RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

Six Years of cross-disciplinary studies at the Astro-Glaciology
Research Unit: Astronomical signatures in polar ice cores
Yuko Motizuki∗1
Our Astro-Glaciology Research Unit (AGU), established on July 1, 2011, is the only group worldwide investigating the new cross-disciplinary research ﬁeld ofastro-glaciology, which combines astrophysics, climate
science and solar-terrestrial physics, and glaciology,
both experimentally and theoretically (Figs. 1 and 2).
Gamma rays, X-rays, and UV radiation arising fromsolar activity and supernova explosions in our galaxy
cause changes in the chemical composition of thestratosphere (from ∼10 − 50 km altitude). The eﬀectsof radiation on the stratosphere are then recorded in
the chemical and isotopic composition of terrestrial ice.
An ice core is therefore a time capsule. The cylindrical ice core drilled by the Japanese Antarctic Research Expedition (JARE) at the Dome Fuji station in
Antarctica is 3,035 m long and corresponds to a time
period from 720,000 years ago to the present (Fig. 3).
We have been studying the uppermost ∼100 m of anice core that was recovered from the same hole as thedeep ice core shown in Fig. 3 as well as another shallow ice core; both cases correspond to the last 2,000years. A newly approved project to obtain an ice core
corresponding to a 1,000,000 year period has alreadycommenced.
There are signiﬁcant advantages in using Dome Fuji
ice cores when studying astronomical phenomena of
the past, because the eﬀects of radiation on the stratosphere recorded in ice cores in the Dome Fuji area are
more pronounced than in ice cores obtained from elsewhere on the earth.
Anions and cations in our ice cores were analyzed atRIKEN using an ultra-high-sensitivity ion chromatography techniquea) (Fig. 4). The sulfate ion (SO2−4 )concentration spikes indicate volcanic eruptions andcan provide time markers for dating ice cores. TheSO2−spikes (Fig. 5) observed in Dome Fuji ice cores4
are an order of magnitude higher than those in a standard ice core drilled in West Antarctica.b) This demonstrates an important advantage when using our cores,
since high volcanic SO2−spikes imply a high level of4
inclusion of stratospheric components.
Collaboration between AGU and the National Institute of Polar Research (NIPR, Tokyo) involves analyzing ionic and isotopic concentrations in Dome Fuji icecores. Our aim is to reveal in the ice cores:(1) signatures of past solar cycles(2) relationships between past solar activity and climatic changes(3) footprints of past supernova explosions in ourgalaxy, in order to understand better the rate ofgalactic supernova explosions.

Fig. 1. Our AGU group in the low temperature room(−30◦ C and −50◦ C) at NIPR (2012).

Fig. 3. A deep ice core drilled in 2007 at Dome Fuji station
in inland Antarctica (photograph courtesy of NIPR).
Fig. 2. Dr. Yoichi Nakai sampling an ice core in the lowtemperature room (−30◦ C) at NIPR (2012).

a)

∗1b)

Research Unit Leader, Astro-Glaciology Research Unit,
RIKEN Nishina Center (2011−present)

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High-resolution chemical analysis of ice core samples fromDome Fuji station, Antarctica: Y. Motizuki, K. Takahashi
et al.: Geochemical J. 51, 293-298 (2017) [Press-release]
2000-yr record of volcanism: M. Sigl, Y. Motizuki et al.:Nature Climate Change 4, 693-697 (2014) [Press-release]Research Activities 1991–2016

RIKEN Accel. Prog. Rep. 50 (2017)

Fig. 4. Dr. Kazuya Takahashi with our ﬁrst ultra-highsensitivity anion chromatography detector (ICS2000system) in a clean room at RIKEN (2006).

Fig. 6. Left to right: Prof. Marc Besancon (French Director, Toshiko Yuasa Laboratory (TYL)), Yuko Motizuki(AGU leader), and Prof. Junji Haba (Japanese Director, TYL) at TYL Gold Prize ceremony (Feb., 2017).

sphere induced by solar proton events [SPEs: the bombardment of the atmosphere by high-energy (∼10 −500 MeV) protons originating in solar ﬂares], in collaboration with the National Institute of Environmental Science (Tsukuba). To do this, we developed a 1D
box model, taking into account 752 reactions including
ion chemistry.1) We are also developing a 3D chemical
climate model with a view of studying changes in thechemical composition of the atmosphere and the effects on climate pertaining to past and future SPEsand supernovae in our galaxy.
Cross-disciplinary collaboration between AGU andthe RIKEN Center for Advanced Photonics has been
initiated in order to develop an automated laser icecore melting system. The system will make it possible
to analyze deeper ice cores at a higher temporal resolution (∼1 month), and it will thus become possible toinvestigate the galactic supernova rate and solar activity of the past 1,000,000 years.
The initial 3-year research project was supportedby the Funding Program for Next Generation WorldLeading Researchers (NEXT Program, Grant Number
GR098) of the Council for Science and Technology Policy (CSTP). The project has been awarded an S-gradeby CSTP (2015) within the NEXT Program.
Our research unit leader was awarded the Toshiko
Yuasa Gold Prize in 2017 (Fig. 6), the theme being “Pioneering a new interdisciplinary research ﬁeld regarding Antarctic ice cores and exemplifying leadership inacademia ; and our leader was also granted Nice Step
Scientist Certiﬁcate by MEXT National Institute Science and Technology Policy (NISTEP) in 2015.
Two postdoctoral researchers have participated, andfour undergraduates and two postgraduate students
have received degrees during this research project.

Fig. 5. SO2−
concentrations shown versus year (AD) for4
two Dome Fuji ice cores and a standard ice core drilledin West Antarctica (WAIS Divide, USA). The temporal
resolution is ∼1 year. Spikes represent volcanic activity.

We have succeeded in extracting periodicities thatcorrespond with solar cycles from the annuallyresolved nitrate ion (NO−3 ) concentrations in a Dome
Fuji ice core, and we have found candidate NO−
3spikes that correspond with supernova explosions inour galaxy. Since the dates of several NO−
3 spikes almost match the dates of galactic supernova explosions
that have occurred in the last millennium, we conjecture that yearly-scale NO−
3 concentration spikes foundin Dome Fuji ice cores can be treated as a candidate
proxy for supernova explosions that have occurred inour galaxy. We have also discovered a strong statisticalcorrelation (r = 0.84) between variations in solar activity and variations in the temperature proxy (18 O/16 O)
in a segment of our ice core that corresponds to a preindustrial period.
Theoretically-based numerical simulations and experimental evidence will be combined. We have simulated chemical compositional changes in the strato-References
1) see, e.g., Y. Nakai, Y. Motizuki et al.: in this volume.

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