The events shown in 10Be and 36Cl records

The time series of 10Be, 14C and 36Cl which are studied herein are displayed in Fig. 2a–c and Fig. 3a–c where the NGRIP, NEEM and WDC chronologies were adjusted to fit the 14C peaks found by Miyake et al.12,13 (Methods). The natural background level, that is, the contribution of galactic cosmic rays on the production of radionuclides, was established for each record as the average 10Be and 36Cl flux values as well as the average 14C production rate values prior to and following the peaks. The peaks result from a combination of production and deposition effects leading to an apparent temporal broadening of the measured events. We thus assume that the values exceeding 3σ of the natural background level around the AD 775 (filled areas in Fig. 2a–c) and AD 994 peaks (filled areas in Fig. 3a–c) are related to the two events. The amplitudes which we inferred from the peak areas above these natural background levels are displayed in Fig. 2d–f and Fig. 3d–f where error bars were calculated in order to take in consideration measurement uncertainties as well as a background variability of 1σ (due to noise in the data and the 11-year solar modulation variability).

Figure 2: The AD 774/5 event in view of 10Be, 14C and 36Cl. Time series for AD 760–810 (a) of 10Be flux from the NEEM-2011-S1, NGRIP and WDC ice cores in addition to the inferred average 10Be flux (thick blue curve), (b) of modelled 14C production rate based on previously published measurements12 and (c) of 36Cl flux21 in addition to an inset with a longer series spanning AD 500–1500 for 36Cl where the grey rectangle represents the time slice investigated. The dashed lines represent the natural background levels which are set as the average values prior to and following the filled areas. The filled areas represent the estimated production enhancements caused by the cosmic-ray event of AD 774/5. The 10Be and 36Cl series have been corrected for a temporal offset between ice-core and tree-ring chronologies (Methods). The right panel shows radionuclide production enhancements caused by the AD 774/5 event in atoms cm−2 s−1 for 1 year for (d) 10Be, (e) 14C and (f) 36Cl. The radionuclide increases are illustrated with arrows corresponding to the ratio between the inferred flux/production enhancements stacked over 1 year (coloured rectangles) and the estimated background levels (white rectangles). Uncertainties are based on error propagation including measurement errors and a background variability of 1σ. Full size image

Figure 3: The AD 993/4 event in view of 10Be, 14C and 36Cl. Time series for AD 985–1015 (a) of 10Be flux from the NEEM-2011-S1 and NGRIP ice cores in addition to the inferred average 10Be flux (thick blue curve), (b) of modelled 14C production rate based on previously published measurements13 and (c) of 36Cl flux21 in addition to an inset with a longer series spanning AD 500–1500 for 36Cl where the grey rectangle represents the time slice investigated. The dashed lines represent the natural background level which is set as the average value prior to and following the filled areas. The filled areas represent the estimated production enhancement caused by the cosmic-ray event of AD 993/4. The 10Be and 36Cl series have been corrected for a temporal offset between ice-core and tree-ring chronologies (Methods). The right panel shows radionuclide production enhancements caused by the AD 993/4 event in atoms cm−2 s−1 for 1 year for (d) 10Be, (e) 14C and (f) 36Cl. The radionuclide increases are illustrated with arrows corresponding to the ratio between the inferred flux/production enhancements stacked over 1 year (coloured rectangles) and the estimated background levels (white rectangles). Uncertainties are based on error propagation including measurement errors and background variability of 1σ. Full size image

