Earth’s Ediacaran Period, roughly 630 to 540 million years ago, has always been something of a magnetic minefield for scientists.
During earlier and later time periods, tectonic plates kept a steady pace, climate zones were normal, and the planet’s magnetic field wobbled modestly around the north and south poles (while occasionally reversing).
But the physics of the Ediacaran don’t seem to add up. Over several decades of study, scientists have found the Ediacaran to be enigmatic, with wild fluctuations in the magnetic signatures preserved in rocks from that age — a variability not seen in rocks that are older or younger. Such seemingly chaotic behavior has presented researchers with great challenges, for example, in using ancient magnetism in rocks (“paleomagnetism”) to produce maps of the drifting tectonic plates from that period.
Theories about the cause of the Ediacaran fluctuations have varied. Some scientists suggest it was due to unusually rapid movement of tectonic plates; other scientists say it may have been caused by a fast shifting of the Earth underneath its spin axis (a process called “true polar wander”).
But what if the variations in magnetism during the Ediacaran weren’t random at all? What if they had a global geometry with some order amid the chaos?
That’s the finding of a new study in the journal Science Advances from an international, Yale-led team of researchers.
Evans conducted fieldwork in Morocco.
“We are proposing a new model for the Earth’s magnetic field that finds structure in its variability rather than simply dismissing it as randomly chaotic,” said David Evans, a professor of Earth and planetary sciences in Yale’s Faculty of Arts and Sciences and co-author of the new study. “We have developed a new method of statistical analysis of Ediacaran paleomagnetic data that we think will hold the key to producing robust maps of the continents and oceans from that period.”
For the study, the researchers focused on a region known as the Anti-Atlas, a mountain range in Morocco, where co-authors from Cadi Ayyad University had identified a series of volcanic layers from the Ediacaran that are exceptionally well preserved and exposed.
The team conducted a layer-by-layer study of the rocks’ magnetism, collected in the field as oriented samples that were brought to Yale for measurements by highly sensitive laboratory equipment.
Cylinder cores and larger rock specimens from Morocco.
“Previous studies of rocks from this time period often employed traditional analytical tools that assumed the Earth’s magnetic field behaved similarly in the past as it does now,” said the study’s first author, James Pierce, a Ph.D. student in Yale’s Graduate School of Arts and Sciences.
“We took a fresh approach. We were able to determine precisely how fast the Earth’s magnetic poles were changing by sampling for paleomagnetism at high stratigraphic (layer-by-layer) resolution and determining precise ages for these rocks,” Pierce said.
Co-authors from Dartmouth College, as well as research institutions in Switzerland and Germany, provided additional information on the rock layers and high-precision dates that demonstrated the dramatic magnetic changes occurred over intervals of thousands of years, rather than millions.
The data ruled out theories of rapidly shifting tectonic plates and “true polar wander,” Evans and Pierce said. Those processes would have required much longer time periods to develop.
Pierce, who conducted field work in the Anti-Atlas mountain range, is first author of the new study.
In addition to documenting the speed of magnetic variability, the researchers found that the variability had an ordered — if unusual — structure. With such a structure in mind, the researchers devised a novel statistical approach to determine shifts in the planet’s magnetic poles that appear to have tumbled all the way around the planet, rather than merely wobbling about the spin axis. The new mathematical method provides a framework for reconstructing the Ediacaran world in future studies.
“My entire career has been dedicated to charting the motions of continents, oceans, and tectonic plates over the Earth’s surface, throughout its history,” said Evans, who is also director of the Yale Paleomagnetic Laboratory.
“The Ediacaran Period in particular, has posed a major barrier in that long-term goal, because global paleomagnetic data just didn’t make much sense,” he said. “If our proposed, new statistical methods prove to be robust, we can bridge the gap between older and younger time periods to produce a consistent visualization of plate tectonics spanning billions of years, from the earliest rock record to the present day.”
The research was funded, in part, by grants from the National Science Foundation.
