The Sirius Debate

A glacial-geological controversy | The Sirius Debate | The view from East Antarctica | Summary | References | Comments |

This page was contributed by Professor Michael Hambrey, and all figures and photographs are copyright Mike Hambrey.

A glacial-geological controversy

Geological timescale

Antarctica displays a long-term (multi-million year) record of glacial history that is unsurpassed on Earth.  From offshore drilling on the continental shelf we know that Antarctic glaciation began around 34 million years ago, if not earlier. The land-based record of glaciation comes from two main mountain ranges, the Transantarctic Mountains and Prince Charles Mountains, but this record is sketchy and incomplete. In the case of the Transantarctic Mountains, the glacial sediments have generated enormous controversy regarding their age and significance in terms of understanding past ice-sheet stability. This controversy is sometimes referred to as the Sirius Debate, after Mount Sirius, where their deposits were first discovered.

The Sirius Debate

Map of Antarctica, showing the Sirius Group. Courtesy of Mike Hambrey

The Sirius Group, as it is formally known, occurs throughout most of the length of the Transantarctic Mountains, at high and low elevations, as far south as 86°S. The rocks we are concerned with are a diamictite (a poorly-sorted sediment with a wide range of stone shapes and sizes). Geologists have undertaken detailed studies of these diamictites, and all are agreed that they were laid down by glaciers as “till” under conditions much warmer than those of today; in fact, more like the polythermal glaciers of the Arctic. The most remarkable thing about them is that they are associated with mats of vegetation, especially a shrubby form of southern beech, which today is found in New Zealand, Patagonia and Tasmania. We therefore believe that these glacial deposits were formed under conditions with mean annual temperatures of around -5°C (or around 25°C warmer than those of today).

Everyone pretty much agrees with all this. The controversy centres around the age of the deposits and their significance in terms of stability of the East Antarctic Ice Sheet. Two main hypotheses have emerged:

  1. The Stabilists. That the rocks are at least 15 million years old and indicate that the East Antarctic Ice Sheet has remained cold and stable over this whole period. The chief proponents of this view are George Denton, David Marchant and David Sugden, and they have presented a very strong body of evidence based on geomorphological mapping and dating on various land surfaces 1-4.
  2. The Dynamicists. That the rocks are much younger, a mere 3 million years, old, and belong to the Pliocene Epoch, when the Earth reached a level of warmth not matched since, but is one which we are likely to reach by the end of this century through global warming. This view, presented forcefully by Peter Webb and David Harwood in the 1980s 5-9 relies on dating the sediments using diatoms (tiny marine micro-organisms). The implications which follow from their data are serious; that the East Antarctic Ice Sheet was much reduced in the Pliocene Epoch and triggered massive sea level rise, under conditions shortly to be reached in the future.
Upper Shackleton Glacier map

Upper Shackleton Glacier map

After years of controversy, when some scientists unprofessionally attacked the opposite view (although, to be fair, not the chief protagonists), the weight of evidence on land has been building for a stable ice sheet, in the absence of new palaeontological data. However, recent research from offshore drilling has shown that the West Antarctic Ice Sheet at least was subject to rapid fluctuations in the Miocene and Pliocene epochs, and that the now ice-filled fjords in the deep South may have become glacier-free.

My own take on the Sirius question, following fieldwork in the Shackleton Glacier around 83°S with Webb and Harwood 10, is that the Sirius Group represents at least three cycles of sedimentation, spread over many millions of years. The older glacial events may even have predated the current topography we see today. Their age, however, has not been determined from this area. Unfortunately, the fossil beech from nearby Beardmore Glacier does not constrain the age of the deposits precisely enough.

A science fiction novel by Kim Stanley Robinson, called “Antarctica”, has drawn part of its inspiration from the Sirius debate, following the author’s attachment to our field programme at Shackleton Glacier. In this novel, he thus addresses an important geological controversy, and explains the nitty-gritty of how geologists do their work under arduous conditions!

The view from East Antarctica

Prince Charles Mountains

The Prince Charles Mountains, East Antarctica

The second main area for work on ancient Antarctic glaciation is the Prince Charles Mountains, a remote mountain range only discovered in 1948. Here, Russian and Australian geologists have been busy unravelling the glacial history (with a little bit of help from this writer). The sediments, although glacial, are present along the margins of a major rift, the Lambert Trough, but have been uplifted from below sea level to as much as 1400 metres 11. These sediments are known as the Pagodroma Group after the snowy petrel which rests there 12-17.

Working with Barrie McKelvey and Jason Whitehead, we found that the oldest strata had been uplifted the most, and that their ages were Oligocene, Middle Miocene and Pliocene 17. The well-preserved marine diatoms provide incontrovertible evidence of age, and the sediments suggest strongly fluctuating glaciers in fjords, similar to conditions in Greenland today.

