Quaternary Geochronology 3 (2008) 208–212 Research paper Transition of solar cycle length in association with the occurrence of grand solar minima indicated by radiocarbon content in tree-rings Hiroko Miyahara a,Ã , Kentaro Nagaya b , Kimiaki Masuda b , Yasushi Muraki b , Hiroyuki Kitagawa c , Toshio Nakamura d a Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan b Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan c Graduate School of Earth and Environmental Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan d Center for Chronological Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan Received 3 April 2007; received in revised form 21 November 2007; accepted 26 November 2007 Available online 29 January 2008 Abstract In this paper, we review the variation of the 11-year solar cycle since the 15th century revealed by the measurement of radiocarbon content in single-year tree-rings of Japanese cedar trees. Measurements of radiocarbon content in absolutely dated tree-rings provide a calibration curve for accurate dating of archaeological matters, but at the same time, enable us to examine the variations of solar magnetic activity in the pre-historical period. The Sun holds several long-term quasi-cyclic variations in addition to the fundamental 11-year sunspot activity cycle and the 22-year polarity reversal cycle, and it is speculated that the property of the 11-year and the 22-year solar cycle varies in association with such long-term quasi-cycles. It is essential to reveal the details of solar variations around the transition time of solar dynamo for illuminating the mechanisms of the long-term solar variations. We therefore have investigated the property of the 11-year and 22-year cycles around the two grand solar minima; the Maunder Minimum (1645–1715 AD) and the Spoerer Minimum (1415–1534 AD), the periods of prolonged sunspot minima. As a result, slight stretching of the ‘‘11-year’’ and the ‘‘22-year’’ solar cycles was found during these two grand solar activity minima; continuously during the Maunder Minimum and only intermittently during the Spoerer Minimum. On the contrary, normal or slightly shortened 11-year cycles were detected during the interval period of these two minima. It suggests the inverse correlation between the solar cycle length and solar magnetic activity level, and also the change of meridional flow during the grand solar activity minima. Further measurements for the beginning of the grand solar minima will provide a clue to the occurrence of such prolonged sunspot disappearance. We also discuss the effect of solar variations to radiocarbon dating. r 2007 Elsevier Ltd. All rights reserved. Keywords: Radiocarbon dating; Tree-ring; Cosmic rays; Maunder Minimum 1. Introduction The magnetic activity of the Sun, which can be clearly recognized by the number of the sunspots appearing at the surface, has kept more or less stable 11-year cyclicity since the 18th century. However, long-term variation of the Sun has caused serious weakening of magnetic acti- vity and consequential prolonged sunspot absence around 1645–1715 AD (Fig. 1). This spell is referred as the Maunder Minimum (Eddy, 1976). Variation of radio- carbon content in tree-rings with decadal resolution (Fig. 2) has revealed that such prolonged sunspot minima with several decades to 100-year timescales have occurred several times in the past (Stuiver and Quay, 1980), and that such weakening of solar activity have played some role in the cold spell so-called the Little Ice Age (Mann et al., 1999). Radiocarbon is mainly produced by the incoming galactic cosmic rays modulated by solar wind and the interplanetary magnetic field, and hence its abundance in the atmosphere and in tree-rings as well as beryllium-10 in ARTICLE IN PRESS www.elsevier.com/locate/quageo 1871-1014/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2007.11.005 Ã Corresponding author. Tel.: +81 3 5841 4317; fax: +81 3 5841 8318. E-mail address: miyahara@eps.s.u-tokyo.ac.jp (H. Miyahara).