The slipperiness of ice isn’t usually newsworthy. But around 1 million years ago, ice sheets in the Northern Hemisphere became less slippery—and that may have been a driving factor behind a major shift in the way Earth’s climate cycles are structured, according to a study recently published in the Proceedings of the National Academy of Sciences of the United States of America.
Up until 1 million years ago, Earth’s glacial-interglacial cycles lasted for approximately 41,000 years. Since then, they have lasted for about 100,000 years, and the glacial periods are characterized by thicker ice and more intense cold. Exactly what caused this Mid-Pleistocene Transition is the subject of much debate among geologists.
Scientists typically expect such a transition to be triggered by variability in Earth’s orbit around the Sun—whether the shape of the orbit, the angle of the axis, or the direction of the axis of rotation—and thus the amount of energy our planet receives from the star. During the Mid-Pleistocene Transition, however, none of those variables changed significantly.
“Something had to change within the Earth system,” said Steven Goldstein, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory and an author on the new paper.
While pursuing her Ph.D. at Lamont-Doherty, Maayan Yehudai, Goldstein’s advisee, turned to the Atlantic Meridional Overturning Circulation, known as AMOC, for clues.
The AMOC, often described as a “conveyer belt,” is a system of ocean currents driven by differences in water temperature and salinity. As warm water from the tropics flows northward, it cools and sinks as evaporation increases its salinity and density. This mass of cold, deep water then travels south and eventually returns to the surface through a process called upwelling, where the cycle starts over again.
Yehudai, now a postdoctoral fellow at the Max Planck Institute for Chemistry in Mainz, Germany, described the AMOC as the planet’s heat-driving engine. “It drives heat from low latitudes to high latitudes and vice versa,” she said. “[The AMOC] has a huge impact on Earth’s climate.”
Getting Down to the Bedrock
To reconstruct how the AMOC was behaving during the Mid-Pleistocene Transition, Yehudai and her coauthors looked at ratios of neodymium isotopes—stored in shelled, single-celled foraminifera and fish debris—in ocean sediment cores taken from five locations running from north to south in the Atlantic Ocean. The researchers hoped that this approach would allow them to pinpoint whether the triggers leading to the Mid-Pleistocene Transition originated in the Northern Hemisphere or the Southern Hemisphere.
Goldstein and his colleague Leopoldo Pena previously found, in a 2014 study, that the AMOC began to run amok during the Mid-Pleistocene Transition. “All of a sudden there was this major crash in the ocean circulation,” Goldstein said. Their data, however, were limited to just two sites near Cape Town, South Africa.
Yehudai’s expanded data set finds strongly negative neodymium isotope ratios in North Atlantic sample cores just before the first 100,000-year glacial cycle and AMOC crash. “These values are very unusual,” Yehudai said. “What contributes negative neodymium to the ocean is weathering.”
The authors say the new data support what geologists call the regolith hypothesis—the idea that prior to the Mid-Pleistocene Transition, ice sheets in the Northern Hemisphere remained relatively thin because of their precarious position atop a thick layer of loose rock and soil (regolith). Over time, repeated glacial cycles gradually eroded that malleable soil layer and wore it down to the bare bedrock.
“There was clearly a major erosional event that occurred by the ice sheets in the Northern Hemisphere continents just prior to the cyclicity shift,” Goldstein said.
Once ice sheets began forming directly atop the bedrock, the increased friction made the ice sheets “stickier” and able to become thicker and more resistant to melting. The influx of fresh water from thicker ice sheets could, in turn, have weakened the AMOC by diluting the salty water that previously formed the dense, deepwater mass.
“The data presented in Yehudai et al. is an exceptionally valuable contribution towards understanding the Mid-Pleistocene Transition,” said Aidan Starr, a paleoceanography researcher at Cardiff University in Wales who was not involved in the study. There may be other explanations for the findings, however. For example, Starr said, the ice sheet dynamics the paper describes “may represent a response, rather than a cause, of the Mid-Pleistocene Transition.”
Goldstein, too, said many questions still remain. “It’s kind of a skeletal view of what happened in the past because every analysis that we do is labor intensive,” he said. Looking forward, his team hopes to continue filling in the blanks in the Mid-Pleistocene Transition timeline. It is, after all, a slippery subject.
—Clara Chaisson, (@clarachaisson), Science Writer