Saturday, August 3, 2019

Why is Australia hot and dry?

The center of Australia today is dry--it wasn't always so

When people first came to Australia, about 55-60,000 years ago, the landscape was very different from today. The first Aboriginal settlers came from Indonesia, during a glacial period when sea levels were much lower than they are now. The distance between Australia and the northern islands could easily be crossed by primitive boats. Within only a few thousand years, people had settled along most of Australia’s vast coastlines as well as the interior that we know today as the Outback, a dry, hot extreme environment with limited plant and animal life. When humans first arrived, the continent was home to many species of giant marsupials, monotremes (mammals that give birth from eggs like the platypus), birds and lizards. These large beasts are called Megafauna.
            What happened to Australia’s megafauna? Scientists have been studying the causes of major animal extinctions in Australia and North America for many decades. Typically, paleontologists (scientists studying past animals and plants) look for fossils, records of pollen from ancient plants, and study the sediments to figure out the living history of an area. In Australia, there are rich beds of fossils that have been recovered in caves, around lakeshores, and in river bank sediments. Because Australia has been hot and dry for so long, the sediments from the Outback don’t have pollen in them anymore, so scientists have little idea what kind of plants used to live here.
Outback landscape recently burned. Grasses sprouting up are spinifex types--generally unpalatable for animals.
            There are a few ideas about what happened to the Megafauna. Some scientists believe that they went extinct as the climate changed naturally. They believe that after a cold glacial period, they never came back. Other scientists think that hunting of the Megafauna slowly depleted their populations until they no longer could survive. The idea that my colleagues and I have been investigating is whether human use of fire was the main reason for wiping out the Megafauna. How can fire destroy almost all of a country’s large animals? What does fire do to the plants and the climate? Can human actions like burning the landscape, as an aid to hunting and food gathering, change the climate of an entire continent?
            Earth’s climate has changed drastically over time. In the past 2 million years, the climate has swung between ice ages and hot houses roughly correlated to subtle differences in how the Earth revolves around the sun. These differences turn out to be cyclical in nature, occurring on periods of 25, 40 or 100 thousand years called Milankovitch cycles. There was very little detailed understanding of the periodicity and magnitude of these cycles until scientists began measuring very small changes in isotope patterns found in carbonates in marine sediment cores. As more and more sediment cores were brought into isotope labs and measured, scientists began to compare and correlate isotope patterns across major ocean basins. Oxygen isotope patterns in a sediment core are “wiggly lines” shifting as sea ice accumulates and global temperatures increase or decrease. Using radiogenic isotopes, like uranium, some of the cores were dated, then scientists matched the wiggly lines allowing them to provide a global pattern of climate change over time (Shacklton and Opdyke, 1973).
         Marine isotope measurements of oxygen record paleo-sea levels and paleotemperatures. The carbon isotope patterns of marine carbonates reveal insights about primary productivity and decomposition, Ice Ages, warm periods, and the activity of the biological pump. Terrestrial records of carbon cycling primarily are studied in laminated lake sediments by analyzing various parameters going down core and associating them with sediment age. In the early 1990s, Ed Hare and his colleague Allison Brooks, an archaeologist at George Washington University, developed techniques for dating fossil ostrich eggshells, which are ubiquitous in many African archaeological deposits (Brooks et al., 1990). As opposed to bone or tooth, eggshell holds onto its protein matrix and is not affected by groundwater leaching even over 100,000s of years. Von Schrinding, van der Merwe and J. C. Vogel (1982) measured carbon isotopes in ostrich eggshell to demonstrate that the eggshells hold a signal of the bird’s diet at the time it laid its egg.

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