Fig 5 - uploaded by Frank Wenzhoefer
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1.1: Examples of screen shots of online echograms of the PHF 19 kHz of PARASOUND showing the hydroacoustic anomalies at the Amsterdam Mud Volcano.
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... Data used in this study, including hydroacoustic records, seafloor video material, sediment and gas samples, water column temperature data and methane efflux rates, was collected during four cruises to the NDSF: R/V Meteor M70/3 in 2006, R/V Pourquoi pas? (Medeco2) in 2007, and R/V Maria S. Merian MSM13 Legs 3 and 4 in 2009. The overall goal of these cruises was to decipher key processes at eastern Mediterranean cold seeps and to investigate associated chemosynthetic ecosystems Wenzhöfer et al., 2011). ...
High acoustic seafloor-backscatter signals characterize hundreds of patches of methane-derived authigenic carbonates and chemosynthetic communities associated with hydrocarbon seepage on the Nile Deep Sea Fan (NDSF) in the Eastern Mediterranean Sea. During a high-resolution ship-based multibeam survey covering a ~ 225 km2 large seafloor area in the Central Province of the NDSF we identified 163 high-backscatter patches at water depths between 1500 and 1800 m, and investigated the source, composition, turnover, flux and fate of emitted hydrocarbons. Systematic Parasound single beam echosounder surveys of the water column showed hydroacoustic anomalies (flares), indicative of gas bubble streams, above 8% of the high-backscatter patches. In echosounder records flares disappeared in the water column close to the upper limit of the gas hydrate stability zone located at about 1350 m water depth due to decomposition of gas hydrate skins and subsequent gas dissolution. Visual inspection of three high-backscatter patches demonstrated that sediment cementation has led to the formation of continuous flat pavements of authigenic carbonates typically 100 to 300 m in diameter. Volume estimates, considering results from high-resolution autonomous underwater vehicle (AUV)-based multibeam mapping, were used to calculate the amount of carbonate-bound carbon stored in these slabs. Additionally, the flux of methane bubbles emitted at one high-backscatter patch was estimated (0.23 to 2.3 × 106 mol a− 1) by combined AUV flare mapping with visual observations by remotely operated vehicle (ROV). Another high-backscatter patch characterized by single carbonate pieces, which were widely distributed and interspaced with sediments inhabited by thiotrophic, chemosynthetic organisms, was investigated using in situ measurements with a benthic chamber and ex situ sediment core incubation and allowed for estimates of the methane consumption (0.1 to 1 × 106 mol a− 1) and dissolved methane flux (2 to 48 × 106 mol a− 1). Our comparison of dissolved and gaseous methane fluxes as well as methane-derived carbonate reservoirs demonstrates the need for quantitative assessment of these different methane escape routes and their interaction with the geo-, bio-, and hydrosphere at cold seeps.
The Amon mud volcano (MV), located at 1250 m water depth on the Nile
Deep Sea Fan, is known for its active emission of methane and
non-methane hydrocarbons into the hydrosphere. Previous investigations
showed a low efficiency of hydrocarbon-degrading anaerobic microbial
communities inhabiting the Amon MV center in the presence of sulphate
and hydrocarbons in the seeping subsurface fluids. By comparing spatial
and temporal patterns of in situ biogeochemical fluxes, temperature
gradients, pore water composition and microbial activities over three
years, we investigated why the activity of anaerobic hydrocarbon
degraders can be low despite high energy supplies. We found that the
central dome of the Amon MV, as well as a lateral mud flow at its base,
showed signs of recent exposure of hot subsurface muds lacking active
hydrocarbon degrading communities. In these highly disturbed areas,
anaerobic degradation of methane was less than 2% of the methane flux.
Rather high oxygen consumption rates compared to low sulphide production
suggest a faster development of more rapidly growing aerobic hydrocarbon
degraders in highly disturbed areas. In contrast, the more stabilized
muds surrounding the central gas and fluid conduits hosted active
anaerobic hydrocarbon-degrading microbial communities. Furthermore,
within three years, cell numbers and hydrocarbon degrading activity
increased at the gas-seeping sites. The low microbial activity in the
hydrocarbon-vented areas of Amon mud volcano is thus a consequence of
kinetic limitations by heat and mud expulsion, whereas most of the outer
mud volcano area is limited by hydrocarbon transport.
