Aridity in Australia: Age, origins and expression in aeolian landforms and sediments

Abstract

The early evidence of aridity in Australia is provided by the contraction of large lakes and extensive faunal changes towards the end of Tertiary time. Although not well dated some indication of probable age is derived from the trends of sea-surface temperature in the Southern Ocean which played an important part in the evolution of the continental climatic pattern. Early changes probably date from the growth of the west Antarctic ice cap about 10–17 m.y. ago. By 2.5 m.y. a substantial cooling had taken place and Australia's climatic pattern probably had begun to resemble that of today. Whilst the events of Tertiary time prepared the way, the climatic oscillations of Quaternary age have been responsible for present expression and diversity of arid features recorded in the dunes, lakes and soils over large areas of the arid and semi-arid zone. Examination of events spanning the last 40 000 years provides a glimpse into the manner in which the Australian environment responded to major climatic changes of glacial and post-glacial age. Before 25 000 B.P. lake levels were generally high and desert dunes were relatively stable. By 25 000 an important change marked the onset of the last major arid phase over the southern part of the continent. Lake levels fell and increasing salinities assisted in the early construction of clay-rich dunes. The trend reached a peak in the interval 18 000–16 000 B.P. when gypsum and clay dunes were constructed on the eastern margins of lakes from Western Australia, South Australia, N.S.W. and Victoria, simultaneously with expansion of desert linear dunes. By 13 000 B.P. the stresses had relaxed; dunes became stabilized and the landscape had acquired many new features that have remained almost unchanged to the present day. The causes of major aridity are to be sought in greatly intensified atmospheric circulation aided by increased continental extent corresponding to glacial low sea levels and reduced seasonal precipitation. The correlation between glacial maxima and dune building episodes suggests the earliest arid landforms in southeastern Australia date from at least 0.3 m.y. Desert dunes were almost certainly present in Central Australia by that time and perhaps considerably earlier. The desert dust component in marine sequences, especially off the northwestern shelf may provide the best method of confirming and extending the chronology of arid depositional episodes back to and beyond the last low sea-level interval.