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Supernovae and Single-Year Anomalies in the Atmospheric Radiocarbon Record

Published online by Cambridge University Press:  01 August 2016

Michael Dee ,
Benjamin Pope ,
Daniel Miles ,
Sturt Manning  and
Fusa Miyake
Michael Dee*
Affiliation:
University of Oxford – RLAHA, South Parks Road, Oxford OX1 3QY, United Kingdom
Benjamin Pope
Affiliation:
University of Oxford – Physics, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, United Kingdom
Daniel Miles
Affiliation:
University of Oxford – RLAHA, South Parks Road, Oxford OX1 3QY, United Kingdom
Sturt Manning
Affiliation:
Cornell University – Cornell Tree-Ring Laboratory, Ithaca, New York, USA
Fusa Miyake
Affiliation:
Nagoya University – ISEE, Nagoya, Japan
*
*Corresponding author. Email: michael.dee@rlaha.ox.ac.uk.
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Abstract

Single-year spikes in radiocarbon production are caused by intense bursts of radiation from space. Supernovae emit both high-energy particle and electromagnetic radiation, but it is the latter that is most likely to strike the atmosphere all at once and cause a surge in 14C production. In the 1990s, it was claimed that the supernova in 1006 CE produced exactly this effect. With the 14C spikes in the years 775 and 994 CE now attributed to extreme solar events, attention has returned to the question of whether historical supernovae are indeed detectable using annual 14C measurements. Here, we combine new and existing measurements over six documented and putative supernovae, and conclude that no such astrophysical event has yet left a distinct imprint on the past atmospheric 14C record.

Keywords

SupernovaMiyake eventgamma flux
Type
Rapid Event in the Natural Atmospheric 14C Content
Information
Radiocarbon , Volume 59 , Special Issue 2: Proceedings of the 22nd International Radiocarbon Conference (Part 1 of 2) , April 2017 , pp. 293 - 302
Copyright
© 2016 by the Arizona Board of Regents on behalf of the University of Arizona 

INTRODUCTION

The rate of natural radiocarbon production is primarily dictated by the abundance of thermalized neutrons in the atmosphere. Their concentration is at its highest in the stratosphere, where they are a secondary product of the incessant cosmic-ray (particle) bombardment (see Lal and Peters Reference Lal and Peters1967; Burr Reference Burr2013). Neutrons of appropriate energy may also be liberated by photonuclear reactions, the most prominent of these effects being the giant dipole resonance (Baldwin and Kleiber Reference Baldwin and Kleiber1947; Povinec and Tokar Reference Povinec and Tokar1979; Pavlov et al. Reference Pavlov, Blinov, Vasilyev, Vdovina, Volkov, Konstantinov and Ostryakov2013), which involves electromagnetic radiation inducing the collective oscillation of all protons against all neutrons in the nucleus. Neutron yields from this effect reach a maximum from photons in the γ-ray region, around 25 MeV (Povinec and Tokar Reference Povinec and Tokar1979; Pavlov et al. Reference Pavlov, Blinov, Vasilyev, Vdovina, Volkov, Konstantinov and Ostryakov2013). Indeed, it has recently been conjectured that terrestrial gamma-ray flashes (TGF) make a minor contribution to atmospheric neutron yields in this fashion (Carlson et al. Reference Carlson, Lehtinen and Inan2010). 14C is formed by the capture of such neutrons by nitrogen [14N(n, p)14C]; other mechanisms are known [such as 16O(n,3He)14C], but their impact is negligible in comparison (Lingenfelter Reference Lingenfelter1963; Masarik and Beer Reference Masarik and Beer2009).

Another potential source of high-energy radiation comes from near-Earth (or galactic) supernovae (SNe). The charged particles emitted by SNe, however, are subject to perturbation by magnetic fields en route to Earth and thus become significantly dispersed and retarded (Güttler et al. Reference Güttler, Adolphi, Beer, Bleicherd, Boswijke, Christl, Hogg, Palmer, Vockenhuber, Wacker and Wundereet2015; Melott et al. Reference Melott, Usoskin, Kovaltsov and Laird2015). In contrast, the γ-ray flux is not impeded in this way and arrives in unison with the visible light, which would have appeared as a new star to premodern observers. Many types of supernovae exist and their luminosities vary widely—commonly between 1046 and 1049 erg (Povinec and Tokar Reference Povinec and Tokar1979; Miyake et al. Reference Miyake, Nagaya, Masuda and Nakamura2012; Güttler et al. Reference Güttler, Adolphi, Beer, Bleicherd, Boswijke, Christl, Hogg, Palmer, Vockenhuber, Wacker and Wundereet2015; Melott et al. Reference Melott, Usoskin, Kovaltsov and Laird2015). A further complication is that SNe may emit γ-rays isotropically or in a highly collimated fashion, making estimation of their impact on Earth even more difficult.

