[Nmcaver] Article in Science News this week on science of Cave Formations

Thu May 4 11:52:07 EDT 2006

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 Science News Online

 Week of April 29, 2006; Vol. 169, No. 17

 Buried Treasures

 Constructing&#151;and deconstructing&#151;cave formations

 Sid Perkins

 In the summer of 2003, geologist Leslie A. Melim and two of her
 undergrads were exploring a 6-meter-by-12-m chamber deep within New
 Mexico's Carlsbad Caverns. As the team members were on their hands
 and knees conducting a detailed survey of the cave floor, one of the
 students blurted out, "Hey, what's this?"

 [IMAGE]  Although scientists have long understood the chemical
 processes that sculpt many cave formations, they've only recently
 come up with a mathematical model that explains some of the features'
 shapes. Researchers also find that bacteria contribute to some types
 of mineral deposits in caves.
 iStockphoto

 "This," says Melim, was an oddly configured carpet of white minerals
 lining a shallow basin that had long ago, when the region's climate
 was wetter, held water. The sharp, wispy peaks of the carpet were
 less than 1 centimeter tall and 1.5 cm across at their bases,
 resembling crests atop a lemon meringue pie.

 "It was unlike any cave formation I'd seen or heard of, and I was
 pretty sure no one else had seen it before either," says Melim.
 Considering the location of the find and its appearance, the team
 from Western Illinois University in Macomb dubbed the mineral coating
 "pool meringue." The group has since found similar formations in two
 other dried pools in Carlsbad's lower reaches.

 As at Carlsbad Caverns, many treasure vaults lie deep beneath Earth's
 surface in cathedral-like chambers, accessible only through narrow
 passages, often with fanciful names such as Contortionist's Delight
 or Fat Man's Squeeze. In these pitch-black and usually humid
 confines, mineral formations range from iciclelike deposits that can
 weigh tons to delicate crystals that shatter at the slightest touch.
 Although scientists have long understood the chemical processes that
 sculpt many cave formations, they've only recently come up with a
 mathematical model that explains some of their shapes.

 An even fuller explanation of the intricate decor will also have a
 biological component, according to researchers who are characterizing
 the unusual bacteria that live in caves. In Mexico's Cueva de Villa
 Luz, as in many others, moist surfaces bear gleaming coats of
 mineral-depositing bacteria, filmy ribbons of their microbial kin
 undulate in cave-floor pools and streams, and damp globs of
 microorganisms hang from the walls. Some types of mineral deposits
 that form in caves appear to be produced by communities of bacteria
 or by the environmental conditions that they create. On the opposite
 side of the coin, some of the same organisms are threatening
 prehistoric cave art.

 Dangling deposits

 Cave formations come in numerous shapes and sizes, and many of their
 names&#151;popcorn, cave bacon, and soda straw, for example&#151;seem
 to have sprung from the minds of imaginative&#151;and
 hungry&#151;cavers. The two formations best known to noncavers,
 however, bear the names stalactite and stalagmite. Pity the grade
 school test taker who forgets that the c in stalactite stands for
 "ceiling" and the g in stalagmite stands for "ground."

 [IMAGE]  ROCKY HANG-UP. Scientists have only recently developed a
 mathematical model that explains a stalactite's carrotlike profile.
 iStockphoto

 Stalactites, stalagmites, and other cave deposits form when water
 picks up minerals as it percolates through sediments and then seeps
 into a cave. If the water has traveled through limestone on its
 journey, it typically is saturated with calcium carbonate and carbon
 dioxide, says Raymond E. Goldstein, a physicist at the University of
 Arizona in Tucson. This seepage can also contain trace amounts of a
 wide variety of elements.

