[M-IS09] Biogeochemistry

[MIS09-P06] Determination on the triple oxygen isotopic compositions of carbon dioxide emitted from terrestrial ecosystem

*Fumiko Nakagawa¹, Keiya Nakamura¹, Urumu Tsunogai¹, Masanori Ito¹, Takanashi Satoru², Ayaka Sakabe³, Takuya Saito⁴ (1.Graduate School of Environmental Studies, Nagoya University, 2.Forest Research and Management Organization, 3.Kyoto University, 4.National Institute for Environmental Studies)

Keywords:carbon dioxide, triple isotopic compositions, terrestrial ecosystem

Carbon dioxide (CO₂), which is well known as the primary greenhouse gas, has been increasing in the troposphere since the beginning of the industrial era, because of rising anthropogenic CO₂ emissions such as fossil fuel combustion and deforestation. The tropospheric CO₂ growth rates are less than the emissions, because most of tropospheric CO₂have undergone interactions with marine and/or terrestrial ecosystems, and some of the CO₂ has been taken up by them. Therefore, a quantitative understanding of the carbon cycle, such as what mechanism determines the absorption rates of CO₂ by the ocean and terrestrial ecosystems and how this will change in the future, has become an extremely important issue. In this study, we used the Δ¹⁷O (= ln(1 + δ¹⁷O) - 0.5229 × ln(1 + δ¹⁸O)) value, the relative ratio of the three oxygen species (¹⁶O, ¹⁷O, ¹⁸O) in CO₂, as a new tracer to identify the major sources of tropospheric CO₂. We determined the Δ¹⁷O value of CO₂ derived from terrestrial ecosystem respiration (soil respiration and plant respiration) together with the other major sources (i.e. fossil fuel combustion), using our newly developed ultra-high precision Δ¹⁷O measurements for atmospheric CO₂, and concluded that the Δ¹⁷O values are different among those major sources. CO₂released by soil respiration and plant respiration showed high Δ¹⁷O values of +71 × 10^-6 and +42 × 10^-6, respectively. The high Δ¹⁷O values could be reasonably explain by considering oxygen isotope exchange with natural water, isotope fractionation by evapotranspiration, and isotope fractionation by diffusion. Atmospheric observations were also conducted at two sites: the urban site in the Nagoya University campus, and the forest site (the Kiryu Hydrological Test Site) in Shiga Prefecture using a forest flux tower. At the forest site, atmospheric samples of upwelling and downwelling streams were separately collected using the eddy accumulation method (Lukas & Anas, 2019). By comparing the Δ¹⁷O values of upwelling CO₂ (considered as forest origin) and downwelling CO₂ (considered as tropospheric origin), the upwelling CO₂ showed significantly higher Δ¹⁷O values than those from downwelling. The results suggest that about 30 % of the CO₂ entered in the forest canopy will interact with terrestrial ecosystem before released into the atmosphere. On the other hand, atmospheric CO₂ at urban site showed a wide range of Δ¹⁷O values from -100 × 10^-6 to +52 × 10^-6. The lower Δ¹⁷O values can be explained by simple mixing of CO₂ from fossil fuel combustion. High Δ¹⁷O values, however, were significantly higher than the background tropospheric CO₂. The higher Δ¹⁷O values observed at the urban site reflect CO₂ emissions from terrestrial ecosystems, suggesting that CO₂ is actively interacting with terrestrial ecosystems even within the urban area. We concluded that CO₂ emitted from terrestrial ecosystems can be clearly distinguished from its marine origin using Δ¹⁷O values as tracer.