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Mantle gases and atmospheric
mysteries
Tiny teeth gnaw on theories
Devils dust off martian geology
Sea level changes may trigger
volcanism
Copyright denied to Kansas
Declining CO in the mid-Atlantic
Taiwan quake in perspective
New evidence of isotopic composition of xenon in carbon dioxide well gases, reported in the September 24 Science, suggests a primitive reservoir of noble gases caught in certain areas of the mantle are still slowly making their way through the mantle and ultimately into the atmosphere 4.6 billion years later.
The study’s discovery of variations in xenon’s light isotopes also helps
solve another mystery behind the timing of the atmosphere’s formation—a
mystery that has divided scientists over differing theories for decades.
The question over the development of the atmosphere has been a controversial
debate since the early 1960s, says Marc Caffee and Bryant Hudson researchers
at the Lawrence Livermore National Laboratory in California and senior
scientists of the study, which involved scientists from Arizona State University,
the University of Minnesota and the University of Wollongong, Australia.
In 1949 it was recognized that Earth’s atmosphere had a different noble gas signature than the sun, a discrepancy considered the result of a rapid degassing of Earth’s mantle. “But not everybody accepts that the outgassing was rapid or complete,” Caffee says.
“The salient result of our study was to find out to what extent the mantle is outgassed,” he says. Xenon seemed to be the clue. Produced in stars, perhaps billions of years before the formation of our own solar system—or as a product of decay from other elements such as iodine, uranium or plutonium—the nine stable isotopes of xenon wield considerable leverage in understanding the chronology of the early solar system. However, there was an apparent problem: Plutonium-derived xenon in Earth was missing!s
Collecting carbon dioxide from McElmo Dome in Cortez Colo.(July 1985). CO2 is often piped for use as a solvent to help recover petroleum. E. Calvin Alexander Jr. |
According to the model of an outgassed mantle, if xenon derived from iodine, which has a half-life of 16 million years, was found in abundance, then xenon derived from plutonium, which has a half-life of 82 million years, should also be found in the mantle. Simply put, “it better be there!” Caffee says. But until a recent identification of plutonium-derived xenon in mid-ocean ridge basalts, no one had identified it in terrestrial reservoirs. One of the reasons such reservoirs eluded researchers was that each isotope is produced in slightly different proportions, and distinguishing plutonium-derived xenon from uranium-derived xenon was especially difficult. By studying xenon from carbon dioxide well gases with mass spectrometers sensitive to noble gases in |
It seemed strange to many scientists that terrestrial evidence of xenon’s light isotopes, which are not plutonium derived but are produced directly in stars, were systematically depleted relative to other solar system reservoirs such as moon rocks and meteorites. It was as though some light isotopes on Earth had been blown away. “Something happened to make it mass fractionate,” Caffee says.
Since the 1960s scientists have debated whether the event that depleted the light isotopes, causing mass fractionation, occurred before or after Earth accreted. Pre-accretion would leave no vestige of xenon with an isotopic pattern that is otherwise found in the solar system. For those who proposed post-accretion theory, the lack of terrestrial evidence was disturbing. “Why don’t we have any when everything else in the solar system does?” Caffee asks.
Caffee and his colleagues studied the noble gasses associated with samples of carbon dioxide piped out of Earth’s mantle and crust. While looking for the isotopic signatures of xenon derived from plutonium or uranium they found variations of xenon’s light isotopes 124, 126 and 128 that did not match the isotopic signature of air. “It is the first time this variation, correlated with the variation of xenon 129, has ever been seen in terrestrial samples,” Caffee says. Excesses of these light isotopes did not come from decay of radionuclides, he adds, but were incorporated when Earth accreted. “It turns out we do have a solar-like reservoir, only something happened to change the isotopic abundance after the formation of Earth and its atmosphere.”
The discovery by Caffee and his colleagues showed that xenon with an isotopic signature of solar xenon was incorporated into Earth’s mantle while it was still forming, indicating that the mass fractionation event happened after Earth accreted. The study collaborates with previously controversial findings of solar neon components in Earth’s mantle. It also lends credit to last year’s finding by Robert Pepin, of the University of Minnesota, of solar argon in Earth’s mantle.
