The Plio-Pleistocene boundary has been located at the place where certain northern species of marine invertebrates first appear in the continuous Plio-Pleistocene sections of Italy. The section at Le Castella, near Crotone, southern Italy, consists of clays with diatomaceous layers and a few thin sand layers. The Plio-Pleistocene boundary there is clearly marked by the appearance of Anomalina baltica and other northern species of Foraminifera. Calabnan (lower Pleistocene) sediments 210m thick overlie the boundary, and upper Pliocene sediments 1405 m thick underlie it. The fossil fauna of the Plio-Pleistocene section consists essentially of abundant pelagic and benthonic Foraminifera, and diatoms. The benthonic microfauna indicates a depositional depth of about 500 m. There is no evidence of turbidity currents and submarine slumping.
A section 167.80 m thick, including the paleontologically defined Plio-Pleistocene boundary, was logged and sampled at close stratigraphic intervals. Oxygen isotopic analyses of different species of pelagic and benthonic Foraminifera and of shell fragments of benthonic mollusks at successively higher stratigraphic positions in the section have revealed numerous temperature oscillations. Surface temperatures, probably representing summer averages, as indicated by Globigerinoides rubra and G. sacculifera, ranged from 21°C. to more than 30°C. in the late Pliocene, from 16°C. to more than 30°C. in the late Pliocene-early Pleistocene, and from 12° C. to 28° C. in the late Pleistocene. Temperatures at some depth and/or at a season other than the summer, as indicated by Globigerina bulloides and G. inflata, ranged from 20°C. to 28°C. in the late Pliocene, from 11°C. to 22°C. in the late Pliocene-early Pleistocene, and from 9°C. to 18°C. in the late Pleistocene. Temperatures given by benthonic Foraminifera ranged from 14°C. to 20°C. in the late Pliocene and from 11°C. to 22°C. in the late Pliocene-early Pleistocene (no data available for the late Pleistocene). Temperatures above 30°C. may be due, in part, to isotopic effects of the sea water. The above figures clearly indicate a major shift of the temperature ranges toward lower values from the late Pliocene to the late Pleistocene.
No major temperature change seems to have occurred across the paleontologically defined Plio-Pleistocene boundary. Since the secular temperature minima of the late Pliocene are somewhat lower than present average summer temperature, an areal extension of the ice somewhat greater than that at present is suggested. Such extension probably increased at the times corresponding to the even lower temperature minima of the early Pleistocene. Major continental glaciations, however, may not have started until later, because none of the temperature minima in the section at Le Castella appear to have reached values as low as those obtaining during the glacial ages in the eastern Mediterranean.
The temperature changes in the section at Le Castella are paralleled by important changes in the microfauna.