An international team of scientists has suggested that changes in Earth’s orbit that led to warming may have played a role in triggering the rapid global warming that occurred 56 million years ago. This event, known as the Paleocene-Eocene Thermal Maximum (PETM), is an analogue of modern-day climate change.
said Lee Kump, professor of earth sciences at Penn State University. “There has been a lot of interest in finding a better resolution of this history, and our work addresses important questions about the cause of the event and the rate of carbon emissions.”
The team of scientists studied core samples from a well-preserved record of PETM near the coast of Maryland using tulipology, a method of dating sedimentary layers based on orbital patterns that occur over long periods of time, known as Milankovitch cycles.
They found that the shape of Earth’s orbit, or eccentricity, and wobble in its rotation, or magnitude, favored warmer conditions at the beginning of the Betem period and that together, these orbital configurations may have played a role in triggering the event.
“An orbital trigger may have triggered the release of carbon that caused many degrees of global warming during the PETM period rather than the currently more popular explanation that supervolcanoes released carbon and triggered the event,” said Coombe, John Lyon, Dean of the School of Earth and Mineral Sciences .
The results published in the journal
“This study allows us to refine our carbon cycle models to better understand how the planet reacts to an injection of carbon over these timescales and to narrow down the possibilities for the source of the carbon that drove the PETM,” said Mingsong Li, assistant professor in the School of Earth and Space Sciences at Peking University and a former assistant research professor of geosciences at Penn State who is lead author on the study.
A 6,000-year onset, coupled with estimates that 10,000 gigatons of carbon were injected into the atmosphere as the greenhouse gases carbon dioxide or methane, indicates that about one and a half gigatons of carbon were released per year.
“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.
The study was funded by the National Key R&D Program of China and the Heising-Simons Foundation.
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