Nature
PUBLISHED
Ancient flooding of the continental margins reveals that at certain times in Earth’s history the planet has shifted relative to its axis more rapidly than it does at the moment. Although we don’t have the capacity to trace these movements for 90 percent of the Earth’s existence, hints of accelerated movement can be seen over the last 320 million years. In that time, there are signs of four periods where the movement was much greater than background.
The Earth is not the calmly spinning body primary school accounts might suggest. Along with precession and subtle changes to timing, which force us to use leap seconds, there’s a phenomenon known as true polar wander (TPW).
TPW is caused by changes to the distribution of the Earth’s mass, for example through plate tectonics. Just as rotating objects (or people in the case of ice skaters) spin faster when their mass is concentrated close to the axis of spin, turning spheres are most stable when they’re densest around the equator. If weight moves to higher latitudes, the ball will shift around the axis to regain stability.
The rate of TPW varies because sometimes the movements of continents roughly balance each other, while at other times mass gets redistributed in ways that force the planet as a whole to move compared to its axis. The current rate of TPW is about 10 centimeters (4 inches) a year, generally considered insignificant on human timescales.
Was ancient TPW much higher?
Knowing something of how continents have moved in the past indicates the rate of TPW must have varied over geologic timescales, but geologists are divided on how much.
Dr Mathew Domeier and colleagues at the University of Oslo think they have found a way to measure ancient rates of TPW, and their results are unexpected.
“These findings refute the view of TPW as negligible or persistently slow,” the authors write, “and highlight the need to consider TPW as an episodic control on sea level change and likely other global environmental and biological dynamics.”
The authors note that when the poles wander, the centrifugal forces on the Earth’s oceans change, causing sea levels to rise in some parts of the world and fall in others. If TPW is rapid, they argue, the effect should be large enough to show up in evidence of where continental margins were swallowed by the oceans.
When TPW occurs it has its own axis, which runs between two points on the equator on opposite sides of the Earth. For the purposes of the sea level changes, this creates four poles – the familiar north and south poles, and those on the TPW axis. There will be no change in forces on the ocean at any of these. However, the locations furthest from any pole (ie latitudes close to 45° and at right angles to the TPW axis) will experience rises or falls in sea level proportional to the rate of TPW.
When dinosaurs roamed
The authors note four periods where signs of sea level change fit a pattern of TPW being great enough to stand out from the noise of other influences.
For example, during the mid-Cretaceous, 100-90 million years ago, when Spinosaurus and Argentinosaurus roamed Earth, after adjusting for climate effects, sea levels rose around North America and Europe but fell around South America, southern Africa and east Asia. That’s what we would expect if TPW was anticlockwise around a pole at 53° East, which is approximately what geologists have predicted based on what we know about continental movements a little earlier.
Although the dates don’t match perfectly, the authors think there’s enough uncertainty in various datasets to consider this a win for their model.
An approximately reversed pattern is seen even more strongly between 150 and 140 million years ago during the Late Jurassic (when Stegosaurus and Diplodocus reigned supreme), except that parts of Antarctica take the place of southern Africa, consistent with clockwise rotation around a TPW pole at 58° East.
Two other periods, from 200 to 190 million years ago and 30-20 million years ago, are less clear, but still show statistically significant patterns. In the older case, the lack of clarity might be attributed to the geologic record being partially erased. However, the authors admit the rate of TPW for the most recent of their four periods must have been quite modest to produce such a barely significant pattern.
TPW matters for more than slight changes in local sea level: it can change the climate at particular latitudes, as well as mess with Earth’s magnetic field.
Studies of the magnetization of ancient rocks have been used to propose much faster rates of TPW in the past, but measurements from different sites are contradictory. Some, but not all, of the periods of increased TPW scientists have proposed based on paleomagnetic readings match the ones Domeier and co-authors find from sea level changes.
The study is published in Science.
