Chloroplast DNA polymorphism in four oak species (Quercus serrata, Q. mongolica var. crispula, Q. dentata and Q. aliena) was studied using collections from a total of 127 localities in Japan and South Korea on the basis of five intergenic spacers (trnD-trnT, trnT-trnL, rps14-psaB, trnS-trnT and trnQ-trnS). Although no variation existed in sequences among the four species, a single nucleotide (T/C) substitution in the trnQ-trnS intergenic spacer was found in all the four species, resulting in two haplotypes (T- and C-type). Phylogenetic analyses of the four species and related species showed that the C-type is derived and even likely of monophyletic origin, while the T-type is ancestral. Geographically, the T-type is widespread from South Korea to Japan, whereas the C-type is restricted to eastern Japan with rare exceptions. "Eastern Japan" approximately coincides with the distribution range of the boreal conifer forest during the last glacial maximum. Overall evidence suggests that the mutation from T- to C-type occurred in an individual of one of the four oak species and then was transferred to all the species by hybridization in eastern Japan, and that the Kanto District provided individuals with the C-type with a refugium during the last glacial maximum.

Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth's atmosphere. Several rapid 14 C increases in the past, such as the 775 CE and 994CE 14 C spikes, have been reported to originate from extreme solar proton events (SPEs). Another rapid 14 C increase, also known as the ca. 660 BCE event in German oak tree rings as well as increases of 10 Be and 36 Cl in ice cores, was presumed similar to the 775 CE event; however, as the 14 C increase of approximately 10‰ in 660 BCE had taken a rather longer rise time of 3-4 years as compared to that of the 775 CE event, the occurrence could not be simply associated to an extreme SPE. In this study, to elucidate the rapid increase in 14 C concentrations in tree rings around 660 BCE, we have precisely measured the 14 C concentrations of earlywoods and latewoods inside the annual rings of Japanese cedar for the period 669-633 BCE. Based on the feature of 14 C production rate calculated from the fine measured profile of the 14 C concentrations, we found that the 14 C rapid increase occurred within 665-663.5 BCE, and that duration of 14 C production describing the event is distributed from one month to 41 months. The possibility of occurrence of consecutive SPEs over up to three years is offered.www.nature.com/scientificreports www.nature.com/scientificreports/ respectively, calculated at every latitude taking account of a geomagnetic cut off rigidity 35 from their fluence spectra and the altitude production yields. The production rate of SPE56 was consistent with the value 2.90 × 10 6 [atoms/cm 2 ] by Kovaltsov et al. 23 . Based on our calculation, the ~660 BCE event is 52-53 times larger than SPE56, and 230-240 times larger than SPE72 if the energy spectrum of the ~660 BCE event is comparable to the SPEs.

A new compact accelerator mass spectrometry (AMS) system has been installed in the Kaminoyama Research Institute at Yamagata University (YU). The AMS system is based on a 0.5MV Pelletron accelerator developed by the National Electrostatics Corporation. An automated acid-alkali-acid (AAA) treatment system and an automated graphitization line were also installed in the same facility for sample preparation. Performance tests of the YU-AMS system were carried out by measuring the C-series standard samples (C1–C8) and HOxII provided by IAEA and NIST, respectively. We evaluated the YU-AMS system by comparing the radiocarbon ages of Japanese tree rings with dendrochronologically determined calendar ages with calibration data. We also carried out some performance tests using a control serum and a 14C-labeled drug (oxaliplatin).

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