Ⅱ-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 49 (2016)

Diagnose oscillation properties observed in an annual ice-core oxygenisotope record obtained from Dronning Maud Land, Antarctica
Y. Hasebe,∗1,∗2 Y. Motizuki,∗1,∗2 Y. Nakai,∗1 and K. Takahashi∗1
Ice cores are time capsules that consist of snow accumulated in layers over a range of years; the deeper an
ice core is, the older the age of it is. Analysing water
isotopes in ice cores is one of the ways to understandclimate change in the past. In particular, the oxygenisotope ratio δ 18O in water, H2 O, has been establishedas the temperature proxy in glaciology1) . Here, δ 18O
is defined as}{ 18 16( O/ O)Ice core
δ 18O =−1× 1000 [‰], (1)( 18O/ 16O)SMOW
where SMOW (Standard Mean Ocean Water) indicatesan international standard of the water isotope ratio.
The objective of this work is to investigate the relationship between the temperature and solar activity byanalyzing the oscillation properties of the δ 18O data.
Figure 1 shows the δ 18O time-series record from AD
1025 to 1997. These data were obtained from ice cores
drilled in Dronning Maud Land (DML), East Antarctica2) . The temporal resolution of the record is annual. In Fig. 1, the DML δ 18O data are translated
into the temperature deviation from the mean using
the commonly-accepted proportional relationship1) .
Figures 2 and 3 show the results of our time series analysis using Fourier analysis (FT) and autoregressive model (AR)3) . Here, the AR method has
higher resolution than FT. As the first step, we analysed the data from AD 1825 to 1997. This is because
the dating of this portion of the ice core is considered
to be relatively precise. As shown in Fig. 2, we obtained a clear signal of a 21-year periodicity using theAR. This peak almost corresponds to the peak of FT
within the range of the FT’s step size. Next, we analysed the overall data from AD 1025 to 1997. As shownin Fig. 3, we obtained signals of periodicity aroundFig. 2. Power spectra obtained from analysing the δ 18O
record from AD 1825 to 1997. The solid blue line
and green circles represent the results obtained using
the auto-regressive model (AR) and by Fourier analysis(FT), respectively.

Fig. 3. Power spectra for the data from AD 1025 to 1997.

102, 126, and 194 years using the AR.
Furthermore, we also analyzed the measured surface
temperature record in the southern hemisphere fromAD 1850 to 2015 (HadCRUT4)4) . We then confirmedthat these observed surface temperatures have a 21year oscillation periodicity. This result is the sameas that of DML (Fig.2). It is thus confirmed that theDML δ 18O data actually reflected temperature history.
From the observed sunspot numbers and 14C content in tree rings, ∼11, ∼22, ∼90, and ∼200 years
are known to be solar activity cycles. In our analysis,
nearly 22 and 200 year cycles were obtained. Therefore, the temperature modulation in the DML ice core
record is suggested to be related with the solar activity cycles. The significance analysis for the obtained
signals is in progress.
References

Fig. 1. Oxygen isotope ratio (δ 18O) time series record (leftvertical axis) in DML2) and corresponding temperaturedeviation (right vertical axis) are shown.
∗1∗2

RIKEN Nishina Center
Department of Physics, Saitama University- 131 -1) V. Masson-Delmotte et al.: Journal of climate 21, 33593387 (2008).
2) W. Graf et al.: Annals of Glaciology 35, 195-201 (2002).
3) D. B. Percival and A. T. Walden: Spectral analysis forphysical applications (Canbridge University Press, England, reprinted 1998).
4) C. P. Morice et al.: Geophys. Res. 117, D08101 (2012).