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Estimating the age of komainu with carbon radioisotopes

January 24, 2025

Canine research at UTokyo

Canine research at UTokyo

Veterinary surgery, ethology, robotics, archaeology, chronological dating, law and animal assisted intervention, veterinary epidemiology, classic literature, and contemporary literature – specialists in these nine fields introduce their canine-related research activities.

Dogs and chronological dating

Estimating the age of komainu with carbon radioisotopes

Yusuke Yokoyama

Professor, Atmosphere and Ocean Research Institute

Yusuke Yokoyama

Professor Yokoyama has been conducting research into historical changes in the climate and ocean environment using accelerator mass spectrometry (AMS). An encounter during his fieldwork led the professor to engage in a new initiative involving the dating of cultural assets.

Using a geochemistry method to date old wooden statues

I have been conducting geochemical research to study environmental changes in the climate and ocean that have occurred over the past two million years. The key to this research method is isotopes, and I pay special attention to the carbon-14 radioactive isotope. The older a sample is, the lower its concentration of carbon-14. You can therefore date samples by comparing their carbon-14 concentration with past records of carbon-14 in the atmosphere using AMS. Also, as oceans absorb atmospheric carbon-14 in a different manner according to location, you can estimate the habitat of a marine creature by examining the carbon-14 contained in a sample.

My past research achievements include finding a huge coral reef that is more than 400 years old off the coast of Kikaijima in Kagoshima Prefecture; summarizing the important points of resource management in Hokkaido based on findings regarding the otoliths of Alaska pollock fish; finding previously unknown volcanic activity of Mt. Fuji by examining the sediment of Lake Yamanaka in Yamanashi Prefecture; and discovering that a two-and-a-half-year-old catfish was caught 4,000 years ago after rising to the ocean’s surface by examining the otolith of a catfish unearthed at a historic site of the Indus Civilization in South Asia. On the Oga Peninsula in Akita Prefecture, I examined the local lake sediment to study historical climate change events in the area. While engaging in this research, I became acquainted with a member of the board of education of Oga City, who asked me about dating old komainu statues possessed by a local shrine called Akagami Shrine. I agreed to date the statues for them because they were made of wood, which is quite unusual. With stone statues, it is impossible to measure the concentration of carbon-14.

Komainu statues at Akagami Shrine
Komainu are paired statues of mythical creatures called “lion-dogs.” The photo shows the komainu at Akagami Shrine in Akita Prefecture. The dog statue on the left has a closed mouth, and the lion statue on the right has an open mouth.

In 2015, with the Shrine’s priest in attendance, I used a knife to scrape trace amounts of samples from the statues. To increase the precision of calendar-year calibration using the Wiggle Matching method, I took samples from the annual growth rings of the wood at 10- to 20-year intervals and analyzed them using AMS. As a result, I found that the wood used for the statues came from sometime between the sixth and 11th century. This was the dating of the wood – not when the statues were carved – but it would be unusual for a statue to be made from wood that had been kept for centuries. As the Shrine is said to have been built in the year 860, the date range for the wood in the komainu statues can thus serve as data supporting that assumption. Using a chronological approach to examine these wooden komainu statues would have been difficult, so by providing quantitative dating results, I hope that I have contributed to the further protection of these cultural assets. I still have several questions, however. For example, how did the komainu statues, which should have been delivered to Nara and Kyoto from continental Asia, end up in Akita? Was the transportation route from Nara/Kyoto to Akita already established at the time? Or was the Oga Peninsula one of the international trading bases? The survey stimulated my imagination a lot.

Wanting to communicate the genuine thrill of engaging in scientific inquiry such as the above, I am leading a project called “School for Marine Sciences and Local Hopes in Amami” to establish an educational base in Amami (Kagoshima Prefecture) in collaboration with the local community. I hope to be able to use carbon-14 dating in our research to determine the ages of habu snakes in the area.

Small AMS
Small AMS system installed at the Laboratory for Accelerator Mass Spectrometry of the Atmosphere and Ocean Research Institute
Samples dated using AMS
AMS can be used to date even a trace sample (an amount equal to the tip of the lead of a mechanical pencil). Whale fins and human teeth can also be dated using this system.


* This article was originally printed in Tansei 47 (Japanese language only). All information in this article is as of September 2023.

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