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. |
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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