H7
Solar activities recorded in tree rings

Some large disks of Yaku cedar have been stored in the Institute for Cosmic Ray Research of the University of Tokyo and we have exhibited one of them in Chronosphere hall. The tree rings are not just materials for accurate radiocarbon dating, but they contains various valuable data on the history of environment and solar activities. That is why tree samples were taken and studies by the Institute for Cosmic Ray Research.

The disk in Chronosphere has about 1650 rings in 130 cm, suggesting average width of rings is less than one mm. Yaku-shima Island is volcanic geology and mountains as high as 1963 m above sea level at Miyanouradake, is the highest peak in Kyushu region. This area has much precipitation but is no suitable for plant growth. That is why Yaku cedar has long life and many ring in them, which typical life span of cedar is several hundred years.

Dr. Kunihiko Kigoshi of Gakushuin University wrote that tree large timbers of cedar was taken from Yaku-shima Island to ICRR in 1960 by Dr. Minoru Oda, who is well known as a pioneer of X-ray astronomy (Kigoshi 1978), and this disk is highly probably originated from one of them. The radiocarbon activities in tree rings were measured for making calibration curve and the exactly same data can be used for investigating the cosmic ray in past. The cosmic ray came from the space change nitrogen into radiocarbon in the upper atmosphere. The amount of cosmic ray is affected by the solar wind and the magnetic field of earth. It means that the weaker solar activity will result in higher production rate of radiocarbon and apparent younger radiocarbon dates in contemporaneous organisms.

This unique record of past solar activity can be investigated in light of historical record of climate change, opening the door of new science field called “cosmic meteorology”. The very high sensitivity of AMS is essential to detect annual fluctuation in solar activity recorded in a single tree ring.

Probably a gigantic cedar tree, so called “Jomon Sugi” on Yaku-shima Island may contain several thousand year records inside. We cannot get its disk now because hewing the Yaku cedar has terminated since 2001. Because the scientific values of old disks is now becoming higher and higher, we are investigating the precise chronological information of this specimen for future studies. (Minoru Yoneda, Takayuki Omori & Hiromasa Ozaki)

References

木越邦彦（1978）『年代を測る―放射性炭素法』中央公論社。

宮原ひろ子（2014）『地球の変動はどこまで宇宙で解明できるか：太陽活動から読み解く地球の過去・現在・未来』化学同人。