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The
Antarctic Ice Sheet
Its Initiation and Evolution
Michael J. Hambrey & Bryn
Hubbard
Huge
glaciers from the East Antarctic ice sheet calve a path through the Transantarctic
Mountains
(Photo:
© M. J. Hambrey)
The Geography of Antarctica
- Antarctica is a continent twice the size of Australia,
or the USA plus Mexico, and 58 times the size of the British Isles, covering
14 million square km.
- Antarctica is noted for being the highest, coldest and
windiest continent.
Persistent
gales whip up loose snow on Battye Glacier and scour the frozen surface of Radok
Lake, East Antarctica
(Photo:
© M. J. Hambrey)
- 98% of the continent is covered by glacier ice. The average
thickness of this ice is 2 km, and in many places thickness exceeds 4 km,
reaching a maximum of 4.8 km.
- The volume of the ice sheet is estimated to be 30 million
cu. km. If this were all to melt, sea level would rise by 56 m according
to recent estimates by the British Antarctic Survey. In addition, the interior
of the continent could rise by as much as 900 m.
- The ice sheet as a whole is divided into three main components:
-
the East Antarctic ice sheet, mainly occupying ground above sea level and
which is therefore relatively stable;
-
the West Antarctic ice sheet, resting on a bed that is mainly below sea-level
and which is therefore vulnerable to catastrophic collapse; and
-
the Antarctic Peninsula ice sheet, a relatively small volume of ice covering
the land of the same name that stretches towards South America.
- The main ice-free areas occur in coastal fringes and in
largely buried mountain ranges, notably the Transantarctic Mountains which
stretches right across the continent (including the 'Dry Valleys'), and the
Prince Charles Mountains which border the largest outlet glacier from the
East Antarctic ice sheet.
Taylor
Valley and saline Lake Bonney, one of the Dry Valleys - a true polar desert
with small 'alpine' glaciers
(Photo:
© M. J. Hambrey)
- The Antarctic ice sheet exerts one of the major controls
on the climate of the southern hemisphere.
- The floating ice shelves, which border much of the coastline,
generate large volumes of cold water, which by flowing at depth for long distances,
influence global oceanic circulation and thus global climate.
- A wide fringe of sea ice encircles Antarctica in winter,
effectively doubling the ice-covered area, but in summer much of this disappears.
- The big questions are: How stable is the Antarctic
ice sheet, and what effect will global warming have upon it?
Aberystwyth research in Antarctica
Two members of the Centre for Glaciology have undertaken
fieldwork in the Antarctic. Prof. Mike Hambrey has spent 8 seasons there, working
as a visiting scientist with the following government research programmes: New
Zealand (3 times), Australia (once), Germany (once), USA (once) and UK (once),
as well as the Ocean Drilling Program (once). Most of his work has focused on
the unravelling the long-term glacial history of Antarctica, both through participation
of offshore drilling programmes such as New Zealand’s CIROS-1 project, the ODP
and the international Cape Roberts project, and also through field projects
on unique glacial sequences preserved at high elevations in the Transantarctic
Mountains, the Prince Charles Mountains and on James Ross Island. Michael Hambrey
has also worked at modern ice margins with New Zealand colleagues in the Dry
Valleys, attempting to unravel glacial debris transport and depositional processes.
Geological
investigations in the Shackleton Glacier area of the the Transantarctic
Mountains, supported by the United States Antarctic Research Program (Photo:
© M. J. Hambrey)
Dr Bryn Hubbard has also spent one season working with New
Zealand colleagues, also in the Dry Valleys. He has undertaken investigations
at Taylor Glacier, including the relative flow-behaviour of debris-rich basal
ice and the overlying debris-poor glacier ice, and (ii) the sedimentology of
basal, ice-marginal and proglacial debris in order to elucidate the interdependency
and ‘evolutionary’ progression of these deposits.
Field camp, Taylor Valley, supported by the New Zealand Antarctic Programme
(Photo:
© Bryn Hubbard)
Some of the results of these activities are highlighted below.
Principal collaborators
Prof. Peter Barrett, Victoria University of Wellington, New
Zealand
Prof. Werner Ehrmann, University of Leipzig, Germany
Dr Sean Fitzsimons, University of Otago, New Zealand
Prof. David Harwood, University of Nebraska, USA
Dr Wendy Lawson, University of Canterbury, New Zealand
Prof. Regi Lorrain, Free University of Brussels, Belgium
Dr Barrie McKelvey, University of New England, Armidale,
NSW, Australia
Prof. Ross Powell, University of Northern Illinois, USA
Dr John Smellie, British Antarctic Survey, Cambridge, UK
Prof. Peter Webb, Ohio State University, USA
Dr Jason Whitehead, University of Tasmania, Australia
Modern glacial processes in Antarctica
Most glaciers in Antarctica terminating on land are probably
‘cold’, that is the ice temperature is below the pressure melting point throughout.
