Analyses of buried glacial landforms and methane seepage in the North Sea and Baltic Sea

Climate change is a major challenge our society faces today and in the future. Intergovernmental efforts aim to reduce greenhouse gases in the atmosphere, such as carbon dioxide and methane, that have the potential to accelerate climate change. The robust quantification of the sources and sinks of greenhouse gases is essential for future climate models. Yet, marine sources of methane are still not well constrained. To evaluate the distribution and magnitude of methane seepage, the subsurface plays a critical role. In northwestern Europe, the landscapes have been shaped by ice sheets during the major glaciations of the Pleistocene and modified to its present state during the Holocene. A better understanding of Pleistocene deposits thus facilitates the quantification of methane seepage. It is also essential for the assessment of constructions sites, groundwater resources, geohazards and coastal protection. In this thesis, I make use of marine acoustic methods of different frequency spectrums to quantify methane seepage from a typical post-glacial basin and to analyse buried glacial landforms in the North Sea to understand their history of formation. A novel survey strategy with highly reduced survey speeds allowed to show that gaseous methane seepage by single gas bubbles can reach 1,900 μmol m-2 d-1 in the Eckernförde Bay, Baltic Sea. The ebullition of single gas bubbles is not limited to geological structures and occurs widespread throughout the bay, where shallow gas is present. Due to a diameter between 5 and 10 mm and the shallow water depth, the gas bubbles likely reach the sea surface with approx. 50% of the initial methane content. The widespread seepage of single gas bubbles thus provides an efficient transport mechanism of marine seabed methane into the atmosphere. This phenomenon is likely triggered by storms and may have been overlooked by studies in the past due to a focus on hot spots and technical limitations during storms. Such gas seepage is likely to occur in larger regions of the Baltic Sea, possibly on a global scale where shallow gas is present. As a result, methane seepage by single gas bubbles in shallow seas is probably a highly underestimated source for the release of marine methane to the atmosphere. The analyses of 2D reflection seismic data in the southeastern North Sea between Amrum and Helgoland shows that buried glacial landforms dominate the shallow subsurface. Large tunnel valley systems (1 to 4.5 km wide) with deep incisions (130 to 380 m deep) into Quaternary and Neogene sediments have been identified and traced. Their formation is explained by high hydraulic heads below ice sheets during the Elsterian glaciation. In direct proximity, a large glaciotectonic complex (Helgoland Glaciotectonic Complex; HGC) has been identified based on a number of thrust faults and thrust sheets (up to 100 m thick). Based on the incision of tunnel valleys into the HGC, an early- or pre-Elsterian age for the HGC is proposed. The orientation of these glacial landforms was used to outline several ice margins in the study area. Whether underlying salt structures had an influence on their formation is speculative, despite their morphological influence on pre-Quaternary strata. Cross-cutting relationships as well as cut-and-fill structures indicate that at least five major ice margins were present in the study area during the Pleistocene, which contradicts the traditional model of ice advances into the North Sea limited to the major glaciations of the Elster and Saale glaciations. The results of my thesis show that the transfer of marine seabed methane to the atmosphere via widespread single gas bubble release contributes significantly to the methane budget of shallow seas. Accordingly, the quantification of marine methane release from shallow seas needs to be addressed in future studies for larger areas supported by long-term monitoring stations. Furthermore, this thesis derived the history of formation of glacial landforms in the southeastern North Sea based on geomorphological and structural analyses. The results indicate that the HGC may be evidence of a pre-Elsterian ice advance in the southeastern North Sea, that has the potential to partly revise the Pleistocene stratigraphy for the area. Future studies could validate these findings by direct sampling of suitable interglacial deposits and the determination of absolute ages. This thesis highlights the importance of high-resolution and high-density geophysical surveys to derive the geological history of previously glaciated regions. Combining such investigations with water column imaging allows to identify the controls on methane seepage in these regions to increase the accuracy of marine methane source estimations. These analyses are critical to improve climate models and to be prepared for the long-term consequences of climate change.


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