The complexity of paleomagnetic pole flipping

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Scott Johnson writes at Ars Technica about the Brunhes-Matuyama boundary: “The last magnetic pole flip saw 22,000 years of weirdness”.

The researchers interpret this additional data as showing a major weakening of the magnetic field starting 795,000 years ago before the pole flipped and strengthened slightly. But around 784,000 years ago, it became unstable again—a weak field with a variable pole favoring the southern end of the planet. That phase lasted until about 773,000 years ago, when it regained strength fairly quickly and moved to the northern geographic pole for good.

The Brunhes-Matuyama paleomagnetic reversal is conventionally recognized as the boundary between the Early and Middle Pleistocene. When we talk about recognizing geological time periods, it is important to realize that our understanding of the boundaries is limited by the precision of our geochronological methods, and the physical processes that give rise to geological changes themselves.

This is an example where a boundary has 22,000 years of wiggle room that we might not have expected. In the span of 780,000 years, that’s not a long time, but if we want to examine whether two events are simultaneous or one caused the other, it’s a long time.