Effects of Global Geomagnetic Field Variations Over the Past 100,000 Years on Cosmogenic Radionuclide Production Rates in the Earth's Atmosphere

Abstract

The production rates of cosmogenic radionuclides, such as 10Be, 14C, and 36Cl, in the Earth's atmosphere vary with the geomagnetic field and solar activity. For the first time, the production rates of several cosmogenic nuclides are estimated for the past 100 ka based on global, time‐dependent geomagnetic field models and a moderate solar‐activity level. In particular, the production rates were high with no notable latitudinal dependence during the Laschamps geomagnetic excursion (41 ka BP). The mean global production of 10Be over the Laschamps excursion was more than two times greater than the present‐day one, whereas the increase was 1.9 times for the Norwegian‐Greenland Sea excursion (∼65 ka), and only 1.3 times for the Mono Lake/Auckland excursion (∼34 ka). All analyzed geomagnetic field models covering the past 100 ka, including the modern and Holocene epochs, lead to hemispheric asymmetry in the production rates, persistent overall time ranges, and reflected in the time‐averaged nuclide production rates. Production rates predicted by the geomagnetic field models are in good agreement with actual measurements from ice cores and sediment records. These global, long‐term production rates are important for a wide range of studies that employ cosmogenic nuclides as a proxy/tracer of different Earth system processes.

Modeling Atmospheric Transport of Cosmogenic Radionuclide 10Be Using GEOS‐Chem 14.1.1 and ECHAM6.3‐HAM2.3: Implications for Solar and Geomagnetic Reconstructions

“…For the subtropical and polar regions instead, an attenuation of up to 35% is observed. The deposition also shows different hemispheric responses to the geomagnetic modulation (up to 10%) due to a persistent hemispheric asymmetry in production rates leading to larger amplitude variations in Northern Hemisphere (Panovska et al, 2023). This asymmetric deposition signal in polar regions is not shown using the H09 mixing scenario from Heikkilä et al (2009) (Figure S6 in Supporting Information S1) as they simply consider the stratosphere source as one well-mixed box that mitigates the asymmetric production signal.…”

“…Finally, they assumed 10 Be is well mixed in the stratosphere and traced the stratospheric 10 Be production as one source box. Thus the H09 mixing scenario does not allow an investigation of potential preservation of hemispherical production differences in deposition due to large‐scale geomagnetic field asymmetries (Panovska et al., 2023).…”