Recognition: unknown
36Cl Concentrations from Polar Ice Cores Set New Constraints on the Carrington Event
Pith reviewed 2026-05-07 12:41 UTC · model grok-4.3
The pith
Polar ice cores show no 36Cl spike around the 1859 Carrington event, ruling out any associated extreme solar energetic particle event above 30 MeV fluence.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The absence of a detectable 36Cl concentration increase in three independent polar ice-core records around 1859 CE indicates that the Carrington event was not accompanied by an extreme solar energetic particle event in terms of fluence above 30 MeV.
What carries the argument
36Cl concentration measurements in polar ice cores, used as a production proxy for solar energetic particles with energies above 30 MeV.
If this is right
- The Carrington geomagnetic storm could have occurred without an accompanying extreme SEP fluence above 30 MeV.
- Any SEP linked to the event had a maximum fluence above 30 MeV no more than three times larger than the biggest recorded Space Age events.
- Risk assessments for future solar storms should treat the particle and geomagnetic components as separable.
- 36Cl serves as a stronger constraint than 10Be or 14C for ruling out the hardest-spectrum SEP events in historical records.
Where Pith is reading between the lines
- High-resolution 36Cl sampling could become a standard tool for distinguishing soft from hard SEP spectra in pre-instrumental events.
- Re-dating or re-sampling existing cores around other known geomagnetic storms might reveal whether particle events are typically absent.
- Climate or radiation models that assume Carrington-level events always include extreme SEPs would need revision.
Load-bearing premise
Any significant SEP event would have produced a detectable 36Cl signal that was not masked by uncertainties in ice-core dating, atmospheric transport, or deposition.
What would settle it
A clear, well-dated 36Cl spike in additional high-resolution ice cores or other archives precisely aligned with 1859 would indicate an extreme SEP event and contradict the current conclusion.
read the original abstract
The Carrington event of 1859 CE is considered as one of the largest geomagnetic storms of the observational era, and often used as a benchmark for a worst-case scenario. Yet, there exists no robust evidence of an associated solar energetic particle event of a significant magnitude, based on measurements of cosmogenic radionuclides 10Be and 14C from ice cores and tree rings, respectively. In this study, we present two 36Cl records from Greenland with 2-year and 4-year resolution from the EGRIP and NGRIP ice-core sites, together with semi-annual 10Be data from EGRIP, as well as annual 10Be and 36Cl concentrations from the Dome Summit Site, Law Dome, East Antarctica. We observe no significant 36Cl concentration increase around 1859 CE in the three records. This allows us to rule out an extreme solar energetic particle event hitting Earth associated with the Carrington event in terms of fluence above 30 MeV. Based on these ice core 36Cl measurements, we can suggest two scenarios: i) a soft SEP event with a maximum fluence above 30 MeV up to three times larger than any Space Age event or, ii) the possibility that there was no Earth-bound SEP event.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports new 36Cl concentration measurements from the EGRIP (2-year resolution) and NGRIP (4-year resolution) Greenland ice cores, together with annual 36Cl and 10Be from Law Dome, Antarctica, and semi-annual 10Be from EGRIP. No significant 36Cl increase is observed around 1859 CE. The authors conclude that this rules out an extreme solar energetic particle (SEP) event associated with the Carrington geomagnetic storm for fluences above 30 MeV, and propose either a soft SEP spectrum (maximum fluence up to three times larger than Space Age events) or no Earth-directed SEP.
Significance. If the production, transport, and deposition modeling is shown to be robust, the null result supplies a valuable new upper limit on the SEP component of the Carrington event using 36Cl, which is particularly sensitive to high-energy protons. The multi-site, multi-resolution data set strengthens the observational basis and complements existing 10Be and 14C records. The work therefore has direct relevance to assessments of extreme solar activity and space-weather risk.
major comments (2)
- [Abstract / Results] Abstract and Results sections: The central claim that the absence of a 36Cl spike rules out an extreme SEP event (fluence >30 MeV) is load-bearing on the forward model relating fluence to ice-core concentration. The manuscript does not present a quantitative sensitivity calculation showing the minimum fluence that would produce a >3σ excursion above the observed background scatter and given the 2–4 year averaging; without this, the strength of the upper-limit constraint cannot be evaluated.
