Environmental Engineering Reference
In-Depth Information
ously demonstrate the biogenic origin of methane formed by microbiological
reduction of CO
2
with hydrogen [99]. Extremely low content of heavy gaseous
homologues of methane in gaseous fluids of seeps also suggested biogenic ori-
gin of methane. The analysis of our samples (Table 14) shows that the content
of these homologues does not usually exceed 0.001% [71]; content of heavy
hydrocarbons in gas samples collected on the Bulgarian shelf was 0.1-0.4%
[26]. Therefore, the biogenic origin of methane from cold seeps is confirmed
by (i) the carbon and (ii) hydrogen isotopic composition of emitted methane,
and by (iii) the composition of methane homologues.
13
C and
Table 14.
Chemical composition of gases and the isotopic composition of methane (δ
δ D) from cold methane seeps of the Dnieper Canyon [71].
13
C
Station
Depth, m
Composition of gases, vol %
δ
δ
D
CH
4
H
4
N
2
CO
2
(PDB), SMOW,
‰‰
14
182
95.0
-
-
-
-68.2
-
21
225
99.0
0.17
0.90
0.00
-68.1
-
33
324
95.1
0.25
4.60
0.26
-70.5
-144
38
230
99.0
0.02
0.90
0.80
-64.0
-132
46
111
99.1
0.01
0.00
0.90
-65.0
-
51
371
94.9
0.02
5.10
0.05
-63.0
-
55
226
99.6
0.00
0.17
0.26
-66.5
-201
61
64
96.2
0.01
3.60
0.10
-65.0
-
68
321
98.8
0.02
1.10
1.10
-68.5
-169
85
230
96.8
0.01
3.20
0.00
-62.4
-
There is no reason to believe that all methane is produced in modern sedi-
ments of the Black Sea. Methane distribution with depth in sediments down to
3.0-3.5 m revealed a constant increase in methane concentrations up to 50-75
mM in pore water [54]. Rates of methanogenesis measured with NaH
14
CO
3
have showed that microbial methanogenesis occurs in the whole sequence of
Holocene sediments down to a depth of at least 2.6 m [46]. Methanogenesis in
sediments of the western Caspian Sea have been detected to sediment depths
as high as 100 m [47]. Many researchers associated the distribution of methane
seeps with fractures in the sea bed. Biogenic methane emitted along these
fractures comes from the sapropel layer rich in organic carbon.
Mud volcanoes and sub-sea gas hydrates in the lower part of the continental
slope (700-750 m water depth) and in the deep basin are the third source of
the Black Sea methane [33, 41, 61]. Mud volcanoes are located on tectonic
dislocations, which suggests that methane emitted from volcanoes can be of
thermocatalytic origin. The data on the isotopic composition of methane from
