Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：UNIV ARIZONA DEPT GEOSCIENCES  

Radiocarbon (C-14) 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 C-14 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 C-14 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 C-14 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 C-14 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 C-14 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.

DOI： 10.1017/RDC.2020.41

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Language：English   Publishing type：Research paper (international conference proceedings)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth's atmosphere. Several rapid 14C increases in the past, such as the 775 CE and 994CE 14C spikes, have been reported to originate from extreme solar proton events (SPEs). Another rapid 14C increase, also known as the ca. 660 BCE event in German oak tree rings as well as increases of 10Be and 36Cl in ice cores, was presumed similar to the 775 CE event; however, as the 14C 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 14C concentrations in tree rings around 660 BCE, we have precisely measured the 14C concentrations of earlywoods and latewoods inside the annual rings of Japanese cedar for the period 669-633 BCE. Based on the feature of 14C production rate calculated from the fine measured profile of the 14C concentrations, we found that the 14C rapid increase occurred within 665-663.5 BCE, and that duration of 14C 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.

DOI： 10.1038/s41598-019-57273-2

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/2514-3433/ab404ach6

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/2514-3433/ab404ach5

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DOI： 10.1088/2514-3433/ab404ach7

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/2514-3433/ab404ach1

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Language：English   Publishing type：Research paper (scientific journal)  

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41598-019-53296-x

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/2041-8213/ab42e4

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Language：English   Publishing type：Research paper (international conference proceedings)  

Language：Japanese   Publishing type：Research paper (other academic)  

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union ({AGU})  

DOI： 10.1029/2018GL080475

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of Science  

Ion-induced nucleation by cosmic rays influenced by solar modulation is of interest, and several studies with radioactive sources or accelerator beams have been performed. We focused on ionization with high-linear energy transfer (LET) particles, such as protons, neutrons, and ions, in secondary cosmic rays and the ion density of the secondary cosmic rays near the ground using the excel-based program for calculating atmospheric cosmic-ray spectrum (EXPACS). According to the calculation, not only muons but also protons and neutrons leave substantial traces of solar modulation at an altitude of 3 km from the ground. To verify the ion-induced nucleation by these secondary high-LET cosmic rays, a chamber experiment was conducted at the accelerator facility HIMAC. Using a chamber with a capacity of 75 L, experiments were conducted by irradiating protons and nitrogen ion beams with a constant energy of 180 MeV/u with varying intensity. The experimental results confirm that the ion density and aerosol density increased as the beam intensity increased. The aerosol density was found to be proportional to the ion density, but irrelevant to the ionization density. It was concluded that the ion-induced nucleation by high-LET secondary cosmic rays, such as protons and neutrons, provides no evidence of enhancement owing to ionization density and cannot account for the claimed variation of cloud formation due to the solar modulation.

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of Science  

Terrestrial cosmogenic nuclides (e.g., 14C, 10Be, and 36Cl) are primarily produced by galactic cosmic rays. However, a certain amount of these nuclides is also produced by solar energetic particles (SEPs) derived from sporadic solar events such as solar flares and coronal mass ejections. Cosmic ray increase events in AD 774/775, AD 993/994 (or AD 992/993), and ~BC 660 have been discovered using 14C data in tree rings (Miyake et al. 2012, 2013; Park et al. 2017). It is considered that the most plausible cause of these events was an extreme SEP events with very hard energy spectra based on 14C analyses of tree rings and 10Be and 36Cl analyses of ice cores (e.g., Mekhaldi et al. 2015; Miyake et al. 2015, 2019; Büntgen et al. 2018; O'Hare et al. 2019). These SEP events are estimated to be several dozens of times larger than the largest events seen in direct observations, and an event of that size might have a serious impact on modern society. Therefore, it is important to investigate the occurrence rate of past extreme events. In recent years, surveys of past SEP events have been actively conducted via cosmogenic nuclide measurements with high time resolution (~one-year resolution). Here the detected past SEP candidates and a further survey of similar events are reviewed.

