Evidence for large century time-scale changes in solar activity in the past 32 Kyr, based on in-situ cosmogenic C-14 in ice at Summit, Greenland

摘要

We present results of estimates of cosmic ray flux in Greenland at the Summit (3200 m.a.s.l, 72.6 degrees N, 38.5 degrees W), during the past 32 Kyr. We derive these estimates based on concentrations of in-situ cosmogenic C-14 produced in ice crystals. Based on the secular equilibrium concentration of in-situ produced C-14 in quartz in terrestrial rocks, we find that on century time-scales, the cosmic ray production rate of C-14 at the Summit was close to its estimated long-term average production rate, except during 3 periods: (i) during 8500-9500 yr B.P. and 27,000-32,000 yr BY, when the production rate was higher by about a factor of 2, and (ii) during 12,000-16,000 yr BY, when the production rate was lower by a factor of similar to 1.5. The observed variation in cosmic ray flux at the polar site is best attributed to changes in solar activity resulting in variable modulation of terrestrial cosmic ray flux. Changes in the geomagnetic field in the past do not affect the cosmic ray flux at polar latitudes. Likewise, climate changes do not affect the in-situ C-14 record in ice.During the first two epochs, the solar activity must have been very low, as during Maunder Minimum (virtually no sunspots), resulting in essentially no modulation of the cosmic ray flux by the solar plasma. During the low cosmic ray flux epoch, 12,000-16,000 yr B.P., the observed decrease in cosmic ray flux corresponds to high solar activity as seen in 1958 (sunspot number similar to 190).We discuss the proxy evidence from tree ring and sediment based records of atmospheric C-14/C-12 ratios during the three epochs. These records have been used as a measure of changes in cosmic ray flux, and solar activity in the past. However, since they are also appreciably affected by climatic changes, a comparison of the two records is potentially valuable for delineating the nature of past changes in solar activity, and large-scale ocean circulation and air-sea exchange. (c) 2005 Elsevier B.V. All rights reserved.