Deborah S. Kelley is Assistant Professor in the Department of Oceanography at the University of Washington.

Black Smokers: Incubators on the Seafloor Deborah S. Kelley

The Discovery of Black Smokers

In 1977 the scientific community was astonished by the discovery of hot springs on the ocean floor, thousands of meters below sea level. These sulfide chimneys or hot springs supported rich biological communities that thrived in the absenceof sunlight. Equally surprising was the finding that these unusualanimal colonies were sustained by microorganisms that feed onchemicals in hot water. The chemicals are released from magma

In this illustration, the Alvin explores the base of the 15-storytall smoker dubbed Godzilla.

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deep within the oceanic crust, then picked upby superheated seawater which circulates withinthe basaltic rocks that make up the ocean floor.The sulfide chimneys, which emit hot plumesladen with fine, dark particles of sulfide materialbecame known as “black smokers.” Sulfide is aterm for a certain group of minerals that containsulfur. The areas around black smokers areoases for vibrantly colored tube worm colonies,clams, crabs, and other animals in the desert ofthe surrounding deep-ocean environment.

Since that pivotal discovery, numerous underwater hot springs sites have been foundalong most of the major oceanic spreading centers (Figure 1A). Ongoing discoveries associated with these incredible environmentscontinue to astound us. For example, the blacksmoker “Godzilla,” discovered along theEndeavour Segment of the Juan de Fuca Ridgein 1991, was as high as a fifteen-story buildingand towered over the surrounding volcanicterrain before it toppled over in 1996 (Figure2C). It is now also known that these “rocks” are literally alive with microbes that thrive withintheir warm, sulfide-rich, water-saturatedinteriors. Perhaps the most far-reaching idea tocome from these hotsprings is that life itselfmay have originated within these dynamic systems, in which geological, chemical, and biological processes are intimately linked.

What Fuels the Black Smokers?The spectacular development of vigorouslyventing black smokers on the ocean floor isfueled by circulation and heating of seawater atdepths of 2–8 kilometers within the oceaniccrust. The same process also fuels moresubdued, lower-temperature venting systems. Involcanically active areas such as the EastPacific Rise (EPR, Figure 1A), the convection ofheated seawater, or hydrothermal fluid, is drivenby heating from a chamber of molten rock, ormagma, at depths of two kilometers below theseafloor. The temperature of this magma is1,200°C. In contrast, in areas such as theEndeavour Segment (Figure 1A) where there islittle current volcanic activity, circulation isbelieved to be driven by the cracking of hotcrystalline rocks, heated to 500–700°C, deepin the ocean crust. Along their downwardjourney from the ocean floor to depths of 2–8kilometers beneath it, the fluids undergosignificant changes in temperature and chemicalcomposition as they approach the heat source.The fluids obtain their final chemicalcomposition at the deepest point of thiscirculating process (Figure 1B).

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Figure 1A: Global distribution of Black Smokers along themid-ocean ridges. B. Cross-section of hydrothermalcirculation. Cold seawater seeps into the seafloor,circulates near a heat source, becomes hot and buoyant,and flows into focused upwellings at hydrothermal fields.

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Special SeawaterThe composition of the hydrothermal fluid,which is chemically modified, superheated seawater, is determined by three factors: the temperature of the rocks through which thefluids circulate; how much water has previouslypassed through that same crack network; andthe composition of the rock. As cool, denseseawater migrates deep within the crust alongthe abundant large and fine-scale networks ofcracks within these spreading environments,two important changes take place. First, the fluids interact and exchange elements with thesurrounding host rock. Elements such as copper, zinc, iron, lead, sulfur, and silica areleached out of the rocks at temperatures of350–550°C, and are incorporated in thehydrothermal fluid. Other elements such as sodium, magnesium, and calcium are added to the rock, modifying its original compositionand mineralogy. In addition, gases such as hydrogen, methane, and carbon dioxide areadded to the fluid when these compounds aredirectly released from magma chambers orleached from the enclosing host rock (Figure1B). The compounds are critical nutrients formicrobiological development at more shallowlevels. Second, as the downwelling fluidsapproach the magma, their physical propertiesundergo dramatic changes. They becomeextremely buoyant, their viscosity and densitydecrease significantly and their ability to carryheat increases. Similar to the way waterchanges when it’s heated in a pan, the nowbuoyant, metal-rich fluids rise up (throughfractures in the seafloor), drawing in cooler fluids in their wake, and a circulation systemdevelops (Figure 1B). This process is called convection.

