Investigation of hydrogen sulphide eruptions along the Namibian coastline using different remote sensing systems

Abstract

Hydrogen sulphide eruptions with their typical turquoise discolorations at the water surface are a unique phenomenon along the Namibian coastline. The remote sensing techniques of ocean colour sensors and microwave scatterometers were used for the investigation of such events. The studies with ocean colour sensors showed that the turquoise discolorations near the Namibian coast were neither linked to dust deposition into the water column by desert storms nor to the reflection of bright material in shallow water areas. In addition, other coloured marine events like algae blooms and river outflows were differentiable from the hydrogen sulphide eruptions by their special optical properties. Quasi-true colour images and spectral identification methods were utilised to monitor and investigate the spatial and temporal distribution of sulphide events. In the past years, they were sometimes and locally limited discovered. Newest remote sensing observations including our own investigations have established that the occurrence of sulphide events is more frequent and longer lasting. The north-westerly direction of propagation and their velocity between 12 cm s −1 and 15 cm s −1 were derived from an event on 14 April 2004. Lastly, the microwave scatterometer remote sensing was applied to investigate the relation of sulphide events to oceanographic conditions. The events from May 2004 were clearly related to strong coastal upwelling.

Coastal Sulfur Plumes off Peru During El Niño, La Niña, and Neutral Phases

“…The oxygen-poor waters can intercept the Peruvian shelf areas, promoting suboxic and even anoxic conditions in the bottom water layer, which would support the formation of H 2 S and the occurrence of surface sulfur plumes. Such conditions were found in the Callao area during the neutral phases in -2009, and 2011-2012(Graco et al, 2017Gutiérrez et al, 2008). The giant subsurface sulfidic plume observed in January 2009 (Schunck et al, 2013) was most likely the source of the satellite-detected sulfur plumes at the beginning of that year (Figure 3a).…”

“…The flux of H 2 S from the sediment to the bottom water layer is controlled by sulfide-oxidizing bacteria (Gutiérrez et al, 2008;Schulz et al, 1999). During the onset of upwelling favorable winds, the H 2 S-enriched waters can be upwelled at the coastal regions (Ohde, 2009;Weeks et al, 2002Weeks et al, , 2004. Such events manifest themselves by milky turquoise discolored surface waters (Ohde & Mohrholz, 2011;Weeks et al, 2004) caused by the formation of colloidal sulfur (S O ) by the oxidation of H 2 S (Galán et al, 2014;Lavik et al, 2009;Schunck et al, 2013).…”

Interannual variability of sulphur plumes off the Namibian coast

“…This result indicated comparable cloud conditions and thus the same impact on the detection of sulphur plumes. It is established that the sulphur patches disappear within 6 days but the mean lifetime is up to now unknown (Ohde et al 2007b, Ohde 2009). The sulphur plumes were detectable if at least 1 day within the mean lifetime a certain percentage of the pixels in the box were cloud-free.…”

“…This area seems to be driven by a different local dynamic because the local high primary productivity and the local decomposition play a strong role in the development of hypoxic-anoxic-sulphidic conditions (Brown et al 1991, Monteiro et al 2005. The Walvis Bay area is very different and highly active in terms of H 2 S concentration in the sediment and the bottom water layers as well as in the observed crater structures at the seafloor (Emeis et al 2004, Brüchert et al 2006, 2009. Sulphide events were observed by local inhabitants during the period from 23 to 27 December 2007 which caused real problems culminating on 31 December with baby hake washed up in the beginning of January 2008.…”

Seasonal and annual variability of coastal sulphur plumes in the northern Benguela upwelling system

“…Another source could be the S 0 enriched subsurface water coming from the detoxification process [ 10 ]. The H 2 S enriched bottom water can be upwelled at the coast during the onset of upwelling favorable wind forces [ 31 ]. Observations of crater structures at the seafloor [ 9 , 22 ] and remote-sensed offshore sulphur patches [ 16 , 23 ] suggest that massive eruptions of H 2 S can also be a potential source.…”