Hot Springs on the Ocean Floor

In many areas of the ocean floor, wherever magma nears the seafloor, or where lava erupts directly at the seafloor surface, hot springs on the seafloor called hydrothermal vents commonly are found. Vent fields are generally associated with submarine (undersea) volcanoes where lava is erupting or preparing to erupt.

Hydrothermal vents also are found in areas of the ocean floor that are spreading, such as at mid-ocean ridges, where tectonic plates are being pulled apart. This movement allows the molten magma to rise from deep inside the Earth, superheating the cold ocean water around it.

The average temperature of deep-ocean water is only 2°C (36°F). The water coming directly from a hydrothermal vent can reach up to 350°C (662°F) and is rich in dissolved chemicals. The hot spring water forms a plume above the vent, somewhat like smoke rising from a chimney into the air. Temperature-sensing instruments, towed behind research vessels, can detect these hot-water plumes and aid oceanographers in locating hydrothermal vents on the ocean floor.

Vent Circulation and Chemistry

Seafloor hydrothermal systems influence local ocean chemistry because hydrothermal circulation removes some chemical compounds from sea water, such as magnesium and sulfate, and adds many others. Further, the circulation pattern of sea water within the hydrothermal vent system creates the unique landforms and biological communities found in vent fields.

Sea water enters into the seafloor by seeping down through fracture systems. As water percolates through the cracks, it heats as it nears the magma chamber. Subsurface water can heat to 60°C (140°F), warm enough to begin dissolving chemicals and minerals from the surrounding rock. The water becomes rich in chemical compounds of sulfur, iron, and magnesium.

When this vent fluid comes close to the magma chamber, it enters a high temperature reaction zone where temperatures rise to 400°C (752°F). This superheated, high-pressure, mineral-laden vent fluid rises forcefully, searching for fracture systems or other openings to the ocean floor. Hydrothermal vent openings can be as small as several centimeters or as large as a meter or more in diameter. At these openings, the vent fluid becomes focused and expels forcefully into the surrounding cold sea water. At this point, the temperature of the vent fluid varies around 350°C (662°F).

The reaction between the two waters of differing temperature causes some of the dissolved minerals to precipitate out, forming tiny grains in the vent fluid. The particles give the fluid the appearance of a dark cloud. Because a large percentage of the chemical compounds are sulfides , such as hydrogen sulfide, the vent fluid is highly toxic, with a pH near 4.0, or about that of vinegar.

As the vent fluid rises, it slows and begins to mix with the surrounding ocean water, producing a plume of warm water that starts to drift with the currents. The plume tends to hang together as it cools, aiding in its detection. Many of the chemical and mineral compounds begin to drop from the plume, forming metalliferous sediments and iron and manganese crusts on the nearby ocean floor. Because deposits from hydrothermal vent fluid can contain iron, manganese, copper, zinc, and other minerals, vents have relevance to certain types of ore deposits.

Chimneys, Black Smokers, and Mineral Deposits

"Sulfide chimneys" form at some high-temperature hydrothermal vents. They are made of minerals deposited directly from the vent fluid. These chimneys appear as freestanding hollow tubes with one or more openings to the sea. Chimneys consist of sulfur and iron compounds with a variety of glistening metallic crystal deposits lining the interior. Some sulfide deposits contain quantities of copper, zinc, gold, and silver, leading speculation as to their profitability for mining.

Sulfide chimneys exhibiting no discharge are commonly found, indicating past volcanic activity that has ceased, gone dormant, or shifted underground. These can account for ancient sulfide deposits found, even at sites far from present-day volcanic activity.

There are several types of hydrothermal vents and sulfide chimneys found in a vent field. Some vents appear simply as a hole in the basalt rock where a crust of rock has broken away, revealing an open, underground chamber called a lava tube. Others form upright chimneys that expel water, hazy from the high heat and clouded with chemical compounds.

One of the most spectacular of the vents is called a black smoker. It appears as if thick, black smoke is being emitted from the vent. In reality, it is vent fluid filled with tiny mineral grains precipitating when the hot vent fluid comes in contact with the cold sea water.

Chemosynthesis

The chemicals in hydrothermal vent fluid would be toxic to most forms of life familiar to humans; but amazingly, a unique ecosystem has evolved to live near hydrothermal vents. The organisms that are supported by the vents rely on microbes , similar to colonies of bacteria, which grow in the vent fluid and on the surface of the surrounding rocks and chimneys.

At ocean depths, sunlight is nonexistent and photosynthesis is impossible. These organisms are not dependent on sunlight and photosynthesis, but instead rely on chemosynthesis, a process in which certain microbes use chemicals in the vent fluid to produce energy.

Microbes.

In vent communities, chemosynthetic microbes utilize the energy released from hydrogen sulfide oxidation to produce food. Essentially, they survive by breaking down the bonds of the hydrogen sulfide molecules, a compound toxic to most life forms. This primary production forms the base for an entire food chain of animals dependent upon hydrothermal vents. Chemosynthetic microbes grow on the seafloor, within the fracture systems, and even within other animals at the vents.

