Environmental Engineering Reference
In-Depth Information
Figure 13.
Comparative depth profile of sulphate and sulphide concentrations (a) from an
area devoid of free gas (left profile) and (b) from an area where the gas-charged zone almost
intercepts with the sea floor. • (sulphate, multi-core),
◦
(sulphate, gravity core),
(sulphide,
multi-core),
(sulphide (gravity core).
7.2 Methane Ebullition
Apart from catastrophic eruptions, advective transport due to methane ebul-
lition in gas-charged areas would also enhance the exchange between sediment
and seawater. Several studies have demonstrated that gas advection enhances
pore water transport [22, 23, 28]. Co-transport of porewater methane and hy-
drogen sulphide is evident from the concentration profiles in one of the craters
(Fig. 6 and 13b). The porewater profiles are linear just below the sediment
surface and show highly significant correlations between hydrogen sulphide
and methane. These profile shapes suggest mixing and advective transport of
porewater and gas. The pH values of the porewaters range between 7.0 and 7.4.
In this pH range, between 25 and 45 percent of dissolved sulphide is present as
hydrogen sulphide gas, which can be transported together with methane. The
