Toward Reconciling Radiocarbon Production Rates With Carbon Cycle Changes of the Last 55,000 Years

Abstract

Since it is currently not understood how changes in 14C production rate (Q), and in the carbon cycle, can be combined to explain the reconstructed atmospheric Δ14C record, we discuss possible reasons for this knowledge gap. Reviewing the literature, we exclude that changes in the content of atoms in the atmosphere, which produce cosmogenic 14C after being hit by galactic cosmic rays, might be responsible for parts of the observed differences. When combining Q with carbon cycle changes, one needs to understand the changes in the atmospheric 14C inventory, which are partially counterintuitive. For example, during the Last Glacial Maximum, Δ14C was ∼400‰ higher compared with preindustrial times, but the 14C inventory was 10% smaller. Some pronounced changes in atmospheric Δ14C do not correspond to any significant changes in the atmospheric 14C inventory, since CO2 was changing simultaneously. Using two conceptually different models (BICYCLE‐SE and LSG‐OGCM), we derive hypothetical Qs by forcing the models with identical atmospheric CO2 and Δ14C data. Results are compared with the most recent data‐based estimates of Q derived from cosmogenic isotopes. Millennial‐scale climate change connected to the bipolar seesaw is missing in the applied models, which might explain some, but probably not all, of the apparent model‐data disagreement in Q. Furthermore, Q based on either data from marine sediments or ice cores contains offsets, suggesting an interpretation deficit in the current data‐based approaches.

On the Polar Bias in Ice Core 10Be Data

“…(2018) and corrected in Köhler et al. (2022) to a suite of independent data sets. We focus on the Laschamps geomagnetic field minimum around 41 kaBP (Bonhomme & Babkine, 1967) which provides good data coverage from ice core and marine 10 Be records and geomagnetic field reconstructions.…”

“…Since this method is purely statistical, it cannot correct specific production rate oscillations but will remove average biases. One effect of such a correction is, that it increases the amplitude of multimillennial production rate changes obtained from ice cores, bringing also ice core 10 Be and independent 14 C records close together during the glacial, albeit not enough to reconcile current disagreements (Dinauer et al., 2020; Köhler et al., 2022).…”

“…So far, the considerations presented here are merely theoretical but strongly suggest, that a polar bias can be expected to be present in ice core 10 Be records from polar regions. In this section, we compare the 10 Be ice core stack compiled by Adolphi et al (2018) and corrected in Köhler et al (2022) to a suite of independent data sets. We focus on the Laschamps geomagnetic field minimum around 41 kaBP (Bonhomme & Babkine, 1967) which provides good data coverage from ice core and marine 10 Be records and geomagnetic field reconstructions.…”

“…Hence, ice‐core CRN‐records allow the reconstruction of past solar activity and geomagnetic field strength (e.g., Muscheler et al., 2016; Zheng, Sturevik‐Storm, et al., 2021). This approach also provides independent estimates of the relative changes of past 14 C‐production rates which are approximately proportional to those of 10 Be (Poluianov et al., 2016), allowing inferences about past carbon cycle changes (Dinauer et al., 2020; Köhler et al., 2022; Muscheler et al., 2004). However, the underlying assumption for all these applications is that the relative changes of CRN‐fluxes (or concentrations) measured in ice cores are proportional to the global atmospheric production rate changes.…”

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

“…We computed the depositional flux of 10 Be flux in the Northern and Southern polar regions and checked the agreement with the measured values in Greenland and Antarctic ice cores, as shown in Figure 10. In addition, we have also compared the global production of 14 C with the IntCal20‐based 14 C production rates (Köhler et al., 2022) in Figure 10d. Having the nuclide atmospheric production calculated over the whole atmospheric depth and time span of the models, we applied a simple deposition model to calculate the deposition flux in the northern polar region (60°N–90°N).…”