Earth’s newest geologic time interval has been named after a jurisdiction outside Tokyo, Japan. The International Union of Geological Sciences (IUGS) recently ratified the name “Chibanian,” meaning “Chiba age,” for a period of time stretching from 770,000 to 126,000 years ago, during the Pleistocene epoch.
Equating to the Middle Pleistocene subepoch, the Chibanian is named after Chiba, one of Japan’s 47 prefectures, and marks the first time a Japanese name has been used in the geological timescale as well as in a Global Boundary Stratotype Section and Point (GSSP). GSSPs are reference points defining the lower boundary of a time interval in the geological timescale. There are dozens of GSSPs around the world, with most in Europe and some in North America and China.
The name was chosen following the discovery of a stratum in Chiba’s Ichihara City, about 55 kilometers southeast of central Tokyo. The stratum lies in a cliff along the Yoro River flowing through the Bōsō Peninsula, which separates Tokyo Bay from the Pacific Ocean. The stratum consists of thick layers of late Cenozoic marine strata of silt or clay-bearing marine deposits, minerals, and volcanic ash deposits.
“This sedimentary sequence, called the Kazusa Group, has a total thickness of 3 kilometers with an anomalously high deposition rate reaching 2 meters per thousand years on average,” said Makoto Okada, a professor of paleomagnetic studies at Ibaraki University in Mito, Japan. “It is probably the unique case in the world that a deep-sea deposit formed younger than 1 million years ago (especially around the Matuyama-Brunhes geomagnetic reversal boundary) can be observed continuously on land. Moreover, this sequence provides us reliable geomagnetic polarity signals and abundant marine microfossils.”
Magnetic Field Reversal Record
The nomenclature is significant for Japan not only because it puts the country on the geological map but also because of an important event that occurred eons ago. The Chibanian is when the last reversal of Earth’s magnetic poles took place, and the section in Chiba has one of the best records of that event. The Brunhes-Matuyama reversal is named for geophysicists Bernard Brunhes of France and Motonori Matuyama of Japan, who was the first to discover that the north and south magnetic poles had changed places in the past. The polarity era that came before the current one is named the Matuyama Chron in his honor.
“As a Japanese geologist, I am happy they made a good decision,” said Hiroshi Kitazato, a professor at Tokyo University of Marine Science and Technology and an IUGS executive member who participated in the discussions. “The Chibanian section is certainly the most well preserved paleomagnetic reversal transition from Reversal (Matuyama) to Normal (Brunhes).… We are grateful to put a GSSP at the site that is the Brunhes-Matuyama geomagnetic reversal horizon.”
The Brunhes-Matuyama reversal is estimated to have taken place about 775,000 years ago, but its duration is disputed. A 2019 study in Science Advances by researchers in Japan and the United States estimated that the flip took approximately 22,000 years.
Stanley Finney, a professor of geological sciences at California State University, Long Beach and the IUGS secretary general, noted the importance of the Chiba site in light of findings that the current magnetic field is changing, possibly signaling another polarity reversal.
“In that section in Chiba, you have one of the best records of the reversal interval of anywhere in the world,” said Finney. “It’s a significant record of past Earth history that helps us see what may happen now.”
Source of Local Pride
The effort to name the interval after Chiba passed multiple screening processes, overcame charges by a Japanese group that related documents had been falsified, and beat out two rival sites in Italy.
The IUGS ratification has become a source of pride in Chiba, with Ichihara mayor Joji Koide commenting in a special city leaflet, “Above all, I would like to share the joy of becoming Japan’s first GSSP-certified place with our community. We expect worldwide attention in the future. As a city, we will move forward with efforts to prepare the environment for visitors.”
A temporary visitor’s center was erected at the site in December ahead of the construction of a permanent facility. A dedication ceremony will be held at the stratum site to recognize the work of the researchers involved in the naming effort, and a GSSP bronze disk, known as a golden spike, will be placed in the cliff face.
“At many of these sites, we have great monuments for illustrative purposes or panels or geoparks,” said Finney. “These are international geostandards, and you can’t take them into a museum; it’s something there in the field.”
Hornyak, T. (2020), Japan puts its mark on geologic time with the Chibanian Age, Eos, 101, https://doi.org/10.1029/2020EO139453. Published on 30 January 2020.
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