|NEWS NOTES||April 1999|
Where on Earth did all this dirt come from?
AGU tackles climate change
ACEing the sun
Siberian slab buried, not lost
Younger Dryas forces human choice
What's causing coral bleaching?
Oxygen in the solar nebula
New research published in the Jan. 29, 1999, issue of Science suggests that some North American continental basins were dominated by Appalachian-Caledonian source materials as soon as the mountains began forming, and other basins were affected as the orogenic event progressed. In addition, the research suggests that these basins continued to be dominated by Appalachian-Caledonian sources well after the orogeny had ended. Jonathan Patchett and James Gleason of the University of Arizona-Tucson and Gerald Ross of the Geological Survey of Canada have analyzed neodymium (Nd) isotope data to identify the source material of sediments in basins across North America and to determine possible dispersal paths for the material. From this information, the researchers were able to place North American continental sediments in a spatial and temporal context.
Source rocks from a general site, such as the Canadian Shield craton or the Appalachian-Caledonian mountain belt, possess a specific Nd signature. Neodymium in sediment reveals an integrated average of the isotopic compositions and the crust formation ages of source-area rock material. In addition, neodymium and its parent element, samarium, are largely unfractionated by sedimentary processes. Therefore, sedimentary rocks in the continental interior of North America may correlate directly with the rock sources that produced them, even if the source material has been reworked en route.
The neodymium signature for the Appalachian-Caledonian orogeny corresponds to neodymium values of -5 to -13, while Archean crust has an Nd value of less than -20, and more recent rocks have Nd values that can approach +2. The researchers found that the neodymium data reveal that cratonic rocks dominated the continental sediment supply from 600 to 450 million years ago. Appalachian-Caledonian sediments began to dominate over cratonic sediments as soon as the mountains began forming 450 million years ago. The Appalachian-Caledonian sediments continued to dominate, either directly or through the recycling of Appalachian-Caledonian-derived sedimentary rocks, until the Cordillera rose to prominence 300 million years later. Patchett, Ross, and Gleason suggest, therefore, that the Appalachian-Caledonian mountains dominated the sediment supply well after the mountain-forming processes had ceased, and did not relinquish dominance until a new sediment supplier, the Cordillera mountains, arose 150 million years ago.
Patchett and Ashton Embry of the Canadian Geological Survey will next apply Nd analysis to sediments from the Sverdrup basin of the Arctic Islands. The Pennsylvanian-Tertiary sediments within the basin are more than 9 kilometers deep, and may indicate whether the entire Canadian shield was covered with sedimentary material during the Devonian and into the Mississippian. In addition, Patchett and Embry hope to determine whether the sediment cover persisted into the Mesozoic and if so, for how long. If the Sverdrup sediments show that the Canadian Shield was covered by Devonian/Mississippian sediments, then a transgression may have occurred during that time. This research may provide evidence for the “dynamic topography” concept, which suggests that continental crust, in this case the Canadian Shield, has been uplifted on a broad, super-regional scale.
— Joshua A. Chamot
Department of Geology, University of Tennessee-Knoxville
The panel was chaired by Dr. Tamara Ledley, senior scientist at the Center for Earth and Space Science Education at the Technical Education Research Center in Cambridge, Mass. Before the AGU Council unanimously approved the statement last December, the 35,000 AGU members had an opportunity to comment on a draft developed during the year-long process. The speakers at the press conference emphasized that the statement is not intended to present new research on climate change or specific policy recommendations. Instead, it attempts to describe the state of peer-reviewed science on climate change and greenhouse gases. Panelist Tim Killeen of the University of Michigan said the report helps distinguish between facts and scientific uncertainty.
“Atmospheric concentrations of carbon dioxide and other greenhouse gases have substantially increased as a consequence of fossil fuel combustion and human activities,” the statement reads. These elevated levels are predicted to stay in the atmosphere “for times ranging to thousands of years.” While acknowledging the remaining uncertainty of Earth’s response to this influx of gases, the AGU scientists note that our understanding of the processes surrounding global climate change have greatly improved over the past several decades.
