2.2: Exploring the deep ocean floor

Hot springs and strange creatures

The ocean floor is home to many unique communities of plants and animals. Most of these marine ecosystems are near the water surface, such as the Great Barrier Reef, a 2,000-km-long coral formation off the northeastern coast of Australia. Coral reefs, like nearly all complex living communities, depend on solar energy for growth (photosynthesis). The sun's energy, however, penetrates at most only about 300 m below the surface of the water. The relatively shallow penetration of solar energy and the sinking of cold, subpolar water combine to make most of the deep ocean floor a frigid environment with few life forms.

In 1977, scientists discovered hot springs at a depth of 2.5 km, on the Galapagos Rift (spreading ridge) off the coast of Ecuador. This exciting discovery was not really a surprise. Since the early 1970s, scientists had predicted that hot springs (geothermal vents) should be found at the active spreading centers along the mid-oceanic ridges, where magma, at temperatures over 1,000 °C, presumably was being erupted to form new oceanic crust. More exciting, because it was totally unexpected, was the discovery of abundant and unusual sea life -- giant tube worms, huge clams, and mussels -- that thrived around the hot springs.

View of the first high-temperature vent (380 °C) ever seen by scientists during a dive of the deep-sea submersible Alvin on the East Pacific Rise (latitude 21° north) in 1979. Such geothermal vents--called smokers because they resemble chimneys--spew dark, mineral-rich, fluids heated by contact with the newly formed, still-hot oceanic crust. This photograph shows a black smoker, but smokers can also be white, grey, or clear depending on the material being ejected. (Photograph by Dudley Foster from RISE expedition, courtesy of William R. Normark, USGS.)

Since 1977, other hot springs and associated sea life have been found at a number of sites along the mid-oceanic ridges, many on the East Pacific Rise. The waters around these deep-ocean hot springs, which can be as hot as 380 °C, are home to a unique ecosystem. Detailed studies have shown that hydrogen sulfide-oxidizing bacteria, which live symbiotically with the larger organisms, form the base of this ecosystem's food chain. The hydrogen sulfide (H2S--the gas that smells like rotten eggs) needed by these bacteria to live is contained in the volcanic gases that spew out of the hot springs. Most of the sulfur comes from the Earth's interior; a small portion (less than 15 percent) is produced by chemical reaction of the sulfate (SO4) present in the sea water. Thus, the energy source that sustains this deep-ocean ecosystem is not sunlight but rather the energy from chemical reaction (chemosynthesis).

The deep-sea hot-spring environment supports abundant and bizarre sea life, including tube worms, crabs, giant clams. This hot-spring "neighborhood" is at 13° N along the East Pacific Rise. (Photograph by Richard A. Lutz, Rutgers University, New Brunswick, New Jersey.)

The manipulator arm of the research submersible Alvin collecting a giant clam from the deep ocean floor. (Photograph by John M. Edmond, Massachusetts Institute of Technology.)

The size of deep-sea giant clams is evident from the hands of a scientist holding them. (Photograph by William R. Normark, USGS.)

But the story about the source of life-sustaining energy in the deep sea is still unfolding. In the late 1980s, scientists documented the existence of a dim glow at some of the hot geothermal vents, which are the targets of current intensive research. The occurrence of "natural" light on the dark seafloor has great significance, because it implies that photosynthesis may be possible at deep-sea geothermal vents. Thus, the base of the deep-sea ecosystem's food chain may comprise both chemosynthetic and, probably in small proportion, photosynthetic bacteria.

USGS scientist Jan Morton entering the submersible Alvin before its launch to begin a research dive. (Photograph by Randolph A. Koski, USGS.)

The Alvin below water after the launch and en route to the deep seafloor. (Photograph courtesy of the Woods Hole Oceanographic Institution).

A colony of tube worms, some as long as 1.5 m, clustered around an ocean floor hot spring. (Photograph by Daniel Fornari, Woods Hole Oceanographic Institution.)

Close-up of spider crab that was observed to be eating tube worms. (Photograph by William R. Normark, USGS.)
 

Scientists discovered the hot-springs ecosystems with the help of Alvin, the world's first deep-sea submersible. Constructed in the early 1960s for the U.S. Navy, Alvin is a three-person, self-propelling capsule-like submarine nearly eight meters long. In 1975, scientists of Project FAMOUS (French-American Mid-Ocean Undersea Study) used Alvin to dive on a segment of the Mid-Atlantic Ridge in an attempt to make the first direct observation of seafloor spreading. No hot springs were observed on this expedition; it was during the next Alvin expedition, the one in 1977 to the Galapagos Rift, that the hot springs and strange creatures were discovered. Since the advent of Alvin, other manned submersibles have been built and used successfully to explore the deep ocean floor. Alvin has an operational maximum depth of about 4,000 m, more than four times greater than that of the deepest diving military submarine. Shinkai 6500, a Japanese research submarine built in 1989, can work at depths down to 6,400 m. The United States and Japan are developing research submersible systems that will be able to explore the ocean floor's deepest spot: the 10,920-m Challenger Deep at the southern end of the Marianas Trench off the Mariana Islands.

Sketch of the Shinkai 6500, a Japanese vessel that is currently the world's deepest-diving manned research submarine. (Courtesy of Japan Marine Science & Technology Center.)

Contributors and Attributions

W. Jacquelyne Kious and Robert Tilling ("The Dynamic Earth" via the U.S. Geological Survey)