene boundary of Glass and his co-workers is interpreted to represent the Nebraskan and Aftonian Stages. Paleomagnetic evidence indicates that the Olduvai magnetic event probably represents the Gilsa event at about 1.6 to 1.8 m.y. ago.

Advances in paleomagnetic stratigraphy clearly indicate that the magnetic reversals facilitate dating and correlation of marine stages at latitudes where differing paleontologic criteria must be used to delineate the glacial and interglacial episodes. The major paleomagnetic epochs apparently are reliable for stratigraphic correlation; however, the minor events of relatively short duration appear to be less reliable as stratigraphic markers.

A new species of the Globorotalia menardii group is described.

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