We present two new millennium-long tree-ring oxygen isotope chronologies for central and northern Japan, based on 9693 annually resolved measurements of tree-ring oxygen isotopes from 39 unearthed samples consisting mainly of Japanese cedar (Cryptomeria japonica). These chronologies were developed through cross-dating of tree-ring widths and δ18O data from multiple samples covering the periods 2349–1009 BCE (1341 yr) and 1412–466 BCE (947 yr) for central and northern Japan, respectively. In combination with our published chronology for central Japan, the tree-ring δ18O dataset currently available covers the past 4354 yr (2349 BCE to 2005 CE), which represents the longest annually resolved tree-ring δ18O dataset for Asia. Furthermore, the high-resolution temporal record of 14C contents independently developed by Sakurai et al. (2020) was reproduced by our 14C measurements of earlywood and latewood in annual rings for the period 667–660 BCE.

Regional offsets from Northern Hemisphere radiocarbon (14C) calibration curves are widely recognized for monsoon Asia and often hinder accurate 14C dating. In this paper, we explore the possible linkage between summer monsoon intensity and 14C offsets using tree-ring δ18O and 14C data from Thailand. We developed a 297-yr floating tree-ring δ18O chronology comprising seven teak log-coffin samples from the Ban Rai rock shelter site, northwestern Thailand. The outermost ring of our chronology was estimated to date from 358–383 CE, within a 95.4% (2σ) probability range, based on a total of 10 14C measurements that were wiggle-matched against a mixed calibration curve evenly weighted from the IntCal20 and SHCal20 curves. Backward trajectory analysis showed that an intensified (weakened) summer monsoon detected in a modern tree-ring δ18O chronology was most likely to be induced by increased (decreased) air mass transport from the tropical Indian Ocean, which is an area of intense upwelling where the 14C concentration is lower than the atmospheric 14C level. However, partly because of the limited sample size and dating uncertainty, the direct linkage between the tree-ring δ18O series and 14C records obtained from our teak log-coffin samples could not be statistically verified.

Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.

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.

The so-called Millennium Eruption of Baitoushan Volcano is one of the largest of the Common Era but its date has been uncertain. Recently, Oppenheimer et al. (2017) reported the eruptive year as late AD 946 using a new method called carbon-14 spike matching. However, it is necessary to verify their result to confirm the eruptive year, since only one wood sample was used in their study. We verified the eruptive year by measuring 14C contents in tree rings from another wood sample buried during the Baitoushan eruption. We succeeded in reproducing the AD 774–775 14C spike (Miyake et al. 2012), and counted the number of rings from the outermost ring accompanied by bark to the ring possessing the AD 774–775 14C spike. We found the outermost ring was formed in AD 946. Our study supported the result of Oppenheimer et al. (2017), which makes the eruptive year conclusive. Also, we suggest that the 14C spike-matching method can be a prominent dating tool applicable to ancient woods that are difficult to date using the usual dendrochronology techniques.

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.

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.

A rapid yearly increase in the radiocarbon content has been detected for the period from AD 993 to 994. However, this event is supported by the 14C measurements of only one cedar tree sample, and verification is necessary to confirm this event more reliably. For this purpose, this study measured the 14C content in Japanese Hinoki tree rings corresponding to the period from AD 988 to 997 using the accelerator mass spectrometry system at Yamagata University (YU-AMS). The result shows a significant 14C increase from AD 993 to 994, and is consistent with the previously measured data for the Japanese cedar tree. This marks the second case detecting an increased 14C level corresponding to the AD 994 event.

To investigate the radiocarbon concentration of atmospheric CO2 over the past few millennia in Japan, we measured the 14C age of annual rings from 3 Japanese trees with calendar dates ranging from ∼2000 yr old to present, and we compared the tree-ring 14C age with the corresponding 14C age from IntCal09. In some instances, the 14C ages of the annual rings of Japanese trees are not consistent with the IntCal09 data sets. Often, the 14C ages of tree rings are older than those from IntCal09, but younger than those from the SHCal04 data sets. The average shifts in the Nagoya 14C age from IntCal09 data sets and 1σ errors were determined to be +26 ± 36, +24 ± 30, +16 ± 22, +5 ± 21, and +14 ± 22 14C yr for the intervals AD 72– 382, 589–1072, 1413–1615, 1617–1739, and 1790–1860, respectively. The Japanese Archipelago is situated near the boundary of the Intertropical Convergence Zone in summer, and the 14C concentration of atmospheric CO2 over Japan can be influenced by air masses of the Southern Hemisphere with lower 14C concentrations during periods of higher solar activity and heightened East Asian summer monsoons. Our results suggest that the Japanese Archipelago is located in a critical zone where it is difficult to calibrate the 14C age of tree-ring samples using existing calibration data sets. It should be noted that calibration of the 14C dates of Japanese samples with IntCal09 may induce additional systematic shifts of calibrated ages toward older ages by about 30 yr compared with the sample optimum calendar ages.