Geoscience Reference
In-Depth Information
Rockies are sufficiently high to have perennial snow and ice. The Andes have a mean
elevation of about 4,000 m and rise to 6,962 m, while the Rockies are much lower
and are mostly below an elevation of 4,000 m.
One much-debated question is whether or not glacial advances and retreats during
the past 20,000 years have been synchronous or out of phase in both hemispheres.
As far as major ice caps are concerned, the issue seems to be resolved, with ice sheet
expansion over Greenland, North America and north-west Europe coinciding with
expansion of the East and West Antarctic ice sheets. However, mountain glaciers will
often show a far more variable response owing to differences in size, shape and aspect,
as well as local controls over precipitation.
The Holocene glaciers in the Peruvian Andes have several sets of moraines, of
which the two most prominent have provided high-precision cosmogenic
10
Be sur-
face exposure ages. The older moraines date between 10 and 8 ka and are broadly
synchronous with early Holocene glacial moraines in the southern Andes, Norway
and the Austrian Alps but are out of phase with
10
Be-dated Holocene glacial maxima
in New Zealand (Licciardi et al.,
2009
). The younger set of moraines date to the latter
part of the Little Ice Age, dated in Europe to between around 1300 and 1860 AD
(Lamb,
1977
; Grove,
1988
). Licciardi et al. (
2009
) proposed that their results sug-
gested a climatic link between the North Atlantic and the Peruvian Andes, with cold
conditions in the north promoting glacier expansion in Europe and a concomitant
southward shift of the ITCZ bringing increased snowfall to the Peruvian Andes and
glacier expansion. Jean Grove (
2004
) had earlier demonstrated that there were mul-
tiple glacial advances in both hemispheres during the Holocene, so future work needs
to focus on dating a wider array of localities in order to provide a more comprehens-
ive and robust set of data, which would allow more rigorous modelling of climatic
linkages between hemispheres.
If we go slightly further back in time to the terminal Pleistocene, glaciers in the
arid Andes of south-west Peru show signs of a readvance or at least a prolonged
stillstand, with moraines located about midway between present and LGM limits
dated by cosmogenic
3
He to 12.8
0.7 ka (Bromley et al.,
2011
).
Ramage et al. (
2005
) have evaluated the pitfalls involved in using different methods
to determine the ELA in a presently ice-free part of the tropical Andes of central Peru.
They concluded that best estimates for the LGM amount to a lowering of 220-550 m
to between 4,250 and 4,570 m elevation, indicating a modest temperature decrease
of around 2.5
±
C. They also noted that the ELA lowering inferred for the LGM
was little different from that of the most extensive glaciations in these valleys, which
date back to
±
1
°
65 ka. This would suggest that the relative influence of temperature
and precipitation on snow accumulation was not the same at these two times. It is
interesting to note that the estimates for ELA lowering in the Peruvian Andes during
the LGM differ quite markedly from those north of the equator in the Venezuelan
Andes (Stansell et al.,
2006
). Here the ELA levels were around 1,420 to 850 m
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