Environmental Engineering Reference
In-Depth Information
oxidation contributes 25 % and9%tothetotal oxygen consumption on the
inner and total shelf, respectively. This comparison emphasizes the importance
of sedimentary processes for the regulation of water column oxygen levels on
the inner shelf. Although there are considerable uncertainties associated with
the estimation of water column respiration, there is a reasonable match between
the combined sediment and water column oxygen consumption versus primary
production and import of oxygen, which gives further support to our estimates.
Table 2.
Areal and volumetric estimates of Namibian shelf area between 29
◦
S and 17
◦
S,
inventory of oxygen and water column hydrogen sulphide, and areal occurrence of sulphur
bacteria.
Area (km
2
)
Areal estimates
Shelf area (max. 300 m water depth) 29
◦
Sto17
◦
S
85472
Shelf (max. 100 m water depth) 29
◦
Sto17
◦
S
21690
Gas-charged area
1350
Area covered by craters, domes, and disrupted sea floor
380
Hydrogen sulphide, bottom water,
>
1µM, 10 - 30 m thickness
27978
Bottom water dissolved oxygen,
<
44.6µM (1 ml/l)
78565
Bottom water dissolved oxygen,
<
22.3µM (0.5 ml/l)
46954
Bottom water dissolved oxygen,
<
11µM (0.25 ml/l)
31491
Bottom water dissolved oxygen,
<
1µM (0.022 ml/l)
8944
Thiomargarita
>
5gm
−
2
biomass
50555
Beggiatoa,
>
5gm
−
2
biomass
33143
6. SEDIMENTOLOGICAL CONTROL ON SULPHATE
REDUCTION RATES AND METHANE
ACCUMULATION
Three main regions with different sediment patterns were outlined from
the acoustic records. The continental slope consists of soft silts forming a
smooth surface with westward prograding sediment layers. At the shelf break
the sea bottom consists of hardground and consolidated sediments suggesting a
non-depositional environment. Well-stratified deposits with dipping reflectors
are truncated and partly covered with silty/sandy sediments (Fig. 10). Strong
bottom currents prevent the deposition of soft sediments. These sediments are
apparently reworked deposits. They contain only small amounts of reactive
organic material and support relatively low depth-integrated rates of sulphate
reduction that are less than 1.5 mmol m
−
2
day
−
1
[7]. Porewater concentrations
of dissolved sulphide are in the low µM range, and fluxes of hydrogen sulphide
across the sediment-water interface are below detection. In addition, at depths
greater than 150 m, no large sulphur bacteria have been observed [7].
