Geology Reference
In-Depth Information
the sea-ice cover. The maximum thickness of annual (single-year) sea ice varies between
30 cm and 200 cm. If sea ice were not to melt completely during the following summer, as
is the case for over 50% of the Arctic Ocean sea ice, multi-year ice is created. This is
thicker and harder than single-year ice and may exceed 2.5 m in thickness.
Sea ice also includes the large ice masses that enter the polar oceans from ice shelf
break-up (where ice shelves are fl oating ice sheets) or from direct calving of tidewater
glaciers. In the northern hemisphere, Baffi n Bay, parts of Newfoundland and eastern
Canada, and the northern North Atlantic are most affected by icebergs. In the Antarctic,
most single-year ice melts as it fl oats northwards towards the Antarctic Convergence and
the Southern Oceans but multi-year ice forms in the Weddel, Ross, and Bellinghausen
Seas.
Sea ice dominates the surface of both the Arctic Ocean and extensive areas of the
Southern Oceans that surround the Antarctica continent. In Canada alone,
90% of its
Arctic coastline of over 150 0 0 0 km is affected by sea ice. The winter extent in the northern
hemisphere is approximately 15
∼
10
−6
m
2
in the
summer (LeDrew et al., 1992). The extent of Antarctic sea ice is even larger: 20
10
−6
km
2
but reduces to approximately 8
×
×
×
10
−6
km
2
10
−6
km
2
in summer. Superimposed upon this signifi cant seasonal
variability is an inter-annual variability that is now complicated by global warming trends.
For example, in the last 30 years, the thickness of the Arctic sea ice has thinned signifi -
cantly (some estimates indicate a 30% loss) and in Antarctica large parts of the Larsen
and Ross Sea ice shelves have recently broken away.
in winter reducing to 3
×
10.4.2. Sea Ice, Wave Generation, and Sediment Transport
Because of sea ice, wave action on the beach may be restricted to as little as 8-10 weeks
a year in certain sheltered parts of the Canadian arctic (Taylor and McCann, 1976). In
other areas, such as the southern Beaufort Sea and the northern Siberian coast, open-
water conditions may develop for several months of the year (Are, 1998; Hequette and
Barnes, 1990; Hume and Schalk, 1964, 1967). In the North Atlantic, warm Gulf Stream
water keeps parts of Svalbard virtually ice-free for most of the year. Thus, ice cover, or
the nearby presence of the permanent pack, is an important control over the distance of
fetch and the time period over which wave action can operate. The effectiveness of wave
action may be further restricted by the presence of a narrow strip of land-fast ice (the “ice
foot”), which remains frozen to the shore and is unaffected by tidal movements. Thus,
Arctic beaches are essentially low-energy environments in which normal wave action and
coastal processes are limited. However, shore-ice action is especially important in the cold
meso- and macro-tidal environments of the mid-latitudes, where it disturbs the intertidal
zone by pushing and disrupting sedimentation and vegetation growth (Allard and
Tremblay, 1983; Dionne, 1975, 1989; Jahn, 1977).
The magnitude and frequency of periods of storm events assume great importance in
most assessments of coastal conditions. The majority of sediment transport and beach
reworking occurs during the few major events which have a frequency of occurrence of
two or three times a year. This is also true for periglacial environments (Harper et al.,
1988; McCann, 1972; Owens and McCann, 1970; Reimnitz and Maurer, 1979). For
example, observations along the northern Alaskan coast near Point Barrow (Hume and
Schalk, 1967; Hume et al., 1972) indicate that coastal cliff retreat rates between 1948 and
1968 were related to the annual frequency of westerly-wind storms. It was concluded that
a 50% decrease in storms during the open-water season resulted in a decrease in cliff
retreat from approximately 4 m/year to 1 m/year.
