[U-03] Advanced understanding of Quaternary and Anthropocene hydroclimate changes in East Asia

[U03-P05] Investigation of the Atmospheric ¹⁴C Distribution around East Asia Since 1950 Based on Δ¹⁴C and δ¹³C Measurements in Japanese Tree Rings and Meteorological Data

*Kohei Sakamoto¹, Wataru Sakashita³, Ren Tsuneoka¹, Yosuke Miyairi², Takahiro AZE², Toshihiro Miyajima², Yusuke Yokoyama² (1.School of science, The University of Tokyo, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.Forestry and Forest Products Research Institute)

Keywords:Tree-ring, Δ14C, δ13C, Subtropical jet, Atmospheric nuclear weapons tests, Japan

The radiocarbon (¹⁴C) calibration curve during the late 20th century has been developed based on the radiocarbon isotope ratios (Δ¹⁴C) obtained from the atmosphere and tree-ring cellulose samples collected worldwide. A prominent peak in Δ¹⁴C was observed from around AD 1955 to 1967 due to the atmospheric nuclear weapons tests (Bomb peak), and it is known that the peak value differs depending on the region. Thus, previous studies divided the world into five regions (NH zone1, NH zone2, NH zone3, SH zone1~2, SH zone3) and provided summer atmospheric Δ¹⁴C data for each region. Locations of the boundaries between each region were defined based on the atmospheric circulation, such as the position of polar and subtropical jets. However, in the East Asia, the sparse of tree-ring cellulose Δ¹⁴C data in this region from around AD 1950 to 1970 hampered the validation of the boundary position, and the assessment of influence of the subtropical jet (which plays an important role in the summer atmospheric circulation in this region) is still insufficient. Thus, we obtained two tree-ring samples (Hinoki cypress in Kumamoto; Chamaecyparis obtuse [32°49' N, 137°44' E, estimated elevation 100 m] and Sawara cypress in Kiso; Chamaecyparis pisifera [35°46'46.0"N, 137°33' 44.7"E, elevation 1050 m]) from the Kyushu and Chubu regions, and the tree-ring cellulose Δ¹⁴C and carbon stable isotope ratios (δ¹³C) were newly measured. The objectives of this study are 1) to verify whether the carbon in tree-ring cellulose actually records atmospheric information during the growing season based on the climate response of tree-ring cellulose δ¹³C, and 2) to assess the influence of the subtropical jet on the tree-ring cellulose Δ¹⁴C in Japan.
Our assessment of the climate response revealed that tree-ring cellulose δ¹³C of Sawara cypress had a significant positive correlation with the monthly total sunshine hours in June, suggesting that the carbon in tree-ring cellulose records atmospheric information during the main growing season. Then, we compared the tree-ring cellulose Δ¹⁴C of Sawara cypress and Hinoki cypress with previously reported those of Japanese black pine in Ishikawa and red pine in South Korea. This comparison showed that our tree-ring cellulose Δ¹⁴C showed relatively low values in AD 1963 (the onset of Bomb peak). This is probably because the subtropical jet in June 1963 was located between our sampling sites and previous study sites, and our tree-ring cellulose Δ¹⁴C of Sawara cypress and Hinoki cypress may be strongly influenced by the southern atmosphere (which had the relatively low Δ¹⁴C value). The relatively slow northward movement of the strong wind axis at 300 hPa was observed around Japan in June 1963, thus, this interpretation is supported by the actual observation. These findings in this study are expected to be useful for updating the regional boundaries of the ¹⁴C calibration curve in the East Asia region since the 1950s.