Fingerprinting the ocean

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Molluscs from Gillman Mound site, northern Adelaide.

Science has long used tree rings as evidence not only of the tree’s age, but also of the climatic events that took place during its lifetime. 

Scientists at the South Australian Museum’s Minerals Section are using shell middens to determine the average temperature of the local waters in Gulf St Vincent over the last 10,000 years. 

Professor Allan Pring, Senior Researcher in Minerals, is overseeing the research. 

“A mollusc shell takes a fingerprint of the sea water’s chemistry while it’s growing, over the space of 3-4 years. This ‘chemical fingerprint’ changes depending on ocean temperatures,” he says. 

Rather than a continuous change like you would see in a tree ring, this technique is more like a snapshot, due to the relatively short life span of the molluscs.

First the researchers must establish that the sample is from a (chemically) closed system, so they can be certain that no change to shell chemistry has occurred while shell was buried in the midden. An open system, for example, may have rainfall washing through the midden. 

“Shells can be carbon dated, which means we can extrapolate this information from up to 10,000 years ago,” says Allan.

With data from multiple midden sets spanning the years, scientists can determine the change in average ocean temperatures, and see changes in climate over the last 10,000 years.