Australasian Science Magazine

Sediments of the ancient Lake Bungunnia near Rufus River in western NSW. The white horizon is the dust layer marking the start of arid climatic regimes. Photo: Richard Stanaway

By Sandra McLaren & Malcolm Wallace

Lake Bungunnia, a megalake that existed 1–2 million years ago in today’s Murray–Darling Basin, reveals the story and timing of the onset of arid climatic conditions in south-eastern Australia.

The modern challenge of climate change demands a sound scientific understanding of the Earth’s climatic history. Historic temperature and rainfall records are very important, but so is the geological evidence of climate recorded in sedimentary rocks that are formed on the surface of the Earth.

The late Neogene, a period of the Earth’s history beginning around 5 million years ago, was characterised by particularly dramatic climatic change around the globe. In Australia this was the time of change from a warm and wet climate, where lush rainforest covered much of southern Australia, to the arid climate we know today, where one-third of the continent receives rainfall of less than 250 mm per year.

However, despite considerable research, exactly when this change occurred and the reasons why it occurred remain poorly understood. Since the early 1980s geo­scientists have thought that the big “drying out” occurred around 700–800,000 years ago, and that it was related to the build-up of ice in Antarctica and the associated changes in Southern Ocean circulation. Additional data collected since that time has added to our understanding but has not further refined the age or the origin of the change.

What we do know about climatic changes in the Neogene is revealed mainly from sediments preserved in deep sea environments. However, sediments on the continents themselves can also be sensitive archives of climate change.

In southern Australia, sedimentary rocks from the modern-day Murray–Darling catchment may hold our best clues to how climate changed on our continent in the past. This is because it is one of the few places in our country where we find nearly complete sequences of sediments ranging in age from about 3 million years before present until the modern day.

Australia’s “Inland Sea”
The existence of an ancient lake in today’s Murray–Darling Basin, known as Lake Bungunnia, was first recognised by the pioneer Australian scientist Ralph Tate in 1885. Modern topographic data collected by NASA’s Shuttle Radar Topographic Mission (SRTM) since 2000 has provided an unprecedented view of subtle variations in the largely flat and featureless Murray Basin.

Data from this mission has provided an opportunity to study the lake’s geometry and size that would have astounded scientists of Tate’s era. Using these data we can see that the lake covered an area of more than 50,000 km2 – almost three-quarters the area of Tasmania – stretching from Nildottie in South Australia to Swan Hill in Victoria. The sheer size of Lake Bungunnia makes it a true “megalake”, and one of the largest known “within-continent” lakes from any period in the Earth’s history.

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 at a maximum depth of 30 metres. Although initially filled with freshwater, Lake Bungunnia was brackish for much of its history – in many ways fitting the myth of Australia’s famed inland sea!

Geologists now believe that Lake Bungunnia formed as a result of tectonic uplift – the vertical movement of the ground surface that occurs when stresses applied to the tectonic plates overcome the strength of the rocks. This uplift occurred in South Australia along a feature termed the Padthaway High, and was possibly related to major collisions in New Zealand.

The uplift effectively dammed the ancient Murray River and caused all the run-off from the Eastern Highlands to pool, gradually filling Lake Bungunnia from the dam wall back to the east.

Archive of Past Climate Change
The existence of such a megalake is hard to imagine in today’s drought-stricken Murray catchment, and 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 or three times the total rainfall that it receives today.

The many small salt lakes that characterise the Mallee area today are remnants of ancient Lake Bungunnia, and it has been proposed that this dramatic contraction of the lake system heralded the onset of arid climatic conditions in southern Australia. Indeed, Lake Bungunnia was so large, but also so shallow, that once it was formed it would have been particularly sensitive to changes in climate, such as total precipitation and evaporation as well as rates of discharge of feeder river systems.

In a major Australian Research Council-funded study, we have undertaken a complete re-evaluation of the geology of the Basin system, trying to interpret the climate and tectonic history from clues preserved in the sediments themselves. As well as studying the lake using the high-resolution NASA SRTM topographic data, looking at the sediments of Lake Bungunnia in the field has also been an essential part of our research.