While Caffee has demonstrated the “when” for mass fractionation, Pepin has a couple of theories on the “how.” In what is called hydrodynamic escape, two scenarios could rid Earth of light isotopes. The young sun, rich in ultraviolet, high-energy radiation, heats up Earth’s primitive atmosphere and energizes the hydrogen to travel into space, Pepin says. “Xenon is so heavy, it doesn’t escape by itself. But if the escaping hydrogen creates a little wind, more hydrogen rises up to replace it and xenon finds itself in a river of outward flowing hydrogen. Before it knows it, it finds itself out in space—where it never would have gone on its own.” Thus hydrogen escape fractionates planetary atmospheres, he says. The other way to create a hydrogen river would be a giant impact, “like the one that made the moon,” Pepin adds.
While excited over the evidence for post-accretion mass fractionation, Caffee is more cautious over the evidence against a rapid outgassing of the mantle. Plutonium produces 30 times more xenon than uranium as it decays, but the ratio his team found was much lower. “That tells us there is a reservoir that never outgassed still in the earth. Although parts of the mantle may be profoundly outgassed, there are primitive, undepleted parts that may be essentially the same as they were 4.6 billion years ago.” The ratio also indicates that Earth’s accretionary material was more heterogeneous, he says. “But we still can’t say if the mantle degassed fast or slow in the beginning, only that it didn’t have to be rapid.”
Christina Reed
Geotimes
 The lower molars of a mammal dating from the Middle Jurassic. The scale bar represents one millimeter. The Chicago Field Museum of Natural History. |
This miniscule mammalian ancestor, named Ambondro mahabo for
a village near the excavation site, is 25 million years older than any
previously discovered of its kind. A. mahabo comes from a subgroup
of mammals called Tribosphenida that includes marsupials and placentals.
The specimen is also double the age of the oldest mammal previously found
in Madagascar.
“Madagascar is a big blank,” says Andre Wyss of the University of California, Santa Barbara. “We are only beginning to fill in the fossil record.” The discovery of A. mahabo challenges two current theories of mammalian evolution. It is the earliest mammal found from the Middle Jurassic in the southern continents, and is older than any Tribosphenida found in the northern continents. A. mahabo challenges the long held belief that Tribosphenida originated in the Northern Hemisphere. Wyss and his |
The fossil also challenges molecular clock estimates of when marsupials
and placentals diverged. Molecular clock data place the divergence at 172
million years ago, but no fossils have been discovered of either group
older than 110 million years. “The fossil record indicates that the divergences
between many kinds of living mammals are not nearly as ancient as molecular
work suggests,” Wyss says.
Livia Levis
Geotimes contributing writer
Before the mission began, Metzger submitted his proposal that Pathfinder’s camera and meteorology mast should scan Mars’ atmosphere for dust devils, but NASA declined the idea. As a result, the spacecraft took relatively few images during the best observation time for dust devils. But after NASA publicly released Pathfinder’s photos, Metzger, who now has his doctorate in geology, James Carr of the University of Nevada, Reno, and colleagues from the U.S. Geological Survey, the California Institute of Technology and the University of Arizona found conclusive evidence for the existence of martian dust devils using a successful image processing technique.
They discovered at least five dust devils—ranging in size from 14 to 79 meters wide and 46 to 350 meters tall—all within 14 kilometers of the spacecraft. In 1985, the Viking Orbiter took images of Mars’ atmosphere, documenting the first images of dust devils; but Metzger’s images from the surface allowed the team to calculate dust loading. Their discovery is the first documentation of martian dust devils from surface images and is described in the Sept. 15 issue of Geophysical Research Letters.
Scientists had not completely understood how dust became airborne because wind on Mars is typically not strong enough to erode the surface, Metzger says. “Because of their high rotational speeds and their low pressure core, dust devils clearly play a major role in the erosion of the martian surface.” Martian dust devils, which scientists think act similarly to dust devils on Earth, can exceed the erosion velocity threshold, thereby contributing dust to the dust cycle.