Eric Sundquist, a research geologist with the U.S. Geological Survey, emphasized AGU’s role of illustrating “the present-day climate in the context of what we understand about how the Earth’s climate and atmosphere have varied in the past.” The panelists stated that “there is no known precedent for the transfer of carbon from the Earth’s crust to the atmospheric carbon dioxide, in quantities comparable to the burning of fossil fuels, without simultaneous changes in other parts of the carbon cycle and climate system.” Their statement continues by saying that “the world may already be committed to some degree of human-caused climate change, and further buildup of greenhouse gas concentrations may be expected to cause further change,” warning that these changes “could be very disruptive.” Panelists cited increases in global mean rates of precipitation and evaporation, rising sea level, and changes in the biosphere as possible effects of increased greenhouse gases.
Ledley told reporters that science should not be the only source of information on deciding how best to address climate change, but “scientific understanding based on peer-reviewed research must be central to informed decision-making.” Within this framework, Killeen explained that the statement implies that science has advanced enough that policy-makers should not use scientific uncertainty as a reason not to pursue policy options to mitigate climate change. The statement recommends additional research on emissions reductions, carbon sequestration, and adaptation to climate change.
Reaction to the statement, which received widespread press coverage, has been mixed. Because it does not offer specific suggestions for addressing climate change or mention any legislative or international treaties, several environmental groups have criticized the statement as not going far enough. On the other hand, Paul George, of the Competitive Enterprise Institute, charged that “AGU has crossed the line separating science from advocacy.” AGU’s statement was cited by Vice-President Al Gore in a press release on the Clinton administration’s research initiatives on climate change. Gore said, “We have an obligation to act responsibly in assessing potential damages, and to protect our economy and national security by investing in efficient energy technologies. As the AGU reinforced today, the risks of climate change are serious, the costs of potential impacts are large, and the time to act to protect our national interests is now.”
— Kasey Shewey White
AGI Government Affairs Program
“Lots of people go out and try to grab hold of material samples,” says Glenn Mason, a professor at the University of Maryland and one of the researchers using ACE. Instead of sampling rocks from the moon or material from Mars, Mason says, ACE researchers are sampling material from the sun, material from the interstellar medium, and galactic material. The galactic matter could come from supernovae that occurred in other parts of the galaxy. Because it can collect and measure even the rarest of ions, and even individual ions and atoms, in the low-density material passing by it, ACE allows researchers to distinguish between solar, galactic, and interstellar materials and to assemble pictures of their different compositions. “On ACE, we gather our atoms one at a time,” Mason says. “We’re building up samples of these materials that we can look at to understand properties of the sources.”
ACE also serves as a weather forecaster, giving researchers at the Space Environment Center in Boulder, Colo., an hour to warn power and communications companies that a solar storm is approaching Earth.
Ejected solar materials can provide clues about the composition of the sun’s corona. Galactic material from supernovae can help scientists understand the origin of matter, because many of the universe’s heaviest elements originate in supernovae. Results from ACE have so far been surprising, Mason says.
Watching in real time
Using data collected by ACE’s Solar Wind Ion Composition Spectrometer (SWICS), a team of researchers led by George Gloeckler, a physics professor at the University of Maryland, published a paper in the Jan. 15, 1999, issue of Geophysical Research Letters (GRL) about the unusual composition of the May 2-3 coronal mass ejection (CME) — a bubble of gas and magnetic field lines that the sun ejects over the course of several hours. The frequency of CMEs varies with the sunspot cycle. They are sometimes associated with solar flares and prominence eruptions. Coronal mass ejections carry plasma, or ionized gas, away from the sun at speeds approaching 2,000 kilometers per second.
Gloecker’s team had studied other CMEs using the same kind of spectrometer on Ulysses, a NASA spacecraft that explored the sun’s atmosphere over its poles in 1994 and 1995. But ACE, the researchers say, is three times closer to the sun than was Ulysses. “The resulting increase in density yields substantially better statistics,” they write in GRL. ACE measures the elemental and charge-state composition of CME particles, which remain relatively unchanged as they travel from several solar radii to L1, Gloeckler says. “In this sense we can use these measurements to infer general conditions closer to the sun. … Previously, it was not possible to obtain elemental and charge-state composition measurements spanning the entire range of elements and charge states.”
