Geoscience Reference
In-Depth Information
lower than present, indicating that temperatures may have been 8.8
±
2
°
C cooler than
today.
10
Be exposure ages for boulders on the crests of a succession of moraines laid
down by former glaciers in the Rio Guanaco Valley of southern Patagonia at latitude
50
±
°
S show that the last local glacial maximum had ended by 19.7
1.1 ka (Murray
et al.,
2012
). Rapid glacier retreat was underway by 18.9
±
0.4 ka, and more than
half of the upper valley ice retreat was accomplished by 17.0
0.3 ka. An important
conclusion arising from this work is that glacier retreat in southern Patagonia was
linked to high-latitude warming in the Southern Hemisphere, associated with changes
in ocean circulation initiated by the retreat of the large Northern Hemisphere ice
sheets at around 19 ka, thereby generating a 'bipolar seesaw' response (Blunier and
Brook,
2001
; Steig,
2006
).
Garcıa et al. (
2012
) obtained thirty-eight
10
Be exposure ages from moraines of
outlet glaciers in the South Patagonian Ice Field at 51
±
S, which showed that they
advanced during the Antarctic cold reversal (14.6-12.8 ka) and were in rapid retreat
by 12.5 ka, consistent with temperature changes in Antarctica and the Southern Ocean
at that time. The primary source of precipitation was from westerly air masses, which
reached much further north during the LGM, when the Subtropical Front reached
to about 40
°
S, and fluctuated thereafter in response to changes in the Sub-Antarctic
Front and the Polar Front (Garcıa et al.,
2012
).
Turning now to the drier regions of North America, it is reasonably certain that
nowhere else in the desert world is the evidence of former glacial activity more
thoroughly studied. The volume edited by Porter (
1983
) provided a comprehensive
overview of Late Wisconsin mountain glaciation in the western United States during
the late Pleistocene, including the LGM. At the time, chronologies were based primar-
ily on radiocarbon dating, so glacial deposits were in general only dated indirectly.
Increasing use of cosmogenic nuclide exposure dating during the past decade or so
meant that, for the first time, glacial moraines could be directly dated, although here,
too, some degree of caution is necessary, as outlined in
Chapter 6
.
Present evidence shows that mountain glaciers advanced close to their maximum
limits during the LGM, but many individual glaciers reflect the influence of local
topographic factors and do not respond solely to regional climatic controls. Young
et al. (
2011
) obtained
10
Be surface exposure ages from moraines, bedrock and river
terrace gravels relating to the late Pleistocene Pinedale glaciation from three adjacent
valleys in the upper Arkansas River Basin. They found that times of maximum glacier
expansion were not synchronous and ranged in age from 22 to 16 ka. Ice retreat, in
contrast, began at the same time, at 16-15 ka. They concluded from their study of
Pinedale glacial moraines at widely different sites in the western United States that
'glacier fluctuations are ultimately driven by climate change, but the exact position
of a glacier terminus is filtered by nonclimatic factors intrinsic to each glacier valley
system' (op. cit., p. 173).
°
