Environmental Engineering Reference
In-Depth Information
actual fluxes of hydrogen sulphide across the sediment-water interface in the
gas-charged sediments are therefore likely higher than those calculated from
molecular diffusion. However, gas bubbling on the Namibian shelf is apparently
intermittent. An areal and temporal quantification of the fluxes during active
gas emission has not yet been studied and remains difficult, because neither the
rate of advection nor the frequency of such gas emissions is known.
7.3 Bottom Water Hydrogen Sulphide Replenishment by
Production of Hydrogen Sulphide in the Topmost
Sediment Layers
The area of gas-charged sediments is significantly smaller than the spatial
occurrence of bottom water sulphide, and is much smaller than the area outlined
by positive fluxes of hydrogen sulphide across the sediment-water interface
(Table 2 and Fig. 7b). Gas-charged sediments occur over a much smaller area
than hydrogen sulphide-containing waters. In particular, bottom water sulphide
has often been observed as far south as 26
◦
30'S [40], where gas saturation
occurs in more than 6 m sediment depth (Fig. 13a). These observations suggest
that processes other than ebullition and gas eruption are of significance for the
production of water column hydrogen sulphide.
In shelf areas with less than 100 m water depth, sulphate reduction rates
in the topmost 20 cm are high enough to generate a hydrogen sulphide flux
that can produce µM concentrations of hydrogen sulphide in the bottom waters
within a few days. In this process hydrogen sulphide is essentially inexhaustible
because continuous replenishment of fresh deposited organic matter and short
diffusion distance for seawater sulphate maintain the microbial production of
hydrogen sulphide at the sediment surface. Using the areal estimates of 27978
km
2
for bottom hydrogen sulphide and the hydrogen sulphide fluxes calculated
for this area (Table 2), it would take a maximum of 7.5 days to reach a sulphide
concentration of 1 µM in a 10 m thick bottom water layer for the whole area.
However, since the flux estimate is taking the whole area into account, in areas
with significantly higher fluxes, locally bottom water hydrogen sulphide would
accumulate much faster.
Key requirements for this process are a bottom boundary layer water that
is stagnant and that contains very little dissolved nitrate and oxygen. Under
these circumstances, bacterial sulphide oxidation with oxygen and nitrate is
inhibited and the diffusive flux of sulphide from the sediment into the bottom
waters is enhanced. Episodic in-shore movement and upward mixing of the
bottom boundary layer would transport hydrogen sulphide into the oxic part of
the water column where turbulent mixing with oxygen produces the colloidal
elemental sulphur.
