Cold seeps are reducing environments where oxygen-depleted fluids diffuse from sediments along continental margins. They are cold because they are only slightly different in temperature from ambient seawater, and they are seeps because the fluid flow rate is usually slow.
Seeping is related to pressure, such as tectonically-induced pressure on continental margins, artesian pressure on source rocks, and sediment pressure from erosion and submarine slides. Seeps are formed in active continental margins in subduction zones. When the oceanic plate is subducted under the continental plate, the continental plate scrapes off the sediment off the oceanic plate, forming a sediment prism called the accretionary prism. As the sediment in the prism is compressed, cracks form, providing channels for trapped fluid to flow through. Methane and sulphide is produced through the combustion of hydrocarbons using heat, and microbial activity. On passive margins, seeps are formed from salt tectonics. For example, when the Atlantic closed the Gulf of Mexico off in the Jurassic period, evaporation was enhanced and a salt sheet was formed. Over time, sediment accumulated unevenly over the salt sheet, causing the salt sheet to move and form pillars underneath, effectively trapping fluids. When the sediment becomes overcompressed and cracks, hydrocarbons flow through and a seep is formed.
Cold seeps have a special chemistry. For example, methane is very abundant at the seeps. It is produced by methanogenic bacteria or by thermogenic combustion. The resulting gas is dissolved in the sediments, released as bubbles, oxidized into sulphide by anaerobic bacteria or archaea, or sequestered in methane hydrate stores. Methane hydrates are formed when there is high pressure and low temperatures. Methane gas is trapped in a cage of water molecules, forming an ice-like compound. It reduces sediment permeability and restricts gas flow, but also acts as a substratum for organisms such as the ice worm. Sulphides are formed when methane from the sediments is oxidized and sulphate from seawater is reduced. It remains in the sediments, which is why concentrations of sulphides at seeps is usually high. Bacterial mats are a good indication of seeps because they form where sulphide concentration is the highest. The oxidation of sulphide also produces carbonate, which creates hard substrate that serves as a habitat for certain organisms. The chemistry influences the physiology conditions of the environment, which influences zonation. For example, mussels that grow regions nearest to the seep at Brine Pool have a higher growth rate, density, and a more diverse size structure because there is high methane and oxygen concentrations, as well as little to no hydrogen sulphide.
Three groups dominate the seeps: the mussels, the tube worms, and the clams.
Mussels, such as the common Bathymodiolus childressi, have thiotrophic and methanotrophic endosymbionts that it digests to obtain carbon. The symbionts, in turn, are given a place at the mussels’ gills, where methane and oxygen for chemosynthesis is abundant. As mentioned above, the size structure of mussel populations at Brine Pool is dependent on the chemistry: juveniles are more abundant in the inner region of the pool, whereas older mussels are on the fringes. This pattern might also be caused by differences in habitat substratum, as well as secondary settlement, migration, and post-settlement mortality).
Clams are sulphophilic and like to align themselves linearly with cracks or burrow themselves horizontally in the sediment. Two or more species can be found to co-occur at one seep. Organisms are distributed concentrically around seeps, and clams are usually found further away from seeps than bacteria. Thus, they benefit from a moderate concentration of sulphide. As they utilize 100% of the available space within these areas, intra-specific competition might occur.
Vestimentiferans, or tube worms, have low mortality rates and extreme longevity. They are the oldest marine invertebrates. As they have endosymbionts, they lack a digestive tract, and instead rely on these endosymbionts to oxidize sulphide into carbon. Vestimentiferans of different species and at different locations may have different types of endosymbionts. As these worms form a high biomass, when they die, they contribute a significant amount of nutrition towards the vent community. The entangled tubes of vestimentiferans also provide habitats for other species. Thus, vestimentiferans are ecosystem engineers. Other secondary habitats include byssal threads and bivalve shells.