The evidence is preserved on steep spurs of loose rock, commonly sitting on vertical crags of basement rocks, into which the ancient glacier carved a landscape whose remnants can be seen today. It is definitely a challenging area to work topographically, and the strong katabatic winds made fieldwork far from comfortable.

The Prince Charles Mountains thus provide a long record of glaciation between successive uplift events, and demonstrate that this part of the East Antarctic Ice Sheet was dynamic, at least prior to the Pliocene.

Summary

We need to elucidate the past glacial record of Antarctica in order to understand the future response of ice sheets to global warming. The Sirius and Pagodrome groups provide windows on the past, when ice sheets were less stable. They convey an image about what will happen to the ice sheet in the future.

Further reading

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References

1.            Sugden, D.E., Summerfield, M.A., Denton, G.H., Wilch, T.I., McIntosh, W.C., Marchant, D.R. & Rutford, R.H. Landscape development in the Royal Society Range, southern Victoria Land, Antarctica: stability since the mid-Miocene. Geomorphology 28, 181-200 (1999).

2.            Sugden, D.E., Denton, G.H. & Marchant, D.R. East Antarctic Ice Sheet sensitivity to Pliocene climatic change from a Dry Valleys perspective. Geografiska Annaler 75, 155-204 (1993).

3.            Sugden, D.E., Marchant, D.R. & Denton, G.H. The case for a stable East Antarctic ice sheet: the background. Geografiska Annaler 75, 151-154 (1993).

4.            Sugden, D.E. The East Antarctic Ice Sheet: unstable ice or unstable ideas? Transactions of the Institute of British Geographers 21, 443-454 (1996).

5.            Webb, P.-N., Harwood, D.M., McKelvey, B.C., Mercer, J.H. & Stott, L.D. Cainozoic marine sedimentation and ice volume variation on the East Antarctic craton. Geology 12, 287-291 (1984).

6.            Webb, P.-N. & Harwood, D.M. Late Cenozoic glacial history of the Ross embayment, Antarctica. Quaternary Science Reviews 10, 215-223 (1991).

7.            Webb, P.N., Harwood, D.M., Mabin, M.G.C. & McKelvey, B.C. A marine and terrestrial Sirius Group succession, middle Beardmore Glacier-Queen Alexandra Range, Transantarctic Mountains, Antarctica. Marine Micropaleontology 27, 273-297 (1996).

8.            Barrett, P.J., Adams, C.J., McLntosh, W.C., Swisher, C.C. & Wilson, G.S. Geochronological evidence supporting Antarctic deglaciation three million years ago. Nature 359, 816-818 (1992).

9.            Wilson, G.S. The Neogene East Antarctic Ice Sheet: a dynamic or stable feature? Quaternary Science Reviews 14, 101-123 (1995).

10.          Hambrey, M.J., Webb, P.-N., Harwood, D. & Krissek, L. Neogene glacial record from the Sirius Group of the Shackleton Glacier area, central Transantarctic Mountains, Antarctica. Geological Society of America Bulletin 115, 994-1015 (2003).

11.          Jamieson, S.S.R., Hulton, N.R.J., Sugden, D.E., Payne, A.J. & Taylor, J. Cenozoic landscape evolution of the Lambert basin, East Antarctica: the relative role of rivers and ice sheets. Global and Planetary Change 45, 35-49 (2005).

12.          Hambrey, M.J. & McKelvey, B.C. Major Neogene fluctuations of the East Antarctic ice sheet: Stratigraphic evidence from the Lambert Glacier region. Geology 28, 887-891 (2000).

13.          Hambrey, M.J. & McKelvey, B.C. Neogene fjordal sedimentation in the Prince Charles Mountains, East Antarctica. Sedimentology 47(2000).

14.          McKelvey, B.C., Hambrey, M.J., Harwood, D.M., Mabin, M.C.G. & Webb, P.-N. The Pagodroma Group – a Cenozoic record of the East Antarctic ice sheet in the northern Prince Charles Mountains. Antarctic Science 13, 455-468 (2001).

15.          Taylor, J., Siegert, M.J., Payne, A.J., Hambrey, M.J., O’Brien, P.E., Cooper, A.K. & Leitchenkov, K. Topographic controls on post-Oligocene changes in ice-sheet dynamics, Prydz Bay region, East Antarctica. Geology 32, 197-200 (2004).

16.          Whitehead, J.M., Harwood, D., McKelvey, B.C., Hambrey, M.J. & McMinn, A. Diatom biostratigraphy of the Cenozoic glaciomarine Pagodroma Group, northern Prince Charles Mountains, East Antarctica. Australian Journal of Earth Sciences 51, 521-547 (2004).

17.          Hambrey, M.J., Glasser, N.F., McKelvey, B.C., Sugden, D.E. & Fink, D. Cenozoic landscape evolution of an East Antarctic oasis (Radok Lake area, northern Prince Charles Mountains), and its implications for the glacial and climatic history of Antarctica. Quaternary Science Reviews 26, 598-626 (2007).

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