〇Masataka HAKOZAKI, Masaki SANO, Minoru SAKAMOTO (National Museum of Japanese History), Tetsuya SHINOZAKI (The University of Tokyo), Fusa MIYAKE (ISEE, Nagoya University)
１． Introduction
In this study, we reconstruct paleoenvironmental changes in the prehistoric Japanese island region using the oxygen isotopic ratio dendrochronology and the carbon-14 spike-match dating method with a temporal and spatial resolution of one year. In particular, we aim to: 1) determine the age of the Kikai-Akahoya eruption, a catastrophic eruption that marked the Initial Jomon and Early Jomon periods, with an error of 0 year; 2) reconstruct annual precipitation and solar activity over the past 6400 years and clarify their relationship; 3) reconstruct the climate of the Grobal Cooling Event (4.2-4.3 ka event) that occurred 4200-4300 years ago and assess its impact on the archipelago.
For this purpose, we will collect volcanically buried wood from Yakushima Island and reconstruct carbon-14 spikes, conduct comprehensive oxygen isotope ratio and carbon-14 analyses of naturally buried wood and wood excavated from archaeological sites from all over Japan, and reconstruct areal climate change based on comparisons between regions over the same time section. Collaborate with other groups to acquire and date new wood materials to reconstruct older and broader paleoenvironmental changes, and provide feedback of paleoenvironmental and chronological information to other groups.
２．Report on activities in FY2023
2.1 Establishment of Research Environment
Associate Professor Masaki Sano of the Takeshi Nakatsuka Laboratory, Graduate School of Environmental Studies, Nagoya University, a world leader in oxygen isotopic dendrochronology, was hired as a specially-appointed associate professor at the National Museum of Japanese History with funds from this project. He will play a central role in carrying out the above research plan together with the principal investigators.
The National Museum of Japanese History has introduced an automatic graphite generator "Ionplus AGE 3," a stable isotope ratio mass spectrometer "Thermo scientific DELTA Q," and an ultra-high resolution digital microscope "HIROX HRX-01" through the adoption of the FY2023 budget request. This has made it possible to further speed up and improve the efficiency of carbon 14 dating, which has been continued for some time. In addition, the museum is now able to measure oxygen isotope ratios of annual ring cellulose on its own, having previously relied on Nagoya University or Fukushima University. In the future, the National Museum of Japanese History will function as the third center for oxygen isotopic dendrochronology in Japan.
2.2 Survey and Analysis
To date artifacts and obtain paleoenvironmental data, we analyzed excavated wood from the Tsushimaoka Great Site in Okayama Prefecture, the Takayagawa Lowland Site in Chiba Prefecture, the Narisawa 2 Site in Akita Prefecture, the Nishizuka Tumulus in Fukui Prefecture, and the Hizako Site in Saitama Prefecture. To obtain basic data for reconstruction of the paleoclimate of the Kanto region, we collected sleepers from the Takanawa embankment in Tokyo, cedar from Sammu City, Chiba Prefecture, mokkoku and enoki from Sakura City, and bodaiju from Asahi City.
2.3 Publication of Results
Results related to this research were published in one book, four international journal articles, three international conferences, five domestic academic conferences, two reports on buried literature, and three lectures for the general public. In addition, the principal investigator appeared on NHK E-television's "The Backyard" to introduce his research on solar activity reconstruction based on carbon-14 in tree annual rings.
2.4 Educational and social contributions
We provided samples, data, and experimental environments necessary for doctoral thesis research by doctoral students of the Graduate School of Frontier Sciences, the University of Tokyo, and for master's thesis research by master's students of the Graduate School of Education, Tokyo Gakugei University.
3. Activity plan for FY2024
Oxygen isotopic dendrochronology is limited in that it can only date materials from periods and regions for which a "master chronology" exists, which serves as a yardstick for dating. Applied research such as climatic reconstructions and wood production area estimation cannot be performed without a master chronology.
At present, a master chronology for the past 2600 years constructed from cypress trees in the Chubu and Kinki regions (Nakatsuka et al. 2020) and two prehistoric master chronologies (2349-1009 BCE and 1412 -466 BCE) (Sano et al. 2023) have only been published. In order to implement paleoenvironmental restoration with high spatio-temporal accuracy, it is essential to expand the master chronologies.
In FY2024, we will publish a new master chronology for northern Japan based on the data we have accumulated so far. We also aim to construct and publish the longest master chronology in East Asia, which exceeds 3000 BC. These standard dendrochronological curves will enable the dating of wood materials from a wide range of regions and periods, and will promote highly accurate climatic reconstructions based on the data obtained. We aim to achieve the third objective of this study, which is to reconstruct the 4.2-4.3 ka event in the Japanese archipelago.
The year 2024 started from the beginning of the year with a huge earthquake and a massive tsunami. In addition, small- and medium-sized earthquakes caused by slow slip in the Boso Peninsula have frequently occurred in Chiba Prefecture. Thirteen years have passed since the Great East Japan Earthquake and eight years since the Kumamoto Earthquake, and these earthquakes remind us once again that Japan is an archipelago of disasters. There are still many earthquakes and tsunamis in both prehistory and history that have not been precisely dated. Shinozaki, who is in charge of the project, is also preparing for carbon-14 dating and oxygen isotopic dendrochronological dating of tsunami deposits and buried trees in various areas. The C01 group will continue to build a foundation to precisely date environmental changes that have greatly affected human society, such as climate change, volcanic activity, earthquakes, and tsunamis, and to accurately assess their effects.