C&EN Washington

Earlier this year, telltale rumblings picked up from undersea microphones off the coast of Washington and Oregon sent oceanographers scrambling to grab their equipment, locate a ship, and speed out to observe one of the most dramatic but underexplored occurrences on the planet--the eruption of an undersea volcano.

The researchers were tipped off by a network of undersea listening devices first developed by the Navy to locate enemy submarines. These hydrophones now continuously monitor a section of the Juan de Fuca Ridge about 200 miles off the Washington-Oregon coast, where a portion of the Juan de Fuca Plate is sliding under the continental plate. They listen not for submarines, but for seismic activity that indicates the moving plates have forced magma upward into the subocean crust, setting off a swarm of small earthquakes.

Thanks to the hydrophones, the 1998 eruption of Axial Volcano on the Juan de Fuca Ridge was the third time oceanographers have known for certain that they were observing the first stages of an undersea volcanic event. And as that event confirmed, undersea vulcanism is dramatically different in its early stages than it is later on.

"We're learning--from the Juan de Fuca studies and elsewhere--that these systems evolve quite spectacularly with time," says Karen L. Von Damm, professor of geochemistry and of earth, oceans, and space at the University of New Hampshire, Durham. What comes out in the vent fluids very early after an eruption is, in some cases, so different from what has been measured at older active vents that "it's changed our ideas dramatically" about the impact these systems have on the chemistry of the oceans as a whole, she says. In addition, "we've seen chemistries change, and when you see that, you can get a much better handle on what the active processes might be that are occurring and which are the important ones."

New eruptions on the seafloor may be sending up clues to more than just the life cycle of the volcanic systems themselves. Carried along in the plumes of water that pour out of underwater volcanoes may be chemical evidence that beneath the seafloor, in regions not yet reachable by drilling ships, biological in addition to inorganic processes are taking place. The strange environment where these chemical signals come from is one that may be duplicated several other places in the solar system. And increasingly, the message coming from these regions is that--on Earth, at least--this unlikely environment is home to microbial life.

The oceanographers are studying one of the most striking features of the seafloor: the streaming plumes of smoky-black, mineral-rich water that pour out of hydrothermal vents at temperatures close to boiling, which, be-cause of the pressure of the ocean above, can be nearly 400 C. Rich in hydrogen sulfide and reduced metal ions, the water instantly turns black when it spews out into the cold, oxygen-rich seawater and much of its mineral load immediately precipitates into fine-grained particulates that will eventually be deposited on the flanks of ocean ridges.

Since their discovery in 1979, these "black smokers" have fascinated oceanographers, among them chemists who want to understand the reactions taking place within the roiling, smoky vent fluid and to sample the fluid for clues to reactions taking place below the ocean floor.

Black smokers can be found wherever there's volcanic activity beneath the oceans. In particular, the vents dot the midocean ridges, where crustal plates are moving apart, allowing magma to flow upward to form new crust. About 60% of the planet's crust is formed along these ridges, making the processes that take place there fundamental to understanding Earth's geochemistry.

As newly formed crust cools and becomes brittle, it cracks. Seawater flows into the cracks where it reacts with the hot rock, picking up sulfide, metal ions, and heat, which causes the rock to cool further and crack more and the water to penetrate deeper. Eventually the water gets so hot that its buoyancy sends it streaming back to the surface to erupt as a black smoker. The cycle continues, oceanographers believe, long after the magma stops intruding into the crust--until the water extracts all the available heat from the crustal rock.

"In the past 15 years, we've looked at the basic chemistry of these systems and figured out how they got to be that way through field studies, laboratory studies of water-rock reactions, and theoretical studies of the thermodynamic properties of fluids and minerals," says David A. Butterfield, a chemical oceanographer with the National Oceanic & Atmospheric Administration's Pacific Marine Environmental Laboratory (PMEL) in Seattle. "We've gotten pretty far along, so that we can say how these fluids got to be the way they are. They are something like what you would get at equilibrium from a known set of minerals. Now we've started looking at what happens when there is volcanic activity. There have now been several eruptions where we have tried to get out quickly to see what's happening, and there are many changes that occur in the chemistry of the fluids coming out and in the amount of heat coming out of these systems. These are transient things that you can only find out about by rushing out there after an eruption takes place."