Damon et al. (Reference Damon, Kaimei, Kocharov, Mikheevai and Peristykh1995) claimed that a rise in atmospheric 14C levels around 1006 CE was attributable to the well-attested Type 1a supernova at this time, denoted SN1006 (supernova in the year 1006 CE). Their study comprised 75 conventional 14C measurements on annual tree rings between 1000 and 1010 CE. The observed rise in 14C (~6‰) actually peaked some 2–3 yr after the star was first documented (see Table 1). While this offset was perplexing to the authors, it concurs well with recent modeling of 14C transport through the stratosphere and troposphere (Levin et al. Reference Levin, Naegler, Kromer, Diehl, Francey, Gomez-Pelaez, Steele, Wagenbach, Weller and Worthy2010; Pavlov et al. Reference Pavlov, Blinov, Vasilyev, Vdovina, Volkov, Konstantinov and Ostryakov2013; Güttler et al. Reference Güttler, Adolphi, Beer, Bleicherd, Boswijke, Christl, Hogg, Palmer, Vockenhuber, Wacker and Wundereet2015). Only one attempt has since been made to replicate these findings, and it could not discern any significant uplift around 1006 CE (see Menjo et al. Reference Menjo, Miyahara, Kuwana, Masuda, Muraki and Nakamura2005). The study also failed to detect SN1054, the explosion that generated the Crab Nebula. Indeed, the authors doubted whether any historical SNe was energetic enough to be visible in the 14C record, especially given the ebbs and flows of the Schwabe cycle (Menjo et al. Reference Menjo, Miyahara, Kuwana, Masuda, Muraki and Nakamura2005).

Table 1 Historical records of ephemeral stars thought to be galactic supernovae. The observational records come from Tse-Tsung (Reference Tse-Tsung1957) and Green and Stephenson (Reference Green and Stephenson2003); the distances from Earth for SN185 and SN393 come from Damon et al. (Reference Damon, Kaimei, Kocharov, Mikheevai and Peristykh1995) and the remainder from Firestone (Reference Firestone2014), but estimates vary widely.

Attention recently returned to this issue after Miyake et al. (Reference Miyake, Nagaya, Masuda and Nakamura2012) reported a rapid increase in atmospheric 14C levels in Japanese tree rings between 774 and 775 CE. The single-year anomaly was of unprecedented magnitude (~12‰). A year later, the same team reported very similar data for the years 993–994 CE (Miyake et al. Reference Miyake, Masuda and Nakamura2013). Importantly, the uplifts were only apparent when annual sequences of tree rings were measured, as opposed to the more common practice of analyzing decadal blocks (see Figure 1). Furthermore, it has since been established that the anomalies were globally synchronous and approximately uniform in magnitude. The 775 CE spike has already been uncovered in dendrochronological archives from Germany (Usoskin et al. Reference Usoskin, Kromer, Ludlow, Beer, Friedrich, Kovaltsov, Solanki and Wacker2013), the USA and Russia (Jull et al. Reference Jull, Panyushkina, Lange, Kukarskih, Myglan, Clark, Salzer, Burr and Leavitt2014), and New Zealand (Güttler et al. Reference Güttler, Adolphi, Beer, Bleicherd, Boswijke, Christl, Hogg, Palmer, Vockenhuber, Wacker and Wundereet2015). Henceforth, these single-year spikes in 14C concentration will be referred to as Miyake events.

Figure 1 Published Δ14C data on the Miyake event in 775 CE. The four Northern Hemisphere data sets [Japan (Miyake et al. Reference Miyake, Nagaya, Masuda and Nakamura2012); Germany (Usoskin et al. Reference Usoskin, Kromer, Ludlow, Beer, Friedrich, Kovaltsov, Solanki and Wacker2013); USA and Russia (Jull et al. Reference Jull, Panyushkina, Lange, Kukarskih, Myglan, Clark, Salzer, Burr and Leavitt2014) pertain to the left-hand axis, and the New Zealand data (Güttler et al. Reference Güttler, Adolphi, Beer, Bleicherd, Boswijke, Christl, Hogg, Palmer, Vockenhuber, Wacker and Wundereet2015) to the right-hand axis]. The latter is offset by 5‰ to account for the differences in absolute activity in the two hemispheres. Please see online version to view figure in color.

