Cold seeps are spectacular ecosystems on the deep-sea floor with an unexpectedly large biomass and diversity of animals and microbes. The key energy source maintaining these oases of life in the deep-sea desert is methane. Antje Boetius of the Deep Life team and her colleague Frank Wenzhöfer, both from the Max Planck Institute for Marine Microbiogy and the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany, have now compiled a synthesis of data on methane consumption rates at cold seeps published today, 29 August 2013, in Nature Geoscience .
Boetius and Wenzhöfer found that the seep-fueled communities consume two orders of magnitude more oxygen per unit area than non-seep seafloor communities. In particular, microbe-hosting animals living in and around the seeps are aiding the consumption of methane at the seafloor by their specific adaptations, so that only a small amount of methane is lost to the overlying hydrosphere (and, potentially, the atmosphere). The proportion of methane flux consumed before emission to the hydrosphere varies with fluid flow rate, ranging from 80% in seeps with slow fluid flow to less than 20% in seeps where fluid flow is high.
Assuming the presence of a few tens of thousands of active cold seep systems on continental slopes worldwide, the authors estimate that up to 0.03 Gt of carbon is lost from continental slope sea beds annually, due to the consumption of methane and its efflux to the overlying hydrosphere. As much more methane is lost than can be produced by microbial conversion of carbon alone, they conclude that a substantial fraction of the methane that fuels seep ecosystems on continental slopes is sourced from deep carbon buried kilometers under the sea floor.
This comprehensive review highlights the need for a better understanding of the origin and transport of deeply sourced hydrocarbons at continental margins, as well as the role of the enigmatic life populating the deep hot biosphere.
Photo credit: Cold seep community fueled by methane at 1000 m water depth, Indian Ocean. Source: ROV QUEST; MARUM, University Bremen.