On the Polar Bias in Ice Core ¹⁰Be Data

Abstract

Ice cores are invaluable archives to constrain past atmospheric production rate changes of cosmogenic radionuclides (CRNs, e.g., 10 Be, 36 Cl, 14 C). The CRN-production rates depend on the incoming flux of cosmic rays which trigger a nuclear cascade in the atmosphere, eventually resulting in CRN production (Lal & Peters, 1967). The cosmic ray flux inside the heliosphere, in turn, depends on the strength of the interplanetary magnetic field, related to solar activity, and the geomagnetic field. 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. The validity of this assumption is an ongoing topic of debate and different studies based on models and/or data have reached different conclusions on the presence of this so-called "polar bias" (

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

“…In response to solar modulation changes, 10 Be deposition in different latitudes varies proportionally to the globally averaged signal and does not show strong deposition biases (less than 5%). This agrees with previous studies showing no strong solar enhancement of 10 Be deposition in polar regions (e.g., Adolphi et al., 2023; Pedro et al., 2012).…”

“…The 10 Be transport and deposition characteristics derived from E63H23 and GEOS-Chem can be simplified into a 12-box mixing model, which implicitly accounts for the transport and deposition influences on 10 Be deposition (e.g., Adolphi et al, 2023). The deposition concentration in the j deposition box (D j ) could be expressed as follows:…”

“…The 10 Be transport and deposition characteristics derived from E63H23 and GEOS‐Chem can be simplified into a 12‐box mixing model, which implicitly accounts for the transport and deposition influences on 10 Be deposition (e.g., Adolphi et al., 2023). The deposition concentration in the j deposition box ( D j ) could be expressed as follows: $$${D}_{j}=\sum\limits _{i=1}^{12}{M}_{ij}\times {C}_{ij}$$$ Where M ij and C ij are the mixing ratio and the latitudinal weighted production rates of the box i for the deposition box j .…”

“…Adolphi et al. (2023) recently proposed that in polar regions, the solar modulation signal in 10 Be is only enhanced by less than 10% compared to the global signal, whereas the geomagnetic signal is attenuated by up to 37%.…”

A radiocarbon spike at 14 300 cal yr BP in subfossil trees provides the impulse response function of the global carbon cycle during the Late Glacial

“…Although most 10 Be production occurs in the stratosphere characterized by intense horizontal mixing and a relatively long residence time [49][50][51], any deviation from complete homogeneous atmospheric mixing would affect the relative amplitudes of the geomagnetic and solar signals embedded in 10 Be records. Recently, Adolphi et al [43] evaluated that geomagnetic variations are dampened by 23-37% in polar ice records. Such a polar atmospheric dampening of the geomagnetic modulation would not occur for 14 C, which is mixed globally.…”