Environmental Engineering Reference
In-Depth Information
mud volcanoes and gas hydrates of the Black Sea (Table 15) obtained so far
suggest that the interpretation of methane origin in these areas is not straight-
forward. The carbon isotopic composition typical for thermocatalytic methane
(δ
13
C of -30.0‰ to -50.0‰) was detected only over large mud volcanoes in
the central part of the Western gyre and in Sorokin Trench [11, 42] (Table 15).
The isotopic data obtained in other mud volcanoes and gas hydrates showed
much lighter carbon isotopic composition of methane (δ
13
C from -55.0‰ to
-75‰) (Table 15). These values are typical for methane produced during CO
2
reduction with hydrogen by autotrophic methanogens [99]. The data on the
composition of gaseous methane homologues are less unique; their content in
the samples collected in the central Black Sea does not reach several per cents
[42]. Such composition is more indicative of the thermocatalytic methane. In
contrast, samples from Sorokin Trench contained less than one per cent of
these homologues. The authors reported that some samples with light isotopic
composition of methane carbon sometimes contained abundant gaseous homo-
logues of methane [42].
The data on the isotopic and gas composition of mud volcanic fluids are
scarce. Most of them have been published as briefs submitted to various con-
ferences. The fact that mud volcanoes are located along tectonic fracture zones
does not necessarily indicate that gas emitting from these fractures is generated
at greater depths and at high temperatures. Methane of a biogenic origin (formed
in Black Sea sediments of 12-15 km thick during past geological epochs) may
come to the surface along geological fractures. This is true for methane gen-
erated in non-consolidated Pleistocene sediments with high organic carbon
content.
The isotopic composition of methane dissolved in the aerobic and anaerobic
water columns of the Black Sea is poorly studied. Table 16 shows a few data
points published in the literature and data kindly offered to us by Prof. R. Seifert,
which were obtained during GOSTDABS expedition. During this expedition,
dissolved methane and methane collected directly from bubble streams were
sampled by a group of German scientists on board submersible “Jago”. Most
investigations were carried out in the Dniper Canyon with active methane
seeps. The analysis of the isotopic composition of methane sampled at the site
of its outcome and from bubble streams at various depths has been performed.
The results showed that along a gas bubble path, the isotopic composition of
methane is getting heavier because of the selective dilution of the isotopically
lighter methane (Table 16, No. 2 and 3). Dissolved methane was considerably
heavier than methane sampled from bubble streams at the same depths in the
water column (Table 16, No. 3 and 4). This phenomenon can be explained by
active anaerobic oxidation of methane [3], which occurs within the sulfide zone
(see above, Fig. 7).
