Sunspots cause changes in the solar system's magnetic field and corresponding changes to the cosmic ray flux, and hence to the production of production: geomagnetic reversals and polarity excursions.In a geomagnetic reversal, the Earth's geomagnetic field weakens and stays weak for thousands of years during the transition to the opposite magnetic polarity and then regains strength as the reversal completes.
The known fluctuations in the strength of the earth's magnetic field match up quite well with this oscillation: cosmic rays are deflected by magnetic fields, so when there is a weaker magnetic field, more production and an older apparent age.
A secondary oscillation is thought to be caused by variations in sunspot activity, which has two separate periods: a longer-term, 200-year oscillation, and a shorter 11-year cycle.
can also be produced at ground level, primarily by cosmic rays that penetrate the atmosphere as far as the earth's surface, but also by spontaneous fission of naturally occurring uranium.
A polarity excursion, which can be either global or local, is a shorter-lived version of a geomagnetic reversal.
A local excursion would not significantly affect 14C production.
However, in 1958, Hessel de Vries was able to demonstrate that the ratio had changed over time by testing wood samples of known ages and showing there was a significant deviation from the expected ratio.
This discrepancy, often called the de Vries effect, was resolved by the study of tree rings.
ratio having remained the same over the preceding few thousand years.
To verify the accuracy of the method, several artefacts that were datable by other techniques were tested; the results of the testing were in reasonable agreement with the true ages of the objects.
Two different trends can be seen in the tree ring series.
First, there is a long-term oscillation with a period of about 9,000 years, which causes radiocarbon dates to be older than true dates for the last 2,000 years and too young before that.