BERYLLIUM-10 (half-life 1.5 Myr) is produced by spallation of nitrogen and oxygen atoms by cosmic rays in the upper atmosphere. Its production rate is proportional to the flux of cosmic rays, which is modulated by solar activity and the strength of the Earth's magnetic field 1,2. Weakening of the magnetic field allows more cosmic rays to impinge on the Earth's atmosphere, thereby increasing Be-10 production. Here we report that the ocean-wide average accumulation rate of Be-10 in Pacific sediments, which reflects the global average production rate of Be-10 (ref. 3), was at least 25% greater during the height of the most recent ice age (approximately 24,000-16,000 yr ago) than during the Holocene (the past 10,000 yr). The higher production rate of Be-10 records the lower intensity of the geomagnetic field during that period and is consistent with the hypothesis developed to explain the younger C-14 ages of fossil corals compared with ages obtained by U-Th dating 4. These results also point to a more general need to consider variations in production rate in geochronological studies using other cosmogenic nuclides.
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