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41467-018-07636-6

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41467-018-06036-0

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1017/RDC.2018.53

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1017/RDC.2017.75

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：GEOCHEMICAL SOCIETY OF JAPAN  

<p>Recently, the increase of cosmic ray flux was found around 5480 BC using the ¹⁴C measurements for tree rings with 1- to 2-y resolution. It is assumed that it may be caused by the large solar flares but we cannot specify the detail cause of this event. In this study, we aim to elucidate the cause of 5480 BC cosmic ray event by measuring ¹⁰Be and ³⁶Cl in the ice core which was drilled at the Dome Fuji Station in Antarctica, and investigate the variation of cosmic ray flux around 5480 BC. The result of ³⁶Cl concentration variation using the Accelerator Mass Spectrometry (AMS) system in the University of Tsukuba is reported in this presentation. In addition, the performance of ¹⁰Be-AMS and the future plan is also presented.</p>

DOI： 10.14862/geochemproc.65.0_230

CiNii Research

Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：UNIV ARIZONA DEPT GEOSCIENCES  

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 C-14 production. In the 1990s, it was claimed that the supernova in 1006 CE produced exactly this effect. With the C-14 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 C-14 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 C-14 record.

DOI： 10.1017/RDC.2016.50

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：UNIV ARIZONA DEPT GEOSCIENCES  

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 C-14 excursions in the past that have yet to be confirmed. In order to detect more of these events, we measured the C-14 contents in bristlecone pine tree-ring samples during the periods when the rate of C-14 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 C-14 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.

DOI： 10.1017/RDC.2016.54

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Sporadic solar energetic particle (SEP) events affect the Earth's atmosphere and environment, in particular leading to depletion of the protective ozone layer in the Earth's atmosphere, and pose potential technological and even life hazards. The greatest SEP storm known for the last 11 millennia (the Holocene) occurred in 774-775 AD, serving as a likely worst-case scenario being 40-50 times stronger than any directly observed one. Here we present a systematic analysis of the impact such an extreme event can have on the Earth's atmosphere. Using state-of-the-art cosmic ray cascade and chemistry-climate models, we successfully reproduce the observed variability of cosmogenic isotope Be-10, around 775 AD, in four ice cores from Greenland and Antarctica, thereby validating the models in the assessment of this event. We add to prior conclusions that any nitrate deposition signal from SEP events remains too weak to be detected in ice cores by showing that, even for such an extreme solar storm and sub-annual data resolution, the nitrate deposition signal is indistinguishable from the seasonal cycle. We show that such a severe event is able to perturb the polar stratosphere for at least one year, leading to regional changes in the surface temperature during northern hemisphere winters.

DOI： 10.1038/srep45257

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：NATL ACAD SCIENCES  

Radiocarbon content in tree rings can be an excellent proxy of the past incoming cosmic ray intensities to Earth. Although such past cosmic ray variations have been studied by measurements of C-14 contents in tree rings with &gt;= 10-y time resolution for the Holocene, there are few annual C-14 data. There is a little understanding about annual C-14 variations in the past, with the exception of a few periods including the AD 774-775 C-14 excursion where annual measurements have been performed. Here, we report the result of C-14 measurements using the bristlecone pine tree rings for the period from 5490 BC to 5411 BC with 1- to 2-y resolution, and a finding of an extraordinarily large C-14 increase (20 parts per thousand) from 5481 BC to 5471 BC (the 5480 BC event). The C-14 increase rate of this event is much larger than that of the normal grand solar minima. We propose the possible causes of this event are an unknown phase of grand solar minimum, or a combination of successive solar proton events and a normal grand solar minimum.

DOI： 10.1073/pnas.1613144114

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.quaint.2015.04.014

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/2014GL062218

Language：Japanese   Publishing type：Research paper (scientific journal)  

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DOI： 10.2458/56.17769

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/2012JA018320

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Presentation type：Oral presentation (invited, special)  

Audience： Media

Type：TV or radio program