Study traces Earth’s past from prehistoric rock deposits

Researchers at the Indian Institute of Science (IISc) and the University of Tennessee have analysed ancient dolomite (carbonate) deposits in Vempalle, in Andhra Pradesh’s Cuddapah district, and estimated the temperature and composition of a shallow, inland sea that most likely existed in the Palaeoproterozoic era, about two billion years ago.

IISc said on Monday that the study traced conditions of the period that provided the “right ambience” for the emergence and bloom of photosynthetic algae that were mainly responsible for pumping oxygen into the atmosphere and making way for other lifeforms to evolve and populate the planet.

“The story of our planet is written in the different strata of rocks,” Prosenjit Ghosh, Professor at the Centre for Earth Science (CEaS), IISc, and corresponding author of the study, published in Chemical Geology, said.

Various studies of fossils from the Palaeoproterozoic era have shown that some life might have existed even under harsh conditions marked by climatic extremes and carbon dioxide levels too toxic for living creatures.

The large amounts of carbon dioxide in the atmosphere were absorbed by the sea and trapped as carbonates in dolomites, Yogaraj Banerjee, a former PhD student from CEaS and one of the authors, said in an official statement.

“(Dolomite) is a direct precipitate from seawater. It provides a signal not only of seawater chemistry but also of seawater temperature,” Robert Riding, Research Professor at the Department of Earth and Planetary Sciences, University of Tennessee, and another author of the study, said.

The researchers collected dolomite samples from chert — hard rocks formed by the interaction of microbes with seawater — and deposits underneath them called dolomitic lime-mud. They used a state-of-the-art technique known as clumped isotope thermometry to narrow down the deposits’ temperature and composition.

After a two-year analysis, the team figured out from the dolomitic mud that the temperature of the seawater during its original time period was about 20°C. The finding was in contrast to previous studies on chert samples that estimated that the temperature was higher, around 50°C.

The current study showed that light water — the regular form of water found even today — also existed back then. The lower seawater temperature and the presence of light water strongly support the hypothesis that conditions around two billion years ago were just right for photosynthetic algae to emerge, the statement said.