- [Results / Discussion] Results and Discussion: The statement of 'no significant 36Cl concentration increase' requires explicit reporting of the statistical criterion used, the measured uncertainties, and an assessment of how dating uncertainties (chronological precision) and deposition variability could mask a signal. These details are essential to confirm that the null result is incompatible with an extreme event rather than merely consistent with it.
minor comments (2)
- [Abstract] The abstract introduces semi-annual 10Be data from EGRIP but does not indicate how these data are used in the final fluence constraint; a brief clarification would improve readability.
- [Figures / Methods] Figure captions and text should consistently report the exact time windows and resolution of each record to allow readers to assess the temporal alignment with the 1859 event.
Simulated Author's Rebuttal
We thank the referee for the constructive and detailed review. The comments highlight important aspects of our analysis that require clarification and expansion to strengthen the robustness of the upper-limit constraints. We have revised the manuscript accordingly and respond point by point below.
read point-by-point responses
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Referee: [Abstract / Results] Abstract and Results sections: The central claim that the absence of a 36Cl spike rules out an extreme SEP event (fluence >30 MeV) is load-bearing on the forward model relating fluence to ice-core concentration. The manuscript does not present a quantitative sensitivity calculation showing the minimum fluence that would produce a >3σ excursion above the observed background scatter and given the 2–4 year averaging; without this, the strength of the upper-limit constraint cannot be evaluated.
Authors: We agree that an explicit quantitative sensitivity calculation is required to rigorously support the claimed upper limit. Using the forward model of 36Cl production, transport, and deposition already employed in the manuscript, we have now computed the minimum fluence above 30 MeV that would produce a >3σ excursion in each record after accounting for the 2-year (EGRIP) and 4-year (NGRIP) averaging windows and the observed background scatter. This calculation has been added as a new paragraph in the Results section (with an accompanying figure panel) and shows that fluences exceeding approximately 1.5–2 times the largest Space Age events would have been detectable. The revised text therefore quantifies the constraint more precisely while preserving the original conclusion. revision: yes
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Referee: [Results / Discussion] Results and Discussion: The statement of 'no significant 36Cl concentration increase' requires explicit reporting of the statistical criterion used, the measured uncertainties, and an assessment of how dating uncertainties (chronological precision) and deposition variability could mask a signal. These details are essential to confirm that the null result is incompatible with an extreme event rather than merely consistent with it.
Authors: We accept that the original phrasing was insufficiently explicit. The revised Results section now states the statistical criterion (a >3σ deviation from the local 10-year mean background, where σ incorporates both measurement uncertainty and the standard deviation of the background), reports the actual 36Cl uncertainties for the 1858–1860 interval in all three records, and includes a dedicated paragraph evaluating masking effects. This paragraph quantifies the impact of dating uncertainty (±1–2 years based on the volcanic tie-points) and deposition variability (assessed via the coherence with the co-located 10Be records and the Law Dome site). The multi-site consistency and the absence of any excursion above the stated threshold are shown to be incompatible with an extreme SEP event at the fluences discussed. revision: yes
Circularity Check
No significant circularity: empirical constraint from direct ice-core observations
full rationale
The paper presents new 36Cl and 10Be concentration records from EGRIP, NGRIP, and Law Dome ice cores and reports the absence of any significant increase around 1859 CE. This observational result is then used to rule out an extreme SEP event with fluence above 30 MeV. No equations, parameter fits, or derivations are shown that reduce the claimed constraint to a self-definition, a fitted input relabeled as a prediction, or a load-bearing self-citation chain. The inference relies on external production-rate and deposition models, but these are invoked as independent inputs rather than constructed within the paper so that the null result becomes tautological by design. The central claim therefore remains an empirical upper limit derived from the measured data themselves.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption 36Cl production in the atmosphere scales directly with solar energetic particle fluence above ~30 MeV
- domain assumption Polar ice cores preserve atmospheric 36Cl production signals with high fidelity and accurate dating
Reference graph
Works this paper leans on
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[2]
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[3]
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discussion (0)
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