Geoscience Reference
In-Depth Information
of Alaska was ice-free. In Greenland, the ice sheet during the LGM was somewhat
more extensive and thicker than now, mainly in southern Greenland (Siegert,
2001
).
However, ice also filled the Nares Strait, linking the Innuitian and Greenland ice
sheets. Outlet glaciers reached the shelf break.
Steppe-tundra vegetation was thought to have covered Beringia (eastern Siberia
and Western Alaska surrounding the Bering land bridge) (Hopkins et al.,
1982
).
There are fossil fauna of large grazing animals including mammoth. This interpre-
tation is based on terrestrial records and is supported for at least the glacial onset
(23 ka) in the Yukon, eastern Beringia (Zazula et al.,
2002
). However studies of
marine core (Elias et al.,
1996
) show that at least on the former Bering land bridge
there was widespread moist shrub tundra. Studies in Alaska point to less widespread
steppe/tundra than was formerly proposed. Insect evidence for the full glacial inter-
val points to dry heaths and meadows, shrub tundra, and interspersed marshes and
ponds.
10.6
Deglaciation
10.6.1
Fundamental Features
Deglaciation began between 15 ka and 13 ka, depending on the latitude of the area.
Ice retreat was accompanied by changes in vegetation and in the atmospheric circu-
lation. For example, before 15 ka, eastern interior Alaska was dominated by cold,
very dry conditions (Edwards et al.,
2001
). After 14 ka, slight warming allowed
birch and poplar growth. This interval also saw the northward expansion of the
tree line in eastern Siberia. Around 11 ka, the Bering seaway was reestablished
and summer temperatures inferred from insect evidence on the Bering shelf were
slightly higher than now. However, winter temperatures appear to have been some-
what colder (Elias et al.,
1996
). The tundra/steppe extended well into Europe at this
time (Rochon et al.,
1998
).
The fundamental driver of the deglaciation was increasing solar radiation.
However, deglaciation was by no means uniform, pointing to a strong role of
processes within the coupled atmosphere-ocean-ice sheet system. Greenland ice
cores document a number of notable events. The Older Dryas (OD) was followed
by the Bölling-Allerod (B/A), a warm event starting at about 14.7 ka and ending
at about 13 ka (
Figure 10.4
). The B/A has two parts, the earlier Bolling being the
warmest. The subsequent Allerod, although still warm, was somewhat cooler. The
B/A ended sharply with the advent of the YD cold interval, which lasted from
about 12.8 to 11.5 ka. The YD represented a return to near glacial conditions. The
Laurentide and Fennoscandian ice sheets began to re-advance during the YD, as
did smaller alpine glaciers at many sites. The onset and termination of the YD
were rapid, occurring over years or decades. While the YD was a cold period
overall, conditions were nevertheless quite variable both spatially and temporally
(Mayewski et al.,
1993
).
