Environmental settings

High primary production off Namibia is supported by the inflow of nutrient rich, oxygen depleted South Atlantic central water (SACW) from the Angola Gyre in austral summer, and Eastern SACW (ESACW) during the winter season (Hagen et al. 2001). The main driving force for the upwelling of these subsurface water masses is the Ekman offshore transport induced by southeast trade winds, which result in coherence in lowered sea surface temperatures (SST) and offshore wind stress (Giraudeau et al. 2000, Hagen et al. 2001). All near surface water masses over the shelf and slope off Walvis Bay are considered to have a net northward flow, inducing onshore bottom Ekman transport for the inner shelf (Giraudeau et al. 2000). This onshore flow may reach close to the coastline during intense upwelling (Hagen et al. 2001). Thus, the diatomaceous mud belt (DMB) is produced by the onshore flow on the inner shelf (van der Plas et al. 2007). On the outer shelf and slope, the deeper, poleward compensatory flow turns the configuration during the upwelling season (Giraudeau et al. 2000). An offshore bottom Ekman transport prevails, gathering the bulk of particles transported downslope (Giraudeau et al. 2000, Bremner 1978).

On the inner shelf, the flux rate of diatom frustules and organic matter accumulates the DMB from 19 to 25 degrees south, with nearshore amounts of 50 to 80% opal, decreasing to 5 to 40% seawards, and organic matter contents of above 7% (Bremner 1980, 1983). Annual accumulation rates do not normally exceed approximately 1 mm (Struck et al. 2002, Bremner 1978). The landward flank is enriched in aeolian and riverine sand, silt, and terrigenous debris, whereas the seaward flank is enriched in particulate organic matter, clay, planktonic foraminifera, and shell gravel (Bremner and Willis 1993). Depending on local bathymetry and dynamic current intensity (Mohrholz et al. 2008), the landward flank is found at 15 to 104 m water depth, and the seaward flank from 45 to 151 m (Bremner and Willis 1993). Re-suspension by bottom currents is most pronounced at Palgrave Point (near 22 degrees 30 minutes south), where a shoal at 50 m water depth hinders accumulation of opal and clay by wave induced turbulence (Bremner 1980). This causes a single interruption of the DMB within a stretch of 740 km. Seismological analyses revealed a sediment thickness of up to 15 m, most pronounced in between 21 to 23 degrees south (Bremner 1980, Emeis et al. 2004). This range coincides with local oxygen concentrations of the bottom waters below detectability, suggesting denitrification and anammox even in the water column.

An innermost range with free hydrogen sulfide in supernatant waters of multicorers was detected from Cape Cross to Walvis Bay, down to 125 m water depth (Emeis et al. 2007). Gas accumulations of methane and H₂S were observed within the DMB in all areas exceeding 12 m sediment thickness, and in 20 to 60% of the locations characterized by a sediment thickness of 6 to 10 m (Emeis et al. 2004). Gas blow outs of methane and hydrogen sulfide were common in the years 2001 and 2002, producing clouds of elementary sulfur visible from space (Weeks et al. 2002, 2004). Such massive gas extrusions from the sediment column result in biochemical stress for the benthic fauna, as well as in the re-suspension of surface sediment layers (Weeks et al. 2004, Emeis et al. 2007). Even buoyancy of gas laden sediment layers was observed as drifting mud islands' (Emeis et al. 2004). Enhanced advection of slightly oxygenated water masses to the sediment surface provides temporal oxygen availability (Chapman and Shannon 1987), but the benthic consumption rates on the inner shelf are sufficient for subsequent complete reduction (van der Plas et al. 2007). Oxygen deficits of up to 150 µmol l^-1 were recorded, due to local consumption (Mohrholz et al. 2008). Within the uppermost sediment column, sulfate-reducing bacteria were found in all cores sampled by Brüchert et al. (2003) on the Namibian shelf. Their sulfate reduction rates culminate at the sediment surface or within the uppermost centimeters of the sediment column, providing a net flux of hydrogen sulfide from the sediment column into the bottom water (Brüchert et al. 2003). This sulfide is detoxified by blooms of chemolithotrophic bacteria that occur within an anoxic bottom water mass reaching several meters above the sea floor (Lavik et al. 2009).

Despite its widespread occurrence in several 1000 km² of the shelf, this newly recovered bacterial process remains unnoticed in the upper water column. The decreased visibility detected by the ROV of the GeoBio-Center^LMU at 20 m above the ocean floor at station 181 (Table 1) was considered resuspended matter onboard (Appendix). Surprisingly, this did not coincide with the inflow of oxygenated water masses, but rather with the complete anoxia in the bottom water (Table 1). Therefore, the video provided here (Appendix) may as well visualize the dense blooms of chemolithotrophic bacteria described by Lavik et al. (2009). Reduced visibility starts at 27 m above the seafloor, and gradually increases to <1 m near the sediment surface.