Gloeckler’s team used ACE’s location to measure the change of the CME’s composition every 30 minutes, observing how long certain ions and charge states within the coronal material (and the energies that sustain them) lasted as the CME moved from the sun into the solar system. They used this data to reconstruct the history of the solar event.
“We were completely surprised by the highly unusual and unexpected composition in this CME,” Gloeckler says. His team observed, for example, that the density of 4He+ was almost as high as the density of 4He++ for several hours. “Such large 4He+/4He++ ratios, persisting for hours, have never been observed in the solar wind before,” they write. They also observed high increases of helium and heavier ions in the CME plasma. The unusual composition of the CME lasted an exceptionally long time, they write, and indicates that materials formed in cold, hot, and normal coronal temperature regions all existed in the ejection simultaneously. “This is certainly not an average solar wind but an anomalous situation,” Gloeckler says. “Yet such anomalous findings often lead to deeper understandings of physical processes.”
While ACE will help researchers understand the origin of CMEs and the solar wind, it is too far away to pinpoint the exact regions on the sun where the particles originate, Gloeckler says. But such information might come from NASA’s Solar Probe, which will pass as close as three solar radii from the sun’s surface. Solar Probe is scheduled for launch in 2007, Gloeckler says.
Geologists from the University of Michigan and Utrecht University, the Netherlands, have located a piece of Earth’s ancient history buried at least 750 miles beneath the surface. If they are correct, the mantle beneath Siberia harbors part of a long-extinct ocean bed, the oldest section of subducted lithosphere ever identified.
“Originally this piece of Earth’s crust was located at the bottom of the Mongol-Okhotsk Ocean, separating what is now Siberia and Mongolia,” says Rob Van der Voo, geology professor at the University of Michigan. “As the Siberian and Mongolian continental plates converged between 200 million and 150 million years ago, this material was forced down — subducted deep into the Earth. It has been sinking ever since at an average rate of one centimeter per year.”
Van der Voo and Utrecht University scientists Wim Spakman and Harmen Bijwaard used seismic tomographic imaging to identify the slab above and among a “graveyard” of slab remnants in the mantle beneath Lake Baikal. The researchers’ images clearly show material descending to the bottom of the mantle layer, confirming that subducted slabs do eventually reach that depth.
“Improvements in tomographic techniques in the past few years have allowed more detailed visualization of deep fossil slabs, such as those that appear to have resulted from subduction of the Pacific and Tethyan oceans,” they write in the Jan. 21, 1999, issue of Nature. Tomography confines the mantle parameters, using either spherical harmonic functions or local cell slowness functions, the authors explain.
“The sizes of these cells in the latter parametrization have steadily decreased,” they write. Cell-size values of 2°– 0.6° instead of 10°, improvements in travel-time datasets, the use of a larger number of composite rays, and a minimal difference in cell hit counts all have contributed to improving the detail of seismic tomographic images. Even so, says Van der Voo, “we didn’t expect to see such a strong signal at these depths. Whether [the slab’s] visibility is the result of differences in temperature, composition, pressure, or a combination of these remains unclear.”
Siberia’s Lake Baikal
Van der Voo and his colleagues at Utrecht selected the Lake Baikal area for their study because it is the site of an ancient, well-documented subduction zone. It also is located in a part of the world that has an extensive network of seismic monitoring stations. The scientists created the tomographic images — their “CAT scans” — of Earth’s interior from this wealth of seismic information.
The suture from the Mongol-Okhotsk ocean is marked by a P-wave velocity contrast in both the crust and uppermost mantle, the authors explain in Nature. The velocity anomalies are well-resolved at depths greater than 1,200 kilometers, and are pronounced at greater than 1,500 kilometers. At 2,500 kilometers depth, they write, the high-velocity anomalies identifying the inferred Mongol-Okhotsk slab “merge with a broad east-Asian high-velocity area in the lowermost mantle, which has been called a ‘graveyard’ of slabs, an interpretation corroborated by our results,” they write.
