Ahrens, Tanja (2020) Underway CH4 measurements across the North Atlantic Ocean from a ship of opportunity. (Bachelor thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 30 pp.

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Abstract

Human activities have been enhancing the atmospheric mole fractions of many long-lived greenhouse gases including carbon dioxide (CO2) and methane (CH4). Methane has an important effect on the radiative balance, because it absorbs terrestrial radiation, therefore has a high radiative impact, like other greenhouse gases. Methane release sources are divided into three different processes: biogenic, thermogenic and pyrogenic. Biogenic methane is a final product of the decomposition of organic matter with sources like swamps, rice paddies, marine sediments and landfills. Thermogenic methane is formed by the breakdown of organic matter due to heat and pressure (eg. marine and land geological gas seeps and exploitation and distribution of fossil fuels) and pyrogenic methane is produced by incomplete combustion of biomass (eg. biofuel burning and wildfires). Though substantial progress has been made over the past 30 years, the role methane plays in climate change is not fully determined yet. High methane accumulations are found in sediments of rivers, lakes and oceanic shelf regions, from where it is released into water columns and oxidized. In shallow waters, part of the sedimentary methane can reach the atmosphere through mixing or direct ebullition, however. Oxygen-replete surface open-ocean waters are usually slightly supersaturated with methane, which cannot be explained by sedimentary sources, therefore often called the oceanic-methane paradox. This thesis focuses on continuous underway measurements in surface waters autonomously using the Atlantic Cartier, a Ship of Opportunity (SOOP) across the North Atlantic Ocean, which was conducted in 2016. Discrete comparison samples were taken to evaluate the performance of the underway system. This comparison revealed a reasonable agreement between the measurement methods. The CH4 concentrations measured during the investigated time period in October 2016 were among the highest concentrations ever measured in the open ocean. They could be associated with a phytoplankton bloom in the North Atlantic ocean and may indicate that open ocean surface methane concentrations may be more variable than previously observed.

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