400 YEAR RECORD OF DELTA-C-14 IN TREE-RINGS - THE SOLAR-ACTIVITY CYCLE BEFORE, DURING AND AFTER MAUNDER MINIMUM AND THE LONGER CYCLES

The analysis of Delta(14)C in dated tree ring series from 1600 to 1940 AD is presented. The data series is divided into two parts, namely before and after 1780 AD to better study the characteristic of its cyclicities before, during and after the Maunder Minimum period. The data after 1780 AD show the presence of significant cyclical variations in the 10-12 year range, similar to the ones observed in other solar-activity index series. On longer time scales, only the <<Gleissberg>> cycle, which appears to modulate the 11 y cycle, seems to be significant. The origin of this last fluctuation in radiocarbon production is influenced by cosmic-ray intensity variations connected to long-term solar-wind flux variations. The analysis of Delta(14)C in two independent series of dated tree rings during 1564-1780 AD shows that there is an enhancement in radiocarbon level during the Maunder Minimum. Our data show that there are time variations of C-14 production rate even during the Maunder Minimum, as cosmic rays were modulated by the Sun in spite of the very low solar activity.