UW News

July 18, 1998

Brief scientific background on sulfide chimneys (black smokers)

News and Information

Sulfide chimneys are pinnacle-shaped structures that form when super-heated seawater, richly charged with metals and volcanic gases, rises into the bitterly cold deep ocean from hot regions below the seafloor. This occurs along the volcanically active mid-ocean ridges, where new oceanic crust is created at about the same rate as human fingernails grow.

Seawater seeps underground through cracks in the ocean floor along the mid-ocean ridges; once underground, the water is heated to tremendous temperatures and at the same time charged with metals and volcanic gases. When this super-heated water rises up into the frigid waters of the deep ocean, the sudden mixing of hot and cold causes the instantaneous precipitation of the metals and gases suspended in the water. Massive amounts of sulfide minerals (metal-bearing sulfur compounds), such as pyrite, chalcopyrite, and sphalerite, along with silica and the calcium sulfate mineral anhydrite, precipitate out and eventually form towering, chimney-like structures. The chimneys grow as the mineral-saturated water continues to vent through a complex array of channel-ways in their interiors.

When the water temperature is above approximately 300 degrees Celsius, the sulfide minerals precipitate as microscopic particles and give the venting water a billowing, black, cloud-like appearance as it rises into the ocean. This inspired the nickname “black smoker” for those sulfide chimneys that spout this seemingly inky water. The largest known sulfide structure, dubbed “Godzilla” in honor of its size, was measured at 140 feet before its collapse in 1996. Since that time, a chimney of nearly 40 feet has risen from its broken stump. It is thought the sulfide structures may be toppled by the small earthquakes that frequently rock the ocean floor at these ridges.

The hydrothermal sulfide structures are an integral part of a volcanic ecosystem that is unlike any other on our planet, an ecosystem that exists in absolute darkness in the otherwise generally lifeless deep ocean. Unlike most other living things on Earth, the organisms that inhabit hydrothermal vents do not depend on photosynthesis, the process by which plants convert sunlight into energy. Instead, animals at the vents depend on an entirely different and only recently recognized process called chemosynthesis, in which energy and nutrition are produced from volcanic gases, such as hydrogen sulfide, dissolved in the water flowing through the ocean crust.

The discovery twenty years ago of living things in such an extreme environment altered the fundamental assumption that sunlight was essential for life to originate. This, in turn, challenged existing concepts that Earth’s first life forms must have lived near its surface. The idea that life may have begun within seafloor volcanic systems is now a viable hypothesis. The materials recovered by the team may yield dramatic new insights into the fundamental processes by which volcanoes support life without sunlight.

Among the most intriguing of the unique creatures living at the hydrothermal vents is the diverse and little-known microbial community that flourishes in the interiors of the sulfide chimneys. Some of these microbes are heat-loving, or “thermophylic,” and are among the most primitive living things on Earth. The sulfide structures the expedition team has recovered will provide one of science’s most complete glimpses of life below the Earth’s surface, and will present a unique opportunity to relate the distribution of microbial activity to thermal and chemical conditions within the chimney.

The study of the sulfide structures is at the intersection of the physical and life sciences, and epitomizes the interdisciplinary nature of modern science. This is reflected in the make-up of expedition research team, which includes more than twenty scientists from four countries – specialists in mineralogy, fluid chemistry, macro-biology, micro-biology, organic chemistry, geophysics, and geology.

Raising the sulfide chimneys will enable the researchers to study numerous aspects of these structures that have been impossible to understand previously. These include:

Studying the mineralogy, internal structure, and distribution of heat in the sulfide chimneys to learn how they form.
Analyzing the distribution of the many different microbes within the structures to reveal the nature of subsurface life and how it is related to mineralogy, temperature, and fluid composition.
Examining the chimney’s mineral surfaces to understand their role in supporting life.
Studying the composition of the waters flowing from the structure to learn the nature of the subsurface “plumbing system.”
Examining the animals that live on the surface of the sulfide chimneys to understand how they depend on the mineral composition and heat distribution in the vent regions.
Analyzing the precious metal content of the chimney’s minerals to gain a better understanding of how ore deposits form.
Large portions of the structures will occupy a central place in the American Museum of Natural History’s new Hall of Planet Earth (opening spring 1999). They will serve as an example of the multidisciplinary way in which science is conducted today.