One early event is the release of a huge "belch" of hot water. The first of these megaplumes was discovered in 1986 off the Juan de Fuca Ridge when a research vessel was lucky enough to arrive at a hydrothermal system within what researchers now believe must have been the first week after an eruption. Since then, similar plumes have been confirmed as early events at other eruptions.

The hot water forms a squashed spheroid that can be more than 12 miles in diameter, half a mile thick, and contain the equivalent of a year's output of water and heat from a typical hydrothermal vent, explains John E. Lupton, a PMEL oceanographer based in Newport, Ore. Lupton studies the movement of these initial megaplumes as well as other hydrothermal vent fluids by monitoring the gases dissolved in them. Megaplumes are enriched in a number of elements that can serve as chemical tracers, he says, including helium, manganese, dissolved and particulate forms of iron, and methane--although the methane is removed fairly quickly, probably by biologically mediated processes.

Helium's rarest isotope, ³He, is enriched in megaplumes, "so it's a fantastic tracer for looking at where these plumes go, although it's difficult to measure," Lupton says. He has used ³He to track the plumes as they rise and become diluted until their density matches that of the surrounding seawater. Then they begin to spread laterally. "We see these plumes wafting away for thousands of miles from the ridge axis," he says. From the Juan de Fuca Ridge they move southwest toward Hawaii, thus providing information about broad-scale subsurface ocean currents.

How megaplumes form is still unknown. They do not stream up in a jet from vents in the ocean floor but are great blobs of free-floating hot water. "One theory is that lava flow on the seafloor produces them as it cools very quickly," Lupton says. "Or they could be produced when magma coming in breaks up the crust and releases in a hurry a lot of hot water that was already there."

The fluid that comes out of hydrothermal vents is also very different immediately after an intrusion of magma. It's hotter, rich in gases, and so low in salinity that it's nearly fresh. It's more like distilled water than seawater, and many researchers think that's basically what it is.

"We think there is subsurface boiling that occurs before some of these materials reach the surface, so that they actually separate into a brine-enriched fluid and a vapor-enriched fluid. The two phases may come out at separate times or at different places," Lupton explains.

Recognizing the temporal variation in vent fluids has led to some reassessments of the role these hydrothermal systems play in the overall chemistry of the oceans. "We used to think that vent fluids were a net sulfur sink in the oceans," Von Damm says, because seawater flowing into observed hydrothermal systems carries with it more sulfate than can be accounted for in the hydrogen sulfide coming out.

Von Damm has been regularly sampling vent fluids in the East Pacific Ridge off the coast of Mexico since the vents there first became active in 1991. There, she finds, "Early in the eruption cycle you have a lot of hydrogen sulfide going out, much more than you have sulfate going in. If you only looked at the early part of the life cycle of a hydrothermal vent, you might actually think that vents were a net sulfur source to the ocean. One of the reasons we've been trying to study a variety of eruptions is to see if what we've seen at this site is typical."

A problem in understanding the impact of hydrothermal systems on ocean chemistry that has puzzled geochemists since the late 1970s, Von Damm says, is that the fluids coming out of black smokers seem to show that these systems put much more potassium into the oceans than could possibly be generated by leaching it from newly formed basalt. But the low-salinity fluids that now have been seen early in the vent cycle carry very little potassium. "If you take into account the early eruptive period, the calculated values may well drop to something much more reasonable," she suggests.

Hydrothermal vent fluids can also affect overall ocean chemistry after they enter the ocean, says Richard A. Feely, a chemical oceanographer with PMEL in Seattle. Feely studies the chemistry of particulates in the plumes several hundred meters above the vent sites. Here iron and other species react to form iron oxides, manganese oxides, and sulfide minerals that eventually end up as metal-rich sediment on the flanks of the ridges.

"When they get up in the water column, these species have the ability to quickly scavenge a number of chemical species from the seawater, thereby impacting their global geochemical budgets," Feely explains. The effect is particularly striking for phosphorus, a key nutrient that limits the biological productivity of some ocean regions when its concentration is too low to sustain large colonies of phytoplankton. "We have found that this process can account for anywhere from 20 to 40% of the phosphorus cycle in the ocean," Feely says.

In the water column, phosphorus and chemically similar species such as vanadium, arsenic, and chromium form anions with oxygen that precipitate with iron oxide carried up in the buoyant plumes, Feely finds. Most of the rare-earth elements are also scavenged from the water column by a slower mechanism in which they form positive cations that also associate with the iron oxide particles to precipitate farther from the vent.

