Ninth international conference on - Marum

Abstracts of posters 69
Methane and organic matter as sources for excess carbon 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 carbon as organic matter is mineralized in marine sediments by microbial activity using
predominantly oxygen, sulfate, and metal oxides as electron acceptors. Besides this, methane can also be
produced and oxidized aerobically or anaerobically. Both lines of oxidation produce carbon dioxide, and both
carbon dioxide and methane are strong green-house gases that may be liberated from the intertidal surface
sediments into the bottom waters or the atmosphere. The most important anaerobic mineralization process is
bacterial sulfate reduction which is also accompanied by the liberation of carbon dioxide. Different carbonbearing
substrates act as carbon sources for sulfate reduction. Methane is involved in anaerobic oxidation of
methane. The carbon isotopic composition of dissolved inorganic carbon (DIC) is a useful tracer for the
biogeochemical transformations of different carbon sources and is used here to identify the key reactions in the
carbon-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 reduction rates, salinity, pH, sulfate, sulfide, pyrite, AVS, reactive Fe* and Mn*, and
the carbon isotopic composition of DIC and methane. Analytical methods include radio tracer incubation, GC
and IC, ion-selective electrodes, extraction and titration methods, and irmMS with different inlet systems.
In the present study, the pore water composition and stable isotopic composition 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 composition of DIC down to -36 per mil indicates methane as a
source for the oxidized carbon pool, in agreement with chemical pore water gradients. In contrast, DIC is less
enriched in the lighter isotope below oxidized surface sands were oxidation of organic matter via using oxygen
and sulfate as electron 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 accumulations in depths till 1000 m below seafloor. It consists of CO2, H2S,
N2 and some lighter hydrocarbons. The most frequent gaseous hydrocarbon 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 unconsolidated sediments.
Already small amounts of free gas in pore fluids may have a visible effect in seismic images. Common effects
are amplitude anomalies. They can be classified as enhanced reflections or bright spots. Enhanced reflections
appear as extended amplitude anomalies with less defined boundaries. In contrast bright spots appear as spatially
limited amplitude anomalies within consistent reflectors. In the northwestern part of the German North Sea large
bright spot accumulations were detected. They occur above salt diapirs at depth of ~400 m to ~1000 m below
seafloor. These accumulations are of a nearly circular shape and it seems that they are connected to the crestal
fault systems, which developed above the diapirs.
Furthermore gas chimneys, acoustic turbidity, and absent reflections 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 zones of chaotic reflections. The effect of absent or faint reflections is characterized
by zones 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 on 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.