Although studies of the peak in Δ14C in AD 775 (henceforth, M12) have been performed by many researchers since Miyake first discovered M12 (Miyake et al. Reference Miyake, Nagaya, Masuda and Nakamura2012), the cause of the peak and its geographic variability remain under investigation. The dates of peak onset were identified as June–August of AD 774 by Büntgen et al. (Reference Büntgen and Lukas2018) and April–June of AD 774 by Uusitalo et al. (Reference Uusitalo, Arppe, Hackman and Helama2018).
Many causes have been proposed for M12, including a solar proton event (SPE) (Melott and Thomas Reference Melott and Thomas2012; Miyake et al. Reference Miyake, Nagaya, Masuda and Nakamura2012; Usoskin et al. Reference Usoskin, Kromer and Ludlow2013; Jull et al. Reference Jull, Panyushkina and Lange2014; Mekhaldi et al. Reference Mekhaldi, Muscheler and Adolphi2015), a supernova (Miyake et al. Reference Miyake, Nagaya, Masuda and Nakamura2012), a gamma ray burst (GRB) (Hambaryan and Neuhäuser Reference Hambaryan and Neuhäuser2013; Pavlov et al. Reference Pavlov, Blinov and Konstantinov2013), and periods of low solar activity (Neuhäuser and Neuhäuser Reference Neuhäuser and Neuhäuser2015). A supernova and a GRB are now viewed as less likely explanations, whereas an SPE remains under consideration. Obtaining M12 data from a range of latitudes, at subannual resolution, could provide insight into 14C production processes and solar activity in relation to M12, including the underlying mechanisms.
This study presents Δ14C values for earlywood and latewood tree rings from pinyon pine (Pinus edulis) in the period AD 770–780, sampled from Mummy Cave, Canyon de Chelly National Monument, Chinle, Arizona, USA. From the results, we can determine the onset time of M12 more precisely.
SAMPLES AND METHODS
A pinyon pine (Pinus edulis) archaeological sample excavated at Mummy Cave, Chinle, Arizona, USA (N36°14′, W109°22′, ca. 1950 m a.s.l., Figure 1), grew during the M12 event and was analyzed in this study. Its tree rings were dated by dendrochronology (Stokes and Smiley Reference Stokes and Smiley1968) in the Laboratory of Tree-Ring Research (LTRR), University of Arizona. To measure Δ14C in earlywood and latewood, tree rings (AD 770–780) were carefully separated under a binocular microscope.
Pinyon is a two-needle pine species, with a range that encompasses Colorado, southern Wyoming, eastern and central Utah, northern Arizona, New Mexico, and the Guadalupe Mountains in far western Texas. This pine also occurs at moderate altitudes of 1600–2400 m a.s.l.
Tree-ring samples for AD 770–780 underwent an acid-base-H2O2–acid treatment to extract holocellulose and were then burned to produce CO2. Subsequently, the samples were reduced to graphite and the radiocarbon content was measured using the accelerator mass spectrometry facility at the Korean Institute of Geoscience and Mineral Resources in Daejeon, Korea.
RESULTS AND DISCUSSION
The Δ14C values for earlywood and latewood (AD 770–780) pinyon pine samples from Mummy Cave are presented in Figure 2 and were compared with the results of Miyake (Miyake et al. Reference Miyake, Nagaya, Masuda and Nakamura2012). Earlywood is estimated to form from late May to July and latewood from late July to August based on growth monitoring of pinyon pine in the 1960s by Fritts et al. (Reference Fritts, Smith and Stokes1965) at Mesa Verde, ca. 160 km to the northeast. Consequently, earlywood values were simply plotted at the year +0.4 position, and latewood values at the year +0.6 position (e.g., AD 774 earlywood was plotted at AD 774.4 and AD 774 latewood was plotted at AD 774.6). The Δ14C values for the latewood (AD 774, 775, and 776) were all significantly higher than those of the earlywood. This indicates that during the latewood’s growing period (July and August), Δ14C values were elevated in the atmosphere around Chinle, Arizona.
A source region for the excess 14C in M12 at high latitudes in the Northern Hemisphere stratosphere could be analogous to the 14C bomb peak associated with nuclear atmospheric tests. During above-ground testing, 14C was produced mainly at high latitudes in the Northern Hemisphere and was rapidly transported into the upper stratosphere via the nuclear fireball (Nydal and Lövseth Reference Nydal and Lövseth1983). Before the Test Ban Treaty (5 August 1963), many nuclear tests were performed in 1961 and 1962; thus, Δ14C in the atmosphere increased dramatically in 1962 and 1963, where it should be noted that there is a 1-year difference between the time of nuclear tests and the peak of Δ14C in atmospheric CO2 (Nydal and Lövseth Reference Nydal and Lövseth1983). This is due to Brewer–Dobson circulation, which causes an extratropical injection of stratospheric air into the troposphere on an annual basis during the spring and summer (Holton et al. Reference Holton, Haynes and McIntyre1995; Stohl et al. Reference Stohl, Bonasoni and Cristofanelli2003; Butchart Reference Butchart2014). Thus, the main reason for the Δ14C peak in 1963 was the 14C in the stratosphere at high Northern Hemisphere latitudes produced by nuclear atmospheric tests occurring in the second half of 1962. Similarly, 14C released from nuclear tests in 1961 was the main source for the increase in Δ14C in 1962.