Three main types of microbes exist at hydrothermal vents. Mat-forming microbes produce a slimy coating that covers all surfaces exposed to the mineral rich vent fluid. The mat consists of many species, from long filamentous microbes to tiny rod-shaped microbes.

Plume microbes grow within the superheated plume of water being expelled by hydrothermal vents. They can survive in a wide range of temperatures, from the deep ocean average of 2°C (36°F) to a high of 110°C (230°F). When expelled from a vent they form a cloud of white flakes giving rise to the name "snow blower vent."

Symbiotic microbes live within the tissues of some animals, producing food for themselves and their host. Symbiotic microbes account for 75 percent of all food producers within the vent community.

Vent Biocommunities

The unique environment created by active hydrothermal vents plays host to a web of life found nowhere else on Earth. The ability of microbes to create a food source from otherwise toxic chemicals provides for animal species that biologists are just beginning to understand. Because each species can tolerate certain levels of heat and toxicity, the communities form concentric rings around hydrothermal vents with each species existing in its preferred habitat. Animals also are categorized as vent and non-vent species. Some are directly dependent on the vents for survival while others can migrate between vents and the open ocean.

Tubeworms.

Tubeworms are one of the most common and distinctive animals found at hydrothermal vents. Tubeworms attach themselves to the seafloor and have no mouth or anus. Instead, they have symbiotic microbes living inside them. Tubeworms live in colonies consisting of hundreds to many thousands of individuals, with many other smaller vent species living among them. The colonies form mounds of long, white stalks topped with red, branching filaments.

Other Species.

Numerous species of clams are found near the vents, siphoning the warm water to digest plume microbes and detritus (decaying matter) dropping from above.^* Gastropods such as limpets and snails move about the rocks, rasping the mat-forming microbes with their radulas. Other species of annelid worms and sea cucumbers also feed from the mat-forming microbes. Species of crabs and arthropods scrape away at the microbes as well as prey on the sedentary and slow-moving species. Some fish prefer the vent fields and can tolerate the high heat and toxins, whereas others roam between the vents in search of food. Rarely, a small gray octopus will swim by. Termed Bentosoctopus, it is very reclusive and not much is known about the species.

Colonization.

When hydrothermal vents begin to develop, animal communities colonize the vents with a set progression. Plume and mat-forming microbes begin to grow almost immediately. Within a few months, fast-colonizing animals, such as tubeworms, scale worms, snails, and limpets begin to colonize hydrothermal vents. Newly colonized vents typically have few species and few individuals, and if tubeworms are present, they are white and small (generally less than 50 centimeters long, or 19.5 inches). Older, established vents have more species and more animals, and the tubeworms are brown and larger (usually more than 50 centimeters long).

Some sessile (nonmobile) and slow-colonizing animals like sponges, sea fans, and crinoids are mostly found on older lava away from active hydrothermal areas. They are sparsely distributed and colonize new lava flows and vents very slowly, often taking many years. Some species of octopus, crab, and fish are mobile and can move freely from one site to another, and can be found near either old or new lava.

Hydrothermal vents do not remain active forever. Inevitably, the underground magma will cool. Vents begin to cool and go dormant. The nonsessile animals have the ability to migrate to other active vent fields. But those species that are attached or move too slowly will perish as their heat and food source slowly dies out.

see also Biodiversity; Life in Extreme Water Environments; Mid-ocean Ridges; Mineral Resources From the Ocean; Ocean Biogeochemistry; Ocean Chemical Processes; Oceanography, Biological; Oceanography, Geological; Plate Tectonics; Sea Water, Physics and Chemistry of; Volcanoes, Submarine.

Ron Crouse

Bibliography

Cone, Joseph. Fire Under the Sea: The Discovery of the Most Extraordinary Environment on Earth—Volcanic Hot Springs on the Ocean Floor. New York: William Morrow and Company, 1991.

Internet Resources

Exploring the Deep Ocean Floor: Hot Springs and Strange Creatures. U.S. Geological Survey. <http://pubs.usgs.gov/publications/text/exploring.html>.

A Seafloor Observatory at an Active Underwater Volcano. NOAA Pacific Marine Environmental Laboratory, New Millennium Observatory (NeMO). <http://www.pmel.noaa.gov/vents/nemo/index.html>.

Vents Program: Researching the Effects of Underwater Hydrothermal Venting Systems. NOAA Pacific Marine Environmental Laboratory. <http:www.pmel.noaa.gov/vents/>.

THE BEGINNINGS OF LIFE

The discovery of hydrothermal vents in the mid-1970s constitutes one of the great advances of science in the twentieth century. The study of hydrothermal vents and their attendant animal communities is a new frontier for oceanographers and one of the last unexplored regions on Earth. With each research dive, new species are discovered and new information comes to light. The mysterious environment of the hydrothermal vents has caused researchers to speculate on how species are able to exist in this toxic habitat.

An environment of superheated water, toxic substances, and enormous pressure is much like Earth of the geologic past. Hydrothermal vent communities may have been in existence long before life appeared on the Earth's surface. Some oceanographers believe this combination of environmental conditions that occurs at hydro-thermal vents may be what spawned the first life on Earth, and not conditions at the ocean's surface as previously thought.

^* See "Life in Extreme Water Environments" for a photograph of vent crabs and bivalves.