Environmental Engineering Reference
In-Depth Information
reach a concentration of 1µM hydrogen sulphide, the detection limit for the
methylene blue method [12]. Measured hydrogen sulphide concentrations in
the water column have been as high as 40 µM, but have been observed to rise
and disappear much faster than 147 days (see section 5.3).
5. BACTERIAL SULPHATE REDUCTION,
METHANOGENESIS, FLUXES, AND RECYCLING
OF SULPHIDE AT THE SEABED
5.1 Carbon Oxidation Processes and Methanogenesis
Sediments on the Namibian shelf generally contain less than 20 percent
clastic material, which is transported mainly as dust from the Namibian desert.
Minerals containing reactive iron and manganese oxide are minor components
of the sediment, with the consequence that these two anaerobic electron accep-
tors are quantitatively unimportant for carbon oxidation [5]. Rates of nitrogen
removal including denitrification in the shelf sediments range from 0.2 to 2.7
mmol m
2
day
−
1
, averaging 2 mmol m
−
2
day
−
1
(Zitzmann and Bruchert, unpubl.
data). This leaves bacterial sulphate reduction as the dominant terminal organic
carbon oxidation process. Areal rates of bacterial sulphate reduction on the
shelf (28 m to 200 m water depth) vary between 3.1 and 62.7 mmol m
−
2
day
−
1
[7].
On average, more than 90 % of bacterial sulphate reduction takes place in
the top 10 cm of sediment indicating a very reactive pool of organic material
(Fig. 5). However, the amount of remaining organic material below 10 cm
sediment depth is still very large. Sulphate reduction continues below 10 cm
depth at low rates until all sulphate is consumed and methanogenesis starts.
Rapid increases in methane concentration lead to methane saturation only cen-
timetres below the sediment-water interface (Fig. 6). Steep opposing gradients
of porewater methane and sulphate indicate anaerobic oxidation of methane
coupled to sulphate reduction [14]. In the absence of reactive iron oxides, the
capacity for precipitation of dissolved sulphide as iron sulphides is limited, and
slowly forming organic sulphides remain as the only significant sediment sink
for hydrogen sulphide [5, 8]. These conditions explain, why concentrations of
dissolved sulphide in porewaters from Namibian shelf sediments can be as high
as 22 mM at only 10 cm sediment depth (Fig. 6), and consistently exceed 2
mM at 6 cm depth in all currently analyzed sediments between 19
◦
S and 27
◦
S
on the shelf.
5.2 Sulphide Oxidation
In most marine sediments, a significant amount of hydrogen sulphide can
be oxidized with iron and manganese oxides [1, 32, 37]. Low concentrations
of reactive iron and manganese in the Namibian shelf sediments limit the
