quote:Earth's odd rotation may solve an ancient climate mystery
A geologic change might have plunged lush landscapes into arid zones, killing off an array of creatures—and it might happen again one day.
By Maya Wei-Haas
PUBLISHED November 14, 2019
At first, it seems like a case of extinction by climate change: More than 160 million years ago, during the Jurassic period, a fanciful menagerie crept, swam, and flew through the cool, damp forests of what is now northeastern China. Then, almost in a geologic instant, the air grew warmer and the land dried out. As the water disappeared, so too did the life. And yet, researchers have struggled to pin down a climate-related culprit behind this ecological collapse.
Now, a study published in the journal Geology suggests that it wasn’t the climate that changed, but the geographic location of the landscape. Paleomagnetic signatures in the area’s rocks indicate that sometime between 174 and 157 million years ago, the whole region shifted southward by a startling 25 degrees, plunging once lush landscapes into zones of desiccating heat.
The ancient rocky lurch was part of a phenomenon known as true polar wander, in which the topmost layers of the planet, likely all the way down to the liquid outer core, rotate significantly even as Earth continues its daily turn around its usual spin axis.
In the Jurassic, the surface and mantle made this twist around an imaginary line through the crook in Africa’s west coast known as the Bight of Benin. The change would have been massive: If a similar shift were to happen today, a flag planted in Dallas, Texas, would end up where Northern Manitoba, Canada, currently sits. On the other side of the world, the continent of Asia would soar southward.
JURASSIC CLIMATE SHIFT
More than 160 million years ago, northeastern China was home to a wide array of plants and animals known as the Yanliao Biota. A dramatic change in climate from temperate to arid conditions likely led to their demise. But what drove this shift has long been a mystery.
A new study of paleomagnetic data from northeastern China suggests that between 174 and 157 million years ago, the entire surface of Earth rotated a staggering 25 degrees, which would have moved the landscape inhabited by the Yanliao Biota from a cool, humid zone into a hot, dry band.
This colossal change can be explained by true polar wander, which occurs when a mass imbalance causes the entire surface of Earth and its mantle to rotate around the core. While true polar wander has likely occurred throughout the planet’s history, this ancient event was particularly drastic.
Earth has likely experienced smaller amounts of true polar wander throughout its past, and some scientists think it continues today.
“We’re experiencing true polar wander as we speak,” says Dennis Kent, a paleomagnetist at both Rutgers and Columbia University who wasn’t part of the new study team.
To be clear, these more recent forays are not the source of modern-day climate change, which is driven by humans’ relentless release of greenhouse gasses into the atmosphere. In addition, the magnitude of this Jurassic shift—and whether true polar wander is even a real phenomenon—remain under debate.
Studying Earth’s past and present geologic wanderings may not only help resolve the controversy, but also improve our understanding of the planet’s complex machinations.
“It’s so important that there’s still fundamental science being done,” says Lydian Boschman, a geologist at Eidgenössische Technische Hochschule (ETH) Zürich who was not a study team member. “If we don’t understand the foundations, then there’s nothing we can do on top of that.”