Chapter 9 The Oligocene–Miocene Boundary – Antarctic Climate Response to Orbital Forcing

Abstract

Abstract Recent high-resolution Oligocene–Miocene oxygen isotopic records revealed a relatively transient, ca . 2 myr period, 1 ‰ amplitude cyclicity in isotopic values (Oi and Mi events, respectively). Intriguingly, it has been suggested that these isotopic excursions in oceanic δ 18 O were linked to ephemeral growth and decay in Antarctic Ice Sheets. A great deal of effort in the palaeoceanography community has been focused on developing techniques and gathering additional records to determine if the Antarctic Ice Sheet has behaved in such a transient manner in the past and indeed what factors might have led to the rapid growth and decay of ice sheets. Deciphering between temperature and ice-volume influences in the deep-sea isotopic record has proven somewhat difficult. Approaches have included the sampling of sediment from beneath different water masses, development of an independent palaeothermometer using magnesium/calcium ratios and improving the resolution and accuracy of coastal sea-level records. Despite these advances, it is only through the recovery of Antarctic drill core records that we have been able to test the resulting hypotheses. Combined with numerical climate models, ice-volume estimates are also available. The Antarctic Oligocene–Miocene record is most complete in the Victoria Land Basin as recovered in the CIROS-1 and CRP-2A drill cores. The strata recovered in both drill cores are cyclic in nature and interpreted to represent periodic advance and retreat of ice across the Antarctic margin in the western Ross Sea concomitant with sea-level fall and rise, respectively. Environmental data suggest a significant Antarctic climate threshold across the Oligocene–Miocene boundary with cooler temperatures implied in the early Miocene with ice volume and palaeo-sea-level estimates suggesting a significant but transient growth in the Antarctic Ice Sheet to ∼25% larger than present. The Antarctic data are entirely consistent with the predictions from deep-sea records including the suggestion that the glacial advance was relatively short lived and interglacial conditions were re-established within a few hundred thousand years. The duration and transience of the Mi1 glacial expansion and swift recovery in Antarctica likely resulted from the limited polar summer warmth from the coincidence of low eccentricity and low-amplitude variability in obliquity of the Earth's orbit at the Oligocene–Miocene boundary. This was followed by warmer polar summers and increased melt from increased eccentricity and high-amplitude variability in obliquity in the early Miocene, allowing the recovery of vegetation on the craton. Atmospheric CO 2 concentrations remained below a 2× pre-industrial threshold, which promoted sensitivity of the climate system to orbital forcing. While climate and ice-sheet modelling support the fundamental role of greenhouse gas forcing as a likely cause of events like Mi1, the models underestimate the range of orbitally paced ice-sheet variability recognised in early Miocene isotope and sea-level records unless accompanied by significant fluctuations in greenhouse gas concentrations. While tectonic influences may have been secondary, they may well have contributed to oceanic cooling recorded at the Cape Roberts Project site in the South Western Ross Sea.