The Formation of Cold Seeps

The major processes that are known to form seeps are: over pressuring of sediments due to high pressure from the amount of sediments and tectonic plates converging, over pressuring from mineral dehydration and gas hydrate dynamics (Gibson et al. 2005).  These processes cause pore waters found in the sediment that are enriched with gases to release methane from the seabed by pressure filtration (Chanton et al. 1991). The concentration of methane found in the pore waters can be 7 times the amount that is found in the water column therefore there is a concentration gradient causing diffusion of the gas out of the sediment ( Juhl and Taghon, 1993). This increase in methane causes microbial communities to settle (Gibson et al.), Juhl and Taghon (1993) found that the abundances of bacterioplankton and heterotrophs were greater in the bottom water surrounding the ‘pockmarks’ where methane release was the highest. Sulphur is also formed either thermochemically in brine or microbially from metabolic activity (Chanton et al. 1991). The bacteria in the sediment anaerobically oxidises the methane and reduces the sulphur creating bicarbonate and hydrogen sulphide as follows:

CH4 + SO2 = HCO3 + HS + H2O

(Gibson et al. 2005)

High levels of methane oxidation leads to increased microbial mass, therefore an increase in food supply for free living and symbiotic bacteria which provides the basis for the cold seep ecosystems (Gibson et al. 2005). The generation of sulphide supports chemoautotrophic, sulfide-oxidising symbiosis found in many of the marine invertebrates living at the cold seep sites. (Van Dover 2000). Also the depletion of sulphates by reduction is critical as the production of methane by microbes through decomposition can only occur with very small amounts of sulphate present (Chanton et al. 1991).

A diagrammatic representation of the various chemical processes occurring at cold seeps. The diagram shows the formation of sulphide from hydrogen sulphide, the formation of carbonate and the leakage of methane from the sediment. Diagram from IFM-GEOMAR.

There are three types of cold seeps: Hydrocarbon seeps, Groundwater seeps which are found at passive margins and Tectonic Subduction Zone seeps which are found at active margins (Sibuet and Olu, 1998). Hydrocarbon seeps release oil as well as gases the organisms found here also use this oil as an energy source, a well studied hydrocarbon seep is the Louisianna Slope in the Gulf of Mexico (Van Dover, 2000). The methane found in the Louisiana Slope is in the form of methane hydrates ( an ice-like solid) due to low temperature and high pressure; an increase in temperature and a decrease in pressure would cause the dissolution of hydrates causing high methane levels (Gibson et al. 2005). Groundwater seeps are the release of terrestrial groundwater that has traveled through the sediment and picked up nutrients, the direct groundwater discharge that reaches the ocean floor accounts for 6% of the total water influx into the seas (Karpen et al. 2006).  An example of a groundwater seep is Eckernforde Bay in the Baltic Sea. Tectonic subduction zone seeps occur both on accretionary prisms which are compressed wedges of sediments over the subduction zone or occur along erosive margins (Tunnicliffe et al. 2003). The Barbados accretionary prism was formed where the Atlantic plate moves underneath the Caribbean plate, the methane in the pore waters is released due to the compaction of sediment (Van Dover, 2000). Tunnicliffe et al. (2003) state that at some sites the subduction trench is pulling apart which forces the fluid out however seepage is usually unstable.

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