Gindorf, Sonja (2020) Development of a purge + trap system for the quantification of methane variability in the Baltic Sea. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 81 pp.

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Abstract

Methane (CH4) is the second most important anthropogenic greenhouse gas. Generally, open ocean surface waters are at atmospheric equilibrium or slightly oversaturated. Oceanic emissions play only a minor role in the global CH4 budget. However, coastal areas account for up to 75 % of the total CH4 emissions from the marine environment to the atmosphere which is linked to direct CH4 inputs from sedimental methanogenesis driven by the high organic matter sedimentation, especially in eutrophicated systems, such as the Baltic Sea. Highly accurate CH4 measurements are necessary to investigate the small-scale variability of CH4 in the surface water and the related gradients and fluxes. Therefore, the objective of this thesis was to develop a purge and trap (PT) system coupled to a GC-FID to examine the surface and water column CH4 distribution within Kiel Bight at a high temporal and spatial resolution in two cruises in June and September 2018. This work showed that PT measurements outplay static HeadSpace equilibration (HS) measurements as the latter bear a significantly higher error in calibration and sample measurements. The CH4 distribution and fluxes observed during two cruises in June and September 2018 fit well with previous data of the region. The results indicate that the monthly measurements at Boknis Eck time series station are representative for the Kiel Bight. Generally, CH4 concentrations were found to increase within the water column. The surface water was at all times oversaturated with respect to atmospheric equilibrium. Concentrations were in the range of observations from previous studies. Strong CH4 gradients occurred in the upper 1 m of the water column leading to significant differences in flux density calculations depending on the surface depth between 0.1 and 1 m raising the question how suitable (continuous) underway measurements are to estimate CH4 surface dynamics.

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