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65 MEETING GEOLOGICAL SOCIETY OF AMERICA 64 And all this activity has been geologi- 63 cally recent. William Hartmann and Daniel 62 Berman of the Planetary Science Institute 61 Life—Potential, Slow, or in Tucson have estimated from the number 60 of impact craters pocking the surface that 59 Long Dead Cerberus has been active “in the last 10 58 million years, maybe less,” says Hartmann. 57 BOSTON, MASSACHUSETTS—Talk about life—not just rocks—at this That means the magma and its heat are 56 month’s annual meeting of the Geological Society of America was com- probably still down there, and Cerberus 55 mon. Lively topics included the potential for life on Mars, the pace of life could surge again. 54 below the sea floor, and how to decipher the evolution of ancient life. Planetary scientists will soon get a better 53 look at Cerberus. Mars Odyssey could de- 52 The Cerberus Fossae re- moto will report unambiguous signs that tect ice within a meter of the surface when it 51 A Gusher gion could soon become a water flowed out of the southernmost frac- starts to make observations from orbit in 50 on Mars hot spot for planetary sci- ture of Cerberus Fossae and southward February. And Athabasca Valles is one of 49 entists and astrobiologists across the plains. The channel system of four sites in the NASA competition for 48 searching Mars for potentially life-sustaining Athabasca Valles, which stretches south- landings by one of the two Mars Explo- 47 water. At the meeting and in an upcoming westward from the western Cerberus Fos- ration Rovers scheduled for launch in 2005. 46 paper, researchers describe evidence that wa- sae, reminds these researchers of the Chan- Winners won’t be decided until this spring, 45 ter rushed out of the great cracks that make neled Scabland of the northwestern United but all the hot, watery hubbub coming out 44 up 1000-kilometer-long Cerberus Fossae and States, an area scoured by huge floods when of Cerberus Fossae “makes Mars a lot more 43 surged across the landscape within the past an ice dam holding back a glacial lake in attractive from the point of view of astro- 42 10 million years, carrying with it whatever present-day Montana busted near the end of biology,” says Hartmann. 41 may remain of any possible subsurface life, the last ice age. In Athabasca Valles, flat- 40 past or present. The findings make the area a topped mesas 100 meters high, with Bacterial life is often said 39 prime candidate for scientific rovers sched- teardrop-shaped tails hundreds of meters Lazy Deep to be “thriving” under the 38 uled to touch down in 2004. long, point downstream, paralleled by 10- Life most difficult of condi- 37 This is not the first time Cerberus and meter-deep grooves. These features speak of tions: encased in rock 36 water have been linked. In the 1980s, plan- catastrophic water flows reaching 1 million kilometers beneath the surface, buried under 35 etary scientists studied images of the area to 2 million cubic meters per second, the re- millions of years’ worth of ocean sediment, 34 returned by the Viking orbiters. Some searchers say, or more than five times the or floating in cloud droplets. But if Steven 33 thought they saw de- D’Hondt is right, life on the edge is nothing 32 posits laid down by run- like life in the fast lane. He and fellow 31 ning water, whereas oth- oceanographers Scott Rutherford and Arthur 30 ers thought the whole Spivack of the University of Rhode Island 29 area was a dry lakebed. (URI), Narragansett Bay, reported at the 28 Still others saw nothing meeting that, by their best estimate, sub–sea- 27 but fields of lava flows. floor bacteria—which some have claimed 26 Since Mars Global constitute more than half of all bacteria on 25 Surveyor (MGS) went the planet—live at an incredibly slow pace. 24 into orbit in 1997 and be- “Either these things are taking very few 23 gan returning sharper breaths very rarely, or they’re generally inac- 22 pictures and strikingly tive,” said D’Hondt. 21 detailed topography, re- Determining how frenetic deep life is 20 searchers have zeroed in presents an experimental challenge. Rather 19 on the true nature of Cer- than directly measuring the rate at which 18 berus. At the meeting, bacteria break down the organic matter 17 planetary scientists Susan Leaky cracks. The fissures of Cerberus Fossae have alternately buried in sediments, the Rhode Island group 16 Sakimoto of the Goddard spewed water and lava down Athabasca Valles (shaded relief map). gauged how quickly deep sediments are con- 15 Earth Sciences and Tech- suming a key chemical: sulfate. When bacte- 14 nology Center (GEST) at NASA’s Goddard flow of the Amazon River. ria break down organic matter to use the en- 13 Space Flight Center in Greenbelt, Mary- Burr and her colleagues suggest that the ergy stored in its chemical bonds, they shut- 12 land; Shauna Riedel of GEST; and Devon fissures, lavas, and water flows of Cerberus tle electrons through a series of compounds. 