Lake Mungo

Lake Mungo is a currently dry lake basin in the semi-arid zone of southeastern Australia. It lies within a chain of playas in the Willandra Lake system, formerly fed by overflow of the Willandra Creek, a distributary of the Lachlan River which has its headwaters in the eastern Australian highlands. As a playa, the Lake Mungo floor frequently deflated and the sediment accumulated as an arcuate transverse dune system called a lunette on the downwind, eastern margin. The lunette contains a record of human occupation of international importance, including some of the earliest archaeological traces on the continent, such as the world’s oldest known cremation and ritual ochre burial. It also contains the most continuous record of climate change over the last glacial cycle in the Australia desert.

As part of a larger study of the archaeology of the region, I led a team to provide a framework for the interpretation of lake environment from before the arrival of people (>41,000 years ago) until the establishment of the pastoral industry in the area after British colonisation¹. The 3D model below was created using aerial imagery collected for the project and shows some of the main areas studied. Clearly visible are two zones. The first is an area of extensive erosion (the “Walls of China”). The second is a dune complex formed from the deposited sediments from the first area. These dunes extend up to half a kilometre into the desert. The erosion has revealed a series of layers within the lunette core which record the different depositional periods along the lake margin. Overall, it is a flat landscape. The highest point of the lunette is about 40 m above the lake floor.

We aimed to determine the age of the sediments in the lunette to reconstruct the lake level through time. Lake level is linked to what the climate was like at the time. We used a technique called optically stimulated luminescence (optical dating) which measures the last time that quartz sand was exposed to sunlight. We processed 83 optical ages from 34 sites which revealed lake level fluctuations of up to 10 m¹. Some sections reveal a rapidly fluctuating lake, either seasonally or to a cycle like ENSO. Other sections indicate that lake level was high for hundreds or thousands of years at a time.

The ages push back the inception of the lake to at least a third of million years ago, but most of the visible dune sequence was built at the end of the last ice age. There were two main phases of deflation which consisted of sandy clay and silt blown up off the lake bed. The older was from about 33,000 to 24,000 years ago. After a short hiatus when the lake appeared to dry out, the water returned during the coldest phase of the last ice age from about 23,000 to 20,000 years ago. After this, the lake never refilled again. This date is earlier than expected, but may relate to earthquake activity in the region diverting inflows. The erosion that created the Walls occurred much later and has been securely dated to within the 18th century with the arrival of British pastoralists. The “smoking gun” as to the cause of erosion is a sheep skeleton lying on top of a soil layer at the beginning of the modern dune sequence.

Photo 1. Dune sands overlain by deflated clay (20,000-21,000 years old)

Photo 2. Deflated sand and clay (24,000-33,000 years old)

River systems snaking out into the desert from the eastern highlands would have provided a life line to the first people in the region. The lunette is full of evidence of people living in the area. One such area is shown in the banner image at the top of this post. At this site there is an ashen grey layer full of hearths and stone tools overlying the ancient core of the lunette, demonstrating a long period of occupation. Even in the deflation sediments above this, there are numerous hearths, often containing “ear bones” from huge golden perch, indicating the lake stayed full for many years at a time. The filling of the Willandra Lakes during the last ice age is still something of a conundrum. Precipitation may have been only half what it is today in eastern Australia with an expanded desert zone, so how did the water get there? We’ll look at this question in the next post.

1. Barrows, T.T., Fitzsimmons, K.E., Mills, S.C., Tumney, J., Pappin, D., Stern, N., 2020. Late Pleistocene lake level history of Lake Mungo, Australia. Quaternary Science Reviews, 238, 106338.