Kochi University, Kyushu University find Geomagnetic reversal lasted "up to 70,000 years"

An international research team led by Professor Yuhji Yamamoto of the Marine Core Research Institute at Kochi University, Associate Professor Futoshi Takahashi of the Faculty of Science at Kyushu University, Dr. Slah Boulila of the Centre National de la Recherche Scientifique (CNRS) and Sorbonne Université, and Associate Professor Peter C. Lippert of the University of Utah conducted high-precision analysis of palaeomagnetic records preserved in deep-sea sediments deposited approximately 40 million years ago and identified two geomagnetic polarity reversals that occurred at that time. The analysis revealed that these reversals took approximately 18,000 years and approximately 70,000 years to complete, far exceeding the previously accepted understanding "reversals are completed in roughly 10,000 years." The findings were published in Communications Earth & Environment.

Earth's magnetic field is generated by electromagnetic induction (the dynamo effect) driven by convective motion in the liquid metal (iron and nickel) inside Earth and is thought to have already formed approximately 4.2 billion years ago. This magnetic field has varied continuously throughout Earth's history and has repeatedly undergone geomagnetic polarity reversals in which the north and south poles switch positions.

Recent studies based on sediment and volcanic rock records estimate that the four reversals within the past 1.8 million years each required approximately 1,000 to 12,000 years to complete the change in direction. Furthermore, sequential lava flow records capturing seven geomagnetic reversals spanning two time intervals —from 0.78 to 17 million years ago and 180 million years ago— were analyzed. The analysis showed that the reversal process consists of three stages: a precursor phase (approximately 2,500 years), a main transition phase (approximately 1,000 years), and a rebound phase (approximately 2,500 years). It has therefore been generally understood that reversals are completed in roughly 10,000 years.

However, approximately 540 geomagnetic reversals have occurred over just the last ∼170 million years of Earth's history, and detailed analysis has been performed on fewer than 1% of these.

The research team conducted precision palaeomagnetic analysis on middle Eocene (approximately 38.4-43.2 million years ago) deep-sea sediment cores. These cores were collected from Site U1408 during Integrated Ocean Drilling Program (IODP) Expedition 342, which was conducted near the Newfoundland ridges in the northwest Atlantic Ocean.

The Site U1408 sediment core is a continuous sedimentary sequence with a relatively high deposition rate of approximately 2.4 cm per thousand years, with clear lithological alternations confirmed from chemical and physical data. A high-precision geochronological model was constructed by incorporating sedimentary rhythms related to variations in Earth's axial tilt (obliquity) and its approximately-173,000-year astronomical cycle into scanning X-ray-fluorescence-measured Ca/Fe elemental ratios.

Magnetic analysis was performed on discrete samples collected at 2 cm intervals across a reversal interval spanning 8 m of sediment thickness, eliminating the signal smoothing that occurs in continuous sample measurements. The results confirmed that the primary carrier of remanent magnetization is biogenic magnetite, yielding a high-quality dataset that faithfully records the geomagnetic field of that time. By calculating changes in the latitude of the virtual geomagnetic pole (VGP) from palaeomagnetic directional changes and integrating variations in relative palaeointensity (RPI), two polarity transition intervals in the Eocene were clearly identified.

Detailed analysis of the sediment core allowed high-precision reconstruction of two reversal intervals spanning 50 cm and 170 cm of sediment thickness respectively. Based on estimates from the geochronological model, the reversal durations were calculated to be 18,000 ± 3,000 years and 70,000 ± 6,000 years respectively—extraordinarily long reversal processes that far exceed the previously accepted timescale of approximately 10,000 years. In particular, the above-mentioned reversal with a duration of 70,000 ± 6,000 years exhibited complex behavior in which the process of the precursor phase → main transition phase → rebound phase occurred multiple times.

This type of multi-stage behavior has also been reported for the most recent reversal (approximately 770,000 years ago, the Brunhes-Matuyama Transition), suggesting that geomagnetic reversals may be inherently diverse and complex phenomena.

Recent numerical simulations have indicated that reversal durations follow a broad distribution and that prolonged reversals can occur naturally. The research team analyzed 160 reversal events using their own numerical geodynamo model and showed that reversal durations spread in a log-normal distribution. When the simulation results are converted to actual Earth time, the maximum duration may correspond to approximately 33,000-130,000 years.

Both reversals revealed in this study fall within this range, confirming that they are theoretically plausible phenomena. Complex geomagnetic field behavior—including precursory directional changes during the reversal and multiple "failed reversals"—was also observed in both the sediment core records and the model.

The wide variation in reversal durations reflects the intrinsic variability of the geodynamo, and represents an important empirical demonstration that geomagnetic reversals can last far longer than 10,000 years. Because reversal duration has a latitude dependence, it is possible that in mid- to high-latitude regions of the time, reversal states may have persisted for even longer than 70,000 years.

If reversal states or periods of weak geomagnetic field intensity persisted for tens of thousands of years or more, high-energy particles from the sun and outer space are more likely to have reached Earth's surface, potentially affecting the surface environment, as well as biological activity and geochemical cycles, during the Eocene. The relationship between geomagnetic field weakening and environmental change is an important scientific topic that warrants future investigation.

Journal Information
Publication: Communications Earth & Environment
Title: Extraordinarily long duration of Eocene geomagnetic polarity reversals
DOI: 10.1038/s43247-026-03205-8