Australia: The Land Where Time Began

A biography of the Australian continent 

The Last Interglacial – Lake Eyre – A Continental Rain Gauge

At the present Lake Eyre is an ephemeral playa lake of 9,690 km² surface area, and forms the centre of the largest internal drainage basin in Australia that extends over an area of 1,215,000 km² Summer monsoon rain in the north drains towards it and any water that reaches it in times of heavy monsoon rain does so via the Warburton-Diamantina-Georgina and the Cooper-Thompson-Barcoo river systems from the northeast and the Macumba and Neales Rivers from the west. Central Australian rivers, such as the Finke and the Todd Rivers are cut off at the present by the Simpson Desert dunefield, also reached Lake Eyre in the past. Lake Eyre has been called ‘a continental rain gauge’ by geomorphologists John Magee because Lake Eyre is sensitive to changes in precipitation over such a large proportion of the Australian continent.

It is in this context that according to Smith the single most important contribution to the Quaternary history of the desert has been the reconstruction of the palaeohydrology of the lake for the last 130 ka by J. Magee (Magee et al., 1995; Magee, 1997; Magee and Miller, 1998). It was shown by Magee that 130-90 ka there was a deep, permanent water body, by the study of shoreline features and the sedimentology of lacustrine sequences, and the dating and the cross-correlation of these in which he made use of a range of Quaternary dating techniques such as optically-stimulated luminescence (OSL), U/Th, amino acid racemisation (AAR) and radiocarbon (¹⁴C), showing major expansion of the lake during MIS 5. This large interglacial lake that resulted           was up to 25 m deep and covered an area of 25,000 km². The flooding of Lake Eyre that occurred in 1974 (Bonython & Fraser, 1989), by contrast, which was the largest historical filling on record, resulted in an ephemeral lake that was 6 % of the size of the palaeolake that existed in MIS 5. The interglacial lake was mostly saline and had marked salinity stratification and anoxic conditions prevailed at the bottom, though it was permanent.

According to Smith the ‘Inland Sea’ was not resource-rich compared with shallow freshwater lakes such as the Willandra Lakes during the Pleistocene. The beach sediments contain abundant shells and shell fragments, mainly Corbiculina bivalves, and lake-bottom gastropods such as Coxiella and Coxiellada (Magee, 1997), none of which were species of shellfish that were preferred by hunter-gatherers. Pelican eggshell is the most common type of egg shell in beach deposits. There were many bones of small fish, such as vertebrae, head plates and spines, and included some dense, thin, horizontal accumulations of fish bone, which suggests a boom and bust cycle associated with flood events. When Lake Eyre is flooded at the present large quantities of nutrients are delivered to the Lake over a short space of time triggering an invertebrate bloom which leads to booms in fish and bird populations. As the water becomes more saline as the halite crust dissolves the populations of both fish and birds both collapse. Smith suggests the ecology of the interglacial lake is likely to have been characterised by high levels of environmental stochasticity, resulting from any disruption of the salinity stratification of the water in the lake as this has catastrophic consequences for the invertebrates, fish and waterfowl populations.

The lake entered a long unstable phase after 90 ka, during which it switched state frequently – occasionally drying out, or oscillating among conditions when it was shallow and ephemeral, deep and saline, or interludes of brackish or freshwater. At 65-60 ka the lake entered its final phase, the water returned to a still-stand that created a beach that was rich in tiny Coxiellada shells (a ‘coquina’), which represents the last deep-water perennial lake in the basin (Magee et al, 2004: 886). The lake switched to a playa that was dominated by groundwater (Magee & Miller, 1998) – apart from minor lacustrine events around 50-40 ka and between 12 and 4 ka, each of which created a lake that was saline and had a depth of about 5 m, which is smaller than the filling of 1974.

Sources & Further reading

Smith, Mike, 2013, The Archaeology of Australia’s Deserts, Cambridge University Press