Middle Late Miocene Cooling

The middle-to-late Miocene period represents a time of significant ice-sheet expansion in Antarctica (Chapter 10). Interpretations of deep-sea isotope records and observations from geologic data from around the world suggest that the middle-Miocene encompassed a change from a period of warm climatic optimum, approximately 17-15 million years ago, to the beginning of major cooling between ca. 14.5-13.5 million years ago, and the formation of a quasi-permanent ice sheet on East Antarctica (Lewis et al., 2007). One outstanding question revolves around whether this cooling led to an ice sheet in East Antarctica that remained stable and in existence to the present day or underwent large-scale fluctuations. New seismic-stratigraphic data from the Ross Sea reveal at least five major intervals of ice shelf advance and retreat in the middle-Miocene. Much of this ice is sourced in West Antarctica, suggesting the presence of a large and dynamic ice sheet in a part of Antarctica that is conventionally thought to be of lesser importance at this time.

One of the most vexing questions concerns the stability of Antarctic climate and ice during the late Miocene and it has been the subject of almost continuous debate for more than 20 years. A variety of indicators from the McMurdo Dry Valleys suggest the maintenance of stable, hyper-arid, cold-desert conditions since 13 Ma. However, microfossil studies (mainly diatoms) in the Transantarctic Mountains, and sedimentological work within Antarctic fjords are suggestive of significant climatic dynamism extending from the late Miocene through the Plio-Pleistocene. A degree of heterogeneity in climate response is expected considering the size and diverse landscapes of Antarctica. Yet the existing state of knowledge is sufficiently contradictory that the community has evolved into two camps when it comes to describing the ice sheet in pre-Quaternary time: the 'stabilists' and 'dynamicists' (Miller and Mabin, 1998).

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