The Oligocene-Miocene boundary (Chapter 9) marks a significant transition in the development of the Antarctic cryosphere, where small dynamic ice sheets of the late Oligocene rapidly expanded to continental scale in the early Miocene. The transition is recorded in benthic foraminiferal 818O records as a positive 1.0 per mil shift, representing the first of the Miocene glaciations (Mi-1). The climatic significance of this was first outlined by Zachos et al. (2001) who recognised the coincidence of the Oligocene-Miocene boundary and the Mi-1 isotope excursion with an unusual coincidence of low eccentricity and low-amplitude variability in obliquity of the Earth's orbit. Mg/Ca reconstructions imply little or no change in temperature and that the ice-volume increase was equivalent to 90 m of sea level lowering (assuming a
Pleistocene calibration of 0.11m 818O per 10 m of sea level). Sediment cores recovered in the Western Ross Sea indicate orbital modulation of the ice sheet during the transition, and corroborate proxy ocean records (Naish et al., 2001). It is argued that the Mi-1 event occurred as a consequence of the aforementioned unique setup of orbital parameters during an interval of declining CO2 that led to a prolonged period of cold summer orbits, during which time a large ice sheet established. This was then followed by warmer polar summers and enhanced melt from increased eccentricity and highamplitude variability in obliquity in the early Miocene, allowing the recovery of vegetation on the Antarctic craton.
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