Ice with everything - Antarctica: Global Science from a Frozen Continent - page 80

Figure 3.14 The Antarctic coast. This

photograph highlights the impressive land
ice. (Credit: IPEV/P. Le Mauguen)

ocean floor) and carbon sources (volcanic outgassing where carbonate-rich oceanic

crust is subducted beneath moving continental plates). About 50million years ago,
the Indo-Asian collision produced the Tibetan Plateau uplift and a progressive loss
of atmospheric carbon dioxide by weathering. Thus, the atmospheric carbon
dioxide concentration declined over the past 50million years and the climate
cooled, with indications of initial Antarctic glaciation beginning more than
40million years ago. When the Antarctic Peninsula was finally separated from the

South American continent, about 34million years ago, the Drake Passage isolated
Antarctica from its closest neighbouring continent and opened the way for
the establishment of the Antarctic Circumpolar Current, the world s largest

current which continually drives the waters of the Southern Ocean around the
Antarctic continent. It was then possible for the thick ice sheet on East Antarctica to
expand, at the same time as West Antarctic glaciers grew, progressively building the
huge continuous ice sheet we have today. The present East Antarctic ice sheet was
probably formed 14million years ago, while the West Antarctic ice sheet

'

finished
forming later (around 8million years ago) when the Ross and Weddell Sea ice
shelves could act as buttresses to support it. Despite probable minor variations in
volume, the Antarctic ice sheet overall appears to have been rather stable over the
past million years.
However, the matrix of the Antarctic ice sheet does not normally preserve
ice that old. The renewal of ice is due to the vertical thinning and to the horizontal

flow of ice towards the coastal glaciers. Due to this turnover between accumulation
and flow, the average age of the ice preserved in East Antarctica is estimated to be

around 125 000 years, while the modern West Antarctic ice sheet is estimated to
be only 45 000 years old. Of course, these averaged numbers mask very large
contrasts between coastal areas, where the turnover rate is large, and inland areas,
where glaciologists still dream of drilling ice older than a million years at the
stagnant ice domes.