For many researchers, the most intriguing early event in the life of a hydrothermal system is also one of the most dramatic. At three eruption sites oceanographers have seen swirling white clouds of flocculent material rise out of seafloor vents to heights of up to 200 meters. Microscopic and chemical analysis shows that the flocculent is debris from bacterial colonies living below the seafloor.

"During the course of an eruption, the subseafloor plumbing changes so dramatically that bacteria get blown out of their natural habitat below the seafloor or in rock crevices," Feely explains. "They form these 'snowblower' vents that are basically blowing out bacteria. They look like you are in a snowstorm."

Chemosynthetic bacteria that base their metabolism on extracting energy from various chemical reactions, rather than from photosynthesis, have been one of the most fascinating features of ocean hydrothermal vents since the vents were first discovered. But snowblower vents and other accumulating evidence suggest that these bacteria may live not just around the vent itself, but deep within the subocean crust as well.

Although many oceanographers are convinced that bacteria have colonized the regions below the ocean floor to some degree, data remain scarce about how extensive and significant these colonies may be. Hypothetically, at least, microbes might be able to live almost anywhere that water penetrates and temperatures are amenable to microbial life--a range that spans at minimum from the 2 C of seawater at the bottom of the ocean to about 110 C.

"There's a lot of debate going on about how big this biosphere may or may not be," Von Damm says. "I think the microbes are there; in fact, I'm sure they are. But how extensive these communities are, how long they last, how deep they go, and what their actual biomass may be is very much open to debate. Certainly they are fascinating to think about."

Snowblower vents provide "direct evidence that there is a very large biomass of bacteria living below the seafloor," Feely says. "The intriguing thing is that the chemical synthesis that's associated with these bacteria may be occurring on other planetoids, such as Europa, one of the moons of Jupiter."

The microbes also leave evidence of their presence in the vent fluids. For example, Feely finds long filaments of elemental sulfur in the particulate material coming out of vents. Microorganisms that oxidize hydrogen sulfide to elemental sulfur make such filaments, which are structurally quite different from sulfur produced by inorganic oxidation. In regions where there are large numbers of sulfur-oxidizing microbes, he says, "we see a very large enrichment of sulfur in the particulate phases associated with the bacteria and a lower concentration in the fluids." In some cases, bacteria have been detected living on the surface of the sulfur filaments in the water column.

In the zone below the seafloor where the hot fluids are mixing with cold seawater, there's a gradient of temperature and redox conditions that could provide niches for several types of chemosynthetic bacteria. Feely and Butterfield often work together examining vent fluids, looking for confirmation of microbial activity from this zone. "I think there probably is a biological component to this chemistry," Butterfield says, "but recognizing it is the tricky part."

While Feely analyzes particulates in the fluid, Butterfield focuses on dissolved organic molecules. The most abundant of these seem to be simple carboxylic acids. Vent fluids are enriched in formate, acetate, and propionate, he finds. Vents releasing cooler water have higher levels of acetate, Butterfield observes, suggesting that the species may be forming at temperatures between 30 and 150 C, not inconsistent with microbial processes.

"I think bacteria probably need a stable place to live," Butterfield suggests. "They like to colonize surfaces below the seafloor where there is an environment--such as a cavity--where fluids come in and mix, with electron donors and acceptors being held in something like an incubator. That's where you can get a lot of microbial production. It may be related to the geological structure of the seafloor. It will take some work to verify that or to disprove it."

Another chemist with data that would support such a model is Marvin D. Lilley, associate professor of oceanography at the University of Washington, Seattle. "I'm of the camp that believes that there certainly is a significant microbial population there," Lilley says, "but as to the extent of the population, I simply don't know."

Lilley studies the concentrations of volatiles in vent fluids, such as carbon dioxide and methane. Carbon dioxide comes from the magma itself, he says, but methane, like elemental sulfur, can be produced by both inorganic and biological processes. Methane coming from high-temperature vents--those that release fluid at more than 300 C--probably forms inorganically through water-rock interactions, Lilley suggests. But in some areas, he's measured more methane vented in lower temperature fluids than from the high-temperature vents. This fluid comes from regions where seawater and hot fluids have intermingled before reaching the surface. That extra methane, he thinks, is being produced by methane-generating bacteria (methanogens) that thrive at these temperatures.

