Geologists Discover Effects of Climate Change on Life During Paleozoic Era

By analyzing the minerals within a type of limestone, researchers were able to determine how changes in the global temperature linked to different types of flourishing life.

A finger points to a small trilobite fossil from the Ordovician strata in Svalbard, Norway. (Credit: Adam Jost)

(CN) — Scientists announced a method Monday of determining the Earth’s temperature from the early Paleozoic era, allowing them to track the rise and fall of different species as the planet warmed and cooled.

In a study published in the Proceedings of the National Academy of Sciences, geologists revealed they had discovered a way to track the warming and cooling of Earth’s global temperature during the early Paleozoic era, a time spanning between 440 to 510 million years ago.

The research team confirmed the changing temperatures allowed more microbial life to flourish during warmer weather, while a wider range of animals did better when temperatures were cooler.

Researchers were able to determine the rising and falling temperatures through an analysis of carbonate muds, limestone that is created by carbonate-rich sediments found on the seafloor, formed over the course of hundreds of millions of years. This method of tracking the Earth’s temperature can give scientists new insights into how the climate changed during the planet’s ancient history.

“Now that we have shown you can use these carbonate muds as climate records, that opens the door to looking back at this whole other part of Earth’s history where there are no fossils, when people don’t really know much about what the climate was,” said lead author Sam Goldberg, graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, in a statement.

Geologists normally analyze fossils to determine Earth’s historical temperature. In particular, they look at ancient shelled creatures found on the seafloor. As precipitation happens, the change of temperature in the surrounding water changes the shells and the abundance of oxygen-16 and oxygen-18.

“As an example, if carbonate precipitates at 4 degrees Celsius, more oxygen-18 ends up in the mineral, from the same starting composition of water, [compared to] carbonate precipitating at 30 degrees Celsius,” Kristin Bergmann, co-author and professor at MIT said. “So, the ratio of oxygen-18 to -16 increases as temperature cools.”

This method has helped scientists analyze the planet’s ancient temperature, but only to a point of the earliest fossils. Past that, an accurate determination of global temperatures had not been able to be established.

“There is about 4 billion years of Earth history where there were no shells, and so shells only give us the last chapter,” Goldberg said.

Geologists have been hesitant to use carbonate mud to analyze the Earth’s temperature, even though they react to precipitation the same way the ancient shells do.

“People have often overlooked mud. They thought that if you try to use it as a temperature indicator, you might be looking at not the original ocean temperature in which it formed, but the temperature of a process that occurred later on, when the mud was buried a mile below the surface,” Goldberg said.

The research team sought out carbonate muds in western Newfoundland and Svalbard, an archipelago located in the Arctic Ocean. They wanted to determine if the muds were subject to chemical changes or if they could be used as accurate determiners of ancient temperatures.

Analyzing these carbonate muds, the geologists discovered very little chemical changes had taken place since they were first formed. They then compared the muds’ oxygen isotope ratios in the same way they do with fossils. 

What they discovered were similar results between the carbonate muds and fossils, meaning that the muds could be used to accurately determine ancient temperatures. The researchers then used these to map out a timeline to show how temperature played a part in determining diversity of life on Earth.

“We found that when it was warmer at the end of the Cambrian and early Ordovician, there was also a peak in microbial abundance,” Goldberg said. “From there it cooled off going into the middle to late Ordovician, when we see abundant animal fossils, before a substantial ice age ends the Ordovician. Previously people could only observe general trends using fossils. Because we used a material that’s very abundant, we could create a higher-resolution record and could see more clearly defined ups and downs.”

The research team is back in the field, looking for older carbonate muds to analyze and help expand scientific understanding of how ancient Earth temperatures varied more than 540 million years ago.

“To go back beyond 540 million years ago, we have to grapple with carbonate muds, because they are really one of the few records we have to constrain climate in the distant past,” Bergmann said.

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