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Science - Education | Environment - Nature

Researchers observe toxic gas eruption off Namibia

German research vessel FS Humboldt studying toxic gas clouds in Namibian waters

© Institut für Ostseeforschung Warnemünde
afrol News, 30 January
- The German research vessel FS Humboldt is in Namibian waters - conducting studies of the eruptions of "enormous clouds of very toxic hydrogen sulphide," which occurs on a regular basis here - has been able to observe such an eruption. The researchers are hoping to establish whether these gases are stored in the sea-bed or if they created by bacteria in the water.

Barbara Hentzsch of the Baltic Sea Research Institute Warnemünde (IOW) in Germany says that the research vessel FS Humboldt had arrived at its destination in Namibian waters on 6 January and has started on its research project - the so-called NAMIBGAS-Expedition.

The rest of January, the 12-persons team from Warnemünde onboard FS Humboldt, led by oceanographer Hans-Ulrich Lass, has been travelling between different sites on the Namibian continental shelf, doing tests. The team of specialists within oceanography, biology, marine chemistry and geology also includes nationals of South Africa and Namibia.

Tests and samples are to produce new knowledge about the still mysterious regular occurrence of highly toxic "clouds" of hydrogen sulphide in the waters off Namibia, which have disastrous effects on the marine ecosystem when set free. The work is followed closely by the Namibian Ministry of Fisheries and Marine Resources as these regular disasters strongly affect local fisheries.

A broadly accepted hypothesis holds that the source of these toxic gases is found in sediments on the sea-bed. From there, it is believed, they are spilled out into the water - sometimes gradually, sometimes suddenly - for example when the usual thick layers of marine bacteria close to the sea-bed die and no longer can hinder the gases to spill into the water.

The Warnemünde team of oceanographers however are investigating a second theory. From the Baltic Sea, they have gained knowledge of the bacterial creation of hydrogen sulphide in floating water. They are to establish whether these processes also occur in the upwelling and nutrient rich water masses of Namibia.

Off the Namibian and Angolan coasts, the powerful Benguela current System is creating unique ecological settings. Here, the warm Angola current water, poor in nutrients, meets the cold nutrient rich Benguela water masses and very productive conditions are created. This in turn could favour bacterial production, which further could explain the creation of hydrogen sulphide "eruptions".

To be able to conduct direct observations of an ongoing "gas eruption", the researchers team took the vessel to areas of high eruption frequency during a time of year - the southern summer - when eruptions are most likely. "The researchers, in addition to good logistics, had the fortune of luck," Ms Hentzsch reports from Warnemünde.

Using continuously updated satellite photos from South Africa, sent to the research vessel via internet from Warnemünde, the team was able to isolate a coastal area "suspicious of gas eruptions." They took the vessel there.

- In fact, in the deep seas only few nautical miles off this very coastal area, they found a cloud of dissolved hydrogen sulphide, ten of meters thick, 20 nautical miles broad and around 200 kilometres long, tells Ms Hentzsch. "The researchers were witnessing a hydrogen sulphate eruption!" she adds.

An analysis of the chemical composition of these waters showed that the normally high concentration of nitrate had been totally spent. According to the Warnemünde team, "this is a strong indication that the cloud cannot surface alone from the sediments, but that hydrogen sulphide also has been created in the water layers."

In these highly nutrient waters, a lack of oxygen is often observed in deep sea layers due to the bacteria's use of oxygen when consuming dying organic substances, Ms Hentzsch explains. When the storage of oxygen dissolved in water is all spent, bacteria turn to another source. The use the oxygen bound in nitrate, turning this into nitrite.

When even the nitrate source is all spent, bacteria turn to sulphides dissolved in water and reduce this into oxygen and hydrogen sulphide. According to the researchers, bacteria only will turn to sulphides as an oxygen source when all the nitrate has been spent, because they need to use far much more energy to perform the chemical reduction.

The same processes occur in sediments on the sea-bed, explains the German team. "Had the gases come from the sediments, it is most probable that one would have found nitrate in the overlaying waters," they conclude. As this was not the case, they hold that hydrogen sulphide also is created in Namibian water layers, exactly as they earlier had observed in the Baltic Sea.

Meanwhile, the Warnemünde institute informs, the research vessel is changing its crew. FS Humboldt is now being equipped with fishery biologists from the Centre for Tropical Marine Ecology (Bremen, Germany), who are to study the growth conditions fir fish larvae in Namibian waters until 10 February.

On 13 February, the NAMIBGAS-Project however is to resume its work on the research vessel. Then, geologists from the Warnemünde institute are to take a closer look on the sediments on Namibia's continental shelf to determine their part of the responsibility for the toxic gas eruptions here.

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