Geoscience Reference
In-Depth Information
13.10 The glacial record from Australia
The semi-arid Snowy Mountains in south-east Australia were glaciated on a number
of occasions during the late Pleistocene, but the area covered in ice during the most
recent glacial advances did not exceed about 50 km
2
(Galloway,
1963
). Barrows
et al. (
2001
;
2002
;
2004
) obtained
10
Be and
36
Cl exposure ages of 55-65, 32
±
2.5,
19.1
1.4 ka for the youngest glacial advances in the SnowyMountains
and ages between 23 and 16 ka, with a weighted average age of 21.9
±
1.6 and 16.8
±
0.5 ka, for
the much more extensive periglacial deposits in this region. The 19 ka, or Blue Lake
Advance, was also synchronous with the LGM ice cap advance in Tasmania (Colhoun
et al.,
2010
; Colhoun and Barrows,
2011
), which was also the time of maximum
periglacial activity. The 55-65 ka glaciation was more extensive than later ones,
perhaps reflecting an increase in aridity and decrease in snowfall during the late
Pleistocene.
There is an interesting contrast between late Pleistocene glacial and periglacial
activity in Tasmania, which, on a very small scale, mirrors events across NorthAmerica
and Eurasia for similar climatic reasons. In the humid west of Tasmania, the LGM
glaciers reached to within 300 m of the present sea level and 420 m of the LGM sea
level. In the drier east of Tasmania, the LGM snow-line was much higher and dunes
were active close to the coast. Colhoun (
2000
) estimated that the LGM orographic
snow-line in Tasmania was 690-1,000 m lower than it is today, with an average of
830 m for the ice cap on the central plateau.
Galloway (
1965b
) had earlier mapped the lower limits of periglacial solifluction
deposits in mainland south-east Australia. From this, he deduced that the upper limit
attained by trees (the 'tree-line' or 'timberline') during the LGM had been lowered by
at least 975 m. He concluded that because the tree-line today roughly coincides with
the 10
±
C isotherm for the hottest month (in this case, January), the difference between
the mean January temperature at that elevation today and 10
°
°
C is the temperature rise
since the LGM, amounting to at least 9
C. Galloway (
1965b
) concluded that the lower
limit of LGM periglacial solifluction, which primarily reflects temperature control,
had been lowered by around 600 m in both eastern and western Tasmania. As already
noted, this limit is broadly equivalent to the 10
°
C isotherm for the warmest month.
Depending on the lapse rate used, it indicates an LGM temperature lowering during
the warmest month of about 5
°
C. This figure is lower than that estimated for the
Snowy Mountains, where the orographic snow-line was 600-700 m lower, the lower
limit of periglacial solifluction was at least 975 m lower and the temperature in the
warmest month was at least 9
°
C cooler (Galloway,
1965b
).
Deglaciation followed the time of coldest sea surface temperatures in the west
Pacific, and there was no evidence of any glacier readvance during the Antarctic Cold
Reversal (around 14-13 ka) or the Younger Dryas (around 13-11.5 ka) (Barrows et al.,
2001
). Blue Lake (
Figure 13.2
) in the Snowy Mountains was free of ice by 15.8 ka,
°