Van der Voo and his colleagues also identified other Asian subduction zones, and are confident that the high-velocity zones under Lake Baikal are not Cenozoic in age nor related to present-day Pacific subduction. Thus, the deep-mantle portions of the slab are Mesozoic. But it is harder to determine where they started their downward journey because the Asian lithosphere may have drifted away from the mantle formerly below it. By examining the displacements of slabs in the eastern and western Pacific subduction zones, as well as the displacement of the continents during the opening of the Atlantic Ocean, the authors deduced that the Siberian lithosphere “has moved very little in an east-west direction with respect to the deep-mantle slabs.”
“The high-velocity structures under Siberia,” they write, “are logically interpreted as remnants of oceanic lithosphere that subducted before the Early Cretaceous… . Therefore, subducted lithosphere of Jurassic age can still be recognized after penetration into the lower mantle.” Significantly fast anomalies in the deeper mantle seem to be remnants of subduction, they add. “This renders tomography an important tool for testing paleogeographical reconstructions.”
When Ofer Bar-Yosef and his colleague, A. Belfer-Cohen, first suggested this connection 10 years ago, plant remains from relevant archaeological sites that would support the idea hadn’t been found. Since 1989, Bar-Yosef and archaeobotanists Gordon Hillman and Sue Collidge, of the Institute of Archaeology in London, have been investigating collections from two sites in the Euphrates Valley where carbonized plants were well preserved.
Meanwhile, Mordechai Kislev of Bar-Ilan University in Israel conducted similar research, studying a large collection of carbonized seeds from a site in the Jordan Valley. The preservation of plant remains in Neolithic villages is generally good because inhabitants used clay for making bricks, Bar-Yosef says. After the houses collapsed or were abandoned, sedimentation rates were faster than at other types of prehistoric sites.
Hillman discovered plant evidence for cultivation, Bar-Yosef says, and dated the seeds to place them within the Younger Dryas (about 11,000 to 10,000 years before the present). Bar-Yosef investigated evidence of long-lasting villages, which were established when early humans stopped migrating in search of their food; dating also placed his findings within the Younger Dryas. What the newer analyses showed, Bar-Yosef says, is that food cultivation and long-term village dwelling started during the Younger Dryas, instead of immediately afterwards.
Agriculture gradually spread across Asia and Europe, starting at the Fertile Crescent, a narrow strip of land that stretches from the Mediterranean Sea to the Persian Gulf through the Tigris and Euphrates valleys. Agriculture probably started in the western wing of the Fertile Crescent (now Israel, Jordan, and parts of western Syria and southeast Turkey), where cereals were available during the Younger Dryas. The climatic crisis of the Younger Dryas caused major environmental deterioration that threatened the subsistence strategies of a local tribe of grain gatherers, the Natufians. Earth became cooler and dryer during this period of sudden climate change, possibly decreasing the natural production of plants that perform C3 photosynthesis. Paleobotanical studies show that this area, the Levant, housed the progenitors of most cereal species, while the archaeological record shows that communities of cultivators settled there. “It seems that this was the locus for the emergence of agriculture in Western Asia,” Bar-Yosef wrote in the October 1998 issue of the Cambridge Archaeological Journal.
Electrical conductivity measurements of the GISP2 (Greenland Ice Sheet Project Two) ice cores show that the full climate change of the Younger Dryas probably took 45 years to set in. That’s one human generation, Bar-Yosef says — one generation of Natufians who saw the food supply diminish and decided to put cereal seeds in the ground and cultivate them. The Natufians who chose to settle, rather than move on in search of food, would have figured out that planting wild cereal seeds offered an opportunity to harvest what they had planted, instead of relying on reduced amounts of wild cereal elsewhere. “The Younger Dryas forced the Natufians to make a choice,” Bar-Yosef says. “Some decided to stay and cultivate. … It’s a combination of climate change and human choice.”
Bar-Yosef’s work contributes to growing evidence of a close association between major changes in climate and in civilization during the Holocene, says Paul Mayewski of the University of New Hampshire, who also presented a climate research paper during the AAAS meeting. This evidence, he says, shows that “in fact, natural climate variability is an important issue, and the Holocene is not quite as benign as we might have thought just a few years ago.”