The Blanchetown Clay is the main sedimentary rock deposited in Lake Bungunnia. The clay ranges from only a few metres in thickness to a maximum of 30 metres near the tri-state border between South Australia, Victoria and New South Wales.

The Blanchetown Clay is made up mostly of pure clay that is grey-green in colour and remarkably uniform. Surprisingly, the clay doesn’t contain many fossils. Fish vertebrae and marsupial bones have been found at the base of the sediment, but elsewhere the general absence of even microfossil remains suggests that the lake was almost entirely devoid of life – a surprising result in itself.

As well as a study of the geochemistry of the clay, our new work has revealed the presence of a distinctive sediment layer within the Blanchetown Clay across the Murray Basin that had not been previously recognised. This layer, which we’ve called the Nampoo Member of the Blanchetown Clay, consists of thin, often finely laminated silts comprising grains that are 4–60 µm in diameter – intermediate between clay and sand sizes. The grains are quartz in composition and are dominantly angular to subangular – very different in shape to the rounded grains that are typically transported into lake systems by rivers.

The use of a laser to accurately measure the particle sizes shows that most grains are around 20–25 µm in diameter. This range of particle sizes is distinctive of wind-blown dust, a sediment type termed “loess”. Indeed, the range of grain sizes we have reported from the Nampoo Member is very similar to well-known loess deposits in China, which are associated with the widespread formation of deserts in the last ice age.

In the central part of Lake Bungunnia, the dust deposits preserved within the Blanchetown Clay are almost 3 metres thick. It is accepted that the presence of wind-blown sediments is one of the clearest indicators of arid climatic conditions. Thus, the presence of such thick dust storm-type sediment within Lake Bungunnia must reflect significant climate change in southern Australia.

We suggest that the sediments of the Nampoo Member of the Blanchetown Clay mark the transition from the wet climatic regimes responsible for the formation of the lake to arid climatic regimes. 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 Death of Lake Bungunnia
To understand in more detail how this aridification of Australia proceeded, we made innovative use of global positioning systems to obtain accurate elevation data for each locality where the sediments of Lake Bungunnia were found. At each locality, elevations were measured with accuracy of ± 0.5 metres. These data reveal that the entire Blanchetown Clay is around 56 metres above sea level. The data also show that the Nampoo Member dust sediments are at around the same absolute elevation, providing evidence for their deposition at the same time across the whole of the basin.

The sediments of Lake Bungunnia include a thin sequence of carbonate called the Bungunnia Limestone. This type of sediment, however, is only found in the Morgan Sub-basin, the most western part of Lake Bungunnia in South Australia. Our elevation data show, for the first time, that these sediments are not continuous across the lake but are present on a series of terraces that range over more than 20 metres in elevation. The terraces on which the limestone is found are also clearly visible on the NASA SRTM image.

The terraces that we can recognise from the GPS and SRTM data show that lake levels decreased progressively through time, and that Lake Bungunnia began to shrink soon after the dust sediments were deposited within the Blanchetown Clay. 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.

Importantly, the preservation of carbonate minerals such as dolomite, calcite and gypsum on these terraces shows that the climate continued to warm significantly after deposition of the Nampoo Member dust deposits. Environmental conditions in this period must have been similar to the modern Coorong Lagoon in South Australia where carbonate, rather than clay, is being deposited today.

Our work has also provided new constraints on the age of the lake using magnetostratigraphy. This technique uses the fact that the episodic reversals of the Earth’s magnetic field can be preserved in clay-rich sediments and can be read a bit like a barcode. These data show that the oldest Blanchetown Clay, and therefore the formation of Lake Bungunnia, dates from around 2.4 million years ago.

The youngest Blanchetown Clay is probably around 1.2 million years in age, and the thick desert dust deposits of the Nampoo Member were laid down around 1.4 million years ago. The terraces on which we find Bungunnia Limestone are all lower in elevation than any of the Blanchetown Clay deposits, meaning that those sediments are younger still – probably as young as 700,000 or 800,000 years.

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, well over half a million years earlier than previously suggested. This result is highly significant, and suggests that many concepts relating to the aridification of Australia, including the role of circulation in the Southern Ocean, need to be re-assessed.