A scientist from Ben-Gurion University of the Negev has probed a mysteriously quiet period of the earth’s geological history… and discovered that things weren’t so quiet after all.
In a paper published on Sunday in the monthly Nature Geoscience journal, geologist Roi Granot and his fellow researchers Jérôme Dyment and Yves Gallet of the Centre National de la Recherche Scientifique in France described how they trailed magnetometers across the ocean floor to map the magnetic properties of submerged rocks on the seabed. The results shed light on the distant history of the earth’s magnetic field between 121 million and 83 million years ago.
“The oceans are like giant tape recorders. You can tow a magnetometer behind a ship and measure magnetic anomalies,” Granot said. “These anomalies reflect the magnetization of the rocks underneath the ocean and therefore provide a continuous record of the evolution of the geomagnetic field.”
Every few hundred thousand years, the earth’s magnetic field reverses. However, during this period, starting about 120 million years ago, the field was known to not reverse for nearly 40 million years. Granot’s research showed that although for most of the period the magnetic field was stable, there was a brief period of “noise” during which the field reversed several times relatively rapidly.
“If you see the same pattern of magnetic anomalies at different locations around the globe, then you know it’s a geomagnetic field signature. We compared the North Atlantic findings to different magnetic data from widespread oceans — the South Atlantic and the Indian Oceans. We saw that the signal was convincingly similar,” Granot said.
“What it means is that the conditions in the outer core evolved in a way that we didn’t know or expect. We can use it to better model the convection processes in the core and understand why the poles flip over and why they don’t,” he added.
“Traditionally, magnetic anomalies are used to date the oceanic basins. That’s how we date the ocean’s crust. These 38 million years, seemingly without reversals of the magnetic poles, couldn’t be dated. Now we hope we can start dating the crust with these new anomalies. This area represents 25 percent of the Earth’s oceans.”