Radiocarbon Analysis of Single-Year Tree Rings at the Last Glacial-Deglacial Transition

Variations in solar insolation originate mainly from the changes in solar radiation and Earth's orbit shift. The number of sunspots is correlated with the amount of solar radiation, which has been responsible for climate changes. The sunspot scarcity in the Maunder Minimum stage, resulting from extraordinary weakening of solar magnetic activity, is considered to have played an important role in a serious cold spell from the 14th to the 19th century (Little Ice Age; Miyahara et al, 2004). Since historical observations of the sunspots are available only for the last 400 years (Versteegh, 2005), climate changes in pre-historic age cannot be reconstructed by the historical data. Variations of the solar activity can be identified using cosmogenic nuclides that are generated in the Earth's atmosphere under the action of cosmic ray fluxes. The amount of cosmogenic nuclides from samples such as tree rings, ice and sediment cores are a good indicator of the variation of the solar activity for the longer time interval. To clarify the solar activity at the end of the last glaciation, we tried to measure the radiocarbon concentration for single-year interval tree rings in fossil trunks (ca. 15.5 cal kyr BP; Horiuchi et al., 2007) of conifer buried in the Towada-Hachinohe tephra from Towada Volcano, NE Japan. As a primary step of investigating the solar cycle at the last deglaciation, we checked up pretreatment methods and attempted a high-precision 14C measurement. The results of the 14C measurement for 33 single-year interval tree rings, we detected about 7-year cycle and 14- year cycle. In recent solar variations, 22-year solar cycle, double length cycle of 11-year solar cycle, appears accompanied by 11-year solar cycle. Therefore, the length of the solar cycle at the transition from glacial to deglacial stage is slightly shorter than that of present days. Our data show a slightly shorter periodicity in the 33-year interval of the last deglaciation, suggesting the increase in sunspots at this transition. Radiocarbon analysis of tree rings from the buried forest may be a means towards the elucidation of correlation between solar activity and climate changes.