Environmental Engineering Reference
In-Depth Information
tions [48] as well as active mud volcanoes [41] were discovered, the distribution
of methane have only been studied at few stations in the deep-sea. It was re-
ported that methane concentration increases mostly linear down to 500-600
m to 10-13 µmol l
−
1
; below methane concentration is constant [40, 84]. The
shape of the methane profile with depth suggested that methane produced by
microorganisms in bottom sediments diffuses into the water column.
Microbiological investigations of the distribution and geochemical activity
of microorganisms involved in the methane production and oxidation have
started in 1980s [46, 64]; extensive studies have been performed in recent years
[29, 30, 49, 52]. Figure 7 shows main results of these investigations. The data
indicate that methane concentration increases monotonically from the upper
boundary of the sulfidic zone to the bottom at stations in the upper part of the
continental slope (up to 600 m) (Fig 7, A). At deep-sea stations in the western
part of the sea (Fig. 7B), pronounced peak of methane concentration at 400-
600 m was explained by lateral migration of methane from abundant cold seeps
located in the upper part of the continental slope. Methane content increases in
the near-bottom water (Fig. 7A, 7B). At many of deep-sea stations, methane
content in bottom sediments is lower than in the near-bottom water (Fig 7D,
E). This phenomenon was first discovered by Berlin et al. [5] and was later
confirmed in our investigations in different Black Sea regions [29, 30, 52].
Figure 7 shows the vertical distribution of the rates of methanogenesis and
anaerobic oxidation of methane. These rates were estimated using
14
CO
2
and
14
CO
3
COONa for methanogenesis and
14
CH
4
for methane oxidation [30, 51].
Integrated over 1 m
2
data demonstrate that methane oxidation exceeds methane
production in the water column. The integrated microbial production of methane
in the water column is 62.9 10
10
moles per year in the entire anaerobic zone (306
10
3
km
2
, depths more than 200 m). The annual methane oxidation is 77.8 10
10
mol per year. Rates of methanogenesis and anaerobic methane oxidation in Late
Holocene sediments (upper 0-40 cm) are much lower - 27.6 10
7
mol and 24.6
10
7
mol, respectively [52, 72]. The data show that microbial methanogenesis
in the anaerobic water column accounts for more than 80% of the methane
content. However, the water column receives annually no less than 14.8 10
10
moles of methane from various external sources - underwater mud volcanoes
and cold methane seeps [52, 72]. The bulk of methane coming from these two
sources, as well as its origin (biogenic or thermogenic) have not been adequately
investigated. The investigation of the isotopic composition of methane as well as
of its heavier gaseous homologues are the most important isotopic-geochemical
problems of the Black Sea.
The values of δ
13
C of methane from surface [2, 39, 40, 51] and subsurface
sediments (4.8 - 6.3 m; [40]) are shown in Fig. 8. These values indicate a
biogenic origin of methane produced during CO
2
reduction with hydrogen.
According to results of our experiments with radioactively labeled substrates,