Such glaciers are thus frozen to the bed, and it is commonly assumed that such
glaciers are protective of the landscape. Only thick, fast-flowing glaciers
are sliding on their bed. Work by Sean Fitzsimons of the University of Otago,
New Zealand and American scientists have demonstrated that this is not, in fact,
the case. Sean Fitzsimons group has tunnelled into several cold glaciers in
the Dry Valleys region using a chain saw. Large amounts of debris are incorporated
by these glaciers, including boulders, fractured bedrock slabs, and rafts of
frozen sediment. During January 2001, Michael Hambrey joined forces with Sean
Fitzsimons to investigate how sediment was uplifted into the ice in these cold-based
glaciers, transported and deposited. Complex glaciotectonic processes are involved,
particularly folding and thrusting, and these lead to the deposition of prominent
moraine complexes. The role of cold glaciers as erosive agents thus needs to
be re-evaluated.
Two views of debris transport and deposition
by a valley glacier in Taylor Valley:
The
impressive ice cliff of Taylor Glacier with debris-rich basal ice at its foot
(Photo:
© Bryn Hubbard)
Recently
deposited basal till from Taylor Glacier
(Photo:
© Bryn Hubbard)
It should be noted that modern cold glaciers in Antarctic
represent an extreme end-member of a wide range of glacier types. Most actively
depositing glaciers slide on their beds and are associated with greater or lesser
amounts of meltwater. Centre for Glaciology staff have thus been investigating
glaciers in a wide range of other environments from this perspective, including
the Southern Alps of New Zealand, Patagonia, and Svalbard in the High-Arctic.
From the body of data gathered from these modern settings, we are better able
to infer processes and associate climates for past conditions of the Antarctic
ice sheet, especially when it becomes apparent that modern Antarctic glaciers
are poor analogues for those past conditions.
History of the Antarctic ice sheet
Why
is this important to human welfare?
Determination of the scale and
rapidity of the response of large ice masses to climatic change is of vital
importance because ice-volume variations lead to changing global sea levels
on a scale of tens of metres or more. It is thus important to assess the stability
of the cryosphere under a warming climate as noted by the Intergovernmental
Panel on Climate Change, particularly as ice-core records have yielded evidence
of a strong correlation between CO2 in the atmosphere and past temperatures.
This concern seems justified when CO2 levels are compared with those
of the past. Since the Antarctic ice sheet is a major driver of Earth's climate
and global sea level, much effort has been expended in deriving theoretical
models of its past and future behaviour. However, in order to validate such
models, it is necessary that we examine the past record of ice-sheet behaviour
in response to (i) climatic change (inferred from sediments), and (ii) palaeoceanographic
conditions (inferred from palaeoecology) and palaeogeography (as recorded in
landscape evolution). Plausible models of ice-sheet behaviour over the multi-million
year time-scale, embracing all pertinent geological data, are needed if we are
to develop the capability to predict future long-term changes. Considerable
strides have been made in deciphering the history of the ice sheet from the
sedimentary record, both on and offshore, but major questions remain to be resolved.
Evaluating
depositional processes in the Southern Ocean is one of the keys to understanding
the response of the ice sheet to climatic change. Here, the German research
vessel Polarstern, photographed from behind a pressure ridge in the sea
ice of the Weddell Sea, is on an interdisciplinary cruise to examine sea-floor
sediments (Photo: © M. J. Hambrey)
Was the ice sheet stable or unstable?
The East Antarctic ice sheet
has existed, according to many researchers, for approximately 35 million years,
but it has fluctuated considerably and been one of the major driving forces
of global sea level and climate throughout the second half of the Cenozoic Era.
The scale and temporal pattern of these fluctuations, however, have been the
subject of considerable debate. Two main hypotheses have emerged concerning
the stability of the ice sheet in pre-Quaternary time:
·
that it has remained
stable for at least the last 15 million years - the ‘stabilist’ view,
a long-standing opinion developed by American scientists such as George Denton
and David Marchant, as well as by David Sugden from Edinburgh;
·
that it only achieved stability
around 3 million years ago - the ‘dynamicist’ view, developed from recent
ideas by Americans David Harwood and Peter Webb. This view has been described
rather over-dramatically as suggesting ‘Antarctic meltdown’, in other words,
collapse of the ice sheet under climatic conditions similar to those of today,
accompanied by rapid rise in global sea level! However, this is somewhat of
a distortion of the Webb/Harwood hypothesis.
These contrasting views
have emerged from work over two decades in the Transantarctic Mountains, and
both groups of people have presented cogent arguments to support their case.
