Desert glaciations - Climatic Change in Deserts - page 229

An unresolved problem in the study of Quaternary climates is why a long interval of
cooling is followed by a relatively short phase of warming, which ends the glaciation.
Denton et al. ( 2010 , p. 1652) returned to this question in the context of the Last
Glacial Termination and commented that 'the reduction of [the Northern Hemisphere]
continental ice to about its present volume represents one of the largest and most rapid
natural climatic changes in Earth's recent history'. They noted that during the warming
intervals associated with rapid melting of the huge Northern Hemisphere ice sheets,
sea level rose by 120 m, atmospheric CO 2 increased by 100 parts per million by
volume (ppmv) and vast quantities of meltwater entered the North Atlantic, creating
stadial (cold) conditions in the Northern Hemisphere and altering the previous oceanic
and associated atmospheric circulation patterns. They hypothesised that during each
northern stadial, the Southern Hemisphere westerlies shifted poleward, resulting in
pulses of oceanic upwelling and warming that in turn caused deglaciation in the
Antarctic and Southern Ocean. Future work is needed to test these ideas. The key
point here is that many independent factors other than global temperature changes
control the waxing and waning of the ice caps and mountain glaciers, so we should
expect to find evidence of regional variability.
13.3 Evidence of glaciation
Evidence for former glaciations may conveniently be considered under two headings:
the erosional evidence and the depositional evidence. Both sets of evidence are useful
and both have certain inherent limitations when it comes to teasing out a climatic
signal.
For a glacier to develop, a layer of snow needs to persist all year and become
progressively thicker over time. Because cold water can hold more carbon dioxide
in solution than an equivalent volume of warm water, melting snow is slightly acidic
and will corrode the underlying bedrock. The result is a nivation hollow, a feature
that is common above 3,000 m on the northern peaks of Tibesti in the south-central
Sahara but that never developed further to form glacial cirques (Messerli et al., 1980 ).
Ice will form when a deep layer of snow becomes compressed by the weight of the
overlying snow mantle to form a semi-crystalline mass called neve ,or firn ,which
is subsequently compressed to form crystalline ice. Ice flows as a result of several
distinct processes. Individual crystals of ice under pressure melt at the point of contact,
movement occurs under gravity in a generally downslope direction and the glaciers
advance slowly down valley. In the case of tropical and temperate glaciers, seasonal
meltwater can penetrate down cracks or crevasses in the ice to lubricate the base of the
ice at its contact with bedrock. This lubrication can cause quite rapid local movement
of the base of the ice column, with the rest of the ice flowing down en masse. In
extreme cases, subglacial meltwater can erode into bedrock, and when later exposed