 In the first fraction of a second after water seeps from a cavern
 ceiling, some of the gas dissolved in the seepage enters the humid
 air because the cave's air isn't saturated with carbon dioxide. This
 process, a gentle fizzing like that in soda pop, causes the water
 droplet to become less acidic, Goldstein says. As a result, some of
 the calcium carbonate in the droplet crystallizes on the cave
 ceiling, and&#151;voil&#224;!&#151;a stalactite is born.

 As seepage continues, new droplets hang from this bump and leave
 their crystalline residue. When droplets fall to the cavern floor,
 the dissolved minerals that they carry often accumulate there as
 stalagmites. When a stalactite and its underlying stalagmite have
 nearly grown together, the deposits' shapes are almost perfect mirror
 images. Further growth melds the formations into a column with a
 narrow waist, which continues to thicken as the minerals precipitate
 from the water flowing over the surface.

 The growth of a stalactite is incredibly slow&#151;typically, it
 takes a century to add a centimeter of length. At first, stalactites
 grow irregularly, but once they've reached a length of 5 cm or so
 they take on a characteristic shape. From the side, the formation
 doesn't look like a perfect cone but bulges like a plump carrot, says
 Goldstein.

 Although scientists had long recognized this distinct profile, no one
 had explained how stalactites end up with their bulging shape, he
 notes. Now, Goldstein and his colleagues have come up with a
 mathematical model that reproduces a stalactite's silhouette.

 Earlier experiments had shown that the rate of mineral precipitation
 on each small patch of stalactite correlates with the thickness of
 the layer of water that's dribbling down the formation. At the top of
 a stalactite, where the layer of flowing water may be only a few
 micrometers (&#181;m) thick, calcium carbonate is deposited slowly,
 says Goldstein. Near the bottom of the formation, where the flowing
 water is spread over a smaller area, the film of water is thicker and
 crystals form more quickly.

 Because the top of the stalactite has a larger diameter and more
 surface area than the bottom does, more rock is deposited there
 overall. However, the more-rapid mineral deposition at the bottom
 portions widens the formation to create the characteristic carrot
 profile. Goldstein and his colleagues reported their findings in the
 August 2005 Physics of Fluids.

 [IMAGE]  GOLDEN RING. Cave formations called soda straws develop as
 crystals grow from a ring-shaped deposit on the ceiling of a cave. A
 drop of water hangs at this soda straw's tip.
 Anderson/Darklightimagery.net

 When the seepage rate into a cave is extremely slow, mineral-laden
 droplets can hang on the ceiling for a long time before they fall. In
 such a case, the calcium carbonate crystallizes in a ring shape. The
 crystals that precipitate from subsequent droplets extend the ring
 into a delicate tube, generating a so-called soda straw that can grow
 several meters long, says Goldstein.

 Only about 1 percent of the calcium carbonate that's dissolved in the
 water flowing down a stalactite remains on that formation, Goldstein
 and his colleagues estimate. The rest is carried to the cave floor
 within the dripping water. Over time, the deposits from drips can
 produce stalagmites.

 Although his team's model doesn't address those structures'
 formation, Goldstein speculates on how a stalagmite's shape develops.
 As stalactites do, stalagmites grow irregularly at first. The
 crystals accumulate in a broad, random pattern, probably because the
 mineral-rich droplets splash when they hit the cave floor, says
 Goldstein. As the mineral layers thicken and the formation grows
 upward, the distance through which the droplets fall becomes shorter,
 so the droplets splash less when they strike and the stalagmite takes
 on a more predictable carrot shape that's often the mirror image of
 the stalactite above it, he proposes.

 Built by bugs?

 Hundreds of types of rock appear in cave formations. The most
 abundant ones are forms of calcium carbonate deposited when
 mineral-rich waters seep into an open space underground.
 Increasingly, however, researchers are finding that many exotic cave
 formations are in one way or another associated with bacteria.