The often-heated debate continues to this day! What is not in dispute, however,
is that some time in the past (whether before 3 or 15 m.y. ago) the ice sheet
was much warmer and subject to major fluctuations.
Following work
by Michael Hambrey with Barrie McKelvey of the University of New England in
Australia, a new region, the Lambert Glacier basin (the largest ice drainage
basin on the continent) is emerging as an ideal site to test these hypotheses,
and to develop a framework for understanding the evolution of the entire East
Antarctic ice sheet. The work involved logging stratigraphic sections on remote
cliff faces in the Prince Charles Mountains, an area first discovered only in
1948.
Map of
Lambert Glacier system showing study sites in the Prince Charles Mountains and
Prydz Bay
The Pagodroma
Group, an ancient glacial succession in the Prince Charles Mountains, with Barrie
McKelvey (Australia) for scale (Photo: © M. J. Hambrey)
By combining these results
with data from drilling in the sediments offshore by ODP in 1986/7 (in which
Michael Hambrey also participated), it has been suggested that major fluctuations
of the ice margin over several hundred kilometres occurred, and that sedimentation
took place under a much warmer climate than that of today; these fluctuations
could therefore signal major changes in ice-sheet volume up until about 3 Ma.
This lends support to the dynamicists’ view, although complete collapse of the
ice sheet is not envisaged.
Cross
section through the Lambert Glacier - Amery Ice Shelf system, illustrating the
major fluctuations that are inferred for the Cenozoic Era (from Hambrey &
McKelvey 2000)
The older glacial record
The long-term record is
preserved mainly in offshore sedimentary basins, in sequences several hundred
metres thick, which can be used, not only to define ice-marginal positions for
specific intervals of geological time, but also to determine palaeotemperatures
(from siliceous marine organisms called diatoms) and hence the probable temperature
characteristics of the source ice-mass.
Drill-sites
in the western Ross Sea region.
The longevity of glaciation
in Antarctic was first noted in the early 1970s during Leg 28 of the Deep Sea
Drilling Project. Subsequently, drilling by New Zealand teams, culminated in
the recovery of a 702 m long core at the drillsite CIROS-1 in McMurdo Sound
in 1986. This recovered a glacial record going back to late Eocene time (36
Ma), and a unique story of ice-sheet and sea-level fluctuations, and including
the last evidence of vegetation (southern beech, Nothofagus) on the continent.
CIROS-1 drilling was closely followed by ODP drilling in Prydz Bay which also
recovered a sequence from 5 holes on a transect across the continental shelf
going back to roughly the same time. However, none of these sites was able to
demonstrate the timing of onset of glaciation.
Drilling
through sea ice into the sea floor at Cape Roberts, western Ross Sea, funded
by a seven-nation consortium (New Zealand, USA, Italy, UK, Germany, Australia
and The Netherlands). Results are published in the journal "Terra Antartica".
(Photograph: Peter Barrett)
The stratigraphic
record recovered from the Ross Sea region (Hambrey et al. 2002)
Among the recent drilling
activities, the most relevant to the long-term record include ODP Leg 188 in
Prydz Bay in early 2000, has finally established the glacial/preglacial by drilling
on the continental shelf, slope and (http://www-odp.tamu.edu)
rise. In addition, and the 3-season Cape Roberts Project (http://www.geo.vuw.ac.nz/croberts/index.html)
drilled three holes in 1997, 1998 and 1999. The Cape Roberts Project was a seven-nation
enterprise (New Zealand, USA, Germany, Italy, Australia, the UK, the Netherlands)
involving over 60 scientists, and operated by New Zealand under the direction
of Peter Barrett. The final hole, nearly a kilometre in depth passed into preglacial
sediments, but the contact coincided with a major gap in the record, and the
critical transitional strata were missing. Nevertheless, a detailed record of
glacial conditions under a warmer climate than today’s was recovered. Combined
with earlier drilling results, we now have a comprehensive record of glaciation
for the Ross Sea region in particular. Initial results from the ODP leg indicate
that the preglacial/glacial transition was covered and dates from around 34
million years ago.
The drill-core data are
supplemented by the record from ice-free areas, from where landscape evolution
can be determined and matched with the sedimentary record. This landscape record,
being biostratigraphically constrained, can yield vital information concerning
the style and scale of glaciation. We are currently in the early stages of this
type of investigation.
One of the most interesting
aspects of the evolution of the ice sheet concerns how it has changed through
time. First the ice sheet was temperate, with outlet glaciers surrounded by
vegetation, notably Nothofagus, such as one sees today in Chilean Patagonia,
where glaciers descend to sea level through forest. A transition to cooler conditions
followed, and the climate began to resemble that in the Arctic today, with glaciers
still associated with scrubby vegetation. Only in the later stages has the ice
sheet been so cold that it has produced little meltwater and been associated
with areas devoid of vegetation.
