Abstract:
An oxygen and carbon isotopic history based on analyses of benthonic and planktonic foraminifera in three overlapping subantarctic sections is presented for the last 55 m.y. with a sampling interval of less than 1 m.y. Surface temperature at Site 277, on the Campbell Plateau, was about 19 degrees C in the Early Eocene, about 13 degrees C in the Middle Eocene, about 11 degrees C in the Late Eocene, and about 7 degrees C in the Oligocene. Declines in temperature appear to have been rather rapid and are separated by episodes of relative temperature stability. Bottom temperature at Site 277 was about 1 degree C below surface temperature in the Paleocene and about 2 degrees C below surface temperature in the Oligocene. Site 279, on the Macquarie Ridge, records an Early Miocene warming of over 2 degrees C followed by a cooling and a second similar temperature rise in the Middle Miocene. Bottom temperature at this somewhat deeper site was about 3 degrees C below surface temperature and was probably as low as 4 degrees C during part of the Early Miocene. Comparisons between sites 277 and 279 suggest that from the Early Oligocene temperatures of deep water were low like the present day, implying that the mean annual temperature in high southern latitudes was near freezing by the beginning of the Oligocene (but certainly no earlier). From this time glaciers would have descended to sea level, and there would have been sea-ice production. If an ice sheet were present, it could not have been more than a small fraction of its present-day size. Site 281, on the South Tasman Rise, extends the record into the Middle and Late Miocene during which time the major East Antarctic ice sheet accumulated. A significant rise in surface temperature during the Late Miocene did not cause the melting of this ice sheet, demonstrating that by this time it had already achieved its present invulnerability to climatic change. Since temperatures during much of the Miocene were significantly above any Pleistocene values, it is extremely unlikely that any climatic change in the geologically near future will significantly affect the stability of the East Antarctic ice sheet.