 For instance, some of the walls in New Mexico's Lechuguilla Cave are
 coated with gnarly bumps of various sizes, a type of formation that
 cavers have nicknamed popcorn. The smallest bumps are 2 to 3
 millimeters across and look more like bacterial colonies growing in a
 petri dish than like popcorn, says Hazel A. Barton, a microbiologist
 at Northern Kentucky University in Highland Heights. The largest
 knobs of the hard, white rock are thumb-size replicas of the popped
 kernels of the snack that shares their name.

 Microscopic analyses of the bumps and knobs show layers of bacteria
 that have been fossilized in calcium carbonate. Barton and her
 colleagues have scraped samples of unidentified live bacteria from
 cave walls and took them back to the lab. When fed a
 calcium-containing substance, the microbes made crystals of calcium
 carbonate, the same material that encases their fossilized brethren.

 Microbes are present on most moist cave surfaces. However, because
 not all such bacteria can be cultured in the lab, it's often
 difficult to confirm which organisms, if any, are responsible for
 forming cave minerals, says Brian Jones, a geologist at the
 University of Alberta in Edmonton who has studied cave microbes.

 In some instances, bacteria appear to play a more indirect role,
 merely creating the environmental conditions under which dissolved
 minerals are more likely to crystallize. For example, quartz pebbles
 and rocks in many cave-floor streams are coated with a layer of
 manganese oxide&#150;containing minerals that can range from
 fractions of a millimeter to a few millimeters thick, says William B.
 White, a geochemist at the Pennsylvania State University in
 University Park. The predominant mineral in the coatings is
 birnessite, a manganese oxide compound that includes traces of
 sodium, calcium, and potassium.

 Microscopic analyses of the birnessite coatings reveal many
 fossilized bacteria, so White speculates that those organisms derived
 energy from the manganese dissolved in the water that flowed over
 them. During the energy exchange, the manganese ions were converted
 to an insoluble form that then precipitated to form the birnessite.

 At this stage of research, White says, it's unclear whether the
 microbes had a role in the coatings' formation or whether they simply
 inhabited the veneer after it formed.

 The pool-meringue formation in Carlsbad Caverns may represent a
 stronger link between microbes and cave minerals. Scattered
 throughout the meringue are smooth tubes 2 to 3 &#181;m long and less
 than 1 &#181;m in diameter. That's the right size and shape to be
 microbial filaments, says Melim.

 Lab results also hint that the filaments have a microbial origin.
 Compared with the rock that encased them, the microscale tubes are
 more resistant to mild acid and contain slightly more carbon.

 Starvation diet

 Ecologically, caves are some of the world's most isolated
 environments. That seclusion offers stability: In large caves,
 temperatures rarely fluctuate, and the humidity in caves receiving a
 steady flow of mineral-rich groundwater stays close to 100 percent.
 "It's like a sauna, only a lot cooler," says Jones.

 [IMAGE]  ART IN DANGER. The prehistoric art in many European caves,
 such as this bull depicted in France's Lascaux Cave, is threatened by
 microbes that consume the ancient pigments.
 &#169; Archivo Iconografico, S.A./Corbis

 The isolation and stability also make several cave features valuable
 recorders of geologic and even climatic history. The ratio of oxygen
 isotopes in a sample of carbonate can provide clues about the
 temperature at which the cave mineral crystallized. While soil
 temperatures at or near Earth's surface rise and fall with the
 seasons, these fluctuations are tempered in deep caverns. Therefore,
 a deep cave's temperature matches the average annual temperature at
 ground level directly above it.

 Although useful to scientists, a cave's isolation from Earth's
 surface causes problems for organisms living there, says Barton. It's
 pitch-black deep inside a cave, so no food chain can be based on
 photosynthesis. Many microbes obtain energy by breaking down rocks
 and taking advantage of the chemical energy from those reactions (SN:
 11/15/03, p. 315:
 http://www.sciencenews.org/articles/20031115/bob9.asp).