Dust devils supply information about the dust cycle, but they may also reveal information about Mars’ surface. “By the presence or absence of dust devils, we can increasingly infer with high confidence the geologic conditions of an area.” Metzger bases his idea on research in areas where dust devils occur on Earth and on his own research in Nevada of desert erosion by eolian processes. “Because we have a huge dataset of terrestrial work, Earth is a useful analog for Mars,” he says.
“Two types of desert surfaces inhibit dust devils’ formation on Earth: surfaces that are covered with grass, or grass stubble at least a few centimeters high, and well-developed desert pavement where boulders armor the surface,” Metzger says. Large-scale vegetation, however, does not seem to prohibit dust devil formation. “Bushes are the equivalent of large, scattered aerodynamic roughness elements on Mars,” he adds. When wind blows between objects that protrude into the wind stream, considered rough elements, the wind is focused in specific spots and may exceed the velocity needed for erosion, he explains. The rotating winds of a dust devil may erode dust that is normally sheltered by sand grains or larger rough elements.
Scientists can infer different geologic conditions based on the frequency
of the dust devils. “Because dust devils are direct interactions between
the atmosphere and the land, they are able to tell us a lot about the ground,
even at a great distance from a lander’s camera,” Metzger says.
Julia Cole
Geotimes contributing writer
During a symposium McGuire, Chris Kilburn (also of the Grieg Centre) and Gregory Zielinksi of the University of New Hampshire convened during the International Union of Geodesy and Geophysics quadrennial meeting in England, several researchers presented evidence linking global climate changes to volcanic eruptions. The July 28 symposium, hosted by the International Association of Volcanology and Chemistry of the Earth’s Interior, corroborated data on volcanoes from five continents.
McGuire was the principal author for one of the first studies linking
sea-level changes with the frequency of explosive volcanism over the past
80,000 years. The study compared the temporal distribution of tephra layers
in Mediterranean deep-sea cores with established global sea-level curves.
McGuire and his colleagues concluded that the frequency of notable explosive
eruptions at Mediterranean volcanoes could be related to rapid variations
in sea level during the late Quaternary.
Studies presented at the July symposium suggested that this correlation
between climate and eruptions applies to volcanoes from Alaska to Oceania.
McGuire says the age dates for enhanced volcanism events he and his colleagues
observed in the Mediterranean sediment cores and those Zielinski and his
collaborators have retrieved in Greenland ice cores appear to correlate
well.
Sea-level changes might trigger explosive volcanism by several mechanisms, such as pore-fluid pressure changes, which weaken the volcanic rock, and the erosion of a volcano’s flanks. A magma expulsion can also be triggered when a large sea-level rise adjacent to a coastal volcano reduces compressional or even tensional stress conditions in and immediately below the volcano—due to a bending plate effect, McGuire says. Island volcanoes may react differently. “Sea-level falls generate a stress decrease associated with debuttressing that might promote lateral failure and decompressive explosive eruptions,” he says.
According to the research by McGuire and his colleagues, 57 percent of active volcanoes form islands or occupy coastal sites while 38 percent are located within 250 kilometers of a coastline. Therefore, as many as 95 percent of the world’s volcanoes could be affected by the mechanisms of sea-level fluctuations. But, he adds, “It is actually a distinct possibility that each volcano reacts individually to a cocktail of factors related to large, rapid changes in sea level.”
At the same time, because cooling climates and falling sea levels may encourage island volcanoes to erupt, McGuire suggests that a feedback mechanism may exist among the volcanoes, sea-level change and climate. Induced island volcanism may enhance global cooling and accelerate glaciation. “As the planet warms up, bursts of explosive activity will continue to act against the warming trend, making it more difficult to get out of ice age conditions,” McGuire says. He suggests that this trend may provide an alternative explanation for Holocene “cold snaps,” such as the Younger Dryas.
“What we need to do,” McGuire says, “is look for the [sea-level/volcanism] relationship in other parts of the world—Antarctica would be good—and undertake more serious modeling to better constrain likely mechanisms for eruption.”
