Triple oxygen isotope variations in natural and anthropogenic carbon dioxide

Abstract

The oxygen and carbon isotope composition (¹⁸O/¹⁶O and ¹³C/¹²C) of atmospheric carbon dioxide is an excellent tool to investigate the atmospheric CO₂ cycle. In recent years, it has been suggested that the triple oxygen isotope composition (¹⁷O/¹⁶O and ¹⁸O/¹⁶O) of tropospheric CO₂ might be a potential new tracer for the terrestrial gross primary production. This study investigates whether and to which extent this new tracer might complement conventional stable isotope investigations of tropospheric CO₂. <br/> This thesis presents (i) a new high-precision technique for triple oxygen isotope analysis in carbon dioxide; (ii) an experimental study on triple oxygen isotope exchange between CO₂ and water, the most important process controlling the triple oxygen isotope composition of tropospheric CO₂; (iii) experimental data on the triple oxygen isotope composition of combustion CO₂, in particular anthropogenic CO₂ emissions; (iv) observational data of tropospheric CO₂ and a global mass balance model. <br/> In chapter 2, we describe the new high-precision analytical technique that allows detecting small variations in near-surface tropospheric CO₂ and in the various CO₂ sources to the troposphere. The method is based on CO₂-CeO2 equilibration at elevated temperature (T = 685 °C) and subsequent laser fluorination of the equilibrated CeO2. The released molecular oxygen is then analyzed for its triple oxygen isotope composition on a mass spectrometer. The oxygen isotope composition of the sample CO₂ can be inferred from the triple oxygen isotope composition of the equilibrated CeO₂. The analytical uncertainty of triple oxygen isotope analysis of CO₂ has been improved by about an order of magnitude compared to former techniques. <br/> The triple oxygen isotope composition of tropospheric CO₂ is largely controlled by CO₂- water exchange in plant leaves, soils and in ocean surface water. Thus, chapter 3 presents a laboratory study on the equilibrium fractionation exponent for oxygen isotope exchange between CO₂ and water at three different temperatures (2 °C, 23 °C and 37 °C). Our experimental results agree well with theoretical calculations. Knowledge of this equilibrium fractionation process is a prerequisite for estimating the triple oxygen isotope composition of natural CO₂ gross fluxes from the bio- and hydrosphere. <br/> In chapter 4, we investigate the triple oxygen isotope composition of CO₂ from propanebutane and wood combustion, car exhaust and human breath. The experimental studies show that high-temperature combustion CO₂ largely inherits its distinct triple oxygen isotope signature from ambient air O₂. For low-temperature combustion, such as wood combustion, the triple oxygen isotope signature of the released CO₂ is also affected by CO₂-water equilibration or other oxygen sources, such as wood inherent oxygen. The oxygen isotope composition of human breath is solely controlled by isotope exchange with body water. The experimental data demonstrate that the triple oxygen isotope composition of anthropogenic CO₂ emissions can be clearly distinguished from natural CO₂ sources. <br/> In chapter 5, we present the first high-precision triple oxygen isotope data of tropospheric CO₂. The data set of ambient air CO₂ sampled in Göttingen shows a significant temporal variation, which only in part follows the seasonal cycle of the ¹⁸O/¹⁶O ratio of tropospheric CO₂. The triple oxygen isotope composition of tropospheric CO₂ sampled on top of the Brocken Mountain (1140 m, Harz Mountains, Germany) falls within the range observed in Göttingen. The mass balance prediction for the triple oxygen isotope composition of global tropospheric CO₂ only in part agrees with the observational data. The modeling results suggest that the observed temporal variation in Δ¹⁷O cannot be attributed to seasonal variations in plant activity, but it may be that the influx of stratospheric CO₂ significantly affects the temporal Δ¹⁷O variations of tropospheric CO₂. <br/> This study presents the basis for triple oxygen isotope analysis in tropospheric CO₂ and its meaningful interpretation. Yet, not all characteristics of the triple oxygen isotope signature of tropospheric CO₂ can be explained calling for future studies.