In addition to their unprecedented abruptness and scale, Miyake events are also unique because they represent significant increases in 14C. A myriad of geological and oceanographic processes can drive depletions, but no terrestrial process— prior to the nuclear age—could be responsible for such sharp enrichments. On this basis, as well as their global impact, it was deduced that the spikes must have been the result of intense pulses of radiation from space. At first, the Sun was not considered a likely cause, as it was not thought capable of emitting radiation of the required energy, so supernovae and other γ-ray sources were preferred (Miyake et al. Reference Miyake, Nagaya, Masuda and Nakamura2012; Hambaryan and Neühauser Reference Hambaryan and Neuhäuser2013; Pavlov et al. Reference Pavlov, Blinov, Vasilyev, Vdovina, Volkov, Konstantinov and Ostryakov2013). However, the consensus now is that intense solar energetic particle (SEP) events were indeed responsible (Melott and Thomas Reference Melott and Thomas2012; Thomas et al. Reference Thomas, Melott, Arkenberg and Snyder2013; Usoskin et al. Reference Usoskin, Kromer, Ludlow, Beer, Friedrich, Kovaltsov, Solanki and Wacker2013; Güttler et al. Reference Güttler, Adolphi, Beer, Bleicherd, Boswijke, Christl, Hogg, Palmer, Vockenhuber, Wacker and Wundereet2015; Mekhaldi et al. Reference Mekhaldi, Muscheler, Adolphi, Aldahan, Beer, McConnell, Possnert, Sigl, Svensson, Synal, Welten and Woodruff2015). SEPs either arise because of extreme solar flares or interplanetary coronal mass ejections (ICMEs). A supernova origin has now effectively been discounted, on two main grounds. Firstly, no historical observations exist for supernovae around 775 or 994 CE, although the expected galactic SN rate of ~1–2 per century does suggest that many past events have gone undetected (Tammann et al. Reference Tammann, Löffler and Schröder1994). As is shown in Table 1, only a handful of observations do exist, and none of them pertain to the night sky of the Southern Hemisphere. Secondly, no galactic supernova remnant can be attributed to an event at either of these dates. The aim of this study is thus to establish categorically whether any historical SNe can be detected in the past atmospheric 14C record.

METHODS

We combined new and existing 14C measurements on annual tree rings that traversed the following historical astronomical records.

1. Star of Bethlehem (SB)

This short-lived star is mentioned twice in the gospel of Matthew. Its historicity and date have long been debated (Tipler Reference Tipler2005), with recent studies centering on 5 BCE (Kidger Reference Kidger1999). For this project, we measured new single rings of oak (Quercus robur) dendrochronologically dated to the years 6–1 BCE from the Roman–British archaeological site of Hacheston (DWH Miles, personal communication).

2. SN185

The appearance of a kèxīng or “guest star” in 185 CE is recorded in the Houhanshu (History of the Later Han Dynasty of Imperial China). Although commonly referred to as the earliest observation of a supernova, this conclusion is by no means unanimous, and some paleographers believe the text refers to a comet (Chin and Huang Reference Chin and Huang1994; Schaefer Reference Schaefer1995; Zhao et al. Reference Zhao, Strom and Jiang2006; Strom Reference Strom2008; Stephenson Reference Stephenson2015). For this event, we measured new single rings of sequoia (Sequoiadendron giganteum), dendrochronologically dated to the years 183–188 CE, from King’s Canyon National Park, USA.

3. SN1006

The supernova in 1006 CE was widely recorded in both the Eastern and Western hemispheres (Stephenson et al. Reference Stephenson, Clark and Crawford1977; Green and Stephenson Reference Green and Stephenson2003). It is thought to have been the brightest star ever witnessed during the historical period (Stephenson et al. Reference Stephenson, Clark and Crawford1977). We measured new single rings of oak (Quercus robur), dendrochronologically dated to the years 1004–1010 CE, originally cored from beams in Salisbury Cathedral (Miles Reference Miles2002). These results were combined with previously published data from Damon et al. (Reference Damon, Kaimei, Kocharov, Mikheevai and Peristykh1995) and Menjo et al. (Reference Menjo, Miyahara, Kuwana, Masuda, Muraki and Nakamura2005).

4. SN1054

This stellar explosion in the Taurus constellation was observed in China in July 1054 (Green and Stephenson Reference Green and Stephenson2003). Its remnant gas clouds now form the Crab Nebula. For this event, we utilize the published results of Menjo et al. (Reference Menjo, Miyahara, Kuwana, Masuda, Muraki and Nakamura2005).

