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ScienceWise - Mar/Apr 2008

How wombats are getting their teeth into the climate change debate

Article Illustration
Professor Rainer GrĂ¼n, Dr Kathryn Fitzsimmons, Ian Moffat and Tegan Kelly with an electron spin resonance machine uses for dating samples.

The climate history of Willandra Lakes

The Willandra Lakes Region of South Australia comprises quarter of a million hectares of semi-arid landscape incorporating dried saline lakebed plains, sand dunes and grassy woodlands. The region has been World Heritage Listed for both its diverse ecology and its cultural significance to indigenous people. This unique and beautiful landscape has undergone continuous human habitation for almost 50,000 years. For scientists like Professor Rainer Grün, this makes the Willandra Lakes a unique and fascinating place.

Professor Grün’s research team, together with the Department of Environment and Heritage and the Traditional Tribal Groups, have recently received an ARC linkage grant to study the history of the region’s climate, using a wide variety of techniques. Professor Grün explains, “The exciting thing about Willandra Lakes is that it has the potential to provide detailed information about climate variation spanning an earlier warm period through the last glaciation and right into the present warming.” By better understanding climatic variations in the past, the team hopes to glean important information about predicting future climate scenarios.

Many disciplines will be brought together within this project. It is hoped that archaeological studies of human habitation carried out in collaboration with the Three Traditional Tribal Group Elders, will yield information about the diet of the indigenous people of the region. The quantities and types of fish and molluscs being consumed at each period in history closely reflects the water levels in the lakes which are in turn linked to climate. Optically stimulated luminescence dating of sediment samples will be used to calculate how long each layer has been buried which, in turn, gives clues about water levels and flows. And of course isotope-dating studies, including radiocarbon dating, will be conducted on organic material.

One particularly interesting facet of these studies will focus on wombat teeth. Wombats are herbivores so various different types of grass form a large part of their diet. Unlike most other herbivores, a wombat’s teeth grow continuously from the roots and are worn down at the grinding surfaces. Consequently, a typical wombat tooth has enamel ranging from zero to just over one year old. As the tooth grows from the root, the isotopic ratio of elements such as carbon and oxygen that are incorporated into the enamel are controlled by the isotopic ratio of the grass in the wombat’s diet.

There are two common photosynthetic pathways found in grasses and other green plants known as C3 and the more evolutionary advanced C4. Species employing the C3 pathway tend to thrive in areas with plentiful groundwater. C4 plants, on the other hand, enjoy a significant advantage in times of drought. In this way, the relative prevalence of C3 vs. C4 vegetation provides an indicator of local rainfall. Because of the slightly different photochemistry between C3 and C4, the isotope ratio of the plant tissue created by each is subtly different. The practical upshot of this is that in times of drought wombats eat more C4 plants because they are more abundant and in wetter times the reverse is true. Because of the differing isotope ratio in the wombat’s food, the isotopic composition of its tooth enamel provides an indirect measure of local rainfall. Different points along the tooth provide data on how the rainfall varies from month to month which can be put into a broader historical timescale by radiocarbon dating. In this way researchers can deduce which points in history had wet springs, dry summers etc. Wombats are a particularly good animal for this type of study because they generally die within their burrows, so their remains are protected from scavengers and the weather.

By pooling the data from wombat teeth and the various other climate indicators of this study, researchers hope to build up a picture of the climate of the Willandra Lakes region spanning back over 100,000 years. Armed with this data, scientists should be able to refine climate models and better understand how to place in context the changes Australia is currently experiencing.

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