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Ancient sea levels record four fast episodes of true polar wander in the past 190 million years
Mathew Domeier's University of Oslo team finds four fast bursts of true polar wander recorded in ancient sea levels over the past 190 million years. During those bursts, he says, the poles moved faster than most tectonic plates do, fast enough to possibly affect climate.
The Scientist · Science desk
What happened
- Earlier evidence for polar wander came mostly from magnetism locked in ancient rocks, and it gave conflicting answers on how often and how far the poles moved.
- The team looked for a beach-ball pattern in which sea level rises in half of each hemisphere and falls in the other, reversed between north and south, a signature Domeier says no other Earth process makes.
- Each burst lasted roughly 10 million years, at about 20, 90, 140 and 190 million years ago.
- True polar wander is still going on, and Domeier says satellites measure it at about 10 centimetres a year.
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Why it matters
- capability Coastline records date polar-wander episodes using a different physical system from magnetised rock, so episodes proposed from the conflicting magnetic data can now be confirmed or contested.
- constraint Only motion faster than about 0.6 degrees per million years shows up, so four is a count of fast episodes, and slower wander remains outside what this record can detect.
- decision Anyone reading latitude-sensitive climate records from near 20, 90, 140 or 190 million years ago now has a second explanation for a latitude change to weigh: the whole shell slipping faster than most plates drift.
In true polar wander, the moving part is Earth's outer shell of crust and mantle, 2800 kilometres thick, which shifts relative to the spin axis, according to Sabin Zahirovic at the University of Sydney [14]. Plate tectonics and mantle convection slowly move mass around inside the mantle and change how the planet spins [2]. The solid outer layers then shift until Earth settles into a new stable spin, and that moves the geographic poles to new positions on the surface [3]. "The crust and the mantle effectively slip over the core during a true polar-wander event," Domeier said [4].
Sea level can record this because water and rock respond at different speeds. Ocean water adjusts to a change in centrifugal potential faster than the rocky sea floor does [6]. For a while during an event, the sea surface takes its new shape over a floor that has not yet caught up. Domeier's team searched the geological record of sea-level change for the trace of that offset [1].
The effect size is a threshold, not a measurement. "We cannot estimate the exact speed, but we can recognise events where the rate of polar motion exceeded about 0.6 degrees per million years," Domeier said [10]. If that rate held across a full 10-million-year burst [9], it adds up to about 6 degrees of polar motion [1]. Because 0.6 is a floor on the rate, 6 degrees gives only a rough scale for the episodes. The gaps between the four dates are 70, 50 and 50 million years [2], too few intervals to call a cycle.
New Scientist's report says the poles wander more than we had thought [13]. What "we had thought" rests on is the magnetic rock record, which had given conflicting results on how often and how far [5]. The report does not give the journal, the number of sea-level records, a prior estimate, or how far each event moved the pole, which is the figure a plate reconstruction would need.
In my view the method matters more than the four dates it has produced so far. The stakes the researchers name are climate and sea level. "This is a very exciting development that helps us link very deep Earth processes to changes in long-term climate and sea level on the planet," Zahirovic said [15].
What to watch
- Whether the four sea-level dates line up with polar-wander episodes previously proposed from paleomagnetic data, or contradict them.
- Per-event magnitudes in degrees from the full paper, which would show how far latitude-based plate and climate reconstructions need to move.
- Whether the sea-level fingerprint can be read in records older than 190 million years.