Distribution and fate of methane released from submarine sources – Results of measurements using an improved in situ mass spectrometer

Torben.Gentz [ at ] awi.de


Methane (CH4) is the most frequent organic compound in the atmosphere and its influence on the global climate is subject of currently conducted scientific discussion. Despite its limited content in the atmosphere (1787 ppbv in 2003), it contributes to ~15 % of the global warming as a result of its 20 to 40 times higher global warming potential compared to carbon dioxide (CO2) on a 100 year timescale. One source of atmospheric methane is the release of biogenic and/or thermogenic CH4 from the oceans seafloor, which is currently one of the research priorities of the marine geosciences. These submarine sources are characterized by rising gas bubbles or diffusive methane flux into the water column. It is estimated that these point sources release a total of ~30 Tg CH4 per year into the ocean, and after its biological oxidation or dissolving in the water, ~10 Tg CH4 are released into the atmosphere per year. Additionally, due to the warming of the oceans, an increasing release of methane can be expected as a result of the melting of permafrost and gas hydrates. Steep gradients over very short distances (< 20 m) and high time-based variability (few hours) are known from dissolved methane concentrations in the water column above these submarine CH4 sources. Due to the limited number of samples taken by conventional ex situ methods, an accurate quantification of the methane distribution could hardly be estimated. Nevertheless, one objective of the present thesis was the detailed spatial representation of the dissolved CH4 in the water column originates from submarine seeps as well as the study of relevant pathways such as vertical or horizontal transport, dilution and its microbial oxidation. Therefore, the first part of the dissertation deals with the optimization and establishment of a novel underwater mass spectrometer (UWMS, Inspectr200-200, Applied Microsystems Limited™) designed for inline, real time and in situ sampling in high frequency. Analysis and evaluation of several thousand samples per day take place in one step, so that one obtains the measurement result in situ and, unlike using conventional methods, without delay, and thus the sampling strategies can be adapted to the existing environment. Additionally, through the use of this novel analytical tool, potential sources of errors that occur during sampling or transport to Summary 11 the laboratories are eliminated. In order to be able to use the potential of this mass spectrometer for scientific research questions, it was necessary to optimize the detection limit for the trace gases that were to be determined. For this purpose, a Stirling cooler was applied, which serves as a trapping system for water vapour and thus leads to optimized conditions for the analysis. In particular for CH4, the detection limit could be decreased from more than 100 nmol L-1 to 16 nmol L-1. Within the framework of this thesis two gas ebullition areas were studied in detail. While one, which is located in the continental shelf northwest of Spitsbergen, is in the center of scientific attention, the gas ebullition area that was studied in the North Sea has not yet been examined until now with regard to the methane release into the water column and its subsequent pathways. The global climatic change in the Arctic regions can be increasingly monitored. Thus years an increase in ocean temperature was observed in the last 30, which potentially leads to destabilization of gas hydrates and reduced methane storage capacity of the sediments. In the studied area with a size of 175 km² and an average water depth of 245 m, 10 gas ebullition locations, which were possibly induced by these climatic changes, were detected by using hydroacoustic methods. In this study, the release of dissolved CH4 into the water column and its subsequent lateral and vertical transportation, microbial oxidation and dilution were investigated. For this purpose, methane concentrations, isotopic ratios and oxidation rates were determined and compared with hydroacoustic and oceanographic data. The study area is influenced by the northwards flowing West Spitsbergen current (WSC), which leads to a stratification (about 20 m above ground) of the water column due to salinity differences. With the help of detailed sampling and a subsequent modeling, we determined that ~80 % of the methane contained in the ascending gas bubbles is dissolved under the pycnocline into the surrounding water and leads to a locally increased CH4 concentration. Even though rise heights of up to 50 m under the sea surface were detected by means of hydroacoustics, a direct transport of methane into the atmosphere via gas bubbles can be excluded due to the fast dissolution of the gaseous CH4 into the ambient water. Summary 12 Therefore, the vertical transport of dissolved CH4 was studied in more detail as a potential source for atmospheric methane. The observed pycnocline represents a limitation for the vertical transportation of dissolved CH4. Therefore, dissolved methane is mostly laterally transported underneath the pycnocline to greater depths and is, as shown by the determinations of the oxidation rates, microbiological oxidized over time. As long as there is a stratification of the water column, the transport of methane in the gas bubbles (about ~20 % of the entire amount of CH4) into the water masses above the pycnocline and the subsequent dissolution of the gas bubbles represent the only potential pathway of methane into the atmosphere. The investigations at the gas seepage area in the Netherland economic zone in the North Sea reveal significant differences regarding to the fate of dissolved CH4 in the water column. It has been shown in previous studies that most probably the gas source is a gas reservoir at a sediment depth of 600 m, which reaches the sediment surface through fractures (gas chimneys) in the sediment structure. Our video observations indicated 113 gas streams with an estimated release of 35.3 + 17.65 t CH4 yr-1. The dissolved methane concentration in various depths was measured in high resolution with the optimized in situ mass spectrometer. This data set, consisting of 11.900 single measurements sampled in between 24 hours, represents the dissolved CH4 concentration above a gas seepage in up to 750 times higher temporal and spatial resolution than would be possible by conventional methods. Highest methane concentrations (~3.5 μmol L-1) were detected in the surrounding water of the gas bubble streams and the inventory in the entire water column was calculated to a total amount of ~6.4*105 μmol. Due to the unique measurements, these results are not yet comparable with other gas seep areas. During the time of measurement the oceanographic data reveal a pycnocline, which we indicated as the main forcing factor for the pathway of methane at the gas seepage in the shelf area offshore West Spitsbergen. However, due to the short distance to the seabed (~10 m), a high grade of gas bubble dissolution takes place in the mixed layer above the pycnocline, and we consider that ~40 % of the total seabed released methane could enter the Summary 13 atmosphere via this indirect pathway. Additionally, we measured direct CH4 transport via gas bubbles of ~25 % by discrete gas bubble sampling at the sea surface, which emphasize, that 65 % (23 + 11.5 t CH4 yr-1) of the entire seabed released CH4 potentially contributing to the atmospheric methane budget, which is far above most studied gas seepages. With the help of the optimized mass spectrometer it became possible for the first time to obtain distribution patterns of dissolved CH4 in the water column in high resolution. With respect to the geochemical functionality of these increasingly important methane sources, the research conducted in this dissertation contribute to improve our knowledge of the entry of CH4 into the water column as well as its fate. Therefore, the applied novel technique can contribute to “revolutionize our understanding of the behavior of seep plumes” as suggested by Judd and Hovland (2007).

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Gentz, T. (2013): Distribution and fate of methane released from submarine sources – Results of measurements using an improved in situ mass spectrometer , PhD thesis, University of Bremen.

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