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

A planet’s temperature is related to the diversity of life it can support. MIT geologists have now reconstructed a timeline of Earth’s temperature during the early Paleozoic Era, between 510 and 440 million years ago – a pivotal period when animals became abundant in a previously microbial-dominated world.

In a study published today in the Proceedings of the National Academy of Sciences, the researchers map dips and peaks in global temperature during the early Paleozoic Era. They report that these temperature swings coincide with the changing diversity of life on the planet: Warmer climates favored microbial life, while cooler temperatures allowed more diverse animals to thrive.

The new record, more detailed than earlier timelines of this period, is based on the team’s analysis of carbonate silt – a common type of limestone that forms from carbonate-rich sediments deposited and compacted on the seafloor over hundreds of millions of years.

“Now that we’ve shown that you can use this carbonate mud as climate records, that opens the door to looking back to this whole different 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, a graduate student at MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).

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

Beyond fossils

To estimate the Earth’s temperature many millions of years ago, scientists are analyzing fossils, in particular remains of ancient enveloped organisms that have precipitated from seawater and either grew on the sea floor or sink to the bottom. When precipitation occurs, the temperature of the surrounding water can change the composition of the shells, changing the relative amounts 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 starting water composition,” [compared to] carbonate precipitates at 30 degrees Celsius, “Bergmann explains.” So the ratio of oxygen-18 to -16 increases as the temperature cools. “

In this way, scientists have used ancient carbonate shells to track back the temperature of the surrounding seawater – an indicator of Earth’s overall climate – when the shells first precipitated. But this approach has cost scientists so far, down to the earliest fossils.

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

A clumped isotopic 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 try to use it as a temperature indicator, you might not be looking at the original ocean temperature in which it formed, but at the temperature of a process that took place later, when the mud became buried a mile below the surface, ”says Goldberg.

To see if carbonate mud could retain the signatures of their original ambient temperature, the team used ‘clumped isotope geochemistry’, a technique used in Bergmann’s lab that analyzes sediments to clump or link two heavy isotopes: oxygen-18 and carbon 13. The likelihood of these isotopes pairing in carbonate sludges depends on temperature, but is not affected by the ocean chemistry in which sludge formation occurs.

Combining this analysis with traditional oxygen isotope measurements places additional limitations on the conditions a sample experiences between its original formation and the present. The team reasoned that this analysis could be a good indication of whether carbonate suspensions have remained unchanged in composition since their formation. By extension, this could mean that the oxygen-18 to -16 ratio in some mud accurately reflects the original temperature at which the rocks formed, allowing them to be used as a climate record.

Ups and downs

The researchers tested their idea on carbonate mud samples extracted from two locations, one in Svalbard, an archipelago in the Arctic Ocean and the other in western Newfoundland. Both sites are known for their exposed rocks that date back to the early Paleozoic Era.

In 2016 and 2017, the teams traveled first to Svalbard and then to Newfoundland to collect samples of carbonate mud from layers of deposited sediment over a period of 70 million years, from the mid-Cambrian, when animals on Earth began to flourish, through the Ordovician. . periods of the Paleozoic.

When they analyzed the samples for clumped isotopes, they found that many of the rocks had undergone few chemical changes since their formation. They used this result to put together the oxygen isotope ratios of the rocks from 10 different early Paleozoic locations to calculate the temperatures at which the rocks formed. The temperatures calculated at most of these locations were comparable to previously published lower resolution fossil temperature records. Ultimately, they mapped a timeline of temperature during the early Paleozoic era and compared it with the fossil record of that period to show that temperature had a profound effect on the diversity of life on the planet.

“We found that when it was warmer at the end of the Cambrian and early Ordovician, there was also a spike in microbial abundance,” Goldberg says. From there it cooled down to the mid to late Ordovician, when we see abundant animal fossils, before a substantial ice age ends the Ordovician. Previously, humans could only observe general trends with fossils. Since we were using a material that is very abundant, a create a higher resolution record and see more clearly defined ups and downs. “

The team is now looking to analyze older mud, predating the appearance of animals, to measure Earth’s temperature changes prior to 540 million years ago.

“To go back more than 540 million years ago, we have to grapple with carbonate mud because it is really one of the few records we have for limiting the climate in the distant past,” Bergmann says.