Josh Chamot
Geotimes contributing writer
In response to changes made in the Kansas standards, the American Association for the Advancement of Science, the National Research Council and the National Science Teachers Association have revoked permission for selected portions of their nationally developed science standards to be used in the Kansas standards. As a result, Kansas will have to rewrite its standards to eliminate references to the national standards.
“The Kansas standards effectively eliminated consideration of any aspects of evolution that examine origins of the Earth and life and processes that may give rise to the formation of new species,” the organizations say. “By deeming that only certain aspects of the theory of evolution should be taught, the State Board of Education adopted a position that is contrary to modern science and to the very visions and goals that the Kansas Science Education Standards claim to espouse.”
Even though the standards will now be rewritten, the Kansas State Board of Education still stands behind its original position regarding evolution, says a spokeswoman for the board.
Once changed, the Kansas Science Education Standards will be available
at: <http://www.ksbe.state.ks.us/outcomes/science_stds99.html>
Christina Reed
Geotimes
A paper published in the Sept. 15 issue of Geophysical Research Letters confirms recent findings of a global trend in decreasing carbon monoxide and a reduction in U.S. carbon monoxide emissions during the same time period. The authors of the study are Kristen A. Hallock-Waters, Bruce G. Doddridge and Russell R. Dickerson of the University of Maryland, Shane Spitzer of the Shenandoah National Park and John D. Ray of the National Park Service.
The team collected and analyzed data from 1988 to 1989 and from 1994
to 1997 at a monitoring station at Big Meadows in Shenandoah National Park,
Va., a site considered representative of the mid-Atlantic region’s air
quality. Their data showed that carbon monoxide decreased by an average
of 4.8 parts per billion by volume (ppbv) each year during the nine-year
period, for a total decline of 22.9 percent in the annual mean values from
1988 to 1997. The Environmental Protection Agency measured an 18.3 percent
decrease in carbon monoxide emissions for the United States over the same
time period, and the National Oceanic and Atmospheric Administration Climate
Monitoring and Diagnostics Laboratory (NOAA/CMDL) determined that carbon
monoxide levels worldwide decreased at a rate of 2 ppbv per year from 1990
to 1995. In contrast, studies show that from the 1950s to the 1980s carbon
monoxide increased at a rate of 1–2 ppbv per year in the Northern Hemisphere.
Air quality monitoring site at Big Meadows, Shenandoah National Park Va. Bruce G. Doddridge |
Since 1984, worldwide emissions from automobiles have been decreasing
because countries are adopting new engine and fuel technologies, says Paul
Novelli, lead researcher of the NOAA/CMDL studies.
The NOAA/CMDL studies determine the baseline level of carbon monoxide
throughout the world. The Shenandoah site, on the other hand, “is a rather
unique site, because at [an altitude of] 1,100 meters, it is sometimes
representative of a regionally averaged air quality,” Doddridge says. Removed
from local pollution sources, the air sampled there shows the influence
of regional metropolitan areas such as Washington, D.C., Baltimore and
Philadelphia.
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Jennifer M. Wang
Geotimes contributing writer
“The eastern part always has earthquakes but they are offshore,” says Char-Shine Liu of the U.S. Geological Survey in Menlo Park, Calif. “Most seismologist in Taiwan were expecting a large earthquake in the western part, not the central part, of the island.”
The Central Weather Bureau of Taiwan, Liu says, had posted warnings for the island’s western half, where a distribution of earthquakes surrounds what is known as the Peikang basement high. The epicenters of the earthquakes in this area create a half circle on land that opens west. The area is created from the Luzon Island Arc where the Philippine Sea Plate collides with the Eurasian plate.
“The basement high is an old feature, part of the Eurasian continent, and is a stronger solid body,” Liu says. The collision of the plates is like pushing soft mud with a stick around a rock, he says. “The mud experiences intensive deformation and strain accumulation, and the central range area of Taiwan is like the softer mud.” The recent earthquake in Taiwan was at the tip of the seismicity zone on the eastern end. “To me, it is reasonable to suspect anywhere around the basement high would have an earthquake,” Liu says.
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