Ⅱ-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 47 (2014)

Measurement of nitrogen and oxygen isotope ratios
in considerably low nitrate concentration ice core samples
S. Okamoto,1 K. Takahashi,1 Y. Nakai,1 Y. Motizuki,1 A. Makabe,2 K. Koba,2 and H. Motoyama3
Nitrate (NO3-) concentration in polar regions is caused byrelevant stratospheric sources1), related extraterrestrial
fluxes of energetic particles, and solar irradiation. In theTalos Dome ice core (Antaractica), NO3- data exhibit highly
significant agreement with cosmic ray flux reconstructions2).
Nitrogen and oxygen isotope ratios (δ15N and δ18O) of NO3in the polar ice core are expected to reflect the difference of
isotope fractionations through photochemical reactions inthe stratosphere caused by cosmic ray and solar irradiation.
Our final object is to clarify the history of solar activity andcosmic events, on the basis of precise analyses of δ15N andδ18O in the ice core. However, it is difficult to measure theisotope ratios of δ15N and δ18O in NO3- in the Antarctic icecore, because NO3- concentrations are low (typically < 20µg l-1) and the sample volume is limited.
In this study, we examined the method of measuringδ15Nand δ18O with high sensitivity for 11 ice core samples from
Dome Fuji drilled in 2010, corresponding to relatively highNO3- concentration (average 22.3 µg l-1) using thedenitrifier method4) and we successfully obtained accurate
data. In this method, 10 ml of each sample was used andNO3- in sample water was quantitatively converted to N2O,
utilizing denitrifying bacteria (Pseudomonas aureofaciens)that lack N2O-reductase. The isotopic composition of N2O
is then measured using the mass spectrometer(IsoPrime100) in RIKEN. The results of the measurementsof δ15N and δ18O are summarized in Fig.1. Each sample isreferenced to the internationally recognized standard
USGS32, USGS34 and USGS35. In Fig. 2, the blue frame
indicates the maximum and minimum values in themeasured 11 ice core samples. These standards were diluted

with ice core water in which NO2- and NO3- were removed
using the ion exchange resin for minimizing the effect of
the exchange of oxygen atoms between the nitrogen oxideintermediates and water3). We corrected the backgroundN2O associated with the medium for bacteria, atmosphereand ice core water used for dilution of standard. The errorof the δ15N and δ18O in 11 samples were calculated by thepropagation of errors including uncertainties in thebackground and in sample measurement. The maximum
errors of the δ15N and δ18O were ± 0.75‰ and ± 0.34‰,
respectively.
The δ15N and δ18O range from 201.7‰ to 258.5‰ andfrom 45.9‰ to 64.1‰, respectively (Fig.2). The variationsof δ15N and δ18O show 24% and 34% for 20% NO3- change,respectively. δ15N values are inside a certain range except
for one sample (201.7). High positive δ15N values may be
attributed to the nitrate post-depositional effect because of
low accumulation rate in Dome Fuji4). NO3- and δ18O show
significant correlation (r = -0.69, p < 0.05), while there is nocorrelation between NO3- concentration and δ15N.
We have successfully established the method to measure
δ15N and δ18O of NO3-. Further detailed measurements areexpected to contribute to elucidate the origin of NO3produced by photochemical reactions in the stratosphere.

Fig. 2. NO3- concentration, δ15N and δ18O of NO3- in DomeFuji ice core. Red arrow indicates the range of δ15N in
Dome C ice core, inland Antarctica5). The unit of NO3concentration in Dome C converts assuming ice density 850kg m-3.
Fig. 1. The δ15N and δ18O of ice core samples andinternational standards USGS32, USGS34 and USGS35.
RIKEN Nishina CenterTokyo University of Agriculture and Technology
3National Institute of Polar Research
12References

1) M. R. Legrand and S. Kirchner: J. Geophys. Res. 95,p3493-6507 (1990).
2) R. Traversi et al.: Solar Phys. 208, p237-254 (2012).
3) K. L. Casciotti et al.: Anal Chem. 74, p4905-4912(2002).
4) J. Erbland et al.: Atmos. Chem. Phys. 13, p6403-6419 (2005).
5) H. D. Freyer et al., Tellus, 48B, p93-105 (1996).

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