regions near the surface of the Sun, cosmogenic isotope variations occur because magnetic fields in the extended solar atmosphere in interplanetary space (the heliosphere) modulate the flux of galactic cosmic rays that reach Earth’s atmosphere (see Bard et al., 2000; Crowley, 2000; Webber and Higbie, 2003). Thus, the exact relation between the two is far from clear.

Even so, several recent studies document a relatively close connection at millennial timescales between regional climate proxies and nuclide variations (colored heavy lines in the smoothed records in Figure 3-5). Correlations have been found in a number of Holocene records from regions influenced by the Indian and Asian monsoons, in cave deposits from Europe, in marine sediments from the North Atlantic and the Gulf of Mexico, and in records of precipitation from southwestern Alaska (Bond et al.,

FIGURE 3-5 Changes in the cosmogenic nuclides (a) ¹⁰Be and (b) ¹⁴C over the last 12,000 years. These changes are taken as proxies of solar activity. Up-pointing peaks indicate reduced activity. ¹⁰Be from GRIP/GISP ice cores and ¹⁴C from treering measurements. Light black lines are the detrended raw records; heavy colored line represents the same data subject to a binomial smoothing to bring out millennial variability. SOURCE: Adapted from Bond et al. (2001).