Geoscience Reference
In-Depth Information
cap in North America at around 19 ka caused changes in the ocean circulation in the
Atlantic, resulting in warming at high latitudes in the Southern Hemisphere, followed
by the melting of ice caps and glaciers in Patagonia.
10
Be exposure ages obtained from moraines of outlet glaciers in the South Patago-
nian Ice Field at 51
S showed that they advanced during the Antarctic cold reversal
(around 14.6-12.8 ka) and were retreating rapidly by 12.5 ka, consistent with temper-
ature changes in Antarctica and the Southern Ocean at this time (Garcıa et al.,
2012
).
Precipitation dominantly came from westerly air masses, which reached much further
north during the LGM, when the Subtropical Front was at about 40
°
S, and fluctuating
after that in response to changes in the SubAntarctic Front and the Polar Front (Garcıa
et al.,
2012
).
Ramage et al. (
2005
) assessed different methods of determining the former snow-
line or equilibrium line altitude (ELA) in what is today an ice-free part of the tropical
Andes of central Peru. They concluded that during the LGM, the ELA had been
220-550 m lower than it is today, pointing to a slight temperature decrease of about
2.5
°
C. The ELA lowering estimated for the LGM was similar to that of the
most extensive glaciations in these valleys with ages in excess of 65 ka. The obvious
inference from this is that the relative influence of temperature and precipitation on
snow accumulation was not the same at these two times. These results from the
Peruvian Andes are in strong contrast to the results obtained by Stansell et al. (
2006
)
for ELA lowering during the LGM in the Venezuelan Andes north of the equator,
which showed that the ELA levels were about 1,420 to 850 m lower than present,
indicating that temperatures were possibly 8.8
±
1
°
±
C cooler than at present. A possible
reason for this difference is that the Peruvian Andes are much drier than the northern
Andes, limiting the potential accumulation of snow and ice.
Using cosmogenic
3
He, Bromley et al. (
2011
) have shown that there was a read-
vance or prolonged standstill of glaciers in the arid Andes of south-west Peru in the
very late Pleistocene, with moraines located about midway between the LGM and
present-day limits dated to 12.8
2
°
0.7 ka. Two sets of Holocene moraines in the
Peruvian Andes have yielded high-precision cosmogenic
10
Be surface exposure ages
(Licciardi et al.,
2009
), with the older moraines dated between 10 and 8 ka and the
younger ones being coeval with the latter half of the Little Ice Age of Europe, which
is dated between about 1300 and 1860 AD (Lamb,
1977
; Grove,
1988
). Jean Grove
(
2004
) has shown in abundant detail that there were multiple glacial advances in both
hemispheres during the Holocene, so it is likely that future work will provide a more
complex pattern of glacial advances and retreats in the Peruvian Andes.
Several ice caps in the central Andes have yielded a useful record of late Pleisto-
cene and Holocene fluctuations in temperature and precipitation, notably the Illimani
and Sajama ice caps in Bolivia and the Quelccaya and Huascaran ice caps in Peru
(Thompson et al.,
1995
; Thompson et al.,
1998
). Analysis of the isotopic compos-
ition of the ice (
±
18
Oand
D) indicates full glacial conditions at 20-18 ka and an
inferred drop in temperature of 8-12
°
C followed by early Holocene warming. One
