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
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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.
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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
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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)