36Cl concentrations from polar ice cores set new constraints on the Carrington event

The Carrington event of 1859 CE is considered 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 (SEP) 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- 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 SEP 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. This article is part of the Theo Murphy meeting issue 'Radiocarbon and cosmic radiation events'.