How Black Smokers FormThe exact physical and chemical processes bywhich hydrothermal vents begin to develop are

still poorly understood. In young hydrothermalsystems, plumes of hot water that rise throughthe crust beneath the seafloor have to displacethe surrounding cooler seawater saturating theshallow portions of the oceanic crust. In orderfor the high-temperature fluids to reach theseafloor, the channels through which this waterrises must become progressively insulated bydeposition of minerals. Once a venting systemis established, however, the black smokersgrow and evolve as the high temperature fluidsvent onto the seafloor. The metal-rich fluids,heated to 350–400°C, mix turbulently withoxygen-rich, cold (2°C) seawater. This drastictemperature change causes solids to precipitatefrom the fluid. This process generates particlesof sulfide minerals such as pyrite, chalcopyrite,and sphalerite, and sulfates such as anhydriteand barite.

Much of the fine-grained sulfide particles arecarried upward into the buoyant, jet-like plumesthat spew out of the vent at more than a meterper second; the particles are carried 100–200meters up into the overlying ocean water,forming broad, extensive hydrothermal plumes.Some of these particles sink back to the oceanfloor, oxidize and become sediment, and someare scavenged by microbial communities thatlive within the plumes. Sulfide and sulfateminerals that are not carried away in thehydrothermal plumes are deposited at theopening of the vent, causing the vent to growupward over time. The chimney walls fractureperiodically, which allows hot fluids to ejectalong the sides of the chimneys, and causesoutward growth. These fluids may engulf andeventually fossilize tube worm colonies growingon the outer surfaces of the vents. In manyoceanic systems, the high temperature (over350–400°C), acidic, oxygen-poor, and sulfur-and metal-rich hydrothermal fluids boil as theyrise up through the plumbing system beneath

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the black smokers. This boiling generates gas-rich vapors. In other, cooler systems, ventsemit metal-rich water that may contain up totwice as much salt as the surrounding seawater. All of these processes come togetherat the seafloor in the formation of black smoker chimneys.

Different Formations Reflect DifferentConditionsThe spacing, growth rates, and mineralogicalevolution of black smoker chimneys arecomplex and not well understood. Blacksmokers’ shapes vary depending on differentspreading environments, even when only a fewhundred meters separate them (Figure 2). Alongthe mid-ocean ridge system, one of the largestsingle known deposits occurs on the Mid-Atlantic Ridge, a slow-spreading environment.The entire deposit, known as TAG (Figure 1A), isa large sulfide mound measuring 250 meters indiameter, and 50 meters high; it was probablyformed by individual venting structures thatcombined into one deposit over time. Activeventing is maintained by a black smoker

complex located on the top of the mound thathosts multiple, spire-shaped chimneys up tofifteen meters high, with fluid temperatures ofaround 370°C (700°F). White smokers on themargins of the mound discharge fluids at265–300°C. Their venting fluids are whitebecause the dark sulfide minerals precipitatewithin the mound before the fluids exit thechimney. Scientists think that in this area ofactive faulting, large intersecting faults havechanneled flow to the hydrothermal moundepisodically over a 50,000-year period.

In contrast to the large sulfide mounds that may typify deposits on the slow-spreading Mid-Atlantic Ridge, black smokers at the fast-spreading East Pacific Rise commonly occur as

Figure 2A: The Mothra Hydrothermal Fields host at least fivesulfide chimney clusters, such as “Faulty Towers.”

Figure 2B: The “Smoke and Mirrors” black smoker chimney.

Figure 2C: “Godzilla”, a towering 15-story black smoker.Note that although each of these hydrothermal vents islocated along the Juan de Fuca Ridge, their shape andstructure vary greatly.

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small, discrete individual structures rarely morethan fifteen meters tall. Individual venting sitesmay be spaced 200–300 meters apart, andmost venting occurs within shallow, fault-bounded central valleys, called grabens, whichare located at the crest of the summit. In this volcanically active area, the small size of thedeposits is probably due to at least two factors.First, molten basaltic rock frequently erupts andcovers the hydrothermal vents and associatedbiological communities. Second, magma movesup from the magma chamber into the shallowunderlying crust, which disturbs the fluid flowchannels that had been established to feedhydrothermal vents. This causes the vents toshut down until the channels become reestab-lished, or even to relocate on a new site.

