Tiny fossil shells unveil details about ancient Earth's climate

Scientists have discovered tiny fossil shells that unveil details about the Earth's climate over half a billion years ago.

The research, published in the journal Science Advances, suggests that early animals diversified within a climate similar to that in which the dinosaurs lived.

An international collaboration of scientists, led by the University of Leicester in the UK, has investigated by combining climate models and chemical analyses of fossil shells about 1mm long.

This interval in time is known for the 'Cambrian explosion', the time during which representatives of most of the major animal groups first appear in the fossil record.

These include the first animals to produce shells, and it is these shelly fossils that the scientists used.

Scientists have long thought that the early Cambrian Period was probably a greenhouse interval in Earth's climate history, a time when there were no permanent polar ice sheets.

Until now, however, scientists have only had a sense of what the Cambrian climate was like because of the types of rock that were deposited at this time - while it has long been believed that the climate was warm, specific details have largely remained a mystery.

Data from the tiny fossil shells, and data from new climate model runs, show that high latitude ( about 65 degree South) sea temperatures were in excess of 20 C. This seems very hot, but it is similar to more recent, better understood, greenhouse climates like that of the Late Cretaceous Period.

"Because scientists cannot directly measure sea temperatures from half a billion years ago, they have to use proxy data - these are measurable quantities that respond in a predictable way to changing climate variables like temperature," said Thomas Hearing, a PhD student from the University of Leicester in the UK.

"In this study, we used oxygen isotope ratios, which is a commonly used palaeothermometer," said Hearing.

"We then used acid to extract fossils about 1mm long from blocks of limestone from Shropshire, UK, dated to between 515 - 510 million years old," he said.

"Careful examination of these tiny fossils revealed that some of them have exceptionally well-preserved shell chemistry which has not changed since they grew on the Cambrian sea floor," he added.

"Many marine animals incorporate chemical traces of seawater into their shells as they grow. That chemical signature is often lost over geological time, so it's remarkable that we can identify it in such ancient fossils," said Tom Harvey, from University of Leicester.

Analyses of the oxygen isotopes of these fossils suggested very warm temperatures for high latitude seas, probably between 20 to 25 degree Celsius.

To see if these were feasible sea temperatures, the scientists then ran climate model simulations for the early Cambrian.

The climate model simulations also suggest that Earth's climate was in a 'typical' greenhouse state, with temperatures similar to more recent, and better understood, greenhouse intervals in Earth's climate history, like the late Mesozoic and early Cenozoic eras.

Ultimately, these findings help to expand our knowledge of the early animals of the period and the environment in which they lived.

"We hope that this approach can be used by other researchers to build up a clearer picture of ancient climates where conventional climate proxy data are not available," said Hearing.