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obvious from sediment methane concentration profiles presented by Sorokin
[45]. Sediment methane concentration profiles from our investigation (Fig. 2)
show high concentrations at the surface and lower concentrations at the bottom
of the cores indicating that sediments from the slope and the basin are a sink for
water column methane rather than a source. Dissolved water column methane
diffuses into the sediments and is consumed by methanotrophic organisms. In
contrast to the methane concentration profiles measured in this study (low µM
range), Reeburgh et al. [37] measured concentrations in the mM range and
suggested a high flux of methane from the shelf sediments to the water column.
However, this is at odd with our findings and the results by Jørgensen et al. [21],
Friedl et al. [10], and Friedrich et al. [11]. One explanation for the very high
methane concentrations of the sediment core from Reeburgh et al. [37] may be
that sediments were recovered from a seep system, an assumption made earlier
also by Jørgensen et al. [21].
Most likely, sediments are only a source of methane where the gas is trans-
ported by advective processes such as fluid flow and ebullition of free gas. A
high number of gas seeps that have been found close to the Crimea peninsula
[16, 29] and meanwhile all around the shelf of the Black Sea (results by EU
projects CRIMEA; METROL, ASSEMBLAGE) support this hypothesis.
(2) Whether methane is formed in the deep anoxic water column by methano-
genesis is highly debated. Ivanov et al. [19] suggested that methane is formed in
the order of 63 x 10 10 mole per year during the process of organic matter degra-
dation in the water column. Reeburgh et al. [37] argued that methane formation
in the water column should be negligible, because sulfate reducers outcompete
methanogenic bacteria for fermentation products at the presence of sufficient
sulfate. Results from Konovalov et al. ([24], and manuscript in preparation)
show that the profiles of ammonium and sulfide are in agreement with what
would be expected when both constituents were solely derived from organic
matter degradation by sulfate reduction, and that sulfate reduction would bal-
ance the export flux from surface water. This means that there is only very little
place for methanogenesis in the water column and presumably not in the amount
proposed by Ivanov et al. [19]. If methanogenesis is a significant process in
the water column, this should show up in the isotope signature, as well as in
the presence of specific biomarker lipids. We have found only minute amounts
of archaeal biomarkers indicative of methanogenic archaea at site 7605. Here,
they coincide with a substantial increase in the δ
13 C of methane, pointing to a
zone of anaerobic methanotrophy rather than methanogenesis. We, therefore,
conclude that methanogenesis is not a significant process in the water column
compared to methane oxidation.
(3) Several hundred seeps emitting methane to the water column were dis-
covered during the last years especially on the NW shelf and south of Crimea
([2] [18] and CRIMEA Cruise Reports 2003, 2004). These seeps are so com-
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