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Spring 1998

Science Bits


Instituto Geofísico del Peru,
University of Miami,
Northwestern University,
Carnegie Institute,
University of Memphis

GPS tracks yearly growth of Andes

In a landmark study for space-based observation of the motions of the earth's crust, geologists have used Global Positioning System (GPS) satellites to monitor the movement of an entire continent and record the yearly growth of the Andes Mountains in South America to within a fraction of an inch.

The research, reported in Science in January, holds promise for improved earthquake hazard estimation, as it shows that satellite data can help geologists to calculate the accumulating stresses along fault lines as the plates of the earth's crust slide over each other.

Researchers from the Instituto Geofisico del Peru, Northwestern University, the University of Miami, the Carnegie Institute, and the University of Memphis, along with Bolivian scientists, used the NAVSTAR GPS satellites to measure movements at locations across the South American continent over a two-year period. Previously, geologists had to rely on data that traced accumulated plate motions over millions of years' time.

In the 1960s, geologists realized that South America's giant mountains, huge volcanoes, and great earthquakes were all consequences of the fact that the oceanic Nazca plate was sliding under the South American plate. But scientists studying plate tectonics were able to observe changes only as they accumulated in the geologic record over millions of years, because the continent-sized plates move only a few inches each year--about the speed fingernails grow. Not until very recently have scientists learned to use GPS mesurements to achieve subcentimeter accuracy.

Researchers drove large pins into firm ground at 43 locations on the South American continent. They traveled to each site and used a tripod-mounted precise optical plumb system to position an antenna directly over the landmark, then tracked the satellite for a couple of days as the GPS receiver recorded the position of the satellite. The research was supported by NASA and NSF.

The results show that about 3 inches (78 millimeters) of motion per year occurs between the Nazca and South American plates, and is divided three ways. About 1.3 in (35 mm) per year of the Nazca plate slides smoothly under South America, giving rise to volcanoes. About the same amount annually is locked up at the plate boundary, squeezing South America, and is released every hundred years or so in great earthquakes. The rest of the motion, about 0.4 in (10 mm) per year, crumples South America, building the Andes.

The researchers found that their findings compared well with the predictions of NUVEL-1, a Northwestern University model that describes the relative velocities and directions in which all of the earth's plates move. "We've had no idea of how geologic processes over millions of years correspond to those on a time scale of years," said Seth Stein (Northwestern), one of the coauthors of the Science paper. "We're now beginning to understand that they're very similar, so plate motion is a very steady phenomenon."

Washington State University
Peru's coastal desert holds El Nino clues

Where do you look for clues to ancient El Niño floods? In one of the planet's driest spots. Gary Huckleberry, a Washington State University geoarchaeologist, made his first reconnaissance of Peru's hyperarid coastal desert last summer and hopes to return again over the next two years to analyze a series of flood deposits that date to 2,500 years ago. "There are few places in the world where a rich history of complex societies coincides with an area so greatly affected by El Niño," he explained. In fact, the term itself originated among the fishers of Peru, who noticed that a characteristic pool of warm water arrived on their coast from the western Pacific around Christmastime and named it after the Christ Child.

The coast of Peru between the ocean and the Andes Mountains, averaging about 16 kilometers (10 miles) wide, is a cold fog desert. The temperature of the ocean is so cold that the winds coming off it do not pick up enough moisture to produce rain. Rainfall on the coastal desert averages less than 5 centimeters (2 inches) a year, and in some regions no measurable precipitation is recorded in decades.

But during extreme El Niño years, rainfall can be devastating. At Trujillo, 750 km (400 mi) north of Lima, between 1918 and 1925, total rainfall was only 3.5 cm (1.4 in). In March 1925 the region was deluged with 39.5 cm (15.5 in), ravaging the area. Many regions of the country's coast are experiencing those conditions again this year.

Huckleberry hopes to conduct his excavations at a site called Quebrada de Los Chinos in the Moche River Valley, located near Trujillo. Prehistoric farmers tapped the Moche at higher elevations and diverted the water through canals to sections within dry washes (quebradas), where they planted corn, cotton, and other crops. The hyperarid climate made these canals largely immune from floods except during El Niño events, when heavy rains washed the land below 1,000 feet (300 meters) elevation. When those occurred, the debris would collect in areas where the flood waters lost their velocity.

The most promising location that Huckleberry found during his reconnaissance last summer is a 100-meter-long streamcut at the mouth of Quebrada de Los Chinos. Exposed is a 12-foot-high (4-meter) bank of sand, with periodic thin layers of plant residue from irrigated plots. Intermixed with the organic residue are charcoal and cultural materials, primarily pieces of ceramic pottery which archaeologists have documented in detail according to evolving civilizations developing there. Radiocarbon techniques date the age of the charcoal.

Preliminary analysis indicates 11 distinct floods associated with El Niño events over a period of 2,500 years, Huckleberry said. A detailed analysis could provide insight into the frequency of flooding by centuries or even decades. That knowledge could represent a small piece of the enormously complex puzzle of global climate change and perhaps explain how increasing emissions of greenhouse gases will affect the frequency and magnitude of El Niño.

