Gas Promotes Mass: Methane Seeps
By Lisa Levin
Imagine a Jacuzzi with jets set on low, or your soft drink after a good shaking. Now pretend those bubbles are filled with methane rather than air or carbon dioxide. There you have the scene at some of the strangest, and most recently discovered, ecosystems on the seafloor: methane seeps. Here, aggregations of clams, mussels, and tubeworms thrive on the chemicals emanating from the seabed. Prior to the discovery of seeps, scientists thought that chemically driven systems on the deep seafloor were associated only with hot vents. Seeps proved them wrong. First revealed in 1984, cold seeps have since been discovered throughout the world's oceans.
Methane seeps occur from the shallow subtidal zone to the ocean trenches, at depths ranging from 15 meters to more than 7,800 meters. Thus, they are not exclusively a deep-sea phenomenon; however, only those systems below the continental shelf host highly specialized biological communities.
Methane is a clear, highly combustible, odorless gas, familiar to all as a source of energy for our gas stoves and home heating. Natural gas, recovered by drilling, is about 75 percent methane. It smells only because organic sulfur compounds are added so that gas leaks can be detected. Methane is found in the Earth's crust under the ocean. In areas of high primary production, large amounts of organic matter—mainly plankton—are deposited over millions of years in the seabed along the edge of the continents. As the organic material sinks and accumulates on the seafloor, it becomes buried under layers of sediment. Then microbes (or the effect of pressure and heat in certain areas) decompose the organic matter without any oxygen, resulting in the formation of methane.
When the deep-buried methane moves upward towards the seafloor, it is consumed by microbes that interact with other bacteria to produce sulfide. Although sulfide, which smells like rotten eggs, is usually highly toxic, it supports a suite of animals that are specialized in dealing with chemical environments; these are chemosynthetic animals, similar in their body organization to the animals found at hydrothermal vents. The fauna attracted to the methane seepage form true animal oases on a landscape of otherwise relatively featureless, homogeneous sediment in the deep sea. First, bacteria graze on the chemical fluids (methane and sulfide) that seep out of the seafloor. Then, special clams and mussels arrive that house symbiotic bacteria that can harvest the chemicals to produce energy for their hosts. Also present at seeps are tubeworms with sulfide-consuming bacteria; some have very long roots that can reach a meter down under the seafloor to look for sulfide.
Methane formed within the seafloor can be squeezed upward by the subduction of oceanic plates under the continental margins. That's why these new ecosystems were named "methane seeps" when they were first discovered. Since then, scientists have learned that methane does not always "seep" out of the seafloor. It can also be exposed by earthquake-induced landslides. For these reasons methane seeps are common along the entire Pacific rim: off Japan, Alaska, Oregon, California, Costa Rica, Peru, and Chile, all areas where there is a great deal of tectonic activity. Seep communities can also occur in other settings, in association with hydrocarbons such as petroleum oil, tar, or asphalt.
The first animal communities ever seen living at seeps on the deep seafloor were found in 1984 off Florida in the deep Gulf of Mexico, associated with brines containing sulfide. Brine is a very salty water that oozes out of large salt deposits within the Earth's crust. Two hundred million years ago, the Gulf of Mexico became an isolated sea, which dried out entirely, producing an eight-kilometer-thick layer of salt. Later, a passage linked the Gulf to the oceans again. Today, the salt layer is trapped under millions of years of sedimentation, but plate movements called "salt tectonics" cause this buried substance to reach the seafloor, where it sometimes forms distinct "brine lakes."
The global inventory of methane in the deep ocean may be 10 times that of the conventional oil reservoir, gas fields, and coal beds combined. New seeps are discovered every few months and will probably prove to be much more widespread than hydrothermal vents, whose geographical distribution is essentially linked to the volcanic activity at midocean ridges or behind subduction zones.
At high pressure and low temperatures in the deep sea, methane can occur in solid form, known as gas hydrate. The hydrates form by the movement of methane gas upward in the seabed along faults and cracks. Contact with cold water causes crystallization: Methane becomes trapped in a prison of water molecules, forming a solid ice within the seafloor. Massive quantities of methane hydrates occurring along the continental margins could represent a major future energy source, as one liter of methane hydrate contains 168 liters of methane gas. However, the solid form is stable only at high pressure and cold temperatures, posing a challenge for recovery, transport, and implementation as a fuel source. A big problem linked to the use of methane remains; it is a greenhouse gas, warming the atmosphere far more than carbon dioxide. Some theories suggest that massive release of gas hydrates long ago in the Earth's history could have triggered rapid severe warming of the atmosphere. This may have occurred during the Permian-Triassic extinction event 252 million years ago for example, or during the Paleocene-Eocene thermal maximum 55 million years ago, but this idea is still under debate.
Seep systems host a wealth of biodiversity, from microbes to mussels. Strange new microbial interactions and relationships are emerging with every visit to new seeps. The animal communities inhabiting cold seeps are similar to those at hydrothermal vents, but differ in the absence of elevated temperature, and in having greater longevity. Seep emissions occur at temperatures similar to those in surrounding sediments, thus they are sometimes called "cold seeps." Seep fluid emissions, while shifting positions locally, are thought to persist in a particular area for much longer periods of time than many hydrothermal vents (venting is inherently ephemeral, at least where the ridge crest spreads quickly), creating more stable communities with longer-lived organisms. Seep tubeworms, for example, may live for over 200 years!
The study of methane seeps is still in its infancy. We have yet to discover most seeps and perhaps most seep species. We don't yet know how seep animals reproduce, move between seeps, respond to settlement cues, or interact with one another. Better understanding of seep ecosystems may ultimately unlock secrets about climate change, the evolution and maintenance of life in the deep sea, and possibly even life on other planets, where oxygen is scarce and toxic chemicals abound.
Lisa Levin is professor in the Integrative Oceanography Division at the Scripps Institution of Oceanography.