The lack of an increase in Δ14C in the earlywood of the AD 774 ring indicates that air containing the pulse of elevated 14C produced at high latitudes had not yet reached northern Arizona by the time photosynthesis and metabolic processes (Grootes et al. Reference Grootes, Farwell, Schmidi, Leach and Minze1989) initiated tree-ring formation in earlywood. However, elevated Δ14C levels in the AD 774 latewood would allow for the peak high-latitude 14C production to have occurred at the beginning of AD 774 in late winter or spring.
Mixing with low-Δ14C air from low latitudes may also help to explain why the amount of Δ14C in the earlywood for AD 775 was lower than the amount for AD 774 latewood. Driven in part by intense winter cooling at the poles, the Brewer–Dobson circulation (Holton et al. Reference Holton, Haynes and McIntyre1995; Stohl et al. Reference Stohl, Bonasoni and Cristofanelli2003; Butchart Reference Butchart2014) moves air towards the winter pole and to lower altitudes; however, this descending high Δ14C air is blocked within the polar vortex until it breaks up in the spring. If the Δ14C content of low-latitude tropospheric air was low, mixing between mid- and low-latitude air throughout the winter could dilute Δ14C in mid-latitude air, thus lowering the Δ14C content of the AD 775 earlywood. Measurement of the Δ14C content of earlywood and latewood from low latitudes would be crucial for testing this hypothesis.
The mid-latitude location of Chinle lies in a dry area dominated by downward convective flow from the subtropical zone, with low levels of Δ14C. Hence, the low-Δ14C air from the subtropics may explain the low levels of Δ14C in AD775 earlywood, and the relatively small Δ14C increase in AD 775 latewood at this sampling site, compared to the increase found in tree rings at higher latitudes.
The Δ14C of the earlywood for AD 776 was slightly higher than that of the latewood for AD 775, and the Δ14C of the latewood for AD 776 was the highest in our data set. This is similar to the delayed rise to the maximum Δ14C measured in annual tree rings during the bomb peak. The highest Δ14C amount yielded by nuclear tests occurred in 1962, and the peak Δ14C in mid-latitude atmospheric CO2 was reached in 1963; however, the highest Δ14C amount in annual tree rings from mid-latitudes appeared later, in 1964 (Grootes et al. Reference Grootes, Farwell, Schmidi, Leach and Minze1989; Hua et al. Reference Hua, Barbetti and Rakowski2013). The average Δ14C amount of earlywood and latewood in AD 776 was similar to the annual (whole-ring) mid-latitude amounts reported by other researchers (Figure 3).
Particles from supernovae and galactic cosmic rays have higher energies than those produced in SPEs, and so are less influenced by the geomagnetic field; gamma rays from GRBs are entirely unaffected. If the spike in 14C arose from one of these more exotic processes (supernovae and galactic cosmic rays), 14C production would show a far less pronounced peak at the poles than if an SPE were responsible. Although SPEs can penetrate the atmosphere to produce 14C at high latitudes, they are unlikely to produce 14C at low latitudes. Hence, if the posited difference in Δ14C amounts between high- and low-latitude air masses was confirmed, the likelihood that M12 was caused by an SPE would be increased (Uusitalo et al. Reference Uusitalo, Arppe, Hackman and Helama2018). Measurements of Δ14C at subannual resolution in Southern Hemisphere tree rings, particularly from higher latitudes, would therefore be of interest.
The Δ14C values from earlywood and latewood annual rings of pinyon pine (Pinus edulis) from Mummy Cave, Chinle, Arizona, USA, were measured for the period AD 770–780. The results showed a small increase in Δ14C in latewood for AD 774, and larger increases for AD 775 and 776. A small increase in Δ14C at mid-latitudes, beyond the error range of the latewood from AD 774, suggests a spike in 14C production as early as the late winter or spring of that year.
The lower value of Δ14C for AD 775 earlywood, relative to AD 774 latewood, is consistent with a low Δ14C amount in air at low latitudes, but confirmation of this hypothesis will require measurement of low-latitude earlywood and latewood Δ14C amounts around AD 775. Measurements of earlywood and latewood, or annual rings, from higher latitudes of the Southern Hemisphere would also help to determine whether high 14C production occurred in the polar region of both hemispheres. This, in turn, would reduce the number of potential mechanisms responsible for the M12.
This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources, funded by the Ministry of Science and ICT of Korea.
To view supplementary material for this article, please visit https://doi.org/10.1017/RDC.2020.108