Shallow gas in shelf sediments of the Namibian coastal upwelling ecosystem

Abstract

Outbreaks of toxic H 2S gas are a seasonally recurrent feature in the near-shore environment of Namibian shelf. They potentially have a significant economic and societal relevance because of their effects on biota in one of the largest marine coastal upwelling regions. Until recently they were considered to be of local geographical extent and forced by processes in the water column. Here, we report on ship-borne acoustic surveys and core evidence that suggest a significant contribution by eruptions of biogenic gas accumulations in unconsolidated organic-rich diatomaceous oozes on the shelf. Approximately 8% of the mud area (1360 km 2) hosts free gas just decimeters to meters below the sediment–water interface. The gas is a mixture of methane and H 2S. A portion of the H 2S is produced by the reduction of sulfate with methane in the sulfate–methane transition zone, where it reaches pore water concentrations up to 18 mmol/l. The gas is contained in very porous, low density and liquid sediments and is in some cases capped by a denser layer within the sediment. Possible candidates to trigger eruptions are changes in the atmospheric and oceanographic pressure fields, shoaling of the methane–sulfate transition zone caused by decreasing rates of methane oxidation, and precipitation-induced pressure signals from the inland catchment transmitted to the base of the diatomaceous ooze via fossil river beds.