A project undertaken at The University of Melbourne, and supervised by Prof Russell Drysdale.
Australia’s iconic arid zone hosts a uniquely adapted flora and fauna, and a geology and geomorphology of remarkable antiquity and heritage value, but our understanding of its climate history is surprisingly meagre. Part of the reason for this is the poor preservation of the field evidence for past climate changes, and the difficulty of dating that evidence. In this project, we used geochemical and geochronological evidence from cave concretions (speleothems, such as stalagmites and stalactites) of the Flinders Ranges to piece together the moisture history of the region over the past 0.5 million years. Caves are ideal environments for such a study because they are protected from the harsh conditions at the surface (weathering and erosion). This enables these speleothem ‘archives of palaeoclimate’ to be preserved for millions of years.
The results from our work show that multimillennial periods of excessive moisture coincided with intervals when summer insolation was more intensive over the southern tropics. These intervals occur when Earth’s perihelion aligns with the austral summer months, which has a period of about 20,000 years. During these times, higher-than-average insolation intensity over the southern tropical landmasses during the austral summer heats the land surface and neighbouring oceans, strengthening Southern Hemisphere summer monsoon systems, including the Indo-Australian Monsoon system that affects northern Australia. The enhanced convection allows tropical weather systems to penetrate further into the continental interior, leading to more rainfall in the Flinders Ranges. The speleothems that grow in the caves of the Flinders Ranges record these rainfall changes.
But it is more complicated than that. Superimposed over these so-called ‘orbitally driven’ variations in regional moisture are finer-scale millennial changes. For example, during the Last Glacial period (115,000 to 12,000 years ago), global climate was characterised by about 25 major oscillations which were associated with changes in the amount of icebergs and meltwater entering the North Atlantic from the great ice sheets that had accumulated over North America and Eurasia. During ice discharge events, ocean circulation in the North Atlantic slowed down, which altered the heat balance between the hemispheres. One consequence of this was enhanced summer monsoon activity in the Southern Hemisphere. During one iceberg discharge event, known as Heinrich Event 5 (about 47,000-48,000 years ago), Europe’s already glacial temperatures plummeted due to disrupted northward heat transport in the North Atlantic, and the Indo-Australian summer monsoon ramped up a few notches to deliver much more moisture than usual to the Flinders Ranges. This was recorded in speleothems that only grow when the regional groundwater level is high. The timing of this ‘pluvial’ event in Australia’s arid zone coincides with the first recorded arrival of indigenous people to the Flinders Ranges. Given that the moisture increase recorded in the cave was from local rainfall, the entire stretch of land between Australia’s northern coast and the Flinders Ranges must have been much wetter than today. This would have provided a wet corridor to facilitate the southward movement of indigenous people across the continental interior.