5. SN1572 (Tycho’s Supernova)

This supernova is named for the Danish astronomer Tycho Brahe, who witnessed the appearance of the star in β Cassiopeiae in early 1572 CE and published his observations the following year. For this event, we utilize the single-year tree-ring data of IntCal13 (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffmann, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and van der Plicht2013), which extend back to the mid-16th century CE.

6. SN1604 (Kepler’s Supernova)

The last near-Earth SN to be observed on Earth was more than 400 yr ago, in 1604 CE. Although extensively documented around the world, the most renowned observations were made by Johannes Kepler in his publication Stella Nova in Pede Serpentarii (Kepler Reference Kepler1606). Once more, the single-year tree-ring data of IntCal13 (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffmann, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and van der Plicht2013) are utilized for this event.

The tree rings obtained for this work by the Oxford Radiocarbon Accelerator Unit (ORAU) for the SB and SN185 were treated to α-cellulose in accordance with recently published protocols (Staff et al. Reference Staff, Reynard, Brock and Bronk Ramsey2014). The samples for SN1006 were given the standard pretreatment for wood samples (Brock et al. Reference Brock, Higham, Ditchfield and Bronk Ramsey2010). All the cellulosic fractions extracted were combusted, graphitized, and measured on ORAU’s AMS system, as described in Brock et al. (Reference Brock, Higham, Ditchfield and Bronk Ramsey2010) and Bronk Ramsey et al. (Reference Bronk Ramsey, Higham and Leach2004).

RESULTS

The new Δ14C measurements obtained by ORAU, together with all the previously published data used in this study, are given in Tables S1 and S2 in the supplementary online material. The new and existing data are summarized in Table 2, and graphically in Figure 2, for the 5 yr leading up to and 10 yr following each historical observation. Weighted averages were produced for the three data sets available for SN1006. In one sense, this is not the most effective means of determining whether an uplift occurred at this time, as the absolute data come from different species, and different parts of the Northern Hemisphere. However, if a spike did occur, it should be synchronous across the hemisphere so yearly averaging would not affect this pattern. Nonetheless, the three data sets available for SN1006 are also given independently in Table S1 and Figure S1 of the supplementary online material.

Figure 2 New and previously published Δ14C data over historical observations of known or potential near-Earth supernovae. The horizontal axis is divided into the 5 calendar years leading up to the observation and the 10 years after it.

Table 2 The six astronomical records investigated in this study. Where available, data are given for the 5 years leading up to the first observation and the 10 years thereafter. Weighted averages were calculated for SN1006, as multiple data sets were available for this event. The supplementary online material gives details of all the underlying data (Table S1), as well as the new results expressed as conventional 14C ages (Table S2).

DISCUSSION

While the amalgamated data sets presented here do reveal the natural year-on-year undulation in atmospheric 14C concentration, the trends exhibited by the Δ14C traces in Figure 2 stand in stark contrast to the Miyake event depicted in Figure 1. If anything, a leveling or gradual decrease in atmospheric 14C levels can be discerned in the data for the 10 years following each historical observation. It is important to emphasise that the observation dates for all the supernovae in the 2nd millennium CE are exactly known. Thus, any rise in 14C that predates these historical records, as evident for SN1054, cannot be casually linked with the stellar explosion. To reiterate, the gamma flux form a supernova would arrive at the same time as the visible light, and any potential impact on 14C levels would only be evident after this point in time.

Despite the lack of any distinct spikes in the data, the precision of individual 14C measurements remains an issue. It is possible that the γ-ray flux from these SNe did increase 14C production by <1‰, and the resultant shifts are simply not detectable by this approach. Moreover, although improvements to accelerator mass spectrometry (AMS) precision are proceeding apace, distinguishing anomalies at such levels of sensitivity is not thought likely in the foreseeable future. Indeed, it is not possible yet to define which precise radiation-producing events may be detectable by this method. As alluded to earlier, the causes of gamma-ray impacts on the Earth are many and varied and their impacts hard to resolve. For example, even if a more pronounced single-year rise is detected in the future, it cannot automatically be assumed that a supernova is not the cause. On the contrary, Miyake events are thought to represent the upper end of solar emissions (Eichler and Mordecai Reference Eichler and Mordecai2012; Usoskin et al. Reference Usoskin, Kromer, Ludlow, Beer, Friedrich, Kovaltsov, Solanki and Wacker2013; Cliver et al. Reference Cliver, Tylka, Dietrich and Ling2014), which implies that upsurges of greater magnitude may require extra-solar explanations. An intense pulse of γ-rays from a very nearby SN should remain a possible cause, especially when surveying data over kiloyear timescales. Definitive evidence may be found using other proxies. For example, it has long been hypothesized that intense bursts of high-energy γ-flux would also be accompanied by ozone depletion, on account of increased initiation of nitrogen radicals in the atmosphere (Ruderman Reference Ruderman1974). However, the search for geochemical and paleoecological evidence in support of these hypotheses has also proven inconclusive, or implied extremely low rates of occurrence (Reid et al. Reference Reid, McAfee and Crutzen1978; Ellis and Schramm Reference Ellis and Schramm1995; Benitez et al. Reference Benitez, Maíz-Apellániz and Canelles2002; Gehrels et al. Reference Gehrels, Laird, Jackman, Cannizzo, Mattson and Chen2003).

