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horizons

Under the Volcano

Blake Edgar

Two seemingly unrelated scientific discoveries during the past 20 years, one in outer space and the other under the sea, may soon converge in an epiphany of life's extent on Earth and, perhaps, on other worlds. In 1977, the deep-sea submersible Alvin found an array of unknown and unexpected creatures living beside active volcanic vents in the seafloor--thriving in the absence of sunlight and air. Two years later, the Voyager probe spotted volcanoes erupting on Jupiter's moon Io. By studying how species survive solely on chemical energy carried in the hot fluid spewing from seafloor vents and volcanoes, scientists hope to learn how Earth's life may have first evolved nearly four billion years ago and whether similar processes could have repeated the experiment elsewhere.

A good place to start is at the Juan de Fuca Ridge. Located off the coast of Oregon and Washington, and a mile- and-a-half deep, the Ridge forms part of a nearly 43,000- mile-long chain of crests in the planet's crust demarcating boundaries between tectonic plates. Here the Pacific and Juan de Fuca plates are moving apart about as fast as a fingernail grows, causing molten magma to be lifted to the seafloor surface and generating episodes of earthquakes and eruptions--a cycle repeated constantly along other oceanic crust-forming ridges.

"The seasons of the seafloor have little to do with the sun and much to do with the life cycle of volcanoes," says University of Washington geologist John Delaney.

In the short time that they have had access to the Juan de Fuca Ridge, a host of researchers funded by the National Science Foundation's RIDGE program had tantalizing glimpses of an intricate web connecting the seafloor's geology and biology. As Delaney explained in presentations last February at the annual meeting of the American Association for the Advancement of Science, the glimpses leave scientists more eager than ever for a closer, lingering look.

Small quakes seem to be incredibly common along the seafloor ridge, and eruptions that inject lava up to the ocean floor through hardened cracks in the crust called dikes could have been the most frequent volcanic activity throughout the Earth's history. But now there's some compelling, if not yet convincing, clues that diking eruptions also release into the ocean plumes of microbial organisms from the crust below. The temperature difference between deep, near-freezing ocean water and the magma-heated water beneath the seafloor drives the hydrothermal circulation of chemical-rich water through the crust, potentially supporting a vast, subsurface realm of microbes that gain their lifeblood from volcanic gases. The conspicuous seafloor vents could be the tip of the iceberg of a hidden biosphere that, Delaney and others speculate, could rival in mass that of any terrestrial or aquatic habitat.

What's the evidence that primitive life could occupy this third realm? Six years ago, scientists Richard Lutz and Rachel Haymon, during a dive aboard Alvin, stumbled upon the remains of a vent community at the East Pacific Rise, an ocean ridge south of Mexico, that had recently been wiped out by an eruption; they dubbed one area Tube Worm Barbecue for the devastation wrought by fresh lava. Nearby, from cracks in the lava billowed fountains of smoky superhot fluid and a blizzard of bacteria, like spray from a snow- making machine. Before the eruptions had subsided, thick bacterial mats covered a wide area.

Similar flocculated clots of microbes have been observed seven other times, including at sites along the Pacific coast's Gorda and Juan de Fuca ridges. In each case, the plumes began shooting from the seafloor soon after volcanism in the vicinity. Some of the microbes spewing from vents after eruptions belong to an amazing and ancient group of microbes known as Archaea (see "Horizons," Fall 1994), specifically the hyperthermophiles that inhabit the planet's hottest and harshest places.

Maybe the eruptions trigger a microbial bloom similar to seasonal blooms of surface plankton or red tides. "You could call it a `bloom'-bust life cycle," says Delaney. "When the rocks crack, the bugs bloom."

Another possibility, says Delaney, is that the plumes contain Archaea that got flushed by roiling water from their purchase in the crust and into the cold abyss. The Archaea may survive underground by simply clinging to a wall, letting nutrient-laden hydrothermal fluid flow past; once dislodged, perhaps they enter a dormant stage until, if they're lucky, they land at another active vent.

There remain many naging questions about the scale and extent of this previously unappreciated biosphere and the nature of its relationship with volcanoes. No one has the answers yet because the observations have been too few and too fleeting.

Since june 1993, scientists with the Pacific Marine Environmental Laboratory in Newport, Oregon have kept acoustic tabs on deep-sea eruptions using a previously top- secret array of hydrophones deployed by the U.S. Navy to spot submarines. Just four days after researchers began getting access to the data, they detected eruptions near the Juan de Fuca's Axial seamount. A month later, NOAA scientist Bob Embley diverted his oceanographic cruise to the site and found a fresh lava flow. Later that year Embley and Delaney returned with Alvin and discovered a particularly rich area of microbial upwelling.