Style
of glaciation with wooden hillsides in the western Ross Sea during Oligocene
times, about 30 m.y. ago (Hambrey and others 2002).
In deciphering the long-term
glacial history we have been contributing to the aims of an international specialist
group named ‘ANTOSTRAT’ operating under the auspices of the Scientific Committee
on Antarctic Research (SCAR 1999). See http://www.scar.org/.
Summary of key stages
in Antarctic ice sheet evolution
·
The inception of the first full-scale
ice sheet over East Antarctica, either around 34 million years ago (the Eocene/Oligocene
transition), following thermal isolation of Antarctica as the last of the southern
hemisphere continents drifted away from Antarctica, and possibly triggered by
uplift of the Transantarctic Mountains.
·
Major fluctuations of the East
Antarctic ice sheet from 34 to about 15 million years ago (Oligocene to mid
Miocene), accompanied by expansions to the edge, and growth of, the continental
shelf.
·
The ice sheet reaches a maximum
size about 15 million years ago (mid-Miocene) and, according to some scientists,
became stable. The West Antarctic ice sheet may have developed for the first
time also.
·
Large-scale reductions in the size
of the ice sheet according to another group of scientists around 2-4 million
years ago (Pliocene Epoch).
·
Stable East Antarctic ice sheet,
but ‘unstable’ west Antarctic ice sheet throughout the last 2 million years
(Quaternary Period).
Publications
The following scientific papers have recently emerged from the Centre
for Glaciology on the above topics:
Hambrey,
M. J., Barrett,
P.J. & Powell, R.D. in press, 2002. Late Oligocene and early Miocene glacimarine
sedimentation in the SW Ross Sea, Antarctica: the record from offshore drilling.
In Dowdeswell, J.A. & O’Cofaigh, C. (eds.). Glacimarine processes in
high-latitude environments. Spec. Publ. Geol. Soc. Lond.
Hambrey, M.J. & McKelvey, B. 2000. Major
Neogene fluctuations of the East Antarctic ice sheet: Stratigraphic evidence
from the Lambert Glacier region, Geology 28 (10), 887-891
Hambrey, M. J. & McKelvey, B. C. 2000. Neogene
fjordal sedimentation in the Prince Charles Mountains, East Antarctica.
Sedimentology 47, 577-607.
Hambrey, M. J. & Van der Meer, J.J.M. 2000.
Sedimentary environments for CRP-2/2A – Introduction. In Studies from the
Cape Roberts Project, Ross Sea, Antarctica – Scientific Report of CRP-2/2A.
Terra Antarctica 7(3), 311-312.
Hambrey, M. J. 1999. The record of Earth’s glacial
climate over the last 3000 Ma. In: Barrett & Orombelli Geological
records of global change. Terra Antarctica Report no. 4, 73-108 (Siena,
Italy).
Hambrey, M. J. & Wise, S. W. (eds.) 1998
[1999]. Studies from the Cape Roberts Project, Ross Sea, Antarctica, Scientific
Report of CRP-1. Terra Antarctica 5(3), 458pp.
Hambrey, M. J. & Woolfe, K. 1998 [1999].
Sedimentary Environments. In: Hambrey, M. J. & Wise, S. W. (eds.) 1998.
Studies from the Cape Roberts Project, Ross Sea, Antarctica, Scientific Report
of CRP-1. Terra Antarctica 5(3), 337-339.
Powell, R. D. , Hambrey, M. J. & Krissek,
L. A. 1998 [1999]. Quaternary and Miocene glacial and climatic history of the
Cape Roberts drillsite region, Antarctica. In: Hambrey, M. J. & Wise, S.
W. (eds.) 1998. Studies from the Cape Roberts Project, Ross Sea, Antarctica,
Scientific Report of CRP-1. Terra Antarctica 5(3), 341-351.
Acknowledgements
Thanks to Antarctic New Zealand, Australian Antarctic
Research Expeditions, US Antarctic Research Program, British Antarctic Survey
& HMS Endurance, Alfred Wegener Institute & Research Vessel Polarstern
(Germany), the Ocean Drilling Program, the Royal Society and the Transantarctic
Association for funding field research in the Antarctic. Also to the Natural
Environment Research Council and the University of Wales for funding the research
in the UK.
Note
to Prospective Students
If
you are interested in learning more about glacial environments, why not consider
studying BSc Geography or Environmental Earth Science at Aberystwyth? Both these
degree schemes include much of glacial interest among a wide choice of modules.
See http://aber.ac.uk/iges
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