Triple oxygen isotope composition of tropospheric carbon dioxide and terrestrial carbonates

The triple oxygen isotope composition of tropospheric CO₂ is a potential new tracer in urban air studies and for biosphere-atmosphere interactions [1]. In this study, we are analyzing CO₂ from different provenances in order to trace the influx of anthropogenic CO₂ to urban air and to test predictions on the stratosphere-troposphere exchange flux. Since July 2010, we are monitoring the triple oxygen isotope composition of CO₂ in urban air in a two-week interval. For this purpose, carbon dioxide was extracted from ~450L of ambient air on the campus of the University of Göttingen using a Russian Doll type cryogenic trap [2]. The CO₂ was analyzed by CO₂-CeO₂ equilibration at 685°C and subsequent IR laser fluorination of CeO₂ and CF-irmMS [3]. All triple oxygen isotope data are reported as Δ¹⁷O_TFL values relative to the terrestrial fractionation line (TFL) with a slope β_TFL=0.5251 and an intercept γ_TFL=-0.014%. On average, the Δ¹⁷O_TFL value of ambient CO₂ was -0.11±0.05% (SD) with a seasonal cycle of 0.04±0.01%. Lower Δ¹⁷O values were observed during wintertime. In order to test the potential of Δ¹⁷O as a tracer for anthropogenic CO₂, we analyzed CO₂ from different combustion processes. Our results showed that the Δ¹⁷O anomaly of tropospheric O₂ [4] is passed on fully, or partially to the combustion CO₂ [5]. We estimate that elevated anthropogenic emission during wintertime could be responsible for a decrease in Δ¹⁷O of urban air CO₂ of -0.02±0.01%. In order to predict the triple oxygen isotope composition of tropospheric CO₂ on a global scale, we revised the box model calculation from Hoag et al. [1]. For the exponent β for CO₂-water equilibrium, we assume that β_CO2-water=0.522±0.001 [6]. Furthermore, we took into account that the Δ¹⁷O_TFL value of CO₂ released from soils is affected by kinetic fractionation. Thus, we obtained a Δ¹⁷O_TFL value for global tropospheric CO₂ of -0.13%. The model calculation agrees well with the Δ¹⁷O_TFL value determined for ambient air CO₂. The triple oxygen isotope composition of terrestrial carbonates should provide additional information on the formation process of biogenic and abiotic carbonates. In order to determine the triple oxygen isotope composition on carbonates, phosphoric acid decomposition of carbonates was carried out. Subsequently, the liberated CO₂ was also analyzed by the CO₂-CeO₂ equilibration method [3]. The Δ¹⁷O_TFL of CO₂ released from Solnhofen limestone was -0.11±0.09% (SE, t0.95, n=6). We are currently determining the exponent β for phosphoric acid decomposition. [1] Hoag, K.J., et al., Geophys. Res. Lett., 2005. 32: p. 1-5. [2] Brenninkmeijer, C.A.M. and T. Röckmann, Anal. Chem., 1996. 68(17): p. 3050-3053. [3] Hofmann, M. and A. Pack, Anal. Chem., 2010. 82: p. 4357-4361. [4] Barkan, E. and B. Luz, Rapid Commun. Mass Spec., 2005. 19(24): p. 3737-3742. [5] Horváth, B., M.E.G. Hofmann, and A. Pack, 2011. XI Isotope Workshop. Budapest. [6] Hofmann, M., et al., 2011. EGU. Vienna.