Shells as climate archives

16.6.17 | Calcite shells of marine organisms such as scallops or clams provide an important archive for the climate reconstructions in previous ages. Climate studies increasingly also use the shell material thrown away near settlements by humans in prehistoric times. Geologist Peter Müller from the Leibniz Centre for Tropical Marine Research (ZMT) has now discovered a considerable potential for error when shells that have been heated for human consumption are used in paleoclimatic studies. His study has just been published in the journal “Scientific Reports”.

What will our climate be like in future? Climate studies from times gone by can help scientists to answer this question. A tried and tested paleoclimatic method is the analysis of oxygen isotopes within the shells of calcifying marine organisms. They can provide insights into the temperature of the oceans or can be employed in reconstructing the rainfall in coastal regions at the time the calcified shells were developed.

For a long time the calcite shells of tiny foraminifers have been used for climate reconstruction. The shells of these aquatic single-cell organisms can be found in almost all sediments. Lately shells such as scallops or clams have also become the focus of climate researchers.

Similar to trees shells grow annual rings when their calcite exterior develops. Their chemical composition mirrors environmental conditions and gives insights into yearly or sometimes even seasonal climate changes. Large amounts of these shells can be found in particular at coastal archaeological sites, which can include deposits of thousands of years.

It is most likely that shells were heated for consumption as far back as the stone ages – even if their exterior does not necessarily show any signs of this cooking process. But are such shells still reliable climate archives?, asks ZMT- geologist Peter Müller.

Together with his colleagues he studied stone-age shell deposits on the coast of Mauretania and compared them to Venus clams from our time, which Müller cooked, grilled or burnt.

Measurements with a mass spectrometer showed that even a small amount of heat changed the composition of isotopes in the shells rendering them unusable for climate reconstructions. By measuring so-called “clumped isotopes”, a relatively new method, the team was able to differentiate precisely between heated and unheated shells.

“The method is so accurate that it even allows us to draw conclusions about the art of cooking in the Stone Age,” says Peter Müller. “We reckon that in order to open them, shells were put on hot stones that had been heated up in the fire.” Previous studies for climate reconstruction that used such prehistoric materials often did not account for that and might have come to the wrong conclusions, says Müller.

In their studies the origin of the Sahara desert was of particular interest for the team. Archaeological deposits could give an insight into the time sequence. Current theories are talking of both abrupt and slow change from lush savannah landscape to desert. Peter Müller’s dataset generated from untreated shells support the theory of a slow climate change in that respect.