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The Making of Arid Australia

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The Murray–Darling Basin covers more than one million square kilometres of eastern and south-eastern Australia and is one of Australia’s most important agricultural regions. In recent years severe drought has seriously impacted irrigation and production within the basin and is an ongoing problem for surface and groundwater management.

In such dry years it’s hard to imagine the existence of an enormous ‘megalake’ in the centre of the Basin, but in the Neogene period of Australia’s geological history a megalake did exist across South Australia, Victoria and New South Wales. The sediments of Lake Bungunnia reveal the story and timing of the onset of arid climatic conditions in south-eastern Australia and have the potential to help inform our understanding of climatic change.

Drought is an unfortunate part of life in Australia, the driest continent on Earth, where one-third of the continent receives rainfall of less than 250 mm per year. Understanding individual drought episodes as well as long-term climate change demands a sound scientific understanding of the Earth’s climatic history. Historic temperature and rainfall records are therefore very important but so is the geological evidence of climate recorded in sedimentary rocks that are formed on the surface of the Earth.

In many parts of the world the late Neogene, a period of Earth history beginning around five million years ago, was characterised by dramatic climatic change. In Australia this was the time when the arid climate we know today developed. However, despite considerable research effort, exactly when this change occurred and the reasons why it occurred remain poorly understood.Topographic map showing the location of Lake Bungunnia. Outline shows the maximum extent of the Lake 1.2 million years ago.Topographic map showing the location of Lake Bungunnia. Outline shows the maximum extent of the Lake 1.2 million years ago.

In southern Australia sedimentary rocks from the modern day Murray–Darling catchment may hold the best clues to exactly when and how our arid climate developed. The existence of an ancient lake, known as Lake Bungunnia, in today’s Murray–Darling Basin, was first recognised by the pioneer Australian scientist, Ralph Tate, in 1885. New generation topographic data has provided an excellent opportunity to study the lake’s size and geometry – otherwise challenging in the largely flat and featureless Murray Basin. These data show that the lake covered an area of more than 50,000 km2 – almost three-quarters the area of Tasmania – and stretched from Nildottie in South Australia to Swan Hill in Victoria. The sheer size of Lake Bungunnia makes it a true “megalake”.

The sediments that filled Lake Bungunnia are mainly exposed where the modern Murray River has cut down into the ancient lake bed. The best exposures are between Wentworth and the South Australian border where the river has cut right to the base of Lake Bungunnia – a maximum depth of just 30 metres. Although initially filled with fresh water, Lake Bungunnia was brackish for much of its history.

The existence of such a megalake tells us that significantly different hydrologic and climatic regimes were operating when the lake was formed. Indeed, for the lake to fill to the size it did means that the Murray–Darling Basin must have received at least two to three times the modern day rainfall. The many small salt lakes that characterise the Mallee area today, such as Lake Tyrell south of Manangatang, are remnants of ancient Lake Bungunnia.  It is this dramatic contraction of the lake system that has previously been suggested to herald the onset of arid climatic conditions in southern Australia.

The Blanchetown Clay is the main sedimentary rock deposited in Lake Bungunnia. The clay thickness ranges from only a few metres to a maximum of 30 metres near the South Australia, Victoria and New South Wales border. The Blanchetown Clay is made up mostly of pure clay, grey–green in colour and remarkably uniform. Of particular interest, however, is the presence of distinctive silty sediment up to 3 metres thick within the Blanchetown Clay.

Using a laser to accurately measure the particle sizes shows that most grains within the silt unit are around 20-25 microns in diameter. This range of particle sizes is very different from what is normally transported into lakes by rivers and is instead distinctive of wind-blown dust, a sediment type termed “loess”. It is well accepted that the presence of wind-blown sediments is one of the clearest indicators of arid climatic conditions and so the presence of such thick dust-storm type sediment within Lake Bungunnia must represent significant climate change in southern Australia.

Thus, the silty sediments of the Blanchetown Clay mark the transition from the wet climatic regimes responsible for the formation of the lake, to much more arid climatic conditions. The dust-deposits of Lake Bungunnia are therefore the first sedimentary evidence of the development of extensive desert dune systems in Australia and the “aridification” of our continent. The silty sediments probably represent the first, and possibly most severe, drought the Murray–Darling Basin endured.

Magnetostratigraphy is a technique that uses the fact that the Earth’s magnetic field has undergone episodic reversals over geological time. Clay minerals within sediments record these reversals that can then be read back a bit like a barcode, which can be read using sophisticated equipment. Using this technique, we estimate that this first major arid shift occurred around 1.4 million years ago.

The sediments of Lake Bungunnia include a thin sequence of carbonate, called the Bungunnia Limestone. Using high precision GPS elevation data we have shown for the first time that these sediments are not continuous across the lake as previously thought but are instead present on a series of terraces that range over more than 20 metres in elevation. The terraces indicate that lake levels decreased progressively through time and that soon after the dust sediments were deposited, Lake Bungunnia began to shrink. The terraces themselves provide a unique record of the step-wise aridification of southern Australia – a bit like rings on a bath – preserved as the lake system progressively dried up.

Thus, our new results both identify and date major and previously unknown steps in the aridification of the Australian continent. These major changes began around 1.4 million years ago, over 500,000 years earlier than previously suggested. These results provide a new perspective on our understanding of drought periods in the Murray–Darling Basin and suggest that many concepts relating to the aridification of Australia need to be re-assessed.

By Sandra McLaren and Malcolm Wallace, School of Earth Sciences, University of Melbourne.

 This article is based on research published recently in the journal Global and Planetary Change and in Australasian Science.

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