<--Maximum Miocene Ice Extent over Antarctica
The East Antarctic Ice Sheet overrode the whole area during the mid-Miocene when it flowed north-north-eastwards to the outermost offshore shelf of the Antarctic continent. The effects of the ice were highly selective with landforms of glacial and meltwater erosion immediately adjacent to surviving patches of pre-existing regolith. The overriding has been dated stratigraphically with argon-argon dating of volcanic ash falls to between 13.6 and ~15 million years ago. The survival of fragile till deposits (and pre-existing regolith) often on slopes of ~300, is evidence of extremely low rates of erosion for ~13 million years since the mid-Miocene. The low rates of erosion are related to cold polar desert conditions where there is virtually no water action. The implication is that the ice sheet, which is responsible for the present climate, has remained intact and stable for ~13 million years. Such a finding rules out the view that the East Antarctic Ice Sheet experienced massive deglaciation in the Pliocene.
The original argument is developed in:
Is the Dry Valleys story of landscape evolution duplicated elsewhere?. Chris Fogwill, University of Edinburgh, was able to look at the cosmogenic isotopes in bedrock samples collected on an earlier expedition to the Shackleton Mountains, adjacent to the Slessor Glacier which discharges from the East Antarctic Ice Sheet into the Filchner-Ronne Ice Shelf in the Weddell Sea. Interestingly, these analyses revealed old summit surfaces that have not been covered by ice for millions of years. Such a finding places an upper limit of thickening of ice feeding into the Filchner Ronne Ice Shelf during the Pleistocene. We suspect that the area is similar to that in the Dry Valleys. If so, then it implies that the Dry Valley story of evolution is characteristic of a wider area of East Antarctica. The results are in:
<--Old Ice in Beacon Valley
The most dramatic evidence for stability comes from discovery of the world's oldest glacier ice buried beneath a veneer of till containing former polygons with pockets of in situ volcanic ash dated at 8 million years. The discovery showed that the cold polar climate that presently discourages sublimation must have persisted for millions of years. This finding is controversial in that there is a narrow and particular range of conditions that permit the survival of ice for so long. Several theoreticians such as Richard Hindmarsh, Bernard Hallet and Felix Ng have argued that theory suggests the buried ice must be much younger and that there must be another explanation for the overlying volcanic ash deposit. We stick to our original conclusion about the stratigraphy of the ash and, indeed, reckon that the ice characteristics and lithology of rock debris in the ice shows it is a remnant of the overriding Miocene ice sheet and that it must have survived 13.6 million years. Subsequently, we have tried to reconcile the two points of view by detailed work that shows that the evolution of polygons during surface lowering by sublimation introduces a countervailing process; it is that water from melting snow in the polygon troughs percolates to form a layer of isotopically distinct ground ice that covers the underlying glacier ice and reverses the process of sublimation. The net result is to slow down the overall rate of surface lowering due to sublimation. If so, here is a fine example of complexity in which it is the mix of processes at different scales that determines the eventual behaviour of a system.
The original discovery is published as:
Marchant, D.R., Lewis, A., Phillips,W.M., Souchez, R., Denton, G.H., Sugden, D.E., Landis, G.P., 2002 Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica. Geological Society of America Bulletin, 114, 718-730.
Two examples of morphological features associated with subglacial outburst flooding in East Antarctica
Perhaps the most spectacular evidence of the former overriding East Antarctic Ice Sheet is the suite of subglacial meltwater features which are on the same scale as those associated with the sudden drainage of the badlands of the Missoula floods in northwest USA. The features, representing the sudden drainage of large subglacial lakes, are illustrated in:
Helen Margerison has carried out direct dating of the deposits associated with the floods, using cosmogenic isotopes. She has obtained some of the world's oldest surface dates (> 8.6-10.4 million years). This confirms the Miocene age of the overriding event. Details are in: