Measuring the isotopic composition of methane as archived in polar ice cores: a tool to constrain paleoclimatic changes

Bock, Michael (2010). Measuring the isotopic composition of methane as archived in polar ice cores: a tool to constrain paleoclimatic changes (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Methane (CH4) is the third most important greenhouse gas after water vapour and carbon dioxide (CO2). Since the industrial revolution the mixing ratio of CH4 in the atmosphere rose to ∼1800 ppbv, a value never reached within the last 800 000 years. Nowadays, CH4 contributes ∼20% to the total radiative forcing from all of the long-lived greenhouse gases. This CH4 increase can only be assessed compared to its natural changes in the past. Air enclosures in polar ice cores represent the only direct paleoatmospheric archive (besides firn air) and show that atmospheric CH4 concentrations changed in concert with northern hemisphere temperature during both glacial/interglacial transitions as well as rapid climate changes (Dansgaard-Oeschger events). The glacial/interglacial changes in atmospheric CH4 concentrations are characterised by a strong increase from 350 ppbv during the Last Glacial Maximum (LGM) to values as high as 700 ppbv during the early and late Holocene. Looking at stadials and interstadials during Marine Isotope Stage 3 (MIS 3) concentration jumps of 100 - 200 ppbv within a few decades are observed. A concentration gradient with higher values in the northern versus the southern hemisphere during warm stages was reconstructed from ice core methane data from Greenland and Antarctica. This gradient indicates additional sources during warm periods located in the northern hemisphere. However, the underlying processes for theses changes are still not well understood. With tropical and boreal wetlands, biomass burning, thermokarst lakes, ruminants, termites, living biomass and marine gas hydrates all contributing to the natural atmospheric CH4 level, an unambiguous source attribution remains difficult. Also changes in the methane sinks can modify the tropospheric CH4 budget, as trace gases like volatile organic compounds (VOCs) are competing for the major reactant - the ·OH radical. Additionally, the changing global atmospheric methane concentration itself feeds back on its lifetime.
Together with the CH4 interhemispheric gradient, stable hydrogen and carbon isotopic studies on methane (δ D(CH4) and δ 13CH4) in ice cores allow to constrain individual CH4 source/sink changes. This thesis provides the methodological tools to measure δ D and δ 13C of atmospheric CH4 as archived in polar ice cores and presents two data sets. The first high resolution δ D(CH4) record over Dansgaard-Oeschger events 7 & 8 is shown in Chapter 3 and the first complete δ 13CH4 evolution over the last deglaciation is presented in Chapter 5.
Two methods (Chapters 2 and 4) were developed suited to measure δ D and δ 13C on CH4 extracted from polar ice cores with high precision and accuracy. Both methods include gas extraction from ice, preconcentration, chromatographic separation and pyrolysis or combustion of methane, for hydrogen or carbon analyses, respectively. For ice samples of 500 and 200 g with CH4 concentrations as low as 350 ppbv precisions of 3.4‰ and 0.15‰ are obtained for δ D(CH4) and δ 13CH4, respectively. Both methods improve precisions obtained by earlier developments; furthermore, for hydrogen sample consumption could be reduced by more than a factor of two at the same time.
Using a simple box model of the global CH4 cycle driven by source emissions in a forward Monte Carlo mode we are able to constrain the potential changes for individual CH4 sources in the past. This first order approach to interpret our measurements is employed in the discussions of the two presented records.
In Chapter 3 the evolution of δ D(CH4) from 34 - 41 kyr BP (kilo years before present) is discussed. It is unequivocally shown that marine gas hydrates (clathrates) were not the reason for the strong increases in CH4 concentration at the onset of the investigated rapid warming events. No clear evidence is found for catastrophic releases of CH4 from marine gas hydrates during the whole investigated period, however they might play a minor role during and at the end of interstadials. Our steady state model results suggest a slight decrease of background clathrate emissions from stadials (∼29 Tg CH4 yr−1) to interstadials (∼25 Tg CH4 yr−1). Furthermore, the role of boreal wetland emissions during Dansgaard-Oeschger cycles is highlighted. Interestingly, we find clear evidence for a climate response in boreal wetland regions that precedes the rapid warming into DO 8. Box modelling reveals that boreal CH4 emissions strengthened from ∼6 to ∼41 Tg CH4 yr−1 from stadial to interstadial conditions, respectively. Tropical wetland emissions turn out to be rather constant over these climate cycles (∼90 Tg CH4 yr−1), while biomass burning emissions show a slight increase in interstadials (∼60 Tg CH4 yr−1) compared to stadials (∼47 Tg CH4 yr−1).
The first δ 13CH4 record over the transition from the Last Glacial Maximum to the Holocene is presented in Chapter 5. It is demonstrated that emissions of boreal wetlands were essentially shut down during cold periods but contributed up to 65 Tg CH4 yr−1 during the preboreal Holocene and the Bølling/Allerød warm phases. In contrast, biomass burning emissions stayed rather constant at ∼45 Tg CH4 yr−1 during the whole investigated time period (10 - 22 kyr BP). The atmospheric lifetime of methane was found to be considerably lower during the investigated cold stages LGM and Younger Dryas.

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