Chapter 3 A History of Antarctic Cenozoic Glaciation – View from the Margin

Abstract

Abstract The scale and antiquity of the Antarctic Ice Sheet was sensed from the time of earliest exploration a century ago. However, significant advances in scientific thinking, along with logistics and technology for gathering data from the continent itself, were required before a clear and consistent framework for ice-sheet history and behaviour could develop and this has emerged only in the last few years. The main features of the present ice sheet were established by over-snow traverses during and following the International Geophysical Year (1957–1958), but the timing and circumstances of its origin remained uncertain. Geological records of post-Jurassic time were largely buried under the ice or the sea floor around the Antarctic margin, though a few radiometric ages from the new K–Ar dating indicated Antarctic glaciation was likely older than the Northern Hemisphere ice ages of the Quaternary Period. New post-World War II techniques in offshore surveying with marine geophysics and ship-based drilling were first applied to the Antarctic margin in the early 1970s, and were immediately productive. The Antarctic continental shelf was found to be underlain by sedimentary basins with the promise of ice-sheet history, and in early 1973, cores from Leg 28 of the Deep Sea Drilling Project (DSDP) in the Ross Sea provided the first physical record of Antarctic glaciation extending back to Oligocene times. DSDP Leg 29 drilled in the Southern Ocean for deep-sea cores from the whole Cenozoic Era, yielding the first set of oxygen isotopic ratios (δ 18 O) from benthic calcareous microfossils, and the first estimates of ice volume. These indicated a two-stage ice-sheet history (cooling and some ice at 34 Ma, and persistent ice from 14 Ma – time-scale of Berggren et al., 1995). About the same time, the Dry Valleys Drilling Project (DVDP) was launched to recover onshore records of Antarctic Cenozoic climate history, and exploratory drilling from fast ice offshore soon followed. Cores from the Ocean Drilling Program in the 1980s from the Prydz Bay Shelf and the Kerguelen Plateau established the timing of the first continental ice sheet at 34 Ma. In the same period, deep continuous coring from sea ice in McMurdo Sound developed from DVDP technology succeeded in providing new detail for its subsequent history. Core recovery at ∼98% yielded lithological evidence of ice margin and sea level fluctuations implied by deep-sea isotope records. Further drilling with improved chronology in the 1990s yielded cores confirming ice margin and sea level changes on Milankovitch frequencies and on a scale of 10–40 m of sea level equivalent. Micropalaeontological and geochemical evidence pointed to a slight cooling from a coastal cold temperate climate, with beech forests during interglacial times. Subsequent development of ice-sheet modelling has indicated that most of the cooling that initiated ice-sheet glaciation was the consequence of a fall in atmospheric CO 2 levels below a critical threshold, allowing ice sheets to form that were highly sensitive to orbital forcing. This claim has been supported by recent work on CO 2 proxies and indicators of a shift in carbonate compensation depth in deep-sea sediments. Since the first measurements in the 1970s, deep-sea isotopic measurements have implied a significant increase in Antarctic ice volume at around 14 Ma that persisted to the present day. However, in the mid-1980s, marine diatoms in glacial deposits in the Transantarctic Mountains suggested periods in Pliocene times when seas invaded the East Antarctic interior, implying dynamic Antarctic Ice Sheets until Quaternary times. Evidence of continued cold in the mountains over the last 14 Ma, glaciological problems with the proposed over-riding scenario, lack of a signal in the deep-sea isotope record for the loss of most Antarctic ice in Pliocene times and possible alternative atmospheric sources for diatoms has shifted the weight of evidence in favour of persistent ice in the east Antarctic interior. However, coastal outcrops in Prydz Bay and a recent deep core from beneath the McMurdo Ice Shelf have shown that in the globally warmer Pliocene, notably around 3–5 Ma, seas around the Antarctic margin were several degrees warmer. Indeed, recent drill cores suggest that the Ross Embayment and perhaps also most of the West Antarctic interior were periodically ice-free in these times. Three decades ago, the Antarctic Ice Sheet was seen as a long-standing feature of the Earth with its origins in early Cenozoic times and its permanency assured by mid-Miocene cooling. Research in the last decade from geological drilling and glaciological remote sensing, supported by ice sheet and climate modelling, indicates the ice sheet is in fact quite responsive to changes in the global climate system, whether natural or human-induced, though at different rates in different sectors. Recent developments in both science and technology outlined here provide opportunities for projecting realistic scenarios for future ice-sheet response on human time scales.