Mud Volcanoes

Mud volcanoes are frequently encountered geo-structures at active and passive continental margins. In contrast to magmatic volcanoes, mud volcanoes are marine or terrestrial, topographic elevation built from vertically rising fluidized mud or mud breccia. Commonly, these structures have a crater, hummocky rime and caldera. Mud volcanism is triggered by various geological processes which lead to a high pore fluid pressure at great depth, sediment instabilities and a subsequent discharge of mud, fluids and gases such as hydrocarbons (mostly the greenhouse gas methane). Although global estimates of methane emissions from mud volcanoes vary over two orders of magnitude, mud volcanism could be an important source for atmospheric methane. However, a substantial fraction of the hydrocarbons are retained in the mud volcanoes surface sediments. Here, the upwelled hydrocarbons fuel a variety of free-living and symbiotic, chemosynthetic communities that oxidize these with electron acceptors such as oxygen or sulphate from the water column or the atmosphere. The activity of the chemosynthetic communities is regulated by the availability of either electron donors (hydrocarbons) or acceptors which, in return, is determined by mass transport processes. Most important in this context are the magnitudes of upward advection of electron donors and the influx of electron acceptors due to diffusion and bioirrigation.