The temperature of a planet is related to the diversity of life it can withstand. MIT geologists have now reconstructed a timeline of Earth’s temperature during the early Paleozoic era, between 510 and 440 million years ago, a crucial period in which animals became abundant in a previously dominated world. by microbes.
In a study published today in the Proceedings of the National Academy of Sciences, researchers represent global temperature drops and peaks during the early Paleozoic. They report that these temperature variations coincide with the diversity of the changing life of the planet: warmer climates favored microbial life, while colder temperatures allowed more diverse animals to flourish.
The new record, more detailed than the previous timelines of this period, is based on the analysis of the carbonate mud equipment, a common type of limestone that is formed from carbonate-rich sediments deposited at the bottom. marine and compacted for hundreds of millions of years.
“Now that we’ve shown that you can use these carbonated muds as climate records, this opens the door to look back on this whole other part of Earth’s history where there are no fossils, when people don’t really know much about what it was climate, ”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, Jr.’s professional 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 analyze fossils, in particular, remnants of ancient shellless organisms that precipitated from seawater and grew or sank to the seabed. When precipitation occurs, the temperature of the surrounding water can change the composition of the shells, altering the relative abundances of two isotopes of oxygen: 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 initial composition of water, [compared to] precipitation of carbonate at 30 degrees Celsius, ”explains Bergmann. “Therefore, the ratio of oxygen-18 to -16 increases as the temperature cools.”
In this way, scientists have used ancient carbonate shells to bring back the temperature of the surrounding seawater, an indicator of the Earth’s global climate, at the time the shells first precipitated. But this approach has only led scientists to the earliest fossils.
“There are about 4 billion years of Earth history where there were no shells, so the shells only give us the last chapter,” Goldberg says.
A grouped isotope signal
The same precipitation reaction in the shells also occurs in the carbonated mud. But geologists assumed that the isotope balance of carbonated sludge would be more vulnerable to chemical changes.
“People have often overlooked the mud. They thought that if you try to use it as a temperature indicator, you will not be looking at the original temperature of the ocean in which it was formed, but the temperature of a process that occurred later, when the mud went be buried a mile below the surface. “, Says Goldberg.
To see if the carbonated sludge could retain signatures from its original surrounding temperature, the team used “clustered isotope geochemistry,” a technique used in Bergmann’s lab that analyzes sediments to agglutinate or pair two heavy isotopes: oxygen. -18 and 13. The probability of these isotopes mating in carbonated sludge depends on the temperature, but is not affected by the ocean chemistry in which the sludge is formed.
The combination of this analysis with traditional measurements of oxygen isotopes provides additional constraints to the conditions experienced by a sample between its original and current formation. The team reasoned that this analysis could be a good indication of whether the carbonated sludge remained unchanged in composition since its formation. By extension, this could mean that the oxygen-18 to -16 ratio in some muds accurately represents the original temperature at which the rocks were formed, allowing their use as a climatic record.
Ascents and descents
The researchers tested their idea on samples of carbonated mud they extracted from two sites, one in Svalbard, an archipelago in the Arctic Ocean and the other in western Newfoundland. Both sites are known for their exposed rocks dating back to the early Paleozoic era.
In 2016 and 2017, the teams traveled first to Svalbard, then to Newfoundland, to collect samples of carbonated sludge from deposited sediment layers that spanned a period of 70 million years, from the middle of the Cambrian, when the animals began to flourish on Earth, through the Ordovician. periods of the Paleozoic era.
When they analyzed the samples to find clustered 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 10 different early Paleozoic sites to calculate the temperatures at which the rocks formed. Temperatures calculated from most of these sites were similar to the fossil temperature records previously published with lower resolution. In the end, 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 effect on the diversity of life on the planet.
“We found that when it was hotter at the end of the Cambrian and early Ordovician, there was also a peak in microbial abundance,” Goldberg says. “From here it cooled through the middle of the late Ordovician, when we see abundant animal fossils, before a substantial glacial era ends with the Ordovician. Previously, people could only observe general trends using fossils. Because we used very abundant material, we could create a higher resolution record and be able to see more defined highs and lows. ”
“This is the best recent isotopic study that addresses the critical question of whether early animals experienced early temperate temperatures,” says Ethan Grossman, a professor of geology at Texas A&M University, who did not contribute to the study. “We should use all the tools at our disposal to explore this important time frame.”
The team is now looking to analyze the oldest mud, which dates back to before the animals appeared, to measure changes in the Earth’s temperature before 540 million years ago.
“To go beyond 540 million years ago, we have to fight carbonated muds, because they are really one of the few records that we have to limit the climate in the distant past,” Bergmann says.
This research was supported, in part, by NASA and the David and Lucile Packard Foundation.