Age determinations by the radiocarbon method provide our primary chronology for both geological and archeological events over the last 40,000 years. It has long been known from measurements on wood samples dated by tree rings that the radiocarbon age scale is not identical to the calendar scale. The reason is that the production rate of radiocarbon changes with time due to variations of the strength of the magnetic shield, which deflects away some of the cosmic rays from their encounter with the Earth's atmosphere. The variations are assodated with changes in both the strength of the Earth's magnetic field and the strength of the magnetic field created by the solar wind. Measurements on tree rings show that because of these changes in cosmic ray irradiation of our atmosphere, the radiocarbon time scale has deviated from the calendar scale by as much as 500 years in the period 5000–9000 calendar years ago. But because the tree ring chronology as yet does not extend further back in time, little information is available about the magnitude of the offset before 9000 calendar years ago.
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