Glacial development and temperature trends in the Antarctic and in South America

Abstract

Variations in surface temperature of the Pacific sector of the Southern Ocean result in similar changes downwind in South America. Northward in South America the influence of the Southern Ocean decreases and that of the Atlantic Ocean increases. Northward retreat of tropical vegetation in southern South America during the Late Eocene and Early Oligocene reflects contemporaneous cooling of the Southern Ocean. The glacial history of southern South America after the onset of major glaciation 3.5 MY ago likewise must reflect Early Pliocene cooling of the Southern Ocean. Oxygen isotopic studies of deep-sea cores suggest that conditions in coastal Antarctica first became frigid at the beginning of the Oligocene ca 37 MY ago, when the sea froze and glaciers extended to sea level. However, other deep-sea core studies suggest that conditions did not become severe until the end of the Oligocene ca 26 MY ago, when ice rafting began and tree species were exterminated. The later date is considered the more reliable because it is based on more direct evidence. According to oxygen isotopic studies, the East Antarctic ice sheet accumulated from 14 to 10 MY ago. If so, the West Antarctic ice sheet, whose situation largely below sea level precludes it ever having been composed of temperate ice, must have formed after 10 MY ago. Its emplacement would have caused a rather abrupt northward expansion of cold Antarctic water; such an event has been noted in Early Pliocene time soon after 5 MY ago. Evidence for an earlier comparably cold episode in Late Miocene time ca 6 MY ago is equivocal. This suggests that the West Antarctic ice sheet first formed during the Early Pliocene. If so, its formation may have been triggered by the ocean freshening caused by massive evaporite deposition in the Mediterranean basis 5.5–5 MY ago. Some geological evidence, however, and some recent reassessments of oceanographic evidence, suggest that the West Antarctic ice sheet was present by about 7 MY ago. Initiation of glaciation in southern South America 743.5 MY ago does not disprove a 7 MY ago for the West Antarctic ice sheet, but it favours a younger age. During the Gauss Epoch 3.3–2.4 MY ago oceanographic studies suggest an unlikely interhemispheric contrast: a return to comparative warmth in high southern latitudes, and the start of major mid-latitude glaciation in the Northern Hemisphere. Unfortunately, little is known about glacial events in South America during this interval. After ∼ 2.1 MY ago repeated glaciations in southern South America undoubtedly correspond to repeated severe coolings of the Southern Ocean, but the individual episodes cannot yet be correlated. During the last interglacial the oxygen isotopic content of ocean water implies less global ice cover than today’s. In southern Chile, deep chemical weathering at that time suggests exceptional warmth. These observations support the hypothesis that the ca + 6 m sea level ca 125 000 years ago resulted from deglaciation of West Antarctica when temperatures rose above the critical level for ice shelves. During the last glaciation world ice volume was greatest ca 18 000 BP, implying that the Northern Hemisphere mid-latitude ice sheets were then largest. In south-central Chile, however, glaciers were largest – and by inference, temperatures were lowest – before 56 000 BP. After a smaller readvance culminating ca 19 500 BP the Chilean glaciers shrank during a major interstade and later readvanced, probably until about 13 000 BP. This oscillation is not evident in the sub-Antarctic Indian Ocean cores, but may be shown by the poorly-dated Byrd Station (Antarctica) core. After 13 000 BP, southern South America warmed rapidly, and by 11 000 BP glaciers were within their present borders, where they remained during the Younger Dryas Stade, ca 11 000–10 000 BP, the final European interval of severe cold. This early recession (compared to the North Atlantic area) is compatible with maximum postglacial warmth ca 9 400 ± 600 BP in the sub-Antarctic Indian Ocean, and strongly suggests that the North Atlantic was a highly atypical part of the world ocean during deglaciation of North America and Eurasia. Former large lakes in South Victoria Land, Antarctica, dammed by grounded Ross Sea ice, imply temperatures at least as high as today’s. A Late Wisconsin age has been suggested, but this seems unlikely, because southern South America was then much colder. The lakes may have formed early in the Southern Hemisphere hypsithermal, starting ca 11 000 BP, when eustatic sea level, which probably controlled the extent of the grounded Ross Sea ice, was still low. In Peru at lat 14° S, at a site dominated by air from the equatorial North Atlantic during the accumulation season, initial results suggested that a minor readvance occurred during Younger Dryas time, but further studies consistently indicate that the advance culminated at least 500 years earlier, ca 11 000 BP. By 10 000 BP the Peruvian glaciers were little, if any, larger than they are today. During Neoglacial time glaciers in southern South America were largest about 4 500 BP, whereas in the Northern Hemisphere a contemporaneous advance was relatively smaller. This suggests that the inferred cooling was caused by an event in high southern latitudes, perhaps greatly increased calving from West Antarctica. A later Neoglacial advance in southern South America culminated ca 2 700–2 200 BP, as in many other parts of the temperate zones of both hemispheres. Throughout this interval in Peru, however, ice was less extensive than it is today.