Changes in Seabed Topography
Due to varying flow of gases from the sediment, varying geological features are formed at cold seep sites. The topography of the seabed has been seen to rise in the shape of mud volcanoes and mud diapirs (ridges) caused by either the destruction of deep gas accumulations or the piercing of the seabed surface by shale diapirs followed by the rise of fluidised mud along faults (Charlou et al. 2003). Mud volcanoes are driven by thermal and density differences between seawater and pore waters which have lower chlorine levels, this moves the majority of the water towards the centre or ‘eye’ of the volcano with the rest permeating through the sediment, this allows methane hydrate dissociation and sulphate reduction to take place (Van Dover, 2000). Mud volcanoes release methane high up into the water column, as a result elevated levels of methane have been measured up to 50m above the volcano (Juhl and Taghon, 1993). The topography of the seabed also shows depressions called pockmarks. Pockmarks are formed by a rapid release of liquid and gas through the seabed, in some cases these pockmarks are filled with hyper-saline brine for example in the Gulf of Mexico. This is caused by a salt diapir found deep in the sediment pushing on a gas reservoir overtime it creates a channel to the surface (Van Dover, 2000). The dissolution and the escape of subsurface salt causes the pool to be filled with the anoxic brine, this brine is still methane rich therefore colonies of mussels still surround the edges of the pool (Van Dover, 2000).
Cold seep sediments consist of quartz sand, carbonate sands and fined grained mud or clays, microbes use the carbonate precipitates to form carbonate platforms which provide a source of hard substrata which some species of Bathymodiolus bivalves prefer (Gibson et al 2005). These carbonate platforms come in the shape of carbonate pavements, chimneys, rings and veins (Stakes et al 1999). Bioherms which are reef-like structures can also be seen around cold seeps, in most cases they occur in the form of aggregations of clams, tube worms and other invertebrates such as galatheid crabs(Gibson et al. 2005). However there are exceptions; in Norway a large reef of cold water corals mainly Lophelia sp. has been found where there is light seepage of hydrocarbons including methane, this shows a link between corals and methane, however there has been no proven evidence (Van Dover, 2000).
Hypogenic caves are another geological feature formed by cold seeps, acidic fluids rich in hydrogen sulphide and methane rise from deep in the sediment and erode the surrounding rocks forming a basin which fills with the acidic water. Organic matter form mats on the surface of the water using methane as their energy source. The metabolic activity of this organic matter produces carbon dioxide which causes erosion of the overhanging rocks creating a cave (Forti et al. 2002). Many scientists use these caves to study the biology of cold seeps as they do not always occur at depth, meaning it is much more cost effective and easier than using a submersible (Forti et al. 2002).