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Cosmogenic radionuclide records from polar ice cores provide unique insights into past cosmic ray flux variations. They allow reconstructions of past solar activity, space weather, and geomagnetic field changes, and provide insights into past carbon cycle changes. However, all these applications rely on the proportionality of the ice core radionuclide records to the global mean production rate changes. This premise has been long debated from a model and data‐perspective. Here, we address this issue through atmospheric mixing model experiments and comparison to independent data. We find that all mixing scenarios, which do not assume complete tropospheric mixing, result in a polar bias. This bias is more prominent for geomagnetic field changes than solar modulation changes. The most likely scenario, supported by independent geomagnetic field records and marine 10Be during the Laschamps geomagnetic field minimum, results in a dampening of geomagnetic field induced changes by 23%–37% and an enhancement of solar‐induced changes by 7%–8%. During the Holocene, we do not find conclusive evidence for a polar bias. We propose a correction function that allows deconvolving the glacial ice core record in order to restore proportionality to the global mean signal. Plain Language Summary: Ice core records of cosmogenic radionuclides (e.g., 10Be) are unique archives providing information on past changes in galactic and solar cosmic ray fluxes. The atmospheric production rates vary in response to changes of the cosmic ray flux, which in turn is modulated by solar and geomagnetic shielding. Typically, it is assumed that the deposition of 10Be on the ice varies in proportion to changes in its atmospheric production rate. However, this assumption has been questioned early on, since the geomagnetic shielding against cosmic rays depends on the geomagnetic latitude, being essentially zero at the ice‐core locations. It was proposed, that ice cores may suffer from a "polar bias" which dampens the geomagnetic field signal and enhances the solar signal in ice core 10Be, thus questioning the use of ice‐core 10Be‐data as proxies for global cosmogenic radionuclide production rate changes. Here, we revisit this hypothesis using mixing model experiments in conjunction with a compilation of 10Be data from high and low‐latitude archives and geomagnetic field models. We deduce that a polar bias is likely present in glacial ice‐core 10Be‐records and provide a simple method to approximately restore the proportionality of ice core 10Be data to global production rate changes. Key Points: Mixing‐model experiments and independent data suggest that ice‐core 10Be‐records do not capture a global‐mean signalDuring the glacial, modulation by geomagnetic field changes is dampened by ∼23%. Effects of solar activity changes are enhanced by ∼7%A transfer function is proposed that restores the approximate proportionality of ice core data to global production rate changes [ABSTRACT FROM AUTHOR]

The threat that solar storms pose to our ever‐modernizing society has gathered significant interest in the recent past. This is partly due to the discoveries of large peaks in the content of cosmogenic radionuclides such as radiocarbon (14C) in tree rings and beryllium‐10 (10Be) and chlorine‐36 (36Cl) in ice cores that were linked to extreme solar storms dated to the past millennia. To better assess the threat that they represent, we need to better quantify the relationship between their energy spectrum and their magnitude with respect to the content of the radionuclides that we measure in environmental archives such as ice cores. Here, we model the global production rate that the 59 largest particle storms coming from the Sun have induced for 10Be, 14C, and 36Cl during the past 70 years. We also consider the deposition flux in 10Be and 36Cl over the high latitudes where all Greenland ice cores are located. Our analysis shows that it is unlikely that any recent solar particle event can be detected in 10Be from ice cores. By relating these values to empirical data from ice cores, we are able to quantify different detection limits and uncertainties for 10Be and 36Cl. Due to different sensitivities to solar energetic particles, we assess that 10Be may only be suitable to detect a limited number of extreme solar storms, while 36Cl is suitable to detect any extreme particle event. This implies that the occurrence‐rate estimates of extreme solar storms, based mainly on 14C and 10Be, relate to a small population of potential events. Plain Language Summary: Solar storms occur when the sun expels charged energetic particles and magnetic material towards Earth. They are a threat to astronauts, spacecraft, and electronic infrastructures on the ground. On rare occasions, solar storms can lead to a significant atmospheric production of radioactive isotopes. If extreme enough, this signal can be retrieved from environmental archives such as ice cores and tree rings. The last known instance of this occurred about a millennium ago. It is therefore paramount to better understand to what extent solar particles can produce these isotopes so that we can, in turn, better identify and quantify the events that generated them. In this study, we combine energy distribution profiles of 59 contemporary solar storms with production functions of the previously mentioned isotopes to model their atmospheric production. This enables us to assess the minimum magnitude of a solar storm required to be detected in ice cores. We also show that one of these isotopes‐chlorine‐36 can be used to detect a considerably wider range of extreme events, relative to the other isotopes. These results have important implications for our current assumptions on the likelihood of an extreme solar storm to hit Earth in the near future. Key Points: We model the production rate of the radionuclides 10Be, 14C, and, 36Cl by 59 ground level enhancement eventsWe provide a sensitivity test for ice‐core 10Be and 36Cl produced by extreme solar storms and infer detection limits36Cl is best to detect past solar storms and its lack of data may lead to an underestimate of the occurrence rate of softer events [ABSTRACT FROM AUTHOR]

