Pliocene-early Pleistocene paleoclimatic history recorded in Antarctic-Subantarctic deep-sea cores

Abstract

Micropaleontological studies have been carried out on seven cores of middle Pliocene to early Pleistocene age (lower to upper Matuyama; t = 0.7 to 2.43 m.y. BP) from northern Antarctic and Subantarctic waters south of Australia and New Zealand. All of the cores contain abundant foraminiferal and radiolarian faunas, and have been dated by both micropaleontological and paleomagnetic methods. Fluctuations of a cold left-coiling Globigerina pachyderma fauna, with a warmer Subantarctic fauna define ten distinct intervals of warming during the Matuyama Reversed Epoch. This compares with six for the Brunhes Normal Epoch ( t = 0.0 to 0.7 m.y. BP). Peaks formed by increased frequencies of warm-water forms and right-coiling G. pachyderma are of lower amplitude than warmer-water peaks in the Brunhes. It is thus inferred that Matuyama climatic fluctuations in the Southern Ocean are of lower magnitude than those in the Brunhes. Furthermore, the foraminigferal evidence indicates that average temperatures during the Matuyama are in general cooler than during the Brunhes. This conflicts with previous paleotemperature determinations based on radiolarian assemblages which suggest average warmer conditions throughout the entire Matuyama Epoch. The disappearance of present-day subtropical radiolarians, Saturnulus planetes and Pterocanium trilobum, near the Brunhes-Matuyama boundary in the Southern Ocean has previously been regarded as strong evidence for a deterioration of climatic conditions in the Brunhes. Throughout the Matuyama, however, these forms occur in association with cold (northern Antarctic-Subantarctic) planktonic foraminiferal assemblages, and thus apparently lost their environmental tolerance for northern Antarctic-Subantarctic waters about 0.7 m.y. BP. The opposite is true for the planktonic foraminifer Globorotalia inflata which adapted to Subantarctic waters approximately 0.7 m.y. BP. Both of these apparent ecological adaptations coincide very closely with the magnetic reversal at the Brunhes-Matuyama boundary. Middle to late Pliocene (lower Matuyama) climatic oscillations within Subantarctic-northern Antarctic waters correspond rather closely with those established for the New Zealand middle to late Pliocene, but appear to be out of phase with equatorial and North Atlantic regions.