With regard to the exact mechanisms behind Miyake events, however, the approach applied here may provide further important information. It has already been speculated that the 775 CE event may be more accurately described as a “superflare.” Using Kepler photometry, Maehara et al. (Reference Maehara, Shibayama, Notsu, Nagao, Kusaba, Honda, Nogami and Shibata2012) showed that superflares are common on sun-like stars. Determining whether this is true also of the Sun, and what might be driving such superflares, is an active topic of research. As noted by Melott and Thomas (Reference Melott and Thomas2012), if the 775 CE anomaly was caused by a solar superflare, a recurrence may pose a significant threat to modern technological civilization, potentially destroying satellites and Earth-bound electrical infrastructure. From Kepler analysis of oscillations in stellar superflares (Balona et al. Reference Balona, Broomhall, Kosovichev, Nakariakov, Pugh and van Doorsselaere2015) and associated starspot-related photometric variability (Notsu et al. Reference Notsu, Shibayama, Maehara, Notsu, Nagao, Honda, Ishii, Nogami and Shibata2013; Maehara et al. Reference Maehara, Shibayama, Notsu, Notsu, Honda, Nogami and Shibata2015), it appears likely that superflares, like lesser flares, are powered by the energy stored in a star’s magnetic field configuration. It is not yet clear, however, if these occur on the Sun as rare events drawn from the same distribution as ordinary solar flares, or if the occurrence of superflares is confined to younger stars (Wichmann et al. Reference Wichmann, Fuhrmeister, Wolter and Nagel2014). A long-term radioisotope record of solar activity, including Miyake events, will help answer this question.

CONCLUSION

In contrast with Damon et al. (Reference Damon, Kaimei, Kocharov, Mikheevai and Peristykh1995), we have uncovered no evidence that SN1006 or any of five other historical or putative SNe caused detectable uplifts in the atmospheric concentrations of 14C. However, this approach still retains enormous potential for elucidating the origin and nature of past radiation impacts on Earth.

ACKNOWLEDGMENTS

The 14C measurements obtained by ORAU this work were funded by the Balliol Interdisciplinary Institute. M Dee is supported by a Leverhulme Trust Early Career Fellowship.

SUPPLEMENTARY MATERIAL

To view supplementary material for this article, please visit http://dx.doi.org/10.1017/RDC.2016.50

Footnotes

Selected Papers from the 2015 Radiocarbon Conference, Dakar, Senegal, 16–20 November 2015

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Figure 0

Table 1 Historical records of ephemeral stars thought to be galactic supernovae. The observational records come from Tse-Tsung (1957) and Green and Stephenson (2003); the distances from Earth for SN185 and SN393 come from Damon et al. (1995) and the remainder from Firestone (2014), but estimates vary widely.

Figure 1

Figure 1 Published Δ14C data on the Miyake event in 775 CE. The four Northern Hemisphere data sets [Japan (Miyake et al. 2012); Germany (Usoskin et al. 2013); USA and Russia (Jull et al. 2014) pertain to the left-hand axis, and the New Zealand data (Güttler et al. 2015) to the right-hand axis]. The latter is offset by 5‰ to account for the differences in absolute activity in the two hemispheres. Please see online version to view figure in color.

Figure 2

Figure 2 New and previously published Δ14C data over historical observations of known or potential near-Earth supernovae. The horizontal axis is divided into the 5 calendar years leading up to the observation and the 10 years after it.

Figure 3

Table 2 The six astronomical records investigated in this study. Where available, data are given for the 5 years leading up to the first observation and the 10 years thereafter. Weighted averages were calculated for SN1006, as multiple data sets were available for this event. The supplementary online material gives details of all the underlying data (Table S1), as well as the new results expressed as conventional 14C ages (Table S2).

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Figure S1 and Tables S1-S2

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