Patterns of Atmospheric 14C Changes

Abstract

Natural atmospheric 14C changes are caused by fluctuations in upper atmospheric 14C production rates (Q) that are related to earth geomagnetic field variations and changes in solar wind magnetic shielding properties. Climate variability may also be responsible for some of the changes because it influences exchange rates of 14C between the various terrestrial carbon reservoirs. Upper atmospheric 14C production rates QM, in at/sec cm2 (earth), were calculated for the past 1200 years from the atmospheric 14C record and a carbon reservoir model. The changes in are compared in detail with the predicted Q variability derived from an Aa solar modulation mechanism and 20th century neutron flux observations. The influence of earth geomagnetic field changes on the magnitude of the solar wind modulation is discussed, and it is shown that the variations in this magnitude agree with the known differences in earth magnetic field intensity during the past 1200 years. The larger calculated QM oscillations during the sixth millennium BP also agree with this concept. Solar wind magnetic as well as geomagnetic forces modulate the incoming cosmic ray flux and explain the main features of the atmospheric 14C record. It is argued that climatic fluctuation is not a dominant cause. The oscillations between 3200 and 3700 BP, as measured by de Jong, Mook, and Becker, differ in rise time from those found for the current millennium.