Something big happened to the planet a million years ago

A new study suggests that a million years ago, glaciers began to adhere more persistently to their bed, triggering cycles of longer ice ages. Here the ice has been discharged from the Icelandic glacier Breiðamerkurjökull en route to the Atlantic Ocean. Credit: Kevin Krajick/Earth Institute

Why did glacial cycles intensify a million years ago? Researchers find clues on the bed of the Atlantic Ocean.

Something big happened to the planet about a million years ago. There has been a major shift in the response of Earth’s climate system to variations in our orbit around the Sun. The change is called the mid-Pleistocene transition. Prior to MPT, cycles between glacial (colder) and interglacial (warmer) periods occurred every 41,000 years. After the MPT, the ice ages became more intense – intense enough to form ice caps in the northern hemisphere that lasted 100,000 years. This gave Earth the regular Ice Age cycles that have persisted in human time.

Scientists have long wondered what triggered this. One likely reason is a phenomenon called Milankovitch cycles – cyclical changes in the Earth’s orbit and orientation to the Sun that affect the amount of energy absorbed by Earth. Scientists agree that this has been the primary natural driver of alternating warm and cold periods for millions of years. However, research has shown that the Milankovitch cycles had not undergone any major changes a million years ago, so something else was probably at work.

Coinciding with the TPM, a vast system of ocean currents that helps move heat around the globe has experienced severe weakening. This system, which sends heat northward across the Atlantic Ocean, is the Atlantic Meridian Overturning Circulation (AMOC). Was this slowdown related to the shift in ice ages? If yes, how and why? These are open questions. A new article published on November 8, 2021 in the journal Proceedings of the National Academy of Sciences offers an answer.

The researchers analyzed deep-sea sediment cores taken from the South and North Atlantic, where ancient deep waters passed and left behind chemical clues. “What we found was that the North Atlantic just before this crash was acting very differently from the rest of the basin,” said lead author Maayan Yehudai, who did the work as a PhD student. student at Colombia UniversityLamont-Doherty Earth Observatory.

Prior to this ocean circulation accident, the northern hemisphere ice sheets began to stick more effectively to their bedrock. This caused the glaciers to become thicker than before. This in turn led to greater global cooling than before and disrupted the Atlantic heat treadmill. This led to both stronger ice ages and a change in the ice age cycle, says Yehudai.

The research supports a long-debated hypothesis that the gradual removal of slippery continental soils accumulated during previous ice ages allowed the ice sheets to cling more tightly to the older, harder crystalline bedrock below, and become thicker and more stable. The results indicate that this growth and stabilization just prior to the weakening of the AMOC shaped the global climate.

“Our research answers one of the biggest questions about the biggest climate change we’ve had since the ice ages began,” Yehudai said. “It was one of the most important climate transitions and we don’t fully understand it. Our discovery identifies the origin of this change in the northern hemisphere and the ice sheets that evolved there as the origin of this shift to the climate patterns we see today. This is a very important step in understanding what caused it and where it came from. It highlights the importance of the North Atlantic region and ocean circulation for current and future climate change.

Reference: “Evidence for a Northern Hemispheric trigger of the 100,000-y glacial cyclicity” by Maayan Yehudai, Joohee Kim, Leopoldo D. Pena, Maria Jaume-Seguí, Karla P. Knudson, Louise Bolge, Alberto Malinverno, Torsten Bickert and Steven L Goldstein, November 8, 2021, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2020260118

The research was also led by Yehudai’s advisor, Lamont geochemist Steven Goldstein, as well as Lamont graduate student Joohee Kim. Other collaborators included Karla Knudson, Louise Bolge, and Alberto Malinverno de Lamont-Doherty; Leo Pena and Maria Jaume-Segui from the University of Barcelona; and Torsten Bickert from the University of Bremen. Yehudai is now at the Max Planck Institute for Chemistry.

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