Environmental Engineering Reference
In-Depth Information
Table 15.
The carbon isotope composition of methane from mud volcanoes and gas hydrates.
13
C,
‰
Description of sites
δ
Reference
1
Large mud volcanic
−
30.0
÷−
50.0
[42]
structures of the
central part of the sea
and Sorokin Trench
−
40.0
÷−
42.0
[11]
2
Small mud
−
55.0
÷−
75.0
[42]
volcanoes of
Sorokin Trench
−
÷−
3
Gas hydrates and
61.0
65.0
host sediments
4
Gas hydrates from
−
62.0
÷−
66.0
[7]
Dvurechensky mud
volcano
∗
5
Sediments of
−
61.8
÷−
63.5
[11]
volcanic structures and
gas hydrates
6
Gas from sediments at
−
63.9
Data from
volcanic structures
CRIMEA Expedition
∗
δ D of methane in the sample varied from -185 to -209 ‰ SMOW.
An active process of microbial methane oxidation causes the increase of the
isotopic composition of residual methane (up to δ
13
C = -19.0‰) in the chemo-
cline (Table 16, No. 3 and 5). In the oxic zone, isotopically light methane (δ
13
C
varied from -40.0‰ to -60.6‰; Table 16) has been detected. This methane is
generated in anaerobic microniches (zooplankton intestines, pellets, and other
particles) due to the activity of anaerobic methanogens. Our experiments with
14
CO
2
and
14
CH
3
COONa have demonstrated that the rate of methanogenesis
in one liter of water from the oxic zone may be as high as 170-180 nl of CH
4
per
day; the excess of microbial methane production over its consumption varies
between 0.6 to 5.4 ml m
−
2
per day for the whole oxic zone [86]. These new
experimental data explain a sharp increase in the methane content within the
oxic zone, which many investigators have described [27, 29, 52]. A bulk of
methane in the sulphidic zone is oxidized upon contact with waters containing
dissolved oxygen. Methane produced during methanogenesis on particles in
the oxic zone is the main source of methane emitting to the atmosphere [86].
Unfortunately, the isotopic composition of methane in the sulphidic zone of
the central part of the Black Sea was not adequately studied. Schubert et al.
[88] reported δ
13
C of methane of -50‰ in the central Black Sea. The heavier