11 Burr of the University of Arizona (UA) in Fossae are the surface manifestation of deep- The favored choice for the last in the line is 10 Tucson, reported how the improved imagery seated magma that could have kept the Cer- dissolved oxygen, when it’s available. But 9 and topography allowed them to trace lava berus subsurface at temperatures hospitable oxygen disappears deeper than a few cen- 8 flows across plains and through valleys in to life for tens of millions of years. The mag- timeters below the sea floor, so bacteria 7 the south, all the way back to fissures in ma rises toward the surface, they suggest, make do with the sulfate ion of seawater, 6 Cerberus Fossae. opening vertical cracks before it. On the way chemically reducing sulfate to sulfide. 5 Clearly, the Cerberus region is largely it encounters water, which apparently can To gauge how fast bacteria are consum- 4 lava-covered, not a lakebed. But it also gush out before or after lava eruptions to cre- ing sulfate and therefore how fast they’re 3 shows signs of massive water flows. In an ate the intermingled channels and flows seen “breathing,” D’Hondt and his colleagues 2 upcoming Geophysical Research Letters pa- in images and topography, then it sinks into compiled measurements of how quickly the

1 per, Burr, Alfred McEwen of UA, and Saki- porous lavas and freezes. concentration of sulfate decreases down the CREDIT: S. SAKIMOTO/MOLA/JPL/NASA

1820 30 NOVEMBER 2001 VOL 294 SCIENCE www.sciencemag.org N EWS F OCUS 65 length of sediment cores retrieved around coral reef–dominated ecosystems seen to- and dominant species than by the traditional 64 the world by the Ocean Drilling Program. day. The success bodes well for the new counting of new and extinct taxa, Todd says. 63 They then combined each sulfate concentra- ecological approach to fossil collection. In 1986, Jackson and Anthony Coates of 62 tion gradient with the rate at which sulfate Just what happened in the tropical western the Smithsonian Tropical Research Institute 61 can diffuse through sediment. That gave Atlantic Ocean and Caribbean Sea as the in Balboa, Panama, began the Panama 60 them the rate at which bacteria are consum- world slid from the relative warmth of the Paleontology Project to “gather the sort of da- 59 ing sulfate and therefore the rate at which Pliocene epoch into the ice ages of the Pleis- ta that an ecologist wants,” says Jackson, who 58 they’re producing energy. tocene has sparked debate for almost began his career as an ecologist. “I’m a per- 57 Deep-sediment bacteria, it seems, are not 2 decades. Variously described as a mass ex- son obsessed with sampling,” he adds. So far, 56 all that energetic. Based on counts of bacteri- tinction or a faunal turnover involving both that has meant going to 463 sites in Panama 55 al cells in deep-sea cores, each cell on aver- appearances and disappearances of species, and Costa Rica (as well as offshore dredge 54 age is metabolizing at a rate one-100,000th the Plio-Pleistocene faunal event seemed to sites) and collecting 202,897 specimens span- 53 that of the least active bacteria in near-shore be related to the gradual closure of the seaway ning the past 12 million years. Unlike most 52 sediments, the URI group reported. “The across what is now other collections—made largely by oil geolo- 51 cells are either adapted to live at rates 105 the Isthmus of gists, says Jackson, who were not interested 50 times lower than anything observed,” says Panama. About the in ecology—these included “everything you 49 Spivack, “or some of what’s being measured can get from” the face of 48 as viable cells is the remnant of dead cells.” a room-size outcrop and 47 Most researchers are willing to believe in several bags of dirt. 46 the cell counts are OK and deep-sediment Classifying fossils by 45 bacteria lead incredibly dull lives. “Very low how they fed and what 44 rates seem fine to me,” says geochemist and they fed on “gives us in- 43 astrobiologist David Des Marais of NASA’s sight to the changing re- 42 Ames Research Center at Moffett Field, gional ecology through 41 California. “These folks are perking along time,” says Todd. 40 at a very low rate.” Oceanographer John With their new eco- 39 