 Most of the nutrients available to support life are those carried in
 by groundwater. Each liter of water seeping into a cave typically
 carries between 15 and 50 micrograms of organic carbon, about
 one-thousandth the concentration that's considered a starvation diet
 for microbes living at Earth's surface. Forced to make a living from
 such slim pickings, the cave-dwelling organisms have developed
 unusual techniques for extracting energy from their surroundings.

 "These bacteria are incredibly starved, but they're incredibly
 diverse," says Barton.

 With that variety in the face of adversity, microbes have evolved
 complex communities that work together to efficiently process the
 minuscule quantities of nutrients that flow their way, she notes.
 These communities take many forms, including slick films on rock
 walls, mats in the cave's streams and pools, and moist globs that
 geologists have dubbed snottites.

 Microbes quickly take advantage of any nutrients brought into the
 cave by human or animal explorers as well as by seeping water, says
 Penelope J. Boston, a microbiologist at the New Mexico Institute of
 Mining and Technology in Socorro. Ropes installed by spelunkers in
 Lechuguilla Cave are being slowly consumed by fungi, she notes.
 Tubing used to siphon drinking water from cave pools, if left
 dangling into the water, soon gains a coating of microbes that derive
 nutrition from organic compounds that leach from the plastic. Even
 the hair and skin cells shed by the occasional caver provide a source
 of nutrition for cave bacteria, says Boston.

 The microbial penchant for consuming any substance with nutritive
 value poses a threat to some of humanity's priceless works of art,
 the cave paintings that are scattered across much of Europe. Some of
 those works date back to the midst of the last ice age. To make their
 paints, the prehistoric artists mixed minerals such as iron oxide and
 organic substances such as charcoal into binders such as vegetable
 oils and fats&#151;appetizing ingredients all, for a starving microbe.

 Recent surveys have found a wide variety of bacteria living on or
 near cave paintings at several sites, says Cesareo Saiz-Jimenez, a
 microbiologist at the Institute of Natural Resources and Agrobiology
 in Seville, Spain. Some of the more-renowned masterpieces under
 attack include cave art in France's Lascaux Cave and Spain's
 Altamira, La Garma, and Tito Bustillo caves. Scientists haven't
 worked out lab methods to grow many of the microbes attacking the
 paintings, so those organisms haven't been studied in detail.
 However, analyses of their DNA provide hints about their family
 relationships and lifestyles.

 For example, cave paintings in Altamira Cave support microbes from
 the group Crenarchaeota, which can be found in many soils but also
 inhabit extreme environments such as near-boiling springs,
 Saiz-Jimenez and his colleagues reported in the January
 Naturwissenschaften. The same paintings also host microbes from the
 genus Acidobacteria, which often thrive in acidic soils. Such
 microorganisms threaten not only the cave art's pigments but also the
 underlying rock.

 Attempts to kill or control the art-loving microbes with
 disinfectants might not work, say some cave-art conservators. They're
 concerned about damage by such cleansers to the rock. Furthermore,
 killing some of the microbes might simply shift the balance of power
 to an even more destructive set of organisms and thus accelerate
 damage rather than prevent it.

 The best hope for the cave art, says Saiz-Jimenez, lies in continued
 research, which may yield insights into how to inhibit the growth of
 microbial communities and how to minimize the damage they're causing.

 If you have a comment on this article that you would like considered
 for publication in Science News, send it to editors at sciencenews.org.
 Please include your name and location.

 References:

 Melim, L.A., et al. 2004. Pool meringue: A new speleothem from
 Carlsbad Caverns of possible biologic origin. Geological Society of
 America meeting. Nov. 7-10. Denver. Abstract available at
 http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_78466.htm.

 Gonzalez, J.M., M.C. Portillo, and C. Saiz-Jimenez. 2006.
 Metabolically active Crenarchaeota in Altamira cave.
 Naturwissenschaften 93(January):42-45. Abstract available at
 http://dx.doi.org/10.1007/s00114-005-0060-3.