Though he was fortunate to mount a rapid response in that instance, Delaney also champions a permanent volcano observatory at Juan de Fuca Ridge as the best way to solve the seafloor mysteries. This would provide a consistent, full-time research presence, so when a sudden eruption occurs, instruments would be ready to collect data bearing on how the ocean ridges work. Delaney compares the traditional approach of sporadic ocean expeditions to trying to understand a person's physiology only by monitoring once a year the heart rate and a few other vital signs. With an observatory, he says, "We would literally be wiring a vent like we wire astronauts in space to monitor bodily functions."

Delaney and his colleague Fred Spiess, of Scripps Institution of Oceanography, envision seismometers and other sensors set up to study the tectonics at the Cleft Segment of the ridge, west of Newport, and a second observatory to document the cycles of hydrothermal activity and its associated biology at vents further north along the Endeavor Segment. Teams of robot subs might one day be called upon to gather samples or relay streams of data to scientists waiting in their homes or labs.

Some instruments have already been deployed at Juan de Fuca Ridge, and more will be in place this summer. Funding would come through the federal RIDGE program, and Delaney estimates the initial cost of running a basic observatory at $2 million per year.

In Delaney's view, the observatory would provide a "telescope to inner space" but it may also illuminate future quests for life in outer space. While many planetary scientists pine for a chance to seek life on Mars, Delaney has his sights set on the Jovian moon Europa. Recent images taken from the Galileo probe suggest that Europa's surface is a flowing crust of water ice which may shield a liquid ocean. And, judging from our planet, oceans and the volcanoes beneath them make great breeding grounds for life. Studying them here would give scientists a head start, should Europa prove to possess seafloor hydrothermal geology--not out of the question, since its neighbor, Io, is the solar system's most volcanically active place.

Perhaps Europa does share a profound similarity with this planet. If life exists there or elsewhere in the universe, contends Delaney, our odds for finding and recognizing it will be improved by whatever insight we can glean from its extreme haunts on Earth.

In a study published in Nature last January 16, University of Chicago paleontologist David Jablonski finally put Cope's rule to a rigorous statistical test. It flunked.

Cope had discerned a connection between size and evolutionary success by studying dinosaurs and large, prehistoric mammals of North America. Jablonski instead examined size trends among Late Cretaceous mollusks that existed on the Gulf and Atlantic coasts during a 16-million-year period. He was already familiar with these particular fossils and knew that they had adequate abundance and preservation for the job.

Previous attempts to test Cope's rule among mammals and sea urchins had sampled relatively few taxa. Jablonski adopted a "brute-force approach" that involved measuring thousands of specimens in several museum collections, a task he returned to off-and-on for ten years. "I just set off, calipers in hand, from museum to museum whenever I could," he says. Eventually, he determined the mean adult shell height and length for all 191 genera of clams and snails (more than 1,000 species total) alive at the time. By sampling such a large fauna over a broad span of time, Jablonski aimed to avoid a bias toward bigger, conspicuous forms that helped to perpetuate Cope's rule in the minds of many paleontologists.

He found that surprisingly similar numbers of mollusks--about 27 percent--underwent net size increases and decreases during those 16 million years. In other words, a roughly equal number supported and refuted Cope's rule. A third prominent size pattern that Jablonski confirmed was increased variance, or small organisms getting smaller and big ones getting bigger. A minority of mollusks showed either an opposing pattern of shrinking size variance or no size change at all. Given these options for evolutionary change, Jablonski concluded in his paper that the "lineages collectively fail to follow a single, predictable size trajectory as their species diffuse or shift through size changes."

Large size still carries a lot of weight for the success of individual organisms on the ground (or in the sea). "Body size determines who eats you, who you can eat, and how many kids you can have," Jablonski points out. "What could be more fundamental to ecology?" But what holds for populations in the short-term may matter much less in the long view of species and lineages.

While Jablonski admits that Cope's tule might hold for some lineages in this or other time periods, he suspects that as more lineages come under scrutiny, the pattern of increased range of body sizes--change with no particular direction--will continue to emerge. Such a pattern suggests that organisms respond to different evolutionary pressures in a variety of ways and that the story is too complex for a single rule to embrace.

Having now slogged through thousands of measurements, Jablonski has a robust set of data for further exploring evolutionary scenarios, and he plans to probe for possible reasons behind each of the distinct patterns of size changes.

"Natural history being what it is, there will always be exceptions to any generalization," he says, "but what we want to do is make statistical statements about how the world tends to work."

Blake Edgar is Associate Editor of California Wild.

summer 1997

Vol. 50:3