Geoscience Reference
In-Depth Information
Teeuw and Rhodes (
2004
) obtained OSL ages for dunes in the savanna region of north-
east Amazonia, with the start of eolian activity between 17 and 15 ka and continuing
until the present. They were unable to decide whether or not this record of gradual
eolian deposition had any climatic significance. Where dunes and loess deposits are
closely associated, as in the Chaco (Iriondo,
1993
), it is more probable than not that the
climate was dry and windy when these eolian sediments were accumulating. During
times of low glacial sea level, the source area for dust from Patagonia doubled, and
considerable volumes of dust reached Antarctica at those times (Maher et al.,
2010
).
The supply of dust from Patagonia diminished rapidly during postglacial times when
meltwater lakes occupied the Patagonian glacial outwash plains at the foot of the
glaciated mountains (Sugden et al.,
2009
).
21.5.3 Glaciations
The southern Andes between latitudes 27
S have a record of glacial activity
extending from the Late Miocene until the present, making it one of the most complete
records on earth, with ages based on magnetostratigraphy and potassium-argon (K-
Ar) dating (Rabassa and Clapperton,
1990
; Clapperton,
1993
). Glacial till has been
found between lavas with K-Ar ages of 7.0-4.6 Ma, and younger tills have ages
of 4.5, 3.68-3.55, around 3.5, 2.6-2.0, 2.05-1.03, 1.36-1.32 and around 1.2 Ma.
This latter Early Pleistocene glaciation seems to have been the most extensive of
all the Patagonian glaciations. The Early and Mid-Pleistocene glaciations have been
distinguished using relative dating methods such as weathering rind thickness on
moraine boulders and the degree of iron and manganese crust formation. Upper
Pleistocene glacial advances have ages of around 70, 20-18 and 15-10 ka, with
the LGM (the Llanquihue Glaciation) being synchronous with the Wisconsin and
Weichsel glaciations of North America and Europe (Rabassa and Clapperton,
1990
)
and corresponding Marine Isotope Stage 2 (MIS 2) glaciations in Australia and New
Zealand (Clapperton,
1990
). Snow-line depression during the LGM amounted to
roughly 1,000 m. The MIS 4 glacial advances were often more extensive than those of
MIS 2, because of lower temperatures and/or greater precipitation during MIS 4 along
the southern Andes.
Much of this earlier work was characterised by outstanding field mapping of glacial
moraines but suffered from the lack of an adequate chronology of glacial advances
and retreats. In the last decade or so, the application of cosmogenic nuclide dating to
boulders within glacial moraines has revolutionised research on past glacial activity
in South America, as indeed elsewhere. Murray et al. (
2012
) obtained
10
Be exposure
ages for boulders on the crests of moraines laid down by former glaciers in the Rio
Guanaco Valley of southern Patagonia in latitude 50
°
S and 55
°
°
S. The LGM ended in this valley
by 19.7
±
1.1 ka and rapid glacier retreat had started by 18.9
±
0.4 ka, with half of
the upper valley ice retreat achieved by 17.0
±
0.3 ka. Retreat of the Laurentide ice
