Environmental Engineering Reference
In-Depth Information
1. INTRODUCTION
Methane is an important greenhouse gas which has increased from a level
of 850 ppb before industrialization to 1.7 ppm today, further increasing with
approximately 1 % per year [6, 34]. Although methane concentration in the
atmosphere is small compared to CO 2 (360 ppm), its impact as a greenhouse
gas in the atmosphere is about 24 times higher [15]. Main methane sources of
the earth today are of human origin. The largest natural sources are wetlands
and termite guts. A large proportion of the methane flux to the atmosphere
comes from anthropogenic sources that are either energy related (i.e. mining
and gas drilling) or agricultural (i.e. ruminants, rice agriculture and biomass
burning beside landfills) [6]. Despite considerable sources of methane in the
seafloor, the ocean generally contributes only a small amount of
5-20 Tg
methane per year ( < 2%) to the atmosphere [6] due to microbial aerobic and
anaerobic oxidation processes in sediments and water column [35, 36]. Recent
research has focused a lot on the role of gas hydrates as the largest reservoir
of methane on earth that has been overlooked before 20-30 years ago [25].
Methane clathrates are now found at almost all continental margins with a
suitable temperature-pressure field. [26] including the north-western part of the
Black Sea [5, 16].
The Black Sea has a surface area of 423,000 km 2 and a maximal depth of
2212 m, and represents the world largest anoxic basin [39]. After a freshwater
period during the last glacial, the Black Sea turned into a brackish basin when
the Bosporus established a full connection to the Mediterranean about 7150 yrs
ago [13, 40]. Due to large freshwater inflow by rivers the surface water now
has a salinity of 17.5-18.5 ‰, whereas the deep water salinity is 22.3 ‰ [31].
Anoxia developed 7500 yrs ago due to the stable stratification of the Black Sea
waters [20]. The aerobic surface waters are separated from anoxic deep waters
by a chemocline at 100-200 m water depth depending on the geographical
location [45]. On the shelf and upper continental slope where large rivers like
the Danube enter the Black Sea the chemocline may even reach down to 300 m
[45]. The water column contains substantial but varying amounts of methane,
which has been attributed to methanogenesis in the water column [17] and
sediments [37], as well as to the release of methane from gas reservoirs such
as methane hydrates [16]. Whereas methane concentrations in the oxic surface
waters are in the nanomolar range, methane concentrations in the anoxic deep
water are much higher, sometimes exceeding 10µM. The inventory of methane
in the Black Sea adds up to 96 Tg methane [37, 44]. It is still not clear which
role the deposited sediments play in the methane turnover in the Black Sea,
i.e. whether they function as a methane source or sink. Reeburgh et al. [37]
have suggested that sediments on the slope emit methane into the water column
whereas basin sediments serve as a methane sink.
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