[A-CC37] [EJ] アイスコアと古環境変動

[ACC37-04] Isotopic constrains on post-depositional processing of snow nitrate in eastern Dronning Maud Land, East Antarctica.

*野呂 和嗣¹、服部 祥平²、植村 立³、福井 幸太郎⁴、平林 幹啓⁵、川村 賢二⁵、本山 秀明⁵、吉田 尚弘²、竹中 規訓¹ (1.大阪府立大学大学院工学研究科、2.東京工業大学、3.琉球大学、4.立山カルデラ砂防博物館、5.国立極地研究所)

キーワード：同位体、硝酸

Stable isotopic ratio of nitrate in deep ice core would be valuable information for paleo-atmospheric analysis. However, post-depositional change should have an effect on the information. In previous study, this process is pointed out as physical release or photochemical reaction of nitrate on snowpack surface. Furthermore, the reactions are rare source of nitrogen oxides(NOx) for clean atmosphere in Antarctica. To determine the mechanism of the process, isotopic composition of N(d15N) is used as an indicator for nitrate photolysis. Nitrate photolysis as post-depositional change enriches d15N for remaining nitrate on snowpack. Thus, the isotope composition(d15N) enable us to estimate how post-depositional process work in the Antarctic Plateau. In order to test spatial variation of isotopic compositions, between coastal site and inland site, here we present latitudinal variation of d15N value of NO₃^- in surface snow in eastern Dronning Maud Land, East Antarctica. Snow samples were collected from the surface to depths of 80 cm and 30 cm at low- and high-elevation sites during the 54th Japanese Antarctic Research(JARE), and the surface to depth 50 cm at coastal sites during 57th Japanese Antarctic Research, respectively. The d 15N of nitrate in snow were considerably increased from coastal to inland based on sample collected in 54th JARE. For the snow pit analysis in 57th JARE from surface to 180 cm, d15N ranged from -8.5 permil to +30.5 permil, and there no appreciably change. In contrast, spatial variability was observed in coastal sites samples within 100 km in JARE57. It suggests that the nitrate source would be different in the small area. The NO₃^- mass fraction f of pit samples on each depths were evaluated using measured d15N values, assuming initial d15N value(-10 permil), NO₃^- concentrations, and atmosphere–snow fractionation constant ¹⁵ε(-60 permil) for δ15N based on a Rayleigh–type process (Frey et al., 2009; Berhanu et al., 2014). From the calculation, f in the pit samples ranged 0.51 to 0.97. This value would be corresponding to nitrogen oxides flux value at the same site in JARE57. In detail, high concentration NO and HONO were emitted from snow surface. NO₂, however, wasn’t detected from snow surface and in the atmosphere.