Paleomagnetic Records and Geomagnetic Excursions

Paleomagnetic records are the remnant magnetisation preserved in rocks, sediments and speleothems that faithfully archive the orientation and intensity of the Earth’s magnetic field through geological time. These archives capture both gradual secular variation and abrupt geomagnetic excursions, during which the field deviates markedly from its normal axial dipole configuration but does not fully reverse polarity. Excursions such as the Laschamp (~41 ka), Mono Lake (~34 ka) and several older Lower Brunhes events are identified by rapid directional swings of tens of degrees and concurrent drops in field intensity to a fraction of the long-term dipole moment. High-resolution records have revealed that excursions often comprise multiple subcentennial directional shifts, suggesting complex non-dipole dynamics in the outer core. Chronological control is achieved through radiocarbon, U–Th dating and astronomical tuning, allowing the synchronisation of palaeomagnetic signals with climate proxies and geomagnetic intensity stacks. Globally distributed records demonstrate that excursions are associated with weakened axial dipole energy, growth of reversed flux patches and enhanced non-axial components. Such findings inform geodynamo models and aid in refining age models for sedimentary and speleothem sequences. Beyond academic interest, understanding excursion behaviour has practical applications in sediment correlation, archaeological dating and assessing past shielding of cosmic radiation.

Research from Nature Portfolio

Recent studies of finely layered lacustrine sediments in central Japan have produced a subcentennial record of the Laschamp excursion and a newly recognised post-Laschamp event. Varve-counted chronology tied to the latest calibration curves reveals that both excursions comprise a sequence of rapid directional swings, with virtual geomagnetic poles forming distinct hemispheric clusters. Simulation of sensor filtering demonstrates that even low-resolution marine and volcanic records capture the dominant features of these excursions, confirming the prevalence of intermittent non-axial dipolar sources during field instability.

High-precision U–Th dating of a continuous speleothem flowstone in northern Italy has elucidated four Lower Brunhes geomagnetic excursions preserved in a single core spanning Marine Isotopic Stages 13–7. Despite overlapping age uncertainties, one of the excursions is tightly constrained to ca. 417 ka. The study highlights the potential of speleothems as complementary archives to sedimentary records and provides direct absolute ages for several hitherto poorly dated excursional events.

Paleomagnetic Records and Geomagnetic Excursions publication trend

The graph below shows the total number of articles in paleomagnetic records and geomagnetic excursions across all publications each year (not limited to Nature Index journals).

Technical terms

Paleomagnetic record: The preserved magnetisation in geological materials that records the past direction and intensity of Earth’s magnetic field.

Geomagnetic excursion: A temporary, large-amplitude deviation of the geomagnetic field direction and intensity that does not culminate in a full polarity reversal.

Virtual geomagnetic pole (VGP): The position on the Earth’s surface where the geomagnetic pole would lie if the field were a geocentric axial dipole, inferred from palaeomagnetic directions.

Relative paleointensity: A proxy for past geomagnetic field strength derived by normalising natural remanent magnetisation against a laboratory-induced magnetic parameter.

U–Th dating: A radiometric technique using the decay of uranium to thorium isotopes to determine the age of carbonate materials, such as speleothems, with high precision.

References

1. Intermittent non-axial dipolar-field dominance of twin Laschamp excursions. Communications Earth & Environment (2022).
2. U-Th dated speleothem recorded geomagnetic excursions in the Lower Brunhes. Scientific Reports (2019).
3. Global Evolution and Dynamics of the Geomagnetic Field in the 15–70 kyr Period Based on Selected Paleomagnetic Sediment Records. Journal of Geophysical Research: Solid Earth (2021).
4. Records of the Laschamps geomagnetic polarity excursion from Black Sea sediments: magnetite versus greigite, discrete sample versus U-channel data. Geophysical Journal International (2020).