[H-TT35_1PO1] 地球人間圏科学研究のための加速器質量分析技術の革新と応用

[HTT35-P12] 波照間島と落石岬における大気中二酸化炭素の放射性炭素同位体比観測

*寺尾 有希夫¹、向井 人史¹ (1.国立環境研究所 地球環境研究センター)

Atmospheric radiocarbon in carbon dioxide (¹⁴CO₂) is a powerful tracer for understanding of carbon cycles, e.g. oceanic and biospheric CO₂ exchanges and CO₂ emissions from fossil fuel combustion. Observation sites for radiocarbon concentrations, ∆¹⁴C, are not many enough to evaluate the global and regional carbon flux. We present an analysis of trends, interannual variability (IAV) and seasonal cycle of ¹⁴CO₂ in background air from July 2004 to December 2012 at two NIES/CGER monitoring stations; Hateruma Island (HAT; latitude 24.06N, longitude 123.81E) and Cape Ochi-ishi (COI; latitude 43.16N, longitude 145.50E). The air samples were collected in 2 L Pyrex glass flasks. The sampling frequency was monthly. CO₂ was extracted from the whole air at NIES and CO2 samples were converted to graphite and analyzed ratios of ¹⁴C/¹²C by accelerator mass spectrometry (AMS, National Electrostatics Corp., 1.5SDH) at Paleo Labo Co., Ltd., Japan. Analytical precision in ∆¹⁴C determined from statistical uncertainty (number of ¹⁴C counts) was ±1.7-2.0 ‰ for most samples. The repeatability of measurements using modern reference air was ±1.9 ‰ . A decreasing trend in ∆¹⁴C was -5‰ yr^-1 in average but large IAV was observed at both stations: large decreases in 2007-2008 and in 2010-2011 (-8 to -9 ‰ yr^-1) and almost zero decrease in 2009. We also observed clear seasonal cycle of ∆¹⁴C. The peak-to-peak amplitudes in the seasonal cycle determined from the smooth curve fits were 7‰ at both stations and the maximum of ∆¹⁴C appeared in July and the minimum in January at HAT, and the maximum in September and the minimum in May at COI. The differences in phase of ∆¹⁴C seasonal cycle between HAT and COI suggested that the atmospheric ∆¹⁴C at COI was influenced by CO₂ emitted from terrestrial biosphere.