The Middle-to-Late Miocene interval is believed to represent a time of significant ice-sheet expansion on Antarctica (e.g. Miller et al., 1991, 2005; Lear et al., 2000; Turco et al., 2001; Billups and Schrag, 2002; Shevenell et al., 2004). The stable isotope record of the deep sea demonstrates that a mid-Miocene ''climatic optimum'', at ~15Ma, was followed by strong enrichment in oceanic 818O and climatic cooling over the next 6 Ma (e.g. Zachos et al., 2001). During this interval, the East Antarctic Ice Sheet (EAIS) is thought to have been a major and permanent ice sheet, although fluctuations in the size of EAIS may still have occurred (e.g. Westerhold et al., 2005).

Denton et al. (1984) proposed that during this time, the EAIS overrode the Transantarctic Mountains (Fig. 10.1). Recent studies from the western Dry Valleys indicate that the atmosphere cooled by as much as ~20°C prior to an erosional event that is linked to the EAIS overtopping the mountains (Lewis et al., 2007). If such a sequence is repeated elsewhere (i.e. cooling and development of cold-based alpine glaciers preceded ice-sheet overriding), then it would suggest that the initial rise in deep-sea benthic 818O (Zachos et al., 2001) reflects deep-water cooling, followed later by ice-sheet expansion. Furthermore, Lewis et al. (2006) propose that immense freshwater floods to the Southern Ocean from large subglacial lakes beneath the expanded EAIS occurred between 14.4 and 12.4 Ma. The discharges are not only considered to have been the erosive force that formed such prominent Dry Valleys features as the Labyrinth, but are also considered through their impact on oceanic circulation to be a cause of mid-Miocene climatic change.

A different explanation for the enrichment in oceanic d18O is that it represents the onset of significant glaciation on West Antarctica (Mercer, 1978; Ciesielski et al., 1982). This is supported by the first occurrence of ice rafted debris (IRD) at Deep Sea Drilling Project (DSDP) Site 325 in the Bellingshausen Sea during the Early to Middle Miocene

Figure 10.1: Map showing geographical locations discussed in the text. Antarctic bed topography also shown from the BEDMAP dataset (Lythe et al., 2000).

Figure 10.1: Map showing geographical locations discussed in the text. Antarctic bed topography also shown from the BEDMAP dataset (Lythe et al., 2000).

(Hollister et al., 1976). However, the relative role that the EAIS and West Antarctic Ice Sheet (WAIS) played in the Middle Miocene Climate Transition (MMCT) has recently been questioned by new seismic-stratigraphic data from the Ross Sea revealing at least five major intervals of grounded ice advance and retreat in the Middle Miocene (Bart, 2003; Chow and Bart, 2003). Much of this ice was sourced from West Antarctica, suggesting the presence of a large and dynamic WAIS prior to the MMCT (Bart, 2003; Chow and Bart, 2003).

The stability of Antarctic Ice Sheets during the Late Miocene and Pliocene has been the subject of almost continuous debate for more than 20 years. The key questions in this argument are when did the EAIS switch from being polythermal and dynamic to predominantly cold based and stable, and could relatively short-lived climatic warm intervals have been sufficient to influence the Antarctic Ice Sheet? A long-standing view is that the EAIS became stable during the Middle Miocene, evidence for which is primarily derived from apparent landscape stability and well-dated surfaces and ash deposits in the Dry Valleys region along the western border of the Ross Sea (e.g. Denton et al., 1993; Marchant et al., 1993a; Sugden, 1996). An alternate view is that terrestrial glacial deposits, known as the Sirius Group, scattered throughout the Transantarctic Mountains, indicate dynamic ice-sheet conditions even during the Pliocene. This conclusion is based on the occurrence of Pliocene (and older) diatoms reworked into glacial deposits (Harwood, 1983, 1986; Webb et al., 1984, 1996; Harwood and Webb, 1986; Wilson, 1995; Wilson et al., 1998). The dynamic nature of the Pliocene Antarctic Ice Sheet is supported by the Pagodroma Group along the flanks of the Lambert Glacier (e.g. Hambrey and McKelvey, 2000a,b). Each view is internally consistent and scientists have been presented with a significant challenge in reconciling the different views. When considering the size and diverse landscapes of Antarctica, we should not be surprised to see a degree of heterogeneity in the climate and environmental response. Yet the current state of knowledge is so contradictory that the scientific community has become polarized into two camps (''stabilists and dynamicists'') over the issue of Middle Miocene to Pliocene conditions on Antarctica (e.g. Harwood and Webb, 1998; Stroeven et al., 1998a,b).

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