Bipolar reconstructions of atmospheric methane and nitrous oxide during the last glacial-interglacial cycle

Baumgartner, Matthias (2013). Bipolar reconstructions of atmospheric methane and nitrous oxide during the last glacial-interglacial cycle (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The focus of this thesis is on atmospheric methane and nitrous oxide, which are the most important greenhouse gases after water vapor and carbon dioxide today. Anthropogenic activity has drastically increased the atmospheric abundance of greenhouse gases through the last 200 years which initiated global warming on Earth. Reconstructions of the past atmospheric composition based on the unique Greenland and Antarctic ice sheet climate archive provide insight into the natural variability of greenhouse gases. This thesis contributes to the establishment of high-quality methane and nitrous oxide records from bipolar ice cores over the last glacial-interglacial cycle. The data are an important basis for modeling studies and allow to estimate the past natural emissions of methane and nitrous oxide. Comparing the past climate variability and variations in greenhouse gas emissions provides important knowledge about changes in the biogeochemical cycles on the Earth.
The main part of this PhD thesis is presented in the form of submitted and peer-reviewed scientific articles (chapters 3–5). Chapter 1 provides a short introduction on the greenhouse gases methane and nitrous oxide. From direct atmospheric measurements over the last decade, which are available from the global network of the National Oceanic and Atmospheric Administration (NOAA), the records of high-latitude northern and southern stations are chosen to discuss the seasonal variations, the interpolar concentration difference, and the natural and anthropogenic sources and sinks. Further, important aspects of the ice core gas archive are introduced.
Chapter 2 gives a short description of a measurement from cutting the ice to the determination of the methane and nitrous oxide concentration. It documents the slight modification in the calibration process of the detectors, the installation of a temperature controlled sampling loop, the replacement of the old by a new standard gas to monitor long-term drifts in the calibration, and reproducibility measurements performed on natural ice samples of various ice cores. It further compares the methane results of recent measurements from Bern and the Laboratoire de Glaciologie et Géophysique de l’Environnement (LGGE), Grenoble.
Chapter 3 presents new bipolar methane measurements from the North Greenland Ice Core project (NGRIP) and the European Project for Ice Coring in Antarctica (EPICA) Dronning Maud Land (EDML) ice cores between 32 and 11 thousand years (kyr) before present (BP). The relative interpolar concentration difference of methane showed positive values (3.7–7.1%) in the time around the Last Glacial Maximum (LGM). This implies that, although large parts of the northern hemisphere were covered by ice sheets during the LGM, the northern hemispheric methane sources still played a dominant role for the global methane cycle. With an atmospheric two-box model approach the methane source strengths in the northern and the southern hemisphere are quantified.
Chapter 4 completes the methane record from the NGRIP ice core over the last glacial-interglacial cycle from 120 to 10 kyr BP. The abrupt variations in methane concentration during the last glacial serve as age markers for synchronisation to other ice core records and are closely coupled to Greenland temperature variations known as Dansgaard-Oeschger (DO) events. We compare the methane record to a temperature record from the same ice core and find the methane-to-NGRIP temperature sensitivity at the onset of DO events to vary between 5–18 ppbv/℃ along with the precessional cycle. The fast methane increases tend to lag the fast NGRIP temperature increases by 56 ± 19 yr on average. Bipolar measurements performed at LGGE yield a relative interpolar concentration difference of 4.4–4.6% in the time interval 80–60 kyr BP.
Chapter 5 focuses on the nitrous oxide record from the NGRIP ice core, which has been measured on the same samples as the methane record described in chapter 4. Concentration artefacts caused by microbial in situ production appear erratically and are separated from the atmospheric concentrations by applying an artefact separation algorithm. The resulting record presents the evolution of nitrous oxide concentration over the last glacial-interglacial cycle in unprecedented temporal resolution (∼ 75yr). We find pronounced relative minima in the nitrous oxide concentration during stadials with Heinrich events. These minima and the following recovery in the concentration are consistent with modeling studies suggesting variations in the oceanic nitrous oxide emissions.
Chapter 6 presents a progress report on a measurement series of the isotopic composition of nitrous oxide and methane over the DO events 19–21, which have been started towards the end of this thesis in the laboratory for the isotopic compositions of trace gases at the University of Bern. Ice from the Talos Dome Ice Core (TALDICE) has been chosen because of its low impurity content and therefore small probability of nitrous oxide in situ production in the ice. The first results are very promising that the signal stored in the ice is of atmospheric origin. The pursued final record has a temporal resolution of 450 years and will allow to investigate changes in the relative contributions of natural sources over strong climatic variations as observed for the DO events 19–21.
Chapter 7 gives an outlook for future investigations. The ongoing automatisation of the wet extraction system including a slight change in the extraction technique will potentially increase the precision of nitrous oxide concentration measurements and will enable to precisely determine the total air content of natural ice samples. A better reproducibility for nitrous oxide will e.g. allow to study weak centennial to millennial scale variability during the Holocene. For methane an important long term goal is to accomplish a reliable record of the interpolar concentration difference over the DO events 5–17 and 21–24, extending the workperformed during this thesis.
The thesis contains two Appendices. Appendix A presents further scientific publications, where our methane and nitrous oxide measurements contributed to the interpretation. Section A.1 is the result of a joint project of the North Greenland Eemian Ice Drilling (NEEM) community members. The methane and nitrous oxide concentration measured in Bern from the deepest part of the NEEM ice core revealed huge concentration spikes, most likely due to the occurrence of surface melt layers during the last interglacial period. The outliers are further characterised by relatively low total air content values. We confirmed the concentration enrichments in methane and nitrous oxide as well as the reduction in total air content by measuring samples from a visible meltlayer from the Holocene in the Dye3 ice core (supplementary information, Figure S9). Section A.2 presents the nitrous oxide record from the TALDICE ice core, section A.3 describes the construction of an age scale of the TALDICE ice core, where methane data from Bern and LGGE Grenoble were published and used for synchronisation. A compilation of climatic data from TALDICE, including methane data, is presented in the publication of the TALDICE community in section A.4.
Appendix B lists the methane and nitrous oxide concentration data measured during this thesis.

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