Geologists are making a new timeline of the Earth’s Paleozoic climate change

CAMBRIDGE – Planet temperature is related to the diversity of life it can support. MIT geologists have now reconstructed a timeline of the Earth’s temperature in the early Paleozoic period, between 510 and 440 million years ago – a crucial time when animals became abundant in a world previously controlled by microbes. .

In a study appearing today in the Proceedings of the National Academy of Sciences, the researchers record dips and peaks in global temperature in the early Paleozoic. They report that these temperature changes correspond to the changing diversity of life on the planet: Warmer climates prefer microbial life, but with colder temperatures allow animals more diverse success.

The new schedule, currently more detailed than previous timelines, is based on the team’s study of carbonate muds – a common type of limestone that comes from sediments rich in carbonate deposited on the seabed. marine and compacted over hundreds of millions of years.

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

Goldberg co-authors Kristin Bergmann, D. Reid Weedon, Jr. Career Development Professor at EAPS, along with Theodore Present of Caltech and Seth Finnegan of the University of California at Berkeley.

Beyond fossils

To estimate the temperature of the Earth many millions of years ago, scientists study fossils, in particular, the remains of ancient sheltered organisms that came from seawater and grew on the seabed or which sank. When precipitation occurs, the temperature of the surrounding water can change the composition of the shells, altering the content of two isotopes of oxygen: oxygen-16, and oxygen-18.

“For example, if carbonate comes down at 4 degrees Celsius, more oxygen-18 ends up in the mine, from the same starting water composition, [compared to] precipitating carbonate at 30 degrees Celsius, “Bergmann explains.” Thus, the ratio of oxygen-18 to -16 increases as the temperature cools. “

In this way, scientists have used ancient carbonate shells to monitor the temperature of the surrounding seawater – a sign of the Earth ‘s overall climate – at the time the shells first formed. But this approach has only taken scientists so far, up to the earliest fossils.

“There are about 4 billion years of Earth’s history where there were no shells, so shells give us only the final chapter,” Goldberg says.

Clumped isotope signal

The same dense reaction in shells also occurs in carbonate pool. But geologists accepted that the isotope balance in carbonate muds would be more vulnerable to chemical changes.

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

To see if carbonate mills could withstand the initial temperature around them, the team used “clumped isotope geochemistry,” a method used in Bergmann’s laboratory, which analyzes sediments for its shaving, or repairing, of two heavy isotopes: oxygen-18 and carbon-13. The appearance of these isotopes in carbonate muds depends on temperature but the chemistry of the ocean in which the muds affect it.

Combining this analysis with traditional oxygen isotope measurements provides additional constraints on the conditions of a sample between its original and present form. The team reasoned that this analysis could be a good indicator of whether carbonate muds have changed unchanged since they were created. By extension, this could mean that the oxygen-18 to -16 ratio in some pipes accurately represents the initial temperature at which the rocks formed, enabling the use as a climate chart.

Ups and downs

The researchers tested their hypothesis on samples of carbonate peaks extracted from two sites, one in Svalbard, an island in the Arctic Ocean, and the other on the west coast of Newfoundland. Both sites are famous for the exposed cliffs dating back to early Paleozoic times.

In 2016 and 2017, teams first traveled to Svalbard, then Newfoundland, to collect samples of carbonate dust from layers of sediment deposited over 70 million years, from the center of the Cambrian, when animals began to emerge. thriving on Earth, through Ordovician times of the Paleozoic era.

When they examined the samples for clumped isotopes, they found that many of the rocks had not undergone much chemical change since their formation. They used this result to compile the isotope oxygen ratios of the rocks from 10 different early Paleozoic sites to create the temperature at which the rocks were. The temperatures measured from most of these sites were similar to previously published lower resolution fossil temperature records. Eventually, they mapped a temperature timeline in the early Paleozoic and compared this to the fossil record from that time, to show that temperature had a major impact on the diversity of life on the planet.

“We found that, when it was warmer in the late Cambrian and early Ordovician, microbial abundance was also peaked,” Goldberg says. “From there it cooled into the middle to the end of the Ordovician, when we see abundant animal fossils, before a substantial ice age reaches the end of the Ordovician. Previously only humans could see general movements with using fossils. Because we used a very abundant material, we made it possible for them to create a record with a higher resolution and it would see a clearer increase and decrease. “

The team is now looking to analyze ancient pipes, dating back to before animals appeared, to measure the Earth’s temperature changes before 540 million years ago.

“To go back more than 540 million years ago, we have to engage with carbonate muds, because they are really one of the few records we need to limit congestion. in the past, “Bergmann says.

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This research was supported, in part, by NASA and the David and Lucile Packard Foundation.