Solar and geomagnetic forcing of the terrestrial radiocarbon cycle.

Abstract

Solar and geomagnetic fields modulate the flux of galactic cosmic rays (GCR) by electromagnetic and turbulent forcing. GCR flux into the Earth's atmosphere produces cosmogenic isotopes such as ¹⁴C. Cosmogenic isotopes then mix into their respective geochemical systems. Cosmogenic isotope archives reflect environmental levels during deposition and provide records of production, GCR flux and solar-geomagnetic modulation. Solar activity indices verify that ¹⁴C activities in tree rings record century-scale solar activity variations including profound solar activity minima, such as the Maunder Minimum, during the past millennium. The radiocarbon calibration tree-ring ¹⁴C data set spans nearly the length of the Holocene providing a well-dated solar-geomagnetic modulation record. Spectral analyses of earlier ¹⁴C data sets have suggested a complex pattern of fundamental, harmonic and combination tones. Many of these features result from a non-stationary higher ¹⁴C variability persisting about 600 years between 1500-year hiati with lower ¹⁴C variability. We have termed this variation the Hallstattzeit cycle (Damon and Jirikowic, 1992a) after a profound episode during the first millennium BC. Two better-known ¹⁴C variations, the ≈210-year Suess cycle, and ≈88-year Gleissberg, respond quite differently to the Hallstattzeit variation. The Suess is strongly amplitude modulated, the Gleissberg is not. Both show evidence of continuous phase change during the high-variability intervals. The Gleissberg and the Suess show amplitude modulation with the inverse of the geomagnetic field intensity. These modulations produce associated side-band spectral peaks confirming the influence of the ≈2100-year Hallstattzeit variance on the Suess and the influence of the Suess cycle upon the phase of the Gleissberg cycle. All periods between ≈2000 and ≈250 years reported by spectral analysis reflect the non-stationary Hallstattzeit variation, most likely through Gibbs phenomena. The Holocene history of solar-wind and geomagnetic modulation of GCR can be reconstructed from the tree-ring ¹⁴C data set. Quasi-periodicity limits extrapolating these variations into the future.