Geoscience Reference
In-Depth Information
Hemisphere. July solar radiation at 65
o
N reached its maximum positive anomaly
around 9 ka when it was 7 percent greater than today. The remaining terrestrial ice
sheets saw dramatic retreat, with the Laurentide Ice Sheet shrinking to a tiny rem-
nant in Quebec-Labrador by about 6 ka. In Eurasia, northward flowing rivers had
formed large proglacial lakes where they were dammed by the ice front, especially
in the Ob basin. Their overflow southward during glacial retreat is thought to have
caused mega-floods. Grossvald (
1999
) suggests these events occurred about 12, 10,
and 7 ka. However, his model demands a much more extensive ice cover over north-
ern Siberia than recent work would support.
The first half of the Holocene contained the most dramatic increase in tempera-
tures. There was a 9
o
C increase in SST in the Greenland-Iceland-Norwegian seas
causing summer air temperatures to be between 10-14
o
C and winter air tempera-
tures between 4-9
o
C. An increased poleward flow of warm Atlantic water reached
the Fram Strait by 9.5 ka (Koc, Jansen, and Haflidason,
1993
). The influx of warm
water affected atmospheric circulation and caused a reduced latitudinal temperature
gradient and poleward expansion of the subtropical gyre.
The so-called Holocene Thermal Maximum (HTM) was characterized by warmer
than present conditions at many sites. D. Kaufman, T. Ager, and N. Anderson (
2004
)
mapped the onset and termination of the HTM for North America based on a vari-
ety of paleoclimate evidence that primarily provide estimates of summer tempera-
tures (
Figure 10.10
). At the terrestrial sites where quantitative estimates could be
obtained, HTM summer temperatures were on average 1.6 C higher than twenti-
eth-century averages. However, both the onset and termination of the HTM varied
widely depending on location. Over northwest Canada and Alaska, onset of the
HTM was quite early, generally between 11-9 ka, about 4,000 years earlier than
for northeastern Canada. The later HTM onset over northeastern Canada is argued
to reflect proximity to the chilling effects of the shrinking Laurentide Ice Sheet and
influences of atmospheric and oceanic circulation. The pattern for the termination
of the HTM over North America largely follows the pattern for onset; over Alaska,
the HTM ended quite early, from between 11-9 ka, compared to as late as 3-4 ka
over northeastern North America.
Boreal forest was present north of the modern treeline between 10 ka to 5 ka in
western Canada, and 5 ka to 4 ka in central Canada but from 4 ka to 2 ka in east-
ern Canada (McDonald and Gajewski,
1992
). However, in Alaska, the Holocene
treeline never extended beyond its modern limit (Anderson and Brubaker,
1994
).
The period from about 6 ka to 4 ka saw the historical maximum amount of drift-
wood occurrence in the Canadian Arctic. As shown in
Figure 10.11
, the Aggasiz ice
core from Ellesmere Island shows maximum pollen concentrations at about 3.1 ka
(Bradley,
1990
).
A notable event interrupting the overall warmth of the early- and mid-Holocene
was a prominent cooling event dated at about 8.2 ka (Alley et al.,
1997
; Kobashi
et al.,
2007
). It is generally believed that this was because of a large pulse of
meltwater to the North Atlantic from glacial lakes Aggasiz and Ojibway, asso-
ciated with the final collapse of the Laurentide Ice Sheet. It appears that average