Our 10Be measurements (Figs 2 and 3, and Table 1) show the existence of the AD 774/5 cosmic-ray event in the Arctic NGRIP and NEEM ice cores as well as in the Antarctic WDC ice core with an average flux enhancement of a factor of 3.4±0.3 (total excess flux related to the average annual background flux). We also report the smaller AD 993/4 event in the 10Be records from NGRIP and NEEM with an average flux enhancement of 1.2±0.2 times the natural background level. The agreement between our stacked 10Be fluxes and modelled 14C production rates (Methods) is comparatively good, especially for the AD 774/5 event which exhibits similar peak amplitudes for both radionuclides. We note, however, that the peak amplitudes in 10Be and 14C differ somewhat at AD 993/4 but agree within the margin of errors (Fig. 3d-e, Table 1). The difference likely is due to small uncertainties in the 10Be measurements (as seen in Fig. 3a with a poorer agreement between the NEEM and NGRIP ice-core data) and/or in the modelled 14C production rates (Methods). Our 10Be records also indicate that the cosmic-ray event around AD 775 was considerably stronger amounting to a threefold multiple of the AD 994 10Be peak, which is consistent with the Δ14C measurements from Miyake et al.12,13. The fact that the AD 994 event was weaker leading to a poorer signal-to-noise ratio also could explain the differences between the NGRIP and NEEM 10Be and the tree ring 14C records for that time period. In general, the agreement with the three 10Be series around AD 774/5 is very good albeit displaying a slightly different structure of the peak itself. This is likely to be related to the fact that a constant sampling resolution per depth (as was used for NGRIP) translates into a somewhat variable temporal sampling resolution due to fluctuations in the annual snow accumulation rates and to different sampling in time for the different sites. Our measurements thus provide first unequivocal evidence of a symmetrical production and deposition of 10Be at both poles (bipolar symmetry) during the AD 774/5 event.

Table 1 Summary of results. Full size table

The fact that the 10Be and 14C increases are imprinted over a time span of 2–3 years despite the probable ephemeral aspect of the cosmic-ray events can be explained by the duration of the transport of the radionuclides from the stratosphere, where they are mostly produced22,23, to the ground. A smaller fraction of radionuclides also can be produced in the troposphere, which would deposit more rapidly than stratospheric counterparts, given cosmic rays that are sufficiently energetic. The induced complex transport24,25 of 10Be, 14C and 36Cl could thus lead to a temporal broadening of the deposition peaks. Also, deposition fluxes at a specific ice-core site can be differently impacted by the involved scavenging processes (for example, proportion of wet and dry deposition) and by atmospheric circulation26. We here assume that the relative peak amplitudes at the different sites investigated are not affected by such system effects as supported by the comparable average 10Be flux and similar relative increases at all three locations (Fig. 2a). In addition, 36Cl can be characterized by some mobility in the snowpack27 as it is deposited to ice caps in the form of gaseous HCl. Although this is not expected to be a major source of uncertainty at high accumulation sites27, this in turn implies that outgassing and migration of 36Cl to upper snow layers can occur, further broadening the signal. The 36Cl data from the GRIP ice core should consequently be regarded as more uncertain. Nevertheless, there are two large peaks present around AD 775 and 994 which are likely to be attributable to the two cosmic-ray events. The time profile insets in Fig. 2c and Fig. 3c, which span from AD 500 to 1500, emphasize that the two peaks represent the two most conspicuous features of the record during this time slice with flux enhancement factors of 6.3±0.4 and 2.7±0.3 for the AD 774/5 and 993/4 events, respectively. The estimates of the production increases in 10Be, 14C and 36Cl are listed in Table 1. Based on our measured 10Be data, modelled 14C production rates and to the lower-resolved 36Cl data, the production of 36Cl was the most enhanced during the two cosmic-ray events in accordance with the expectations for lower energy particles relative to galactic cosmic rays (Fig. 1).

Supporting a solar origin

It was recently suggested that the radiocarbon peak measured at AD 774/5 was caused by cometary dust from the collision of a bolide into the atmosphere16. The authors report a large increase in 14C content in corals from the South China Sea around AD 773. They also note that their measurements are coeval with sightings of a comet and dust event documented in ancient Chinese chronicles. However, it was concluded in other studies that the dimensions needed for a comet to account for this additional injection of radiocarbon would need to be significantly more massive28,29 than the previous estimates16. In consequence, the comet would inevitably have had a considerable and observable impact on the geobiosphere. More problematic for the comet hypothesis is that 10Be and 14C fallouts released from a comet disintegrating in the atmosphere would be, at most, hemispherically redistributed so that the event would only be recorded in either one of the hemispheres30. The 36Cl peaks arising from the French nuclear bomb tests which mainly occurred around the 1960s represent a good analogy to this. The related 36Cl fluxes are significantly larger in the southern hemisphere, where the bomb tests had been undertaken31. Moreover, the fact that the peaks around AD 774/5 and 993/4 are reported around the globe and in a multitude of radionuclide records12,13,16,18,30,32,33 in addition to their large amplitude is indicative of an enhanced atmospheric production triggered by an extraordinary influx of cosmic rays in both hemispheres.

