Major reversal of Earth’s magnetic field 773 thousand years ago marks a new unit of the Geological Time Scale
On 17 January 2020, the Executive Committee of the International Union of Geological Sciences, meeting in Busan, South Korea, ratified the proposal for the Global Stratotype Section and Point (GSSP) that defines the base of the new Chibanian Stage and the Middle Pleistocene Subseries. The stratotype section is located in the Chiba region of Japan and not only includes the boundary but also records the most pronounced reversal of Earth’s magnetic field in the past two million years. Given concerns regarding the unexplained present shift in Earth’s magnetic field, the Chiba record could hold clues as to whether we are now heading for a new reversal.
The geological time scale is based on specific designated intervals of rocks (stratigraphic sections) around the world that are termed chronostratigraphic units (e.g., Jurassic System, Chibanian Stage) and that are the physical records of intervals of respective intervals of geologic time (e.g., Jurassic Period, Chibanian Age). Most important as global geostandards are those sections that best exhibit the features used to define the bases of the standard global chronostratigraphic units, i.e., the GSSPs. At the International Geological Congress in Florence, Italy, in 2004, a group of scientists voted to designate the so-called Brunhes–Matuyama paleomagnetic reversal as the primary guide for the Early–Middle Pleistocene boundary. This reversal represents a 180° switch in the Earth magnetic dipole, by which the magnetic south pole suddenly becomes the north pole and vice versa.
Professor Martin Head of Brock University, Canada, who was co-convener of the group that met in Florence, said “the Brunhes–Matuyama boundary was chosen because it represents a near instantaneous event detectable in marine and terrestrial sediments around the world and even in an ice core in Antarctica. It is the perfect tool for global correlation and the obvious guide for recognising the base of the Middle Pleistocene.”
After years of searching the world for the best section to study this paleomagnetic reversal, the Chiba section in Japan, just a few hour’s drive from Tokyo, emerged as the clear favourite. The Chiba section is exposed in a spectacular, deeply incised gorge, and its sediments, deposited around 774 thousand years ago on what was then the Pacific continental slope, have captured not only the key signals of the Brunhes–Matuyama reversal but also a wide array of geochemical and microfossil data. These allow climatic and oceanographic changes to be reconstructed before, during, and after the reversal.
As research intensified, so it became clear that the Chiba section held a unique record. According to Professor Makoto Okada of Ibaraki University, and the paleomagnetic specialist on the Chiba research team, “I am convinced that Chiba has the highest time resolution record in the world for the Brunhes–Matuyama reversal”. Professor Mark Hounslow, Lancaster University, UK, a recognized expert on paleomagnetism and an author of the proposal, concurred: “the paleomagnetic record from the Chiba composite section is one of the most detailed … records of the M–B geomagnetic field reversal on the globe”.
In detail, the Chiba section and nearby contiguous sections record the directional midpoint of the Brunhes–Matuyama reversal at ~772.9 thousand years ago. The entire directional reversal event was completed within a mere 1.9 thousand years. The base of the Chibanian Stage and Middle Pleistocene Subseries is defined by a regionally extensive ash bed, the Ontake-Byakubi-E bed, dated at 774.1 thousand years ago and occurring just 1.1 m below the reversal midpoint. This distinctive bed gives visible expression to the boundary.
This is now Japan’s first global boundary stratotype section and point, simultaneously defining the Chibanian Stage/Age and Middle Pleistocene Subseries/Subepoch on the ICS International Chronostratigraphic Chart/International Geological Time Scale.
Meanwhile, the present unexplained rapid shift in the north pole’s position and decline in field strength continue to cause concern, not least because a collapse of the Earth’s magnetic field would allow harmful cosmic and solar radiation to reach our planet’s surface. Can research on the Chiba section help us understand the near future? Dr. Yusuke Suganuma, lead proponent of the Chiba research group and scientist at the National Institute of Polar Research, Tokyo, thinks it might. He says: “the Chiba record is key to understanding the nature of the geomagnetic field reversal event, and will probably contribute to a more realistic simulation of the geodynamo in the near future”.
END