"We have isolated hyperthermophilic methanogens from low-temperature vent fluids, which is a pretty strong indication that they are growing below the seafloor at higher temperatures," Lilley points out. He also finds lower than expected carbon dioxide levels in these lower temperature vent fluids. Carbon dioxide is the carbon source for microbial methane generation, he points out. "But we see losses of carbon dioxide that are in excess of what would be required to generate the obvious methane. I think there are other microbial processes consuming carbon dioxide as well."

Laboratory experiments are under way to try to separate out the inorganic and biological sources of the organic molecules found in hydrothermal vent fluids. Geochemist Jeffrey S. Seewald, an associate scientist at Woods Hole Oceanographic Institution, Woods Hole, Mass., for example, is trying to understand what chemical reactions can take place when the mix of organic species and inorganic minerals found beneath the seafloor in natural hydrothermal systems is subjected to the kinds of temperatures and pressures found in those systems. Seewald loads a gold reaction cell with seawater, basalt, and the carbon sources he's interested in studying, heats and pressurizes the system to simulate natural conditions, and then samples fluids from the cell over time.

The experiments generate various dissolved organic species, including formate, Seewald says. The redox state of the system controls the stability of the various organic products. "Thermodynamically, everything should go to carbon dioxide and methane," he notes, "but there are kinetic barriers that allow some more complex organic molecules to be generated. So you may have an abiotic source for a lot of organic compounds."

At Arizona State University, Tempe, chemistry and geology professor John R. Holloway and geology assistant professor Peggy A. O'Day are building a more complex lab model of a hydrothermal vent. In one pressurized chamber, precursor hydrothermal vent fluid is heated to about 400 C and forced through glassy rock collected from a hydrothermal vent. The fluid then flows into a second, much cooler chamber that contains oxygen-rich seawater. Just as on the ocean floor, the resulting change in temperature and redox potential causes pyrite (FeS₂), silica (SiO₂), and chalcopyrite (CuFeS₂) to precipitate from the fluid.

Constructing a high-pressure flow system that generates large amounts of particulates is a tricky engineering proposition, however, and the researchers are still working out problems of precipitate clogging critical tubing in their experimental model. "At this point, we've shown that we can reproduce the inorganic chemistry quite accurately," Holloway says. The next step will be "to see if this kind of system will abiotically synthesize organic molecules."

Another approach is to bring into the laboratory as large a piece as possible of an intact hydrothermal vent. Last summer, a team of researchers from the University of Washington attempted just that (C&EN, July 27, page 11). They sawed off four sulfide chimneys from a portion of the Juan de Fuca Ridge, each weighing some 4,000 lb, and hauled them up to waiting research vessels.

"We wanted to pick up something that was big enough that it wouldn't cool off before it got to the surface," explains John R. Delaney, professor of marine geology at the university and chief scientist on one of the two research vessels that took part in the project. "We basically preserved everything but the pressure."

Although only one of the recovered structures had been colonized by the tube worms, mussels, and other larger animals that live on the outside of hydrothermal vents, all contained evidence of microbial life. Microbiologists immediately took samples for culture and analysis, which are still under way, according to marine geologist Deborah S. Kelley, chief scientist on the second research vessel.

One entire structure and half of each of the other three were sent to the American Museum of Natural History in New York City, cosponsor of the project, where they will be part of a new exhibit opening in the spring. The remaining material was sliced into 10-cm thick slabs for detailed study.

Although results are still preliminary, two of the samples show biological products of some kind within the structures, Kelley says. Among other early surprises was that the main fluid conduit in a structure that was venting 300 C fluid is partially lined with clay, something that's never been seen before. "We're trying to figure out how that forms and what kind of implications it has," Kelley says. "Clay is a very good substrate for microorganisms to grow on as it provides lots of possibilities for different kinds of energy sources." But the temperature is far beyond the range where living organisms have ever been found.

Despite recent discoveries, hydrothermal vents continue to offer surprises. They are unlikely, however, to remain underexplored. As Delaney puts it: "Most people recognize now that the submarine volcanic environment, which we don't understand very well, is a potential site on any planet for life to evolve. That being the case, in-depth studies of these environments will inform both the search for life on other planets and the search for our own roots.

For more on hydrothermal vents, you can visit the following Websites:

NOVA's "Into the Abyss"

Quicktime video of a Pacific hydrothermal vent

Smithsonian exhibit

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Copyright © 1998 American Chemical Society