Search

Annually resolved radiocarbon (14C) measurements on tree rings led to the discovery of abrupt variations in 14C production attributed to large solar flares. We present new results of annual and subannual 14C fluctuations in tree rings from a middle-latitude sequoia (California) and a high-latitude pine (Finland), analyzed for the period 1030–1080 CE, to trace a possible impact of the Crab supernova explosion, occurring during the Oort minimum of solar activity. Our results indicate an increase of Δ14C around 1054/55 CE, which we estimate is higher in magnitude than the cyclic variability due to solar activity at a 2σ significance level. The net signal appears to be synchronized in the studied locations. Several sources of this event are possible including γ-rays from the Crab supernova, an unusually weak solar minimum or a solar energetic particle incident. More data are needed to provide more insight into the origin of this 14C event.

A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-21647-w

Nature Communications, 12 (1), ISSN:2041-1723

Excursions in the radiocarbon (14C) record, which are rapid changes on a scale of a few years are presumed to be caused by an increase of incoming cosmic rays. The excursions at AD 774-775AD and 993-994AD have generated widespread interest and have been reproduced in many different tree-ring records (Miyake et al. 2012, 2013, 2017; Büntgen et al. 2018). Similar structures have also been detailed, such as at 660BC (Park et al. 2017; O’Hare et al. 2019). Other types of change in 14C production may be due to a mix of SPE and different phenomena, such as around 5480BC (Miyake et al. 2017) and 815BC (Jull et al. 2018). We note that a proposed SPE event about 3371BC (Wang et al. 2018) is currently unconfirmed and this emphasizes the need for an anchored dendro-record to determine possible events. Timing of these events is important to understand the underlying recurrence intervals. A considerable number of processes can affect the cosmic-ray flux, including solar events, gamma-ray bursts, geomagnetic shifts and relatively close supernovae. Such studies are providing a wealth of new information through which to characterize new ‘events’ in 14C structure and to begin to understand the processes behind them. These effects also have introduced more complexity to the international radiocarbon calibration curve.This research was supported in part by the European Union and the State of Hungary, co-financed by the European Regional Development Fund in the project of GINOP-2.3.2-15-2016-00009 ‘ICER’.References: Büntgen et al. 2018. Nature Communications 9: 3605; A. J. T. Jull et al. 2014. Geophysics Research Letters 41: 3004-3010; F. Miyake et al. 2012. Nature 486: 282-284; F. Miyake et al. 2013. Nature Communications 4: 1748; F. Miyake et al. 2017. PNAS 114: 881-884; P. O’Hare et al. 2019. PNAS 116: 5961-5966; J. Park et al. 2017. Radiocarbon 59: 1147-1156; F. Y. Wang et al. Nature Communications 18: 1487.

Two radiocarbon (14C) excursions are caused by an increase of incoming cosmic rays on a short time scale found in the Late Holocene (AD 774–775 and AD 993–994), which are widely explained as due to extreme solar proton events (SPE). In addition, a larger event has also been reported at 5480 BC (Miyake et al. 2017a), which is attributed to a special mode of a grand solar minimum, as well as another at 660 BC (Park et al. 2017). Clearly, other events must exist, but could have different causes. In order to detect more such possible events, we have identified periods when the 14C increase rate is rapid and large in the international radiocarbon calibration (IntCal) data (Reimer et al. 2013). In this paper, we follow on from previous studies and identify a possible excursion starting at 814–813 BC, which may be connected to the beginning of a grand solar minimum associated with the beginning of the Hallstatt period, which is characterized by relatively constant 14C ages in the period from 800–400 BC. We compare results of annual 14C measurements from tree rings of sequoia (California) and cedar (Japan), and compare these results to other identified excursions, as well as geomagnetic data. We note that the structure of the increase from 813 BC is similar to the increase at 5480 BC, suggesting a related origin. We also assess whether there are different kinds of events that may be observed and may be consistent with different types of solar phenomena, or other explanations.

Two radiocarbon excursions (AD 774–775 and AD 993–994) occurred due to an increase of incoming cosmic rays on a short timescale. The most plausible cause of these events is considered to be extreme solar proton events (SPE). It is possible that there are other annual 14C excursions in the past that have yet to be confirmed. In order to detect more of these events, we measured the 14C contents in bristlecone pine tree-ring samples during the periods when the rate of 14C increase in the IntCal data is large. We analyzed four periods every other year (2479–2455 BC, 4055–4031 BC, 4465–4441 BC, and 4689–4681 BC), and found no anomalous 14C excursions during these periods. This study confirms that it is important to do continuous measurements to find annual cosmic-ray events at other locations in the tree-ring record.