Parkes of the University of Bristol, U.K., logical insights, Todd and 38 who made the bacterial cell counts, sees his colleagues think they 37 ways for bacteria to get along under the have a much clearer pic- 36 stringent constraints from the sulfate analy- ture of what happened 35 sis. For one, sulfate is probably not the only in the Plio-Pleistocene 34 electron sink in sediments, he says. The iron event, at least in the 33 of sediment minerals, for example, could Losers. Caribbean predatory snails suffered declines in abundance southwest Caribbean. A 32 steal electrons from sulfide over the millen- and species diversity about 2 million years ago. major extinction did 31 nia, which would recycle sulfide back to strike the mollusks, but it 30 sulfate for reuse by bacteria and ever so time the isthmus rose and blocked the west- came around 2 million years ago. “That’s a 29 slightly speed up the pace of deep life. ward current linking the Atlantic and Pacific, million years too late to be a simple, direct 28 There may also be a way for bacteria to the waters of the Caribbean seem to have response to the closing of the isthmus,” says 27 shut down nearly all their metabolic machin- cooled. Judging by the species and genera of Jackson. Lots of new mollusk genera ap- 26 ery without abandoning the vegetative life snails and clams that disappeared, especially peared in the same interval, so that, unlike in 25 and producing spores. Although cells might those preserved around Florida, some paleon- a mass extinction, the extinctions had no ef- 24 on average divide only once in 100,000 years, tologists concluded that this cooling weeded fect on the total diversity of molluscan life. 23 says Parkes, it may be that a tiny fraction of out mollusks that couldn’t take the cold. Oth- There was a dramatic ecological change, 22 them are far more active—dividing, say, once ers blamed the food chain: Mollusks that ate however. The relative abundance of predatory 21 every 10 years—while the rest drop to a phytoplankton (or ate other animals that did) gastropods—snails that feed on clams and 20 “maintenance” level of metabolic activity, faced lean days, thought these paleontologists, other animals—plummeted and the abun- 19 just enough to hold body and soul together when redirected currents caused a drop in the dance of filter-feeding clams living off the 18 but not enough to grow. Microbiologists productivity of the microscopic plants floating plankton declined, but the abundance of 17 don’t know how low bacteria could go or in the overlying waters. clams and snails with other life habits and di- 16 how they would do it, but those in deep-sea Paleontologists Jonathan Todd of The ets did not change. That pattern makes Jack- 15 sediments seem to know how to go slow and Natural History Museum in London, Jeremy son think that a drop in productivity in over- 14 steady enough, if not to win the race, at least Jackson of the Scripps Institution of lying waters, rather than a cooling, drove the 13 to keep them in the game. Oceanography in La Jolla, California, and southwestern Caribbean toward present-day 12 colleagues didn’t think either side could ecosystems in which productivity is centered 11 A massive collection of prove its case on the evidence they had— in coral reefs and shallow seagrass meadows. 10 Starving some 200,000 fossil often museum drawers stuffed with fossil That looks right to paleontologist War- 9 in the shells may have solved a shells. “Unfortunately, museum collections ren Allmon of the Paleontological Re- 8 Caribbean long-standing mystery: are great storehouses of rare taxa,” says search Institution in Ithaca, New York. But 7 What caused the trans- Todd, “but they really underrepresent com- he notes that just what caused productivity 6 formation of an entire Caribbean sea-floor mon taxa.” Paleontologists tend to pick up to change remains unknown, and concur- 5 community a couple of million years ago? one of every species they find, he notes, rent extinctions as far north as the coast of 4 The answer: starvation. The snails and blurring the distinction between rare and Virginia could conceivably have a different 3 clams of the southwestern Caribbean appear common species. Yet scientists can learn far cause. A few hundred thousand more spec- 2 to have run short of food, and that in turn more about why faunal changes occur by imens may be in order.

1 RIGHT):TO CREDITS (LEFT J. TODD/NHM, LONDON; J. DAWSON/UNIV.led OF IOWA to a makeover of bottom life into the studying the changing proportions of rare –RICHARD A. KERR

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