Geoscience Reference
In-Depth Information
are effectively islands, whether surrounded by ice or sea. One striking exception to
this generalisation is provided by the
'
'
of Victoria Land, where ablation
in the incredibly dry air outweighs precipitation, leaving a frigid desert area tens of
thousands of square kilometres in extent.
Antarctica is an ice-bound continental mass surrounded and isolated by a large
extent of cold ocean. This contrasts with the Arctic, where large and ice-free
continental masses surround a polar ocean. This simple geographical contrast
underlies fundamental climatic and biological differences between the two. At any
given latitude, the climate experienced on land in Antarctica is considerably more
harsh, while the extreme isolation from possible refugia and other southern
hemisphere sources of colonists, combined with large-scale (but not complete)
obliteration during Pleistocene and previous glacial maxima, have minimised
colonisation and led to very low levels of diversity for groups present.
Antarctica was a central component of the supercontinent Gondwana. As
Gondwana fragmented, its last continental links were with Australia and South
America, and these were lost 30
Dry Valleys
35 million years ago. Fossil evidence from that
time indicates that Antarctica had a cool temperate fauna and
-
ora, and that
elements of this persisted as a tundra-like assemblage until at least the mid Miocene,
12
14 million years ago. The development of circumpolar ocean currents and
atmospheric circulation patterns had physical as well as biological consequences,
accelerating the continent
-
s gradual cooling and, eventually, glaciation. The extent
of continental cover by ice sheets and glaciers has varied widely over this period,
although Pleistocene glaciation mirrored that of the Arctic. The majority of time
over at least the last 800 000 years has been spent in the glacial phase of glacial
'
-
interglacial cycles, with periods of considerably thicker ice sheets than now, and
with vast ice shelves extending out to the edge of the continental shelf. Periods of ice
maxima are often thought to have led to complete eradication of terrestrial biota
(not least, simply through loss of habitat), followed by waves of recolonisation.
However, recent work across the continent has identi
ed many examples of biota
whose presence is most easily explained as persisting through the ice ages in
refugia rather than as colonists, and reconciling these observations with glacial
reconstructions is a pressing challenge.
At a broad scale, Antarctica
s terrestrial ecosystems and biology are treated as
three components, whose ecosystems are distinctly different. However, while
convenient and useful, this remains an oversimpli
'
cation and is currently an active
area of scienti
c advance.
The richest terrestrial ecosystems are found in the sub-Antarctic, which
consists of a set of isolated Southern Ocean islands and archipelagos, including
South Georgia, the Prince Edward Islands, Îles Kerguelen and Crozet, Heard and
Macdonald Islands, and Macquarie Island. The majority of these lie either close
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