Environmental Engineering Reference
In-Depth Information
upwelling system. Advective transport of methane and hydrogen sulphide from
gas-charged sediments has an intermittent and locally restricted impact on water
column sulphide.
Keywords:
Benguela, coastal upwelling, Namibia, bottom water sulphide
1. INTRODUCTION
Many coastal upwelling systems are characterized by year-round low-oxyg-
en conditions in the water column and seasonal water column anoxia [17, 26,
39]. In contrast, the regular occurrence of wide spread sulphidic waters on
open-ocean shelves is rare. One example is the Namibian shelf between 22
◦
S
and 27
◦
S. Continuous upwelling between 25
◦
30'S and 27
◦
S in the L uderitz
cell feeds high primary production, which contributes to extreme water column
oxygen depletion and episodically occurring sulphidic bottom water in the area
between 25
◦
30'S and 20
◦
S [2, 6, 7, 11]. Turquoise discolorations of the near-
shore surface waters are a regular phenomenon during the austral summer and
spring. These discolorations were traditionally interpreted as coccolithophore
blooms, but more recent interpretations suggest that some of these patches
reflect the presence of dispersed colloidal sulphur - an oxidation product of
hydrogen sulphide [40]. During the occurrence of these patches, the water
column is severely oxygen-depleted and sulphidic up to the photic zone, with
severe repercussions for the living resources (fish and crustaceans) in one of
the largest marine ecosystems on earth [15].
A characteristic bathymetric feature of the central Benguela coastal up-
welling region is a 50 to 150 km broad shelf with two edges, one at 150 m
depth, and the second between 300 and 350 m water depth. Due to the high
productivity and relatively shallow water depth, large amounts of phytoplank-
ton debris reach the sea bottom before they are consumed in the water column.
The large flux of organic matter permits high rates of carbon mineralization in
the sediment. Since oxygen is already largely consumed in the water column,
bacterial sulphate reduction becomes the dominant sediment mineralization
process [7]. Ultimately, even sulphate availability is limited in these sediments,
and methanogenesis starts a few centimetres below the sediment surface, which
leads to the accumulation of free methane gas [14].
It is clear that the high productivity in this upwelling system is ultimately
responsible for the development of free water column hydrogen sulphide. How-
ever, the pattern of hydrogen sulphide occurrence suggests very specific inter-
actions between the dynamic oceanographic conditions, the biogeochemical
processes in the sediment and water column, and the physical processes oc-
curring within the sediment. Emeis et al. [14] suggested a close relationship
between anoxia and eruptions of biogenic methane. Weeks et al. [40] also
observed a sudden development of near-shore water column hydrogen sul-
