Quantifying past changes of the global carbon cycle based on d13CO2 measurements in Antarctic ice cores

Schneider, Robert (2011). Quantifying past changes of the global carbon cycle based on d13CO2 measurements in Antarctic ice cores (Unpublished). (Dissertation, University of Bern, Faculty for Philosophical and Natural Science, Physics Department, Department of Climate and Environmental Physics)

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The current dramatic increase in atmospheric CO2 concentrations can only be explained in the context of severe human influences on the global carbon cycle. Intensive research on polar ice cores allowed for such conclusions. The CO2 evolution during the last 800’000 years was reconstructed from the EPICA (European Project for Ice Coring in Antarctica) Dome C ice core: CO2 concentrations varied between 180 and 290 ppm, with high concentrations at interglacial and low values at glacial times, e.g. the Last Glacial Maximum (LGM) about 22’000 years before present. Still enigmatic are the reasons for past natural CO2 variations and the associated carbon flux changes between source and sink reservoirs as climatic conditions change, for example at a glacial-to-interglacial transition. The stable isotope ratio of 12C and 13C of atmospheric CO2 bears the potential to identify major carbon fluxes between the sources and sinks responsible for past CO2 fluctuations and furthermore to quantify the transferred carbon amount between the reservoirs. Reconstructing this δ13CO2 isotopic ratio for the last 800’000 years from ice cores and combiningwith the already known CO2 evolution, bears the potential to identify the specific carboncycle fluxes, that mainly altered past atmospheric CO2.
This thesis introduces the methodical base to reconstruct past δ13CO2 from ice cores using a sublimation technique off-line coupled to a gas chromatography-isotope ratio mass spectrometer system, after separation of CO2 from other air components such as N2, O2 and N2O. Natural δ13CO2 variations, as archived in ice cores, amount to around 0.5h in 3’000 years, demanding for high precision measurements. The sublimation system which was constructed, tested and perfected during this thesis allows for reproducibilities better than 0.06h. Furthermore, absolute accuracy of δ13CO2 and CO2 data measured from ice cores is important: On the one hand, systematic offsets between ice samples and air standards must be adequately examined and controlled, ensuring an identical treatment of both, sample and standard. On the other hand, the reliability of ice cores to reflect true atmospheric CO2 and δ13CO2 is an issue. Systematic fractionation of δ13CO2 and enrichment in CO2 concentrations along the gas enclosure process in the firn column are theoretically explained, clarified with examples, and finally corrected using measured δ15N2 from the same air extraction; a measurement procedure that was developed during this work. and Moreover, the latter method to measure δ15N2 allows for measurements of the O2/N2 ratio from ice cores, revealing information concerning the gas loss during storage. In addition, organic in situ production was identified to account for CO2 enrichments of 5 to 10 ppm in the EPICA Dronning Maud Land (EDML) ice core.
In a second part of this thesis, δ13CO2 and CO2 measurements from the EPICA Dome C and the Talos Dome ice cores are presented; data are interpreted within expected global carbon cycle dynamics. An early human influence via deforestation and land use change since 7’000 years before present is not important, as natural fluxes already explain the pre-industrial CO2 rise of 20 ppm during the late Holocene.
Subsequently, the complete δ13CO2 evolution for the last 24’000 years is presented, including a detailed discussion on the causes for the so-called mystery interval at 17’000 years BP. The carbon cycle state during the LGM was at a dynamic equilibrium with constant evolutions in CO2 and δ13CO2 of 190 ppm and -6.4h, respectively.
In addition, the first highly resolved measurements of the past δ13CO2 evolution before, during and after the previous interglacial are presented. The record covers a time period between the Penultimate Glacial Maximum (PGM; 150’000 years before present), and the last glacial inception until Marine Isotope Stage (MIS) 4 (65’000 years before present). The data reveal a 0.4h offset in δ13CO2 between the PGM and the LGM. Such an offset must be explained via long term trends of the stable carbon isotopic signature δ13C of the combined ocean/atmosphere reservoir. Furthermore, the steadily constant CO2 evolution at the end of MIS 5.5 (120’000 years before present), despite declining Antarctic temperatures, can be explained via a carbon release from terrestrial biomass and carbonate compensation. Lastly, the first δ13CO2 measurements during Antarctic Isotope Maxima (AIM) events are presented and integrated into the current understanding of the causes for coincidental CO2 maxima and AIM events. An anti correlation between CO2 and δ13CO2 evolutions, with about 0.1h more negative δ13CO2 as CO2 maxima are at full amplitude of 20 to 30 ppm.
In summary, the established sublimation based measurement system to reconstruct past δ13CO2, CO2 and δ15N2 from ice cores, offers sufficient precision and accuracy to investigate the reasons for natural CO2 variations during the last 800’000 years. Hence, the data presented in this thesis are the first robust constraints of δ13CO2 in the atmosphere allowing for specific conclusions on carbon cycle dynamics.

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