The Earth’s magnetic field, generated 3,000km below our feet in the liquid iron core, is crucially important to life on our planet. The magnetic field is constantly changing in both its strength and direction. It has undergone some dramatic shifts in the past.
A long-standing question has been how fast the field can change. Earth’s magnetic fields typically switch every 200 to 300 millennia. Yet, the planet has remained steady for more than twice that now, with the last magnetic reversal occurring about 773,000 years ago.
A new study has offered a better understanding of the geophysical events leading up to the switch and how Earth has responded since then.
The study was conducted by scientists from the National Institute of Polar Research (NIPR).
According to scientists, Matuyama-Brunhes geomagnetic reversal is one of the most studied paleomagnetic events. Geological records of the Matuyama–Brunhes (M–B) geomagnetic reversal facilitate the development of an age model for sedimentary and volcanic sequences and help decipher the Earth’s magnetic field’s dynamics.
Such reversals swap the planet’s magnetic poles, which could have implications for plant and animal life. Such extrapolation to life, including humans and our ancestors, depends on the magnetic flip duration.
Yuki Haneda, project researcher at the National Institute of Polar Research (NIPR), said, “Previous studies by other researchers examined samples taken from lava flows, which offer a good snapshot of moments in geophysical history. However, lava sequences cannot provide continuous paleomagnetic records due to the nature of sporadic eruptions.”
“In this study, we collected new samples and conducted paleo- and rock-magnetic analyses of samples from the Chiba composite section, a continuous and expanded marine succession in Central Japan, to reconstruct the full sequence of the Matuyama-Brunhes geomagnetic reversal. The Chiba composite section is widely considered to contain the most detailed marine sedimentary record of the Matuyama-Brunhes geomagnetic reversal. It serves as the international standard for the lower boundary of the Middle Pleistocene Subseries and Chibanian Stage — when Homo sapiens emerged as a species.”
The study reveals that the geomagnetic field became unstable at least 10,000 years before the magnetic direction change 773,000 years ago. What’s more, it is a full reversal process that took at least 20,000 years.
Haneda said, “Our data is one of the most detailed paleomagnetic records during the Matuyama-Brunhes geomagnetic reversal, offering deep insight into the mechanism of the geomagnetic reversal.”
In the future, scientists will determine how the geomagnetic reversal influenced plankton and plants, using marine microfossil and pollen data found in their samples.
This work was supported by Tabuchi district of Ichihara City, Chiba Prefecture, the Agency for Cultural Affairs, and the Ministry of Education, Culture, Sports, Science, and Technology.
- Haneda, Y., Okada, M., Suganuma, Y. et al. A full sequence of the Matuyama–Brunhes geomagnetic reversal in the Chiba composite section, Central Japan. Prog Earth Planet Sci 7, 44 (2020). DOI: 10.1186/s40645-020-00354-y