Environmental Engineering Reference
In-Depth Information
mon that for instance only during the two cruises linked to the CRIMEA project
more than 1000 new seeps where discovered (CRIMEA Cruise Report 2004).
Gas seepage is not only found on the shelf, but occurs also on the upper and
lower slope [5]. Indications for methane fluxes from seeps at the upper to middle
slope could be seen in the δ
13
C
CH
4
profile of station 064 (Fig. 5). A decrease
13
C
CH
4
values in the water column at 800 to 500 m water depth shows
that methane escaping from seeps located at the deeper shelf and slope leaves
an imprint on the δ
in δ
13
C
CH
4
depth profile. This is supported by higher methane
concentrations in water depths around 600 m, where the methane profile clearly
deviates from other biogeochemical parameters such as NH
4
and H
2
S, hence
indicating an additional methane source (Konovalov, unpubl. model results).
Sorokin [45] showed that the stable carbon isotope composition of methane
seeping out of the Black Sea bottom is
-58 ‰ VPDB and that the age of
the methane as determined by
14
C dating lies between 3.500 to 5000 years BP.
The δ
∼
13
C
CH
4
values at station 064 at around 500 m of
-58‰ VPDB are
actually very close to the values measured for methane escaping the seeps on
the shelf and slope indicating a methane source from seepage. The age of the
methane further confirms the argument that the methane is not formed by recent
methanogenesis in the uppermost sediments, but is delivered from older Black
Sea sediment deposits.
Comparing reference site (064) and seep site (072) on the lower slope,
methane concentrations below 500 m water depth at both sites were more
or less similar with 10-12.5 µM. First it seemed surprising that the methane
plume at the seep site which was traced by acoustical means (echosounding)
from 2000 m water depth up to 800 m water depth was not reflected in the
methane concentration. Most likely, the huge background concentration of 12
µM methane in the deep water masks the signature of the plume. Accordingly,
a plume concentration of around 500 nM as detected at the shallow seep (038)
would not be resolved at a background of 12 µM.
One method to determine the methane input from seeps into the water is
the distribution of noble gases in the water column. The concentrations of
dissolved atmospheric noble gases in lake and ocean water correspond closely
to the equilibrium concentrations determined by the surface water temperature
and salinity that prevailed during gas exchange with the atmosphere [7, 22].
Noble gases are chemically inert, and therefore any observed deviations from
the initial equilibrium concentrations can be used for modeling the purely
physical processes. The release of gas bubbles into the water column stimulates
a secondary gas exchange between the ascending gas phase and the surrounding
water by gas stripping and dissolution and therefore affects the local noble gas
concentrations [50].
Neon concentrations in the deep water were approximately constant with
depth for each of the two profiles, but the mean Neon concentration determined
∼
