A Golden Spike in Time
Along the Yoro River in Ichihara City, a quiet cliff face holds the global standard for a major event in Earth's history. This location, formally known as the Chiba composite section, is the Global Boundary Stratotype Section and Point (GSSP) for the Chibanian Age. The International Union of Geological Sciences ratified this designation on January 17, 2020, making it the first GSSP in Japan. A GSSP is a geological "golden spike" that marks the boundary between two ages in the official geologic time scale. The Chibanian age spans from approximately 770,000 to 126,000 years ago.
The rock layers here record the most recent reversal of Earth's magnetic field, an event called the Matuyama-Brunhes reversal. This was the last time the magnetic north and south poles swapped places. The name is for Japanese geophysicist Motonori Matuyama, who first proposed that Earth's field had reversed in the past, and French geophysicist Bernard Brunhes. The geologic section is part of a nearly two-mile-thick deposit of former deep-sea sediments called the Kazusa Group. These layers of silt and clay, once on the ocean floor, were uplifted by tectonic activity. They accumulated at an exceptionally high rate, averaging around 2 meters per thousand years, which created a detailed record.
Reading the Magnetic Minerals
The reversal is recorded by iron-bearing minerals within the rock. As these sediments were deposited on the ancient seabed, the magnetic minerals aligned themselves with Earth's prevailing magnetic field. When the layers solidified into rock, these minerals were locked in place, preserving a continuous recording of the magnetic field's orientation over thousands of years.
Analysis of the Chiba section shows the entire reversal process took around 20,000 years. This included a period of instability lasting about 10,000 years before the main flip occurred. The exact boundary for the Chibanian age is placed at the base of a specific, 3.5 cm thick layer of white volcanic ash known as the Ontake-Byakubi-E (Byk-E) tephra bed. This layer is a precise marker that can be dated. Using astronomical and radiometric dating methods, the GSSP is timed to 774,100 years ago. The midpoint of the magnetic reversal itself is found 1.1 meters above this ash layer in the strata. The site also contains abundant marine microfossils and pollen, which help scientists reconstruct the environmental conditions of the period.