Solar modulation of cosmogenic nuclide production over the last millennium: comparison between C-14 and Be-10 records

For about the last 30 years it has been recognized that the high frequency component of the tree rings C-14/C-12 record is dominated by the modulation of the cosmic ray flux by the solar wind. In particular, it has been demonstrated that the three most recent periods of low sunspot occurrence were characterized by high values of atmospheric C-14/C-12. During the last millennium other periods of high C-14/C-12 Values were observed but their solar origin is still debatable. In the present work we compare these fluctuations with an independent record of cosmogenic Be-10 measured in ice from the South Pole to check the solar origin of the observed C-14/C-12 variations. In order to compare quantitatively the results obtained on Be-10 and C-14, it is necessary to take into account the different behaviour of these two cosmogenic isotopes, and especially the damping effect of the carbon cycle in the case of C-14. As an input to a 12-box numerical model we used the relative fluctuations of the Be-10 concentrations record measured in South Pole ice and converted it into a synthetic C-14 record. We took into account the fact that Be-10 modulation is enhanced in polar regions due to the orientation of the geomagnetic field. As expected, the fluctuations of the modelled C-14 record are much smaller (a factor of 20) than those observed for the raw Be-10 record. In addition, the variations are smoother and shifted in time by a few decades. The Be-10-based C-14 variations closely resemble the C-14 measurements obtained on tree rings (R = 0.81). In particular, it is easy to identify periods of maximal C-14/C-12 which correspond to solar activity minima centred at about 1060, 1320 (Wolf), 1500 (Sporer), 1690 (Maunder) and 1820 (Dalton) yr A.D. Cross-correlation calculations suggest that there is no significant lag between the Be-10-based C-14 and the tree-ring C-14 records. Our study strongly suggests the dominance of the solar modulation on the cosmonuclide production variations during the last millennium. (C) 1997 Elsevier Science B.V.