Environmental Engineering Reference
In-Depth Information
13
C
bio
13
C
tot
δ
×
M
bio
−
δ
×
M
tot
13
C
sip
+
volc
δ
=
=−
51.3‰
+
M
sip
volc
We calculated the total amount of methane coming from volcanoes (M
volc
)
and seeps (M
sip
):
13
C
sip
13
C
sip
+
volc
×
δ
−
×
δ
M
sip
+
volc
M
sip
+
volc
10
10
moles/year
M
volc
=
=
7,8
×
13
C
volc
13
C
sip
)
(δ
−
δ
The annual flux of methane from external sources (mud volcanoes and cold
methane seeps) should be similar (7.8 and 7.0 10
10
moles/year, respectively).
It is noticebale that our calculation of the annual flux of methane from seeps is
close to the estimation of Egorov and his colleagues (9.8 10
10
moles/year) who
used a different method of calculation [28].
In cold seeps and mud volcanoes, a large-scale anaerobic methane oxidation
is associated with the methane release. This process is followed by accumulation
of the isotopically light organic matter of bacterial mats and methane-derived
carbonate minerals. First analyses which we carried out in 1989 [48] revealed
that the δ
13
C values of aragonite from carbonate constructions ranged between
-35 to -42‰; δ
13
C of organic matter of microbial mats was -82.5‰.
Figure 9 shows the results of more detailed investigations of the isotopic
composition of organic matter and lipid fraction of mats as well as of aragonite
from carbonate constructions and calcite from their base [69, 71, 77]. Insignif-
icant depletion in
13
C of organic carbon compared to that of methane is the
result of fractionation of carbon isotopes during the synthesis of microbial bio-
mass. The lipid fraction is far more depleted in
13
C. Inorganic carbon dissolved
in seawater takes part in the formation of carbonate minerals as well, along
with the isotopically light carbon dioxide produced during methane oxidation.
The carbon of carbonate minerals is therefore less depleted in
13
C compared
to carbon of the microbial biomass. The difference in the isotopic composition
of aragonite from carbonate constructions and of calcite from its base can be
explained by the fact that carbonate minerals at the base contain isotopically
heavy carbonate precipitated from the water column.
7.
CONCLUSIONS
1. The results of detailed investigations of the isotopic composition of hy-
drogen sulfide and sulfates in the water column and bottom sediments of
the Black Sea have confirmed that hydrogen sulfide is produced mostly in
the anaerobic water column due to activity of sulfate-reducing bacteria.
34
S
of hydrogen sulfide has confirmed the results of laboratory experiments
2. A comparison between the data on the rates of sulfate reduction and δ
