ABSTRACT
Reconstruction of glacial age environments in semi-arid southern Australia has demonstrated the reality there of a wet phase (45 000 to 25 000 BP) followed by a long dry interval (25 000 to 14 000 BP). The transition from wet to dry is identified by a fall in lake levels, but the construction of lakeshore clay-rich dunes (lunettes) and by renewed activation of longitudinal desert quartz dunes. Measured by the maximum aeolian activity, the peak aridity occurred between 18 000 and 16 000 BP coincident with the maximum advance of global ice sheets.
The two-phase nature of southern Australian glacial age environments is evident also in the chronologies of other low latitude dry regions, especially in Lake Chad and Lake Abhé in northern Africa.
Loess sequences from cold Pleistocene semi-arid environments of Europe, USSR and North America also reflect a two-stage cycle; the transition there from pedogenic to aeolian environments occurred about the same time as comparable low latitude changes. After 25 000 BP the quantity of long-distance dust reaching Antarctica increased markedly.
The association of intensified aeolian activity with cold glacial conditions can be validated for the past 700 000 years, both in Atlantic deep sea cores, and in the continental loess sequences of Czechoslovakia and China. The consistency of the transformation from wet to dry environments preceding each glacial maximum, points to a major change in atmospheric circulation about that time. However, the last such change about 25 000 BP is not matched by changes of equivalent magnitude either in oxygen isotope or sea surface temperature reconstructions.
Whilst the cause of the changes that occurred about 25 000 BP is not well understood, the Australian evidence suggests intensification in the frequency and velocity of winds emanating from the continental interior. In glacial summers, sites downwind of such winds would have experienced hot dry conditions in which enhanced seasonal evaporation helped produce low lake levels; these seasonal changes amplified the general trend towards aridity already evident due to decreased precipitation.
Widespread dusty environments at high and low latitudes with consequent effects of both increased advective heat transfer and dust-induced thermal 150modifications had characterised each major glacial episode. The repetition of such conditions preceding each glacial-interglacial transition suggests a possible two-way interaction. In helping to produce dry, dusty environments on a global scale, the expanded ice sheets and sea ice of high latitudes may well have been agents instrumental in their own destruction.