California Institute of Technology
Atmospheric nitrous oxide sources largely biological

Nitrous oxide (N2O) is an atmospheric gas known to contribute both to global warming and to ozone depletion. New research suggests that its changing concentration in the atmosphere is largely a result of biological activity.

N2O is of increasing concern because its concentration in the atmosphere has been rising for several decades. It is created in the decay of organic material, principally plants, but is also generated in the manufacture of nylon. Scientists have known for years that it enters the nitrogen cycle, but the ultimate sources and sinks of the gas have been unclear.

"Nitrous oxide is less important as a greenhouse molecule than carbon dioxide, and slightly less important than methane," says Yuk Yung (California Institute of Technology). "But the concentrations have been increasing since good measurements began 20 years ago, and ice core samples in Greenland and Antarctica suggest that it has been increasing since the Industrial Revolution began."

Yung and Charles Miller (now at Haverford College) described their work on isotopes of N2O in Science last December. They specifically looked at isotopes of nitrous oxide once it enters the stratosphere. A careful analysis of isotopic variations is an effective way of tracing substances to their sources. If a nitrogen-based fertilizer has a known isotopic makeup and that same percentage is found in the stratosphere, for example, then it can be concluded that agricultural fertilization is a contributor.

Yung and Miller examined theoretically how isotopes of nitrous oxide interact with ultraviolet light energy. They predicted that, as N2O is destroyed by light, heavier isotopes survive preferentially because molecules comprising slightly heavier isotopes require a bit more energy for the atoms to separate.

From their theory and related atmospheric measurements presented in the same issue by researchers at the Scripps Institution of Oceanography and the University of California at San Diego, Yung and Miller concluded that new chemical sources do not need to be introduced to account for the isotopic concentrations that are indeed observed in the stratosphere. Sources such as the decay of plant life and the burning of rainforests and other biomass burning can account for the signatures that are seen.

"I think the most reasonable explanation for the increase is that we are accelerating biological activity globally," Yung says. "Because of global warming, the use of agricultural fertilizers, and nitrogen made from pollution that acts just like a fertilizer, the biosphere has been stimulated. This fosters the growth-decay cycle which leads to N2O release."

University of Washington
Dissolved organic matter in oceans may mitigate greenhouse effect

Vast amounts of dissolved organic matter in the ocean, once thought to be inert, may play a surprising role in mitigating the greenhouse effect, according to bioengineering researchers at the University of Washington.

Tiny chains, or polymers, of carbon-based molecules compose a significant portion of all the organic material in the oceans but were thought to be too small to register on the marine food chain. Instead, they appear to be spontaneously assembling into molecular networks called polymer gels that would give them a vital role in the carbon cycle, the UW researchers reported in Nature last month. These microgels would provide an unexpected mechanism for dissolved organic matter either to enter the carbon cycle or to abandon it and ultimately remove carbon dioxide from the atmosphere.

"Understanding processes that can potentially affect the carbon cycle is critical to understanding global warming," explained bioengineering professor Pedro Verdugo, who coauthored the Nature report along with graduate student Wei-Chun Chin and postdoctoral fellow Monica Orellana. "Gigantic amounts of CO2 are being produced through burning of fossil fuels and we should be experiencing a tremendous greenhouse effect, but some of the CO2 we're producing remains unaccounted for. Our research raises the possibility that dissolved organic matter in the oceans might be playing an unforeseen role in removing CO2 from the atmosphere."

Microscopic algae in the oceans are among the planet's most important engines of photosynthesis. During summer blooms in some coastal areas and in the polar regions, microalgae can produce jellylike layers composed mostly of carbon polysugars that cover several square miles. These marine photosynthesized compounds, together with organic material discharged by rivers, get dispersed in the ocean into organic particles of various sizes. Larger polymer clusters, called particulate organic matter, can be colonized by bacteria and reenter the food chain. But the smaller polymers of dissolved organic matter have long been considered minor players in the carbon cycle.

"It hasn't been clear what is happening to these dissolved organic matter polymers," Verdugo said. "What we have shown is that they can reassemble as microgels and can reenter the carbon cycle or abandon it. We're not sure yet how much this is happening in the oceans, but chances are it is happening quite a lot and is a very important part of the carbon cycle."

In research funded by NSF's Office of Polar Programs, Verdugo's team analyzed dissolved organic polymers in filtered sea water from Puget Sound, the Arctic Ocean, and the North Pacific Ocean. Using dynamic laser scattering spectroscopy and flow cytometry, the researchers observed the polymers spontaneously assembling into tiny hydrogels.

University of Washington,
University of California at Berkeley

Researchers question the source of high-energy X-rays from the upper atmosphere

X-ray detectors on a balloon sent aloft by scientists over Kiruna, Sweden, last year recorded a mysterious series of bursts of X-rays, coming not from outer space but from the earth's atmosphere itself. The X-rays occurred in seven bursts, each separated by only a few minutes and lasting for a total of half an hour.

Although there are high-energy X-