Lessons from dead giants

Studying ancient birds from central Australia reveals much about the origin of the continent’s deserts.

The United Nations has declared 2006 the International Year of Deserts and Desertification. This is to raise awareness about the encroaching aridity that affects one billion people worldwide by undermining agriculture and exacerbating poverty. The UN is marshalling experts to combat desertification, which it defines as environmental degradation as a consequence of human activity and climate change. To address such a massive problem, policy makers must call on everyone from climatologists to agronomists. But will anyone think to consult an extinct bird from one of the world’s driest continents?


Dr John Magee and Dr Gifford Miller have been collaborating to uncover the origins of Australia's central deserts.

More than 30 years before the General Assembly made combating desertification a key aim, geologist Dr John Magee began to develop a history of Australia’s past climate change. His principal focus has been the Lake Eyre region in South Australia. Today, the lake is a giant salt pan, untouched by major flooding since the early 1970s. But 100,000 years ago, Lake Eyre was a massive permanent body, replenished annually by monsoonal rainfall in the north of the continent. Magee’s mission is to find out more about why the lake dried up, and, in the process, uncover the origin of Australia’s central deserts.

“We look at the deposits left by the lake - beach deposits, lake sediments, and inflowing river sediments - and we can tell from those when the system has become more active and when the lake was full,” Magee says. “With these deposits, we can determine how big the water body was just by measuring the old shorelines. We measure their altitude and we try to find ways to date them so we know what level the lake was at a particular time.”

As Magee and his colleagues began to build a picture of Lake Eyre in time, they realised that its metamorphosis from a sizeable water body into a salt lake occurred with dramatic swiftness.

“In previous interglacial periods (which are the warm, wetter climate periods in Australia in the glacial-interglacial cycles) Lake Eyre has always filled to a relatively large permanent water body. It was probably twice the size of the present salt lake and 25 metres deep 130,000 years ago. But in the present interglacial, which began about 12,000 years ago, there was some reactivation of the lake, but only at a very low level. We see this as quite anomalous.”

In order to uncover more about this anomaly, the researchers needed a way to date geological activity with greater accuracy. In addition to radiocarbon dating and other methods, it was decided that organic materials might be useful for determining the age of sediment. The Lake Eyre region had long been home to two giant birds: emus and Genyornis newtoni. Genyornis, now extinct, was taller and heavier than an emu, and is more closely related to a magpie goose. Bits of eggshell from both species were to be found buried all around the lake, but how to determine the age of the fragments?

Magee turned to geologist Dr Gifford Miller, who is currently a Visiting Fellow at the ANU Research School of Earth Sciences and has had a long association with ANU researchers from his regular position at the University of Colorado. Miller employed a technique called amino acid racemisation, which looks at a process of chemical change within organic material to determine age. After extensive testing, the researchers discovered that the most recent eggshell fragments from Genyornis were 50,000 years old. They were understandably excited, having just dated the extinction of an entire species – and the timeline fit neatly in the period when Lake Eyre began to dry up.

“It wasn’t because the deposits of Genyornis eggs weren’t there, because we can find emus continually up to the present day,” Miller explains. “Previously, we always found the emu shells in among the Genyornis, so that eliminated all the other explanations, and it had to be that the bird suddenly disappeared.”


	*Fragments of Genyornis* eggs.**

To test the hypothesis that climate change had caused the extinction of Genyornis, the team searched for younger eggshell fragments at sites further away from the continent’s arid core. They reasoned that a climate shift would lead to gradual extinction, with those animals in the dry centre dying out much earlier than those in more temperate zones. The search failed to turn up any evidence that Genyornis had survived past the 50,000 year mark anywhere. Miller says that this discovery narrowed the options for an extinction agent.

“There were two possible explanations – it was climate change, or it was human activity. The gradual extinction didn’t occur, and nor does it accord with the last glacial period 30-15 thousand years ago. For human activity to be a candidate, humans had to have been there prior to the extinction. There now seems to be wide evidence that that was the case. Then the really open question is, if it was humans, what are the mechanisms? Three had been proposed: over-hunting, human disease, and modification of landscape.”

To find out which of these extinction causes were most likely, the researchers turned back to the eggshells. By studying carbon isotopes in the fragments, they were able to detect a change in emu diets from a range of leafy shrubs, small trees and nutritious grasses to exclusively scrub plants but very few grasses. This suggests that the ecological landscape was altered dramatically around 50,000 years ago. Magee and Miller believe that the most plausible hypothesis is that the use of fire by humans changed the face of the continent, leading to the extinction of less-adaptable herbivores like Genyornis.

“We know that when humans came to the continent, they would have had controlled use of fire,” Magee says. “There have been quite a lot of people who mistakenly assume that because Aboriginal people used fire in the landscape (‘firestick farming’) at the time of European arrival, the people who first came onto the continent must have used fire in the same way, and that this practice couldn’t have caused an extinction. That’s a completely fallacious sort of argument, because that modern Aboriginal fire use is what has evolved through time. The people that arrived would have brought with them a quite different use of fire, in what may have been a very different environment. I don’t think the modern way people were using fire is a good analogy, and it’s probably dangerous to think of it as such. We don’t really know.”

Magee and Miller are confident that it was human modification of the landscape by fire that led to the extinction of Genyornis and the giant marsupials. Such a transformation of the landscape might also explain what it was that Magee set out to explore in the first place – why Australia’s centre became more arid between the previous and present interglacials.

Dr Gifford Miller

Miller says that the cessation of the monsoonal rains that fed the tributaries heading into central Australia could have been the result of vegetation change too.

“If moisture from the Indian Ocean moving into the interior of Australia encountered trees and shrubs instead of desert scrub, the soils and the plants would do a better job of holding the water. The moisture would then evaporate back into the air and continue moving into the interior. This process is well known from the Amazon basin where roughly half of the rain fall is recycled. Surface roughness is also an issue. If there is only spinifex present it’s a smooth surface, so the wind moves across it more or less in a steady flow. If you can have more variable landscapes like you get in Savannah systems, however, then you start to get turbulence, which is required for convection.”

If the vegetation in central Australia changed from trees and shrubs to desert scrub, Miller reasons, the ability of the soil to hold rainwater, and the amount of vegetation enhanced surface turbulence, would have decreased markedly. The researchers believe this is why Lake Eyre has remained as a giant salt pan during the present interglacial, instead of filling with water as it has in past interglacials. Earlier this year they travelled to Madagascar on a project funded by the National Geographic Society to study the relationship between human impact and the extinction of another giant – the Elephant Bird. This species disappeared around 1,000 years ago, meaning that eggshells should be more abundant and easier to date accurately than the long-disappeared Genyornis. By drawing links between ancient birds, human activity, and the modification of landscapes, Magee and Miller are contributing much to the global understanding of desertification.

ANU Reporter
Winter 2006