The annual 14C data in tree rings is an outstanding proxy for uncovering extreme solar energetic particle (SEP) events in the past. Signatures of extreme SEP events have been reported in 774/775 CE, 992/993 CE, and ~660 BCE. Here, we report another rapid increase of 14C concentration in tree rings from California, Switzerland, and Finland around 5410 BCE. These 14C data series show a significant increase of ~6‰ in 5411-5410 BCE. The signature of 14C variation is very similar to the confirmed three SEP events and points to an extreme short-term flux of cosmic ray radiation into the atmosphere. The rapid 14C increase in 5411/5410 BCE rings occurred during a period of high solar activity and 60 years after a grand 14C excursion during 5481-5471 BCE. The similarity of our 14C data to previous events suggests that the origin of the 5410 BCE event is an extreme SEP event. Plain Language Summary The 14C data of tree rings register extreme storms of solar energetic particles (SEPs), far larger than the largest SEP event ever observed in the Space Era. So far, three extreme SEP events have been reported as a rapid 14C increase in tree rings and 10Be and 36Cl increases in ice cores. In this study, we found a new rapid 14C increase in 5410 BCE using annual 14C content in ring samples from California, Switzerland, and Finland. The 14C signature of 5410 BCE event ties the origin of this 14C excursion to an extreme SEP event similar to three other extreme SEP events (774/775 CE, 992/993 CE, and ~660 BCE) confirmed with radionuclides of ice cores. [ABSTRACT FROM AUTHOR]

Investigations of past climate dynamics rely on accurate and precise chronologies of the employed climate reconstructions. The radiocarbon dating calibration curve (IntCal13) and the Greenland ice core chronology (GICC05) represent two of the most widely used chronological frameworks in paleoclimatology of the past 50 000 years. However, comparisons of climate records anchored on these chronologies are hampered by the precision and accuracy of both timescales. Here we use common variations in the production rates of 14C and 10Be recorded in tree-rings and ice cores, respectively, to assess the differences between both timescales during the Holocene. Compared to earlier work, we employ a novel statistical approach which leads to strongly reduced and yet, more robust, uncertainty estimates. Furthermore, we demonstrate that the inferred timescale differences are robust independent of (i) the applied ice core 10Be records, (ii) assumptions of the mode of 10Be deposition, as well as (iii) carbon cycle effects on 14C, and (iv) in agreement with independent estimates of the timescale differences. Our results imply that the GICC05 counting error is likely underestimated during the most recent 2000 years leading to a dating bias that propagates throughout large parts of the Holocene. Nevertheless, our analysis indicates that the GICC05 counting error is generally a robust uncertainty measurement but care has to be taken when treating it as a nearly Gaussian error distribution. The proposed IntCal13-GICC05 transfer function facilitates the comparison of ice core and radiocarbon dated paleoclimate records at high chronological precision. [ABSTRACT FROM AUTHOR]