Revisiting radiocarbon production and the glacial carbon cycle during the Laschamps geomagnetic excursion

Reconstructions of atmospheric radiocarbon during the Laschamps geomagnetic excursion show a pronounced increase in Δ¹⁴C. The amplitude of this increase remains poorly reproduced by current carbon cycle models driven by independent ¹⁴C-production rates derived from ¹⁰Be ice-core records or geomagnetic field intensity reconstructions. This mismatch has commonly been attributed to uncertainties in cosmogenic ¹⁴C production rates, potentially arising from the underestimation of global production-rate changes in polar ice-core ¹⁰Be records during periods of strongly reduced geomagnetic field intensity.

Here we present a new global compilation of ¹⁰Be records from ice cores and marine sediments spanning the Laschamps event, providing an improved, globally integrated estimate of cosmogenic nuclide production for the period from 30,000 to 60,000 years BP. This compilation overcomes previous limitations of polar-only ice-core records, is more representative of global production, and is consistent with latest geomagnetic field intensity reconstructions. However, while the revised production rate implies larger ¹⁴C production-rate changes than previous estimates, it remains insufficient to reproduce the full amplitude of the observed Δ¹⁴C increase when implemented in carbon cycle models under conservative parameterization.

Using transient tuning of a simple carbon cycle model, we show that the remaining model–data mismatch is closely linked to signals observed in independent climate proxies, in particular ice-core δ¹⁸O records. This similarity suggests that the interactions between climate changes and carbon cycle dynamics during the glacial period are not adequately represented in current models.

Our results indicate that uncertainties in cosmogenic production alone cannot explain the radiocarbon anomaly associated with the Laschamps event. Instead, they point to a need for improved representations of climate–carbon cycle interactions under glacial conditions. This finding highlights the importance of revisiting carbon cycle dynamics, including carbon reservoir sizes, exchange rates, and circulation changes, in glacial climates, and demonstrates the value of globally integrated cosmogenic isotope records for disentangling production and carbon cycle effects in past radiocarbon variations.