The temperature of a planet is linked to the variety of lives it can support. MIT geologists have now reconstructed a timeline of the Earth’s temperature during the early Paleozoic era, between 510 and 440 million years ago – an important period in which animals became abundant in a world formerly dominated by microbes.
In a study published today in the Proceedings of the National Academy of Sciences, the researchers show declines and peaks in world temperature during the early Paleozoic. They report that these temperature variations coincide with the changing life diversity of the planet: Warmer climates favored microbial life, while colder temperatures could allow more diverse animals to thrive.
The new record, more detailed than previous timelines of this period, is based on the team’s analysis of carbonate mud – a common limestone formed from carbonate-rich sediments deposited on the seabed and compacted over hundreds of millions of years.
‘Now that we’re showing that you can use this carbonate mud as climate records, it opens the door to look back at this whole other part of the Earth’s history where there are no fossils, when people do not know much about what the climate was like. . , ”Says lead author Sam Goldberg, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).
Goldberg’s co – authors are Kristin Bergmann, D. Reid Weedon, Professor of Career Development at EAPS, with Theodore Present of Caltech and Seth Finnegan of the University of California at Berkeley.
Outside fossils
To estimate the Earth’s temperature millions of years ago, scientists are analyzing fossils, especially remnants of ancient shelled organisms that have precipitated from seawater and either grown on the seabed. When precipitation occurs, the temperature of the surrounding water can change the composition of the shells and change the relative abundance of two isotopes of oxygen: oxygen-16 and oxygen-18.
“For example, if carbonate precipitates at 4 degrees Celsius, more oxygen-18 ends up in the mineral, from the same initial composition of water, [compared to] Bergaat precipitation at 30 degrees Celsius, ”Bergmann explains. “The oxygen-18 to -16 ratio therefore increases as the temperature cools.”
In this way, scientists used ancient carbonate shells to hold back the temperature of the surrounding seawater – an indication of the Earth’s overall climate – the moment the shells precipitated. But this approach has taken scientists so far to the earliest fossils.
“There are about 4 billion years of Earth’s history where there were no shells, and so the shells only give us the last chapter,” Goldberg says.
A bunch of isotope signal
The same precipitation reaction in shells also occurs in carbonate mud. But geologists assumed that the isotope balance in carbonate mud would be more vulnerable to chemical changes.
‘People have often overlooked mud. They thought that if you tried to use it as a temperature indicator, you might not be looking at the original ocean temperature at which it was formed, but at the temperature of a process that took place later when the mud was a kilometer below the surface is buried. , ”Says Goldberg.
To see if carbonate mud could retain signatures of their original ambient temperature, the team used ‘clumped isotope geochemistry’, a technique used in Bergmann’s laboratory to analyze sediments for the clamping or coupling of two heavy isotopes: oxygen- 18 and carbon- 13. The probability that these isotopes hang together in carbonate mud depends on the temperature, but is not affected by the ocean chemistry in which the mud forms.
The combination of this analysis with traditional measurements of the oxygen isotope offers additional limitations on the conditions that the sample experiences between the original formation and the current one. The team reasoned that this analysis could be a good indication of whether carbonate mud has remained unchanged since their formation. Upon expansion, this could mean that the oxygen-18 to -16 ratio in some mud accurately represents the original temperature at which the rocks are formed, making this possible as a climate record.
Love and loss
The researchers tested their idea on samples of carbonate mud they extracted from two sites, one in Svalbard, an archipelago in the North Pole Sea, and the other in western Newfoundland. Both sites are known for their exposed rocks dating from the early Paleozoic era.
In 2016 and 2017, teams first traveled to Svalbard, then Newfoundland, to collect samples of carbonate mud from low-deposited sediment that stretched over a period of 70 million years, from the central Cambrian, when animals on earth began by the Ordovicus flourished. periods of the Paleozoic era.
When analyzing the samples for clamped isotopes, they found that many of the rocks had undergone few chemical changes since their formation. They used this result to compile the oxygen isotope ratios of the rocks from ten different early Paleozoic sites to calculate the temperatures at which the rocks formed. The temperatures calculated at most of these sites were similar to records previously published with lower resolution fossil temperatures. Finally, they mapped a timeline of temperature during the early Paleozoic and compared it to the fossil record of that period, to show that temperature had a major impact on the diversity of life on the planet.
“We found that when it was warmer at the end of the Cambrian and early Ordovicus, there was also a peak in microbial abundance,” Goldberg says. ‘From there it cooled to the middle to the late Ordovicus, when we see abundant animal fossils, before a considerable ice age ended the Ordovicus. Previously, humans could only observe general trends using fossils. Because we use material that is very plentiful, we can set a record with a higher resolution and see more clearly defined ups and downs. “
“This is the best recent isotopic study that addresses the critical question of whether early animals experienced warm early temperatures,” said Ethan Grossman, a professor of geology at Texas A&M University who was not a contributor to the study. . “We need to use all the tools at our disposal to investigate this important time interval.”
The team now wants to analyze older mud, before the appearance of animals, to determine the temperature changes of the earth before 540 million years ago.
“To go further than 540 million years ago, we have to wrestle with carbonate mud, because it’s actually one of the few records we have in the distant past to limit the climate,” says Bergmann.
This research was supported in part by NASA and the David and Lucile Packard Foundation.