Environmental Engineering Reference
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still unknown, hence the emission of methane from the water column to the
atmosphere related to gas seepage cannot be constrained at this point.
4.2 Variations in Methane Concentration in the Water
Column Over Time
The key publication that deals with methane in the Black Sea arose from
the 1988
R/V Knorr
expedition [37]. Reeburgh et al. [37] showed one methane
profile from the central basin with low concentrations (
<
10 nM) in the oxic
zone above 100 m, increasing concentrations from 100 to 550 m, and very
stable concentrations around 11 µM down to 2200 m. Additionally, Reeburgh
et al. [37] showed methane concentration data from the deep anoxic waters
measured by Scranton [44] that were above 12 µM and speculated that this
might hint to a methane decrease in the time between the two studies, i.e., 1975
and 1988. However, our measurements from 2001 and new measurements
during the CRIMEA cruises in 2003 and 2004 are all between 10.5 and 13.1
µM and therefore close to Reeburgh's et al. [37] data. Hence, we conclude that
any increase or decrease in this range could be reflecting regional variability,
and that methane concentrations appear to be relatively stable over the past 30
years.
4.3 Sources of Methane to the Water Column
There are three potential sources for methane in the Black Sea: (1) methane
is released from the sediments to the water column, (2) methane is produced in
the water column or (3) methane seeps emit methane from deeper reservoirs to
the water column.
(1) Methane contribution from the sediments to the water column: Depend-
ing on the location in the Black Sea the organic matter burial rates are very
different. In front of rivers, high amounts of marine and terrestrial organic ma-
terial are delivered to the sediments and degraded under anoxic conditions by
iron, manganese, and sulfate reduction, eventually leading to methanogenesis.
On the other hand, towards the central part of the Black Sea terrestrial contri-
bution is limited and organic matter input depends on export of phytoplankton
biomass to the seafloor [46]. Lander investigations by Friedl et al. [10] and
Friedrich et al. [11] showed that no or only negligible amounts of methane
were formed during degradation of organic material in sediments underlying
oxic and anoxic bottom waters of the north-western Black Sea shelf. Addi-
tionally, Jørgensen et al. [21] could show on a sediment transect located on
the north-western shelf and including sediments from water depths from 100 to
1200 m that, although methane is produced deeper in the sediments, no methane
reaches the sediment surface and escapes to the water column due to the anaer-
obic oxidation of methane at the sulfate/methane transition zone. This is also
