Oyster shells are made up of calcium carbonate, which forms layers as the oyster grows. Other elements, such as Mg, can also be incorporated into the shell in trace amounts. In many other biogenic carbonates (calcium carbonate that is produced by an animal to make their shell or skeleton) Mg incorporation is dependent upon temperature, so analysis of the Mg/Ca in these carbonates provides a temperature record for when that animal grew.
We wanted to check if this Mg/Ca-temperature relationship holds true for oysters. We grew Sydney Rock oysters (Saccostrea glomerata) on the south coast of New South Wales and southeast Queensland for one year, measuring water temperature continuously. We then measured the Mg/Ca of the oyster shells and found that the Mg/Ca of the incremental oyster shell layers does indeed track the temperature of the water over time.
We also analysed shells that were around 1600 years old from an Aboriginal midden site in southeast Queensland. The Mg/Ca from these shells indicates average annual temperatures when these shells grew were around 3ºC cooler than modern temperatures.