Methane concentration in ice cores: A tool to reveal firn-ice properties and past climate changes

Spahni, Renato (2006). Methane concentration in ice cores: A tool to reveal firn-ice properties and past climate changes (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Polar ice sheets provide an archive of ancient air trapped in small air bubbles that can be analyzed directly for its composition. Deep ice core drillings in Antarctica and Greenland penetrated more than 3’000 m in the ice and provided mostly undisturbed concentration records of the greenhouse gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in the past. Because these long lived trace gases are well mixed in the atmosphere, they reflect large-scale changes of at least hemispheric extent. In the frame of the European Project for Ice Coring in Antarctica (EPICA) two ice cores, providing currently the oldest greenhouse gas records, have been successfully drilled close to bedrock. This thesis includes a study, presenting records of atmospheric CH4 and N2O back to 650,000 years before present along the EPICA Dome C ice core. There is strong evidence that for this period the CH4 and N2O concentrations have never reached today’s values. Another investigation shows that the anthropogenic increases of CO2, CH4 and N2O is exceptionally for at least the last 20 thousand years, based on ice core data and modelling of the air enclosure process during the ice formation. Although the CH4 and N2O results are obtained from parallel measurements on the same sample, the interpretation in this thesis focusses on CH4 as a tool to derive past climatic variations, to study processes in firn and ice, and to synchronize different ice cores from Greenland and Antarctica.
The first introductory chapter gives an overview on the anthropogenic increase of the greenhouse gases CO2, CH4 and N2O over the last 250 years, followed by a brief description of their source and sink terms with a focus on CH4. In a second part the wide range of applications for the CH4 measurements in ice is highlighted on the basis of the involved time scales.
Gas trapping in polar ice is manifold and involves physical processes such as diffusive mixing of firn air and the formation of bubbles. These processes can be simulated using a gas and enclosure model, which is presented in chapter 2. This model is able to explain the attenuation of trace gas concentrations variations, compared to the atmospheric evolution, during the inclusion in the ice. It is applied to CH4 variations during the cold event 8,200 years ago and to the anthropogenic increase of the greenhouse gases with respect to ice core data. Further it helps to analyze fractionation processes on air elemental ratios occurring during the air enclosure in the ice.
High-resolution CH4 and N2O records from Greenland in parallel with fast temperature variations, known as Dansgaard/Oeschger (D/O) events, during the last glacial period are presented in chapter 3. One study shows how closely the CH4 concentration follows the abrupt local temperature rises of up to 0.58◦ C per decade. In a second part new CH4 and N2O data from the EPICA Dronning Maud Land ice core (Antarctica) is synchronized to the Greenland ice core records, to obtain the relative phasing between the temperature signals in the two hemispheres. The work discusses the accuracy of the modelled age scale and the uncertainty of the CH4 match.
The climate of the last million years has undergone several glacial-interglacial cycles with pacings of 41 and 100 thousand years. Temperature variations are paralleled by the greenhouse gases CO2, CH4 and N2O, with higher concentrations during interglacial and lower concentrations during glacial periods. Chapter 4 includes the longest CH4 and N2O records currently available from ice cores that assert that the current atmospheric concentrations are unprecedented during the last 650 thousand years. The second part gives an outlook to even older sections of the ice core record. In addition the CH4 record is compared to even older sediment records.
The studies in the chapters represent the main work of the thesis. However, there have been many fruitful international collaborations detailed in the Appendix that complete the understanding and interpretation of CH4 variations recorded in ice cores.

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