 Schabereiter-Gurtner, C., C. Saiz-Jimenez, et al. 2004. Phylogenetic
 diversity of bacteria associated with paleolithic paintings and
 surrounding rock walls in two Spanish caves (Llonin and La Garma).
 FEMS Microbiology Ecology 47(February):235-247. Abstract available at
 http://dx.doi.org/10.1016/S0168-6496(03)00280-0.

 Short, M.B., J.C. Baygents, and R.E. Goldstein. 2005. Stalactite
 growth as a free-boundary problem. Physics of Fluids
 17(August):083101. Abstract available at
 http://dx.doi.org/10.1063/1.2006027.

 White, W.B. 2004. Manganese oxide minerals in caves: Microbially
 driven heavy metal scavengers. Geological Society of America meeting.
 Nov. 7-10. Denver. Abstract available at
 http://gsa.confex.com/gsa/2004AM/finalprogram/abstract_73813.htm.

 Further Readings:

 Jones, B. 2001. Microbial activity in caves&#151;a geological
 perspective. Geomicrobiology Journal 18(July 1):345-357. Abstract.

 Perkins, S. 2003. Attack of the rock-eating microbes! Science News
 164(Nov. 15):315-317. Available at  
 http://www.sciencenews.org/articles/20031115/bob9.asp.

 ______. 2002. Cave formations yield seismic clues. Science News
 162(Sept. 14):174. Available to subscribers at
 http://www.sciencenews.org/articles/20020914/note13.asp.

 ______. 2001. For past climate clues, ask a stalag-mite. Science News
 160(July 28):55. Available to subscribers at
 http://www.sciencenews.org/articles/20010728/fob8.asp.

 Short, M.B., J.C. Baygents... and R.E. Goldstein. 2005. Stalactite
 growth as a free-boundary problem: A geometric law and its platonic
 ideal. Physical Review Letters 94(Jan. 14):018501. Abstract available
 at http://link.aps.org/abstract/PRL/v94/e018501.

 Stone, D.A., and R.E. Goldstein. 2004. Tubular precipitation and
 redox gradients on a bubbling template. Proceedings of the National
 Academy of Sciences 101(Aug. 10):11537-11541. Available at
 http://www.pnas.org/cgi/content/full/101/32/11537.

 Zimmerman, J., J.M. Gonzalez, C. Saiz-Jimenez, and W. Ludwig. 2005.
 Detection and phylogenetic relationships of highly diverse uncultured
 acidobacterial communities in Altamira Cave using 23S rRNA sequence
 analyses. Geomicrobiology Journal 22(October/November):379-388.
 Abstract available at http://dx.doi.org/10.1080/01490450500248986.

 Sources:

 Hazel A. Barton
 Department of Biological Sciences
 Northern Kentucky University
 Highland Heights, KY  41099

 Penelope J. Boston
 New Mexico Institute of Mining and Technology
 Department of Earth and Environmental Science, MSEC 208
 801 Leroy Place
 Socorro, NM 87801

 Raymond E. Goldstein
 Department of Physics
 University of Arizona
 Tucson, AZ 85721

 Brian Jones
 Department of Earth and Atmospheric Sciences
 Earth Sciences Building, Rm. 2-03
 University of Alberta
 Edmonton, Alberta T6G 2E3
 Canada

 Leslie A. Melim
 Geology Department
 Western Illinois University
 1 University Circle
 Macomb, IL 61455

 Cesareo Saiz-Jimenez
 Institute of Natural Resources and Agrobiology
 Spanish National Research Council
 Apartado 1052
 41080 Seville
 Spain

 William B. White
 Department of Geosciences and Materials Research Institute
 Pennsylvania State University
 210 Materials Research Lab
 University Park, PA 16802

 http://www.sciencenews.org/articles/20060429/bob9.asp

 From Science News, Vol. 169, No. 17, April 29, 2006, p. 266.

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