Hints of large particle events around solar cycle 14 from 36Cl in Greenland ice cores

Extending the observational record of solar energetic particle (SEP) events is key to improve our understanding of exceptionally large solar storms. To achieve this, we can turn to cosmogenic radionuclides such as radiocarbon (14C) in tree rings, as well as beryllium-10 and chlorine-36 (10Be, 36Cl) in ice cores. This is because a cascade of nuclear reactions is triggered when high-energy galactic and solar cosmic rays hit atmospheric constituents eventually leading to the production of radionuclides that are then locked within environmental archives. These archives have revealed several occasions when Earth was irradiated by extreme fluxes of SEPs throughout the past millennia. Among the three radionuclides mentioned previously, the atmospheric production of 36Cl has been shown to be the most enhanced by SEPs. Unfortunately, 36Cl is also the scarcest of these radionuclides in the environment and thus the most difficult to measure. Here we show new 36Cl records, in conjunction with 10Be data, from different Greenland ice cores spanning the turn of the 20th century. These proxy records show years around solar cycle 14 that are strongly enhanced in 36Cl concentrations, hinting towards particularly large SEP events having hit Earth. While solar cycle 14 was one of the smallest in terms of the cycle amplitude since the Dalton Minimum, this cycle was also characterized with at least two geomagnetic superstorms in October 1903 and September 1909. This was attested from the early observations of the solar eruptions, geomagnetic disturbances, and auroral visibilities. Within dating uncertainties, the 36Cl enhancements chronologically coincide with these extreme solar eruptions, although it remains a challenge to robustly assess their link due to the coarse sampling resolution.