The cosmic-ray events around AD 775 and AD 993 : assessing their causes and possible effects on climate

Abstract: Miyake et al. (2012, 2013) discovered rapid increases of 14C content in tree rings dated to AD 774-5 and AD 992-3 which were attributed to unprecedented cosmic-ray events. These extreme particle events have no counterparts in the instrumental and historical record and consequently praised great interest. Indeed, many studies have tentatively associated the two events to solar proton events (SPE), supernovae, gamma-ray bursts (GRB) and to a cometary event which all differ in terms of their energy spectrum. Furthermore, such outbursts of energetic particles have the potential to deplete atmospheric ozone and impact atmospheric circulation and temperature. In consequence, the aims of this project were twofold. The first and most emphasized was to assess the likelihood for the different suggested causes. The second was to investigate the possible effects of the cosmic-ray events on climate. Cosmogenic radionuclides such as 10Be, 14C and 36Cl arise from the nuclear cascade which is triggered when cosmic-rays reach the atmosphere. These radio-isotopes are produced through different reaction pathways which have different energy dependencies. This discriminant feature could consequently help to better constrain the energy of the incident particles and thus the origin of the two events. Nevertheless, only 14C has been measured so far at annual resolution. In that light, new annually-resolved 10Be was measured from the NGRIP ice core and used in complement with available records of other radionuclides. An exhaustive and very highly-resolved dataset of ions and element compounds from the NGRIP ice core in addition to δ18O records from several Greenland ice cores were utilized in order to investigate the potential effects on climate. The results demonstrate that 10Be concentrations and flux from the NGRIP ice core also exhibit large increases in relation to both events and similar to those reported in tree rings 14C content. This symmetric increase in both radionuclides suggests solar proton events as a cause for both cosmic-ray events. Furthermore, the multiple cosmogenic radionuclide records show that both events were measured around the globe in both hemispheres which is also fully consistent with solar proton events. Calculations based on the different production yields of 10Be, 14C and 36Cl indicate that the two probable SPEs were characterized by a hard spectrum and by unprecedented fluences. As a matter of fact, it is found that the solar flare responsible for the larger of the two events (AD 774-5) was an order of magnitude stronger than the hardest instrumental SPE. More startlingly, it was substantially more energetic than the Carrington event of 1859 which is considered as the strongest reported historical solar flare. Coeval peaks in Pb, Cd and Na ice core concentrations suggest an atmospheric circulation response timed with the larger of the two events. The high Na concentrations are interpreted as increased vigor in marine air masses associated with sustained storminess in the North Atlantic synchronous with the exceptional SPE. Finally, the new NGRIP 10Be measurements from this study shed light on the existence of an unexpected early offset in the Greenland Ice Core Chronology 2005 (GICC05) time scale. Lags of 4 years at AD 993 and of 7 years at AD 775 in GICC05 are reported.