Environmental Engineering Reference
In-Depth Information
sulphide concentrations in the methane-containing zone are 15 mM [14]. Areal
integration of the amount of hydrogen sulphide in the water column yielded an
estimated 0.3 to 0.8 x 10
9
moles of hydrogen sulphide. Only between 0.2 and
0.4 % of this reservoir is required to replenish the bottom water with hydrogen
sulphide to concentrations exceeding 1 µM.
We assessed the amount of hydrogen sulphide that would have been released
during a violent sediment eruption, using as an example a crater with a di-
ameter of 250 m and a depth of 10 m, which was mapped during METEOR
expedition M57-3 [42]. The total volume of sediment missing from this crater
is about 490,000 m
3
. If this volume of sediment had an average porewater
hydrogen sulphide concentration of 15 mM and a porosity of 0.8, then the
total amount of hydrogen sulphide in the crater void would have been 5.9 x
10
6
moles. Assuming that the overlying seawater contained at least 6 µmoles
of hydrogen sulphide per litre seawater, about 10
12
litres of seawater could
contain this concentration of hydrogen sulphide. For a water depth of 60 me-
ters, this corresponds to an area covering 16 square kilometres. The calculation
indicates that individual sediment eruptions may have an important local im-
pact on hydrogen sulphide in the water column, but are of limited regional
significance.
Sediments, which contain no free gas or free gas at more than 3 m sediment
depth at the time of the acoustic echosounding surveys, have not likely emitted
gas to the overlying water column in the past years. The size and outline of
the gas-charged areas identified on echosounding lines conducted on cruises in
August 2000, March 2003, and March 2004 coincide closely. Gas-free areas
and areas with free gas at more than 3 m sediment depth all have good sediment
stratification [5], a lack of surface structures, and from the core samples exam-
ined, a deep penetration of dissolved sulphate (Fig. 13a). The profile shape of
pore water sulphate in Fig. 13a suggests that the dominant mode of sulphate
transport has been by molecular diffusion. If these sediments once contained
free gas near the sediment surface, sulphate would have been depleted a few
centimetres below the sediment surface. After the gas release, sulphate would
have diffused downward. It would take approximately 180 years for sulphate
to diffuse over a distance of 3 m by molecular diffusion [18]. In addition,
in sediments with diffusive interfaces between methane and sulphate more
than 95 percent of the methane is oxidized anaerobically with sulphate, which
counteracts the build-up of gas overpressure. For these reasons, it is unlikely
that undisturbed areas surveyed have in the past contributed to methane gas
emission.