Effects of temperature
Bleaching was first reported in the 1980s in Puerto Rico, Jamaica, southern Florida, and other Caribbean areas. As researchers began to collect data on the bleaching, they found that Caribbean water temperature had increased by nearly 2 degrees Celsius between 1986 and 1987. Episodes of bleaching, linked to temperature increases, continued to occur during the next 10 years.
In some cases, it seemed as though El Niño had caused the sea-surface temperature increase. But other past episodes of bleaching were not in El Niño years. Scientists began to consider the possibility that bleaching was the result of an overall global warming trend.
Other factors also could be involved, but high temperatures seem to make the corals more susceptible to stress. “Although other factors such as high light intensity, high ultraviolet light, and lowered salinity have also been implicated in coral bleaching events, their effect in stressing corals is greatly exacerbated near corals’ upper thermal limits,” says Ray Berklemans, monitoring project officer at Australia’s Great Barrier Reef Marine Park Authority. But there seems to be another side to the story.
Pathogens, disease, and natural variability
“There could be more to bleaching than just an increase in temperature,” says Esther Peters, a histopathologist and senior scientist at a consulting company in Fairfax, Va. Other factors can contribute to coral bleaching, including an increase in sedimentation and exposure to toxic chemicals.
“I don’t think we can say that every case of bleaching is due to elevated sea-surface temperature,” says Peters. “We need to continue multidisciplinary research to look at many facets. Of particular interest are whether and how localized conditions affect bleaching patterns.”
A growing number of scientists are looking at other local factors in the coral bleaching epizootic. One long-term study of corals in Mauritius, an island near Madagascar, by I. Fagoonee (University of Mauritius) and colleagues shows that a natural variability in the density of zooxanthellae could play a large role. Bleaching might occur not because the algae are expelled, but because there are fewer of them in the coral colony. And there is considerable fluctuation in the amount of symbiotic algae through the year. According to Fagoonee, “Although there is a correlation between time of year and temperature, the variation in zooxanthellae density is better explained by season than by temperature. …Over and above the effects of season, the zooxanthellae density is positively correlated with nitrate concentration, indicating nitrate limitation” (Science, Feb. 5, 1999).
Coral bleaching could also be increasing due to susceptibility to pathogens. A 1996 paper in Nature by Ariel Kushmaro (Tel Aviv University) and colleagues suggested that a bacterial infection had caused bleaching of Oculina patagonica in the Mediterranean. The scientists in that study collected bleached and unbleached coral, and discovered the presence of bacteria on the tentacular rim of the bleached coral only. When they tested the bacteria on the unbleached coral, that coral also bleached.
C. Drew Harvell, a researcher at Cornell University, ran a session on pathogens at the January meeting of the American Association for the Advancement of Science. She suggests that the increase in ocean temperature spurs an increase in pathogens such as Perkinsus marinus. The warmth allows the pathogens to multiply their range and infect populations of susceptible hosts. The hosts themselves may be more susceptible because of warming.
Peters agrees. “Extensive studies have not yet been done,” she cautions, “but we’re looking at pathogens as another possible cause for some cases of bleaching.” Parasites known as coccidia, similar to the protozoa that infect the gastrointestinal tract in humans, are about the same size as zooxanthellae. The coccidia have been observed to replace the algae, causing infections in the corals’ gastrodermal cells and resulting in the appearance of bleaching.
It’s possible that temperature increases are the spur that makes corals susceptible to everything from natural variability in algae to pathogen infection. Or it could be that the pathogens are increasing of their own accord. The answer remains to be seen, as researchers continue to study the bleaching epizootic. As Peters says, “It’s not a simple thing. It’s far more complex than anyone might imagine.”
Cosmochemists still don’t agree on what process caused excess amounts of the oxygen-16 isotope to accumulate in these CAIs. But recent work by two Japanese planetary scientists suggests that these anomalies were formed by chemical processes within the solar nebula and not, as has been suggested, from an outside source carrying excess oxygen-16 (such as a supernova).
