Summary and Conclusions

Strata recovered from the Antarctic margin indicate a significant glacial advance at the Oligocene-Miocene boundary reaching the south Shetland Islands on the Antarctic Peninsula (Troedson and Riding, 2002) and grounding in Prydz Bay and the South Western Ross Sea as indicated by hiatuses in drill cores (Hambrey et al., 1991; Roberts et al., 2003; Naish et al.. 2008). Ice rafted as far north as Maud Rise (Barker et al., 1988a, b) and the central Ross Sea (Leckie and Webb, 1983) but did not appear to reach the Kerguelen Plateau (Schlich and Wise, 1992). Pre-Oligocene-Miocene boundary strata indicate a late Oligocene Antarctic Ice Sheet (Cape Roberts Science Team, 1999), which expanded to an ice volume of the order of 20% greater than the present ice sheet at the Oligocene-Miocene boundary (Naish et al., 2008). The glacial expansion, however, although significant in extent and volume, must have been relatively transient and neither cold nor extensive enough to extinguish Nothofagus tundra vegetation (Askin and Raine, 2000; Roberts et al., 2003), which persisted across the boundary despite a slight drop in temperature (Passchier and Krissek, 2008). Marine palynomorphs, however, indicate that coastal temperatures did not return to the warmth of the late Oligocene with a much reduced freshwater melt input to coastal regions (Barrett, 2007).

Data from the Antarctic continent are entirely consistent with the shortlived (200 ky) ice-volume increase from 40% of present Antarctic ice volume to 25% greater than present Antarctic ice volume across the Oligocene-Miocene boundary with concomitant oceanic deep-water cooling implied by the Mi1 isotopic excursion recognised in equatorial and Southern Hemisphere deep-sea sedimentary records (Paul et al., 2000). Ice-volume estimates are confirmed by the backstripped stratigraphic records from the New Jersey Margin (Kominz and Pekar, 2001; Pekar et al., 2002), however, accommodating this much ice on Antarctica when global temperatures were presumably warmer than today may prove difficult from a modelling perspective. Warm summer mean temperatures were re-established soon after the Oligocene-Miocene boundary, although a few degrees cooler than pre-Miocene summer mean temperatures. The duration and transience of the Mi1 glacial expansion and swift recovery in Antarctica likely resulted from the limited polar summer warmth from coincidence of low eccentricity and low-amplitude variability in obliquity of the Earth's orbit at the Oligocene-Miocene boundary (Zachos et al., 2001b). This was followed by warmer polar summers and increased melt from increased eccentricity and highamplitude variability in obliquity in the early Miocene, allowing the recovery of vegetation on the Antarctic craton. Atmospheric CO2 concentrations remained below the 2 x pre-industrial threshold, which promoted sensitivity of the climate system to orbital forcing during cold Austral summers. While climate and ice-sheet modelling supports the fundamental role of greenhouse gas forcing punctuated by orbital forcing as a likely cause of events like Mi1 (DeConto and Pollard, 2003a; Huber et al., 2004; DeConto et al., 2007; Palike et al., 2006), 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 (Pollard and DeConto, 2005). While, tectonic influence 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 (Barrett, 2007).

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