It was also suggested that a typical signature of a gamma-ray burst (GRB) on the production of different radionuclides, its ‘isotopic footprint’, would be a distinct increase in 14C and 36Cl but not in 10Be content15. As stated by the authors, the induced secondary neutrons would be at an insufficient energy to initiate spallation reactions on oxygen nuclei and produce detectable amounts of 10Be. Thus, a GRB is inconsistent as a possible astrophysical source for the two events in perspective of our newly obtained 10Be records (Figs 2 and 3). In addition, the two events are rather similar in that they produced abnormal quantities of 10Be, 14C and 36Cl. This leads us to believe that they share the same cause. In purely probabilistic terms, two GRBs striking the Earth within 200 years is unlikely considering the suggested rate of 1 GRB directed at Earth from our galaxy every 125,000 years34. Another diagnostic feature is the above-mentioned bipolar symmetry in the production of 10Be, but also in the production of 14C (ref. 30). This implies that the incoming particles must have been affected and redirected by the geomagnetic field and, thus, that they were charged. This rules out gamma rays (photons) as triggers of the 10Be, 14C and 36Cl peaks at AD 774/5 and 993/4.

Our data, therefore, support the hypothesis that one or several extreme solar proton events are responsible for the radionuclide production peaks measured at AD 774/5 and 993/4 as it is the only option which is in agreement with all available data. Furthermore, the fact that the 36Cl peaks exhibit the largest amplitude mirroring the resonance effect35 shown in Fig. 1 constitutes further evidence for a solar origin, that is, being caused by solar cosmic rays which generally have lower energies than galactic cosmic rays.

Spectral hardness of the SPEs

The conclusion that one SPE (or a series of SPEs) is responsible for the production increase of 10Be, 14C and 36Cl at AD 774/5 (Fig. 2) is of particular significance because it implies that it must have reached an exceptional magnitude. In fact, no solar phenomena, including the Carrington event, have ever been unequivocally associated with a distinct increase in 10Be concentrations in ice cores. Knowledge of the characteristics of this major solar event, such as its spectral hardness and its fluence ≥30 MeV, could consequently help to better estimate the upper limit of the magnitudes of SPEs. The proton fluences of energy ≥30 MeV, or F 30 of an SPE required to yield a given increase in the production rate of a given radionuclide is directly bound to the spectral hardness of the SPE (that is, the proportion of high energy protons compared to low energy protons). For instance, Webber et al.35 have listed the F 30 of observed SPEs and computed estimates of their impact on the atmospheric production of 10Be and 36Cl. They show that the very hard SPE of February 1956 (SPE56), with a F 30 of about 1.8 × 109 protons per cm2 yielded five times more 10Be than the very soft SPE of August 1972 (SPE72) which yet had a F 30 twice as large. Hence it is crucial to ascertain the spectral hardness of the SPEs around AD 775 and 994 in order to reliably evaluate their F 30 . To achieve this, one can exploit the different energy sensitivities of the production rates of cosmogenic radionuclides. For instance, the yield functions of 10Be and 36Cl have very different shapes at low energies (Fig. 1). As such, the production of solar-induced 36Cl nuclides is relatively more sensitive to incident protons at about 30 MeV while the production of 10Be nuclides is, compared to 36Cl, more sensitive to solar protons at about 100 MeV (ref. 35). A small ratio of the relative production enhancement of 36Cl relative to 10Be (36Cl/10Be) would therefore be expected to be indicative of hard SPEs which are characterized by larger amounts of protons ≥100 MeV resulting in a flatter spectrum and vice versa for soft SPEs. As a test, we investigated the relative 36Cl/10Be ratios of notable SPEs which occurred between 1956 and 2005, for which the spectral characteristics are known and for which 10Be and 36Cl production yields have been computed35. The results are listed in Table 2 while the integral fluence spectra of the related solar proton events are plotted in Fig. 4.

Table 2 Relative 36Cl/10Be ratios. Full size table

Figure 4: Event-integrated fluence spectra of recorded SPEs. Integral fluence spectra for 10 notable solar proton events which occurred between 1956 and 2005 (ref. 35). The numbers of the spectra relate to Table 2. The green and blue bands represent the approximate specific peak response energies of 36Cl and 10Be, that is, the incident proton energies at which each radionuclide is mainly produced. The red curves emphasize very hard spectra here defined as leading to a Ground Level Enhancement peak intensity above 1,000% of neutron monitor at sea level on the polar plateau. Modified from ref. 35. Full size image