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The surprising connection between Earth’s orbital patterns and an ancient warming event

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An international team of scientists has found that changes in Earth’s orbit that favored hotter conditions may have helped trigger a rapid global warming event known as the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago.

An international team of scientists has suggested that changes in Earth’s orbit resulting in hotter conditions may have played a role in triggering a rapid global warming event that occurred 56 million years ago. Known as the Paleocene-Eocene Thermal Maximum (PETM), this event is thought to be an analogue of today’s climate change.

“The Paleocene-Eocene thermal maximum is what we have in the geological record, the closest we have to what we are experiencing now and may experience in the future with climate change,” said Lee Kump, professor of geosciences Penn State University. “There has been a lot of interest in better resolving this story, and our work addresses important questions about what triggered the event and the rate of carbon emissions.”

The team of scientists examined core samples from a well-preserved record of the PETM near the Maryland coast using astrochronology, a method of dating sediment layers based on orbital patterns occurring over long periods of time, known as Milankovitch cycles.

Penn State is working on a core sample

Victoria Fortiz (right), then a graduate student at Penn State University, and Jean Self-Trail, a research geologist with the US Geological Survey, work on a core sample from Maryland’s Howard’s Tract. Photo credit: Penn State

They found that the shape of the Earth’s orbit, or eccentricity, and the wobble in its rotation or precession favored hotter conditions early in the PETM, and that these orbital configurations together may have played a role in triggering the event.

“An orbital trigger may have led to the carbon release that caused multiple degrees of global warming during the PETM, contrary to a currently more popular interpretation that massive volcanism released the carbon and triggered the event,” said Kump, the Dean at John Leone College of Earth and Mineral Sciences.

The results published in the journal

“Those rates are close to an order of magnitude slower than the rate of carbon emissions today, so that is cause for some concern,” Kump said. “We are now emitting carbon at a rate that’s 5 to 10 times higher than our estimates of emissions during this geological event that left an indelible imprint on the planet 56 million years ago.”

The scientists conducted a time series analysis of calcium content and magnetic susceptibility found in the cores, which are proxies for changes in orbital cycles, and used that information to estimate the pacing of the PETM.

Earth’s orbit varies in predictable, calculable ways due to gravitational interactions with the sun and other planets in the solar system. These changes impact how much sunlight reaches Earth and its geographic distribution and therefore influence the climate.

“The reason there’s an expression in the geologic record of these orbital changes is because they affect climate,” Kump said. “And that affects how productive marine and terrestrial organisms are, how much rainfall there is, how much erosion there is on the continents, and therefore how much sediment is carried into the ocean environment.”

Erosion from the paleo Potomac and Susquehanna rivers, which at the onset of the PETM may have rivaled the discharge of the Amazon River, carried sediments to the ocean where they were deposited on the continental shelf. This formation, called the Marlboro Clay, is now inland and offers one of the best-preserved examples of the PETM.

“We can develop histories by coring down through the layers of sediment and extracting specific cycles that are creating this story, just like you could extract each note from a song,” Kump said. “Of course, some of the records are distorted and there are gaps — but we can use the same types of statistical methods that are used in apps that can determine what song you are trying to sing. You can sing a song and if you forget half the words and skip a chorus, it will still be able to determine the song, and we can use that same approach to reconstruct these records.”

Reference: “Astrochronology of the Paleocene-Eocene Thermal Maximum on the Atlantic Coastal Plain” by Mingsong Li, Timothy J. Bralower, Lee R. Kump, Jean M. Self-Trail, James C. Zachos, William D. Rush and Marci M. Robinson, 24 September 2022, Nature Communications.
DOI: 10.1038/s41467-022-33390-x

The study was funded by the National Key R&D Program of China and the Heising-Simons Foundation.

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