Hajime Hiyagon of the University of Tokyo and Akihiko Hashimoto of Hokkaido University in Sapporo, Japan, published a report in the Feb. 5, 1999, issue of Science showing that the oxygen isotopic anomaly thought only to exist in calcium-aluminum-rich inclusions also exists in olivine-rich inclusions (OIs). Their discovery is surprising for two reasons. First, because olivine is much more common in chondrites than CAI-forming minerals, few people have paid attention to olivine as a carrier of oxygen-16 excesses, says Hiyagon. “Although it has been observed that some olivine phases, related to CAIs, have oxygen-16 excesses, generally they are considered marginal. Hence, our discovery of OIs was a big surprise in the community.” Secondly, because OIs most likely formed in the solar nebula after CAIs (according to the condensation theory), it is surprising that the olivine inclusions carry the same anomaly displayed by the calcium-aluminum inclusions. That both inclusions bear the same isotopic signature implies that they formed in similar environments of excess oxygen-16, meaning that they evolved from a chemical process in the solar nebula, not from an outside carrier.
Earlier theories had suggested that an oxygen-16 carrier arrived in the solar nebula from a supernova. But Hiyagon and Hashimoto conclude that this possibility is unlikely. The Japanese researchers found the same degree of oxygen isotopic anomaly in olivine as other researchers had found in spinel grains (such as in CAIs in the Allende chondrite). Some kind of process would be needed to transfer all of an outside carrier’s oxygen anomaly from spinel to olivine evenly, in spite of their different condensation temperatures. Hiyagon and Hashimoto point out that such a physical-chemical process is impossible. Thus, their discovery makes it more probable that the excess oxygen-16 formed from a chemical process within the solar nebula, says Mark Thiemens, professor of chemistry and biochemistry at the University of California, San Diego.
“By the chemical process, the anomalies arise only as a result of the chemical reactions, which convert the nebular cloud into solid planetary objects. It requires no alien addition and is simply a consequence of condensation,” says Thiemens. Hiyagon notes that researchers still have not reached a consensus on what the heat source for the formation of CAIs, OIs, and the chondrules found in chondrites was, or where these chemical groupings formed within the nebula.
These oxygen isotopic anomalies probably evolved from mass-independent fractionation. While most isotopes fractionate based on their differing masses, the isotopes contained in these inclusions apparently did not. But no one has determined the mechanisms for this fractionation. Thiemens and co-author John E. Heidenreich first described the mass-independent fractionation of oxygen in the solar nebula in a 1983 Science paper. “Based upon our understanding of the chemical physics of this [mass-independent] effect, it is easily plausible that these reactions were as prevalent in the presolar nebula as they are in Earth’s atmosphere,” Thiemens says.
Analyzing smaller samples
Hiyagon and Hashimoto used in situ microprobe analysis to find the oxygen isotopic anomalies within the olivine-rich inclusions. Analyzing oxygen isotopes with conventional gas mass spectrometry requires samples weighing about 1 milligram. But olivine-rich inclusions are generally only about 100 micrometers in diameter and weigh about one-thousandth of a milligram. With the ion microprobe, it is possible to analyze isotopic compositions in these tiny samples. The researchers analyzed inclusions in the Yamato-86009 chondrite and also in the Murchison chondrite, a very primitive meteorite that fell over Murchison, Australia, in 1969 and defines one of the classes of carbonaceous chondrites.
The Japanese researchers’ work brings some certainty to one of many questions and contradictions recorded in meteorites. Their work rules out the possibility that supernova material contributed to oxygen-16 excesses in presolar CAIs and OIs. “However, this does not mean that supernova-originated materials have a negligible contribution to our solar system,” Hiyagon warns. Solid-phase materials constituting later accretions of the planets have been supplied in the galaxy by supernovae. But the role of supernovae in the presolar nebula is still not completely clear. Most of the presolar oxide grains that emigrated from outside the solar nebula came from Red Giant stars, Hiyagon says. At the same time, presolar grains found in nonoxides usually originated from supernova material. “Further studies are required to understand this apparent contradiction.”