Australia’s western and eastern ecosystems are biodiversity hotspots separated by an arid desert interior. But millions of years ago, many species roamed more freely between connected habitats across the continent.
Us new researchpublished in Geophysical Research Letters, provides insight into ancient climate change that shaped our modern landscapes and ecosystems.
We have developed a new way to reconstruct the timing of major droughts on our planet’s continents. This work helps to gain knowledge about arid regions that are particularly affected by current climatic stresses.
Billions of people live in dry areas
Drylands cover nearly half of the Earth’s land area and are home to about 3 billion people. Dramatic, record-breaking droughts are becoming more common Worldwide.
As the driest inhabited continent, Australia (70% is considered arid or semi-arid) also faces many challenges, including droughts and wildfires. Understanding the history of arid regions and their response to past climate change is important to mitigate the impact of our warming planet’s future.
Iluka Resources, author provided
The Nullarbor Plain in South Australia covers an area the size of Great Britain (about 200,000 square kilometers). However, it is very different in almost every other aspect: extremely flat, very little rain and almost no trees. These conditions and its size make the Nullarbor Plain a natural “biogeographical barrier”, separating rich and diverse ecosystems in Western and Eastern Australia.
Today, the dusty Nullarbor Plain bears little resemblance to its vibrant past. Before about 14 million years ago, it was covered by ocean and harbored reefs. More recently, it is said to have been a lavish home to an exotic menagerie, including the world’s largest cuckoos.
Read more: The world’s largest cuckoos once roamed the Nullarbor plain
Drying the Nullarbor
We know that the Earth is constantly evolving, but we often have a poor idea of exactly when environmental changes occurred in the distant past. Fortunately, some minerals that record past climatic events can be dated.
For most people, rust is something they want to avoid as it damages our cars, fences and steel appliances. But rust can be useful for understanding climate change. In our work we used a ferrous mineral called goethite – most of rust – to unlock timing of drying on the Nullarbor.
We found goethite in rocks about 80 feet below the Nullarbor plain. These rocks mark the former water table. Dating the age of the goethite minerals helps us understand how past water tables changed in response to climate change.
Maximilian Dröllner, author provided
We fired a laser beam at small pieces of goethite, about the size of a grain of salt, to release their atomic building blocks. We then measured the helium isotopes – variants of helium – that had accumulated since the mineral formed. This got us a kind of clock”.
We calculated that groundwater fell dramatically on the Nullarbor Plain between 2.4 and 2.7 million years ago – at the same time as a period of global cooling.
As the climate changed, drying altered local ecosystems, effectively creating a wall for many species. As large parts of Australia changed from forest to dry grassland, habitat and food availability shrank for many species.
Significantly, this barrier cut across the once unbroken connection between the species of southwestern and southeastern Australia.
Species split
The evolution of many known species was influenced by this separation. There’s the yellow-tailed black cockatoo from South East Australia with yellow cheeks, and Carnaby’s black cockatoo with white cheeks in the southwest. Genetically, these two are cockatoos its closest relativesbut today live thousands of miles apart.
Modified after kookr/flickr and jean_hort/flickr. CC BY-NC
Through isolation, the drying of the Nullarbor played a key role in creating the species richness of South West Australia. This region is one of only 35 biodiversity hotspots on Earth and home to over 6,000 endemic species, many of which are found nowhere else.
Measuring the time scales of drying landscapes is important for conservation biology. Many native species are already facing or will face existential problems due to climate change and habitat degradation, including the iconic Carnaby’s cockatoo.
A history locked in minerals
By studying minerals formed during groundwater degradation, we improve our understanding of our continent’s past and its biosphere. These minerals form as a direct result of continental desiccation, often in sediment with interesting fossils.
Previously, we often relied on indirect information such as the chemistry of marine sediments to date continental landscape processes.
Maximilian Dröllner, author provided.
More generally, having information about the timing of drying events in the past could also help test theories of human evolution. Changing landscapes and extreme drought were probably important to the development of our own species.
By determining the timing of environmental changes, scientists can see how these events have affected biodiversity and the evolution of species over time. Studying the past is also essential to understanding how the Earth responds to climate change. Understanding how ecosystems dry out can help us develop strategies to mitigate the damage.
