Poster Sessi<strong>on</strong> <strong>on</strong> September 16 and 18, 16:30 – 18:30 Abstracts of posters (alphabetic order)
Abstracts of posters 69 Methane and organic matter as sources for excess carb<strong>on</strong> dioxide in intertidal surface sands of the North Sea: Biogeochemical and stable isotope evidence A.M. Al-Raei 1 , M. E.Böttcher 1,2 , V. Heuer 3 , Y. Hilker 4 , B. Engelen 4 , K. U. Hinrichs 3 , M. Segl 3 1 Max Planck Institute for Marine Microbiology, D-28359 Bremen, FRG 2 Leibniz Institute for Baltic Sea Research, FRG 3 RCOM und FB Geowissenschaften, University of Bremen, Germany 4 ICBM, Oldenburg University, Germany Reduced organic carb<strong>on</strong> as organic matter is mineralized in marine sediments by microbial activity using predominantly oxygen, sulfate, and metal oxides as electr<strong>on</strong> acceptors. Besides this, methane can also be produced and oxidized aerobically or anaerobically. Both lines of oxidati<strong>on</strong> produce carb<strong>on</strong> dioxide, and both carb<strong>on</strong> dioxide and methane are str<strong>on</strong>g green-house gases that may be liberated from the intertidal surface sediments into the bottom waters or the atmosphere. The most important anaerobic mineralizati<strong>on</strong> process is bacterial sulfate reducti<strong>on</strong> which is also accompanied by the liberati<strong>on</strong> of carb<strong>on</strong> dioxide. Different carb<strong>on</strong>bearing substrates act as carb<strong>on</strong> sources for sulfate reducti<strong>on</strong>. Methane is involved in anaerobic oxidati<strong>on</strong> of methane. The carb<strong>on</strong> isotopic compositi<strong>on</strong> of dissolved inorganic carb<strong>on</strong> (DIC) is a useful tracer for the biogeochemical transformati<strong>on</strong>s of different carb<strong>on</strong> sources and is used here to identify the key reacti<strong>on</strong>s in the carb<strong>on</strong>-sulfur cycle of intertidal surface sediments. Pore waters from intertidal sands of the back-barrier tidal area of Spiekeroog and Sylt islands (southern and eastern North Sea) have been sampled down to 40 cm using different techniques, and water samples and sediments are analyzed for a number of (bio)geochemical parameters as, for instance TOC, TIC, DIC, TA, methane, microbial sulfate reducti<strong>on</strong> rates, salinity, pH, sulfate, sulfide, pyrite, AVS, reactive Fe* and Mn*, and the carb<strong>on</strong> isotopic compositi<strong>on</strong> of DIC and methane. Analytical methods include radio tracer incubati<strong>on</strong>, GC and IC, i<strong>on</strong>-selective electrodes, extracti<strong>on</strong> and titrati<strong>on</strong> methods, and irmMS with different inlet systems. In the present study, the pore water compositi<strong>on</strong> and stable isotopic compositi<strong>on</strong> of DIC is investigated to characterize the different biogeochemical processes in intertidal surface sands below oxic and anoxic surfaces. Below reduced sediment surfaces, the isotopic compositi<strong>on</strong> of DIC down to -36 per mil indicates methane as a source for the oxidized carb<strong>on</strong> pool, in agreement with chemical pore water gradients. In c<strong>on</strong>trast, DIC is less enriched in the lighter isotope below oxidized surface sands were oxidati<strong>on</strong> of organic matter via using oxygen and sulfate as electr<strong>on</strong> acceptors dominate. Gas indicators in seismic data J. Appel 1 , R. Lutz 1 , H. Keppler 1 , C. Gaedicke 1 1 Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germany The term “shallow gas” defines gas accumulati<strong>on</strong>s in depths till 1000 m below seafloor. It c<strong>on</strong>sists of CO2, H2S, N2 and some lighter hydrocarb<strong>on</strong>s. The most frequent gaseous hydrocarb<strong>on</strong> is methane, followed by ethane, propane, and less butane and pentane (with their equivalent alkenes). The gas indicators were examined in 2-D and3-D seismic data located in the German North Sea. They mostly occur in unc<strong>on</strong>solidated sediments. Already small amounts of free gas in pore fluids may have a visible effect in seismic images. Comm<strong>on</strong> effects are amplitude anomalies. They can be classified as enhanced reflecti<strong>on</strong>s or bright spots. Enhanced reflecti<strong>on</strong>s appear as extended amplitude anomalies with less defined boundaries. In c<strong>on</strong>trast bright spots appear as spatially limited amplitude anomalies within c<strong>on</strong>sistent reflectors. In the northwestern part of the German North Sea large bright spot accumulati<strong>on</strong>s were detected. They occur above salt diapirs at depth of ~400 m to ~1000 m below seafloor. These accumulati<strong>on</strong>s are of a nearly circular shape and it seems that they are c<strong>on</strong>nected to the crestal fault systems, which developed above the diapirs. Furthermore gas chimneys, acoustic turbidity, and absent reflecti<strong>on</strong>s are other possible gas indicators in the observed seismic data. Chimneys are vertical or near vertical columnar disturbances in seismic data. Acoustic turbidity is characterized by z<strong>on</strong>es of chaotic reflecti<strong>on</strong>s. The effect of absent or faint reflecti<strong>on</strong>s is characterized by z<strong>on</strong>es of acoustic blanking and mostly occur in high-frequency seismic data. All this potentially gas-indicating effects appear in seismic data of the North Sea. Based <strong>on</strong> 2-D and 3-D seismic data we mapped the occurrences of bright spots in the German North Sea. This map can be used to minimize the risks of geohazards (e.g. blow-out risk) related with shallow gas.
Change language