Ancient sea-level changes provide new evidence that during the Jurassic and Cretaceous, Earth underwent episodes of rapid "true polar wander" (TPW). "These findings refute the view of TPW as negligible or persistently slow," say the authors, "and highlight the need to consider TPW as an episodic control on sea level change and likely other global environmental and biological dynamics." Because Earth is not a perfect sphere, its rotation axis is naturally balanced by the distribution of mass on the planet. When tectonic and mantle processes, like continental drift, redistribute mass, this balance can be disturbed, causing the solid Earth to reorient relative to its spin axis – a phenomenon known as true polar wander. During TPW, the crust and mantle effectively shift relative to Earth's spin axis, while the core and climate belts remain tied to that axis. Large or rapid episodes of TPW could therefore produce major changes in climate, the biosphere, and Earth's magnetic field. However, determining whether rapid TPW has occurred in the past is challenging because tectonic motion can mimic TPW's signature. Previous studies have yielded conflicting interpretations, from negligible or slow TPW to brief episodes of rapid, large-scale reorientation.
Here, Mathew Domeier and colleagues introduce a new way to detect rapid TPW by examining ancient sea-level changes. Because TPW would cause predictable, globe-spanning patterns of continental flooding and exposure, Domeier et al. analyzed maps of these changes at 10-million-year intervals and used statistical modeling to identify patterns consistent with rapid TPW over the past 320 million years. The analysis revealed four intervals with significant TPW signals, including the mid-Cretaceous (100–90 million years ago) and Late Jurassic–Early Cretaceous (150–140 million years ago). These findings broadly corroborate earlier paleomagnetic and plate-motion studies that have suggested rapid TPW. What's more, the findings provide little evidence for rapid TPW during most of the Cenozoic and no significant signal supporting the idea that rapid northward movement of the supercontinent Pangea was driven primarily by TPW during the late Carboniferous and Permian, challenging that proposed explanation.