Somewhere in between these configurations isthe Endeavour Segment of the Juan de FucaRidge, an extremely active hydrothermal area300 kilometers off the coast of WashingtonState. It is one of the best-studied underwaterenvironments. Active faulting (rather thanvolcanic activity) along this segment hasproduced a one kilometer wide, 100–200meters deep graben in which four known ventfields are spaced 2–3 kilometers apart (Figure1B). The fields, which are generally 400–500meters long, host abundant large sulfide structures on top of which stand abundant blacksmokers. Areas where lower-temperature fluidsvent more weakly are also common. In three ofthe fields, the most common sulfide formsinclude large, irregularly-shaped structures (upto eighteen meters tall) that host vigorouslyventing 350–400°C black smoker chimneys ontheir tops (Figure 2B). Lower-temperatureventing of nutrient-rich hydrothermal fluidsthrough the porous chimney walls support richand diverse colonies of tube, sulfide, and palmworms, galatheid crabs, and a variety of snailsand limpets (Figure 3A). On actively ventingstructures, these animal communities are so

dense that they obscure the underlying hostrock. Many structures are characterized bystair-step arrays of large sulfide ledges thatform an almost tree-like structure (Figure 2C).The most spectacular of these structures,“Godzilla” in the Endeavour Segment’s High RiseField, was forty-five meters high and contained15–16 tiers of ledges up to seven meters inlength which trapped 330°C fluids. Thisstructure collapsed sometime in 1996.

In contrast to the steep mound and ledgeshapes that typify most of the sulfide structuresin the three other hydrothermal fields atEndeavour, the recently-discovered MothraHydrothermal Field hosts at least five sulfideclusters composed of multiple, isolated, andfused pinnacles that reach up to twenty-fourmeters in height (Figure 2A). Many structuresare awash in cooler, slower-movinghydrothermal fluids (30–210°C) that supportdense and diverse biological communitiescomposed predominantly of a variety of worms,snails, and crabs. These animals colonize thesteep outer surfaces of active chimneys.Venting fluids from some of these steep,isolated pinnacles reach 305°C. Horizontalfractures, marked by dense colonies of single-celled organisms that include filamentousbacteria and microbial mats, commonly cut the sulfide structures, and toppledstructures are fairly common. Mothra is thelargest hydrothermal field on the Endeavour,reaching over 500 meters in length.

Life in Extreme EnvironmentsWithin the walls of active smokers, where highly-specific combinations of thermal andchemical conditions exist, the limits to life arebeginning to be defined. That is what makes thestudy of these and similar sulfide chimneysparticularly exciting and important tounderwater researchers (Figure 3A). Until twentyyears ago, no one thought life would be

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possible under these conditions. Manymicroorganisms recovered from these extremeenvironments belong to the domain Archaea,considered the most ancient of hyper-thermophilic organisms, those that thrive in100°C water. In addition, scientists believe thatthe sulfide structures provide a window intoprocesses similar to those operating deepwithin the seafloor, but which are not easilyaccessible with our current technologies. Theyare conducting intensive research into the linkedmineralogical and biological interactions thattake place within black smokers and the lower-temperature fluids around them.

The origin of hydrothermal systems is extremelyancient. For several related reasons, scientiststheorize that life on the early Earth may havearisen in environments like these. One reason isthat relentless collisions with large planetarybodies may have rendered both the shallowearly ocean and the proto-continentsuninhabitable. Furthermore, microbial life exists

within hydrothermal systems that rely onchemosynthesis for energy in the absence ofthe sunlight. The study of black smokers iscrucial to a broad range of microbiological andgeological inquiries. Subsurface environmentslike these, both hot and oxygen-free, arebelieved to mimic most closely the conditions of Earth’s earliest subsurface. From thesestudies, it is clear that volcanically activeplanets that contain liquid water may harbor life. Black smokers can thus be viewed asnatural laboratories for studying microbialmetabolisms thought to be ancient on Earth—and perhaps elsewhere.

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Figure 3A: Highly specific combinations of thermal andchemical conditions support dense communities of diverselife forms on the sulfide chimneys. B. Cross-section showinginternal structure of the sulfide chimney. C. Nutrient-richhydrothermal fluids vent through the porous chimney wallssupporting colonies of tube, sulfide, and palm worms,Galatheid crabs and a variety of snails and limpets.

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