Limestones from southern China record 29 polarity zones in 3.75 million years, with reversals reaching about 11 per million years before the end-Guadalupian extinction, roughly ten times the long-term average.
Earth’s magnetic field is not fixed. Over geological time, its north and south poles swap places, sometimes several times in a million years and sometimes not at all for tens of millions of years. Each reversal leaves a signature in rocks that record the direction of the field as they form.
A new study of ancient limestones in southern China suggests that the field became unusually restless around the time of a major mass extinction about 261 million years ago. Published in National Science Review, the research was led by scientists at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, with collaborators from Utrecht University, BOKU University, Nanjing University and other institutions.
The end of a long quiet
For a long stretch of the late Carboniferous and much of the Permian period, the magnetic field held a single, reversed polarity. Geologists call this interval the Kiaman Superchron. It ended near 265 million years ago, and the field then began to reverse again.
Superchrons are rare. The best known is the Cretaceous Normal Superchron, roughly 121 to 83 million years ago, when the field stayed in normal polarity for nearly 40 million years. Why the field sometimes falls quiet for so long and then becomes active again is one of the open questions about the geodynamo, the churning of molten iron in Earth’s outer core that generates the magnetic field.
Twenty-nine flips in a limestone section
The researchers studied the Fengshan section in southern China, a sequence of shallow-water limestones deposited near the ancient equator. They collected more than 870 oriented rock specimens, each marked so that its original position could be reconstructed, and measured the magnetic signals locked inside. Of these, 309 yielded stable directions recording both normal and reversed polarity.
Rocks can record the magnetic field because some of the minerals they contain, such as tiny grains of magnetite, align with the field when the rock forms and lock that direction in place. Laboratory treatments that gradually heat a sample or expose it to alternating magnetic fields strip away later overprints, revealing the original signal. Specimens that do not give a clear, consistent direction are set aside, which is why fewer than half of the samples were used.
Between about 262.5 and 258.75 million years ago, an interval of 3.75 million years, the team identified 29 distinct magnetic polarity zones, alternating bands of rock formed under normal or reversed fields.
The reversals were concentrated around the end-Guadalupian extinction. In the roughly 1.7 million years before that event, reversals occurred at a rate of about 11 per million years, roughly ten times the long-term average. Afterward, the field became calmer for a few million years.
An overlooked extinction
The end-Guadalupian extinction, sometimes called the Capitanian extinction, occurred about 261 million years ago, roughly ten million years before the end-Permian extinction. The later event, sometimes known as the Great Dying, was the most severe in Earth’s history and eliminated most marine species. The earlier crisis is less famous but was also significant, especially for marine life in tropical seas.
The new paper’s figures place the magnetic record alongside other evidence from the period, including volcanic activity and large igneous provinces, sea-level change and the configuration of the supercontinent Pangea, carbon isotope records and trends in species diversity. Large igneous provinces are vast outpourings of lava that can release enormous amounts of gases into the atmosphere, and they have been linked to several mass extinctions.
Clues from deep in the Earth
What would make the magnetic field so unstable? The authors suggest that changes deep in the mantle may have played a role. During the assembly of Pangea, slabs of ocean floor sank into the mantle at subduction zones. As those cold slabs descended toward the boundary between the mantle and the core, they may have altered the flow of heat out of the core, which in turn could have destabilized the geodynamo.
The idea that conditions at the core-mantle boundary help control how often the field reverses has been proposed before, because the rate at which heat escapes from the core influences the convection that drives the dynamo. Rocks recording the field over long intervals offer one of the few ways to test such ideas.
Earth’s outer core lies about 2,900 kilometres below the surface. Iron flowing there generates electric currents, and those currents sustain the magnetic field. The pattern of flow is sensitive to how quickly the overlying mantle draws heat away, and that rate can vary from place to place depending on whether hot upwellings or cold sinking slabs sit above the core.
Correlation, not causation
The authors are careful about what their findings show. The study does not demonstrate that magnetic instability caused the extinction, and direct evidence linking the two remains limited.
One proposed connection involves the ozone layer. During a reversal, the field weakens, which could allow more charged particles from the Sun and from space to reach the upper atmosphere and potentially deplete ozone, increasing ultraviolet radiation at the surface. But the authors note an alternative: halogen gases released by large igneous provinces could also have damaged the ozone layer, without any help from the magnetic field.
Disentangling those possibilities will require more evidence, including precise dating that can establish whether changes in the magnetic field, volcanic activity and extinction happened in a specific sequence.
What the Fengshan record does establish is a detailed picture of a magnetic field that went from tens of millions of years of stability to frantic activity, with the busiest interval falling just before one of the Permian period’s great biological crises. Whether that timing is coincidence or a clue to deeper connections between Earth’s interior and life at its surface is a question the authors say remains open.
The research was supported by the National Natural Science Foundation of China.
Notes
- National Science Review, 2026. DOI: 10.1093/nsr/nwag647.
- Science China Press. “Earth’s magnetic field flipped rapidly across a mass extinction about 261 million years ago.” EurekAlert!, 22 October 2026. https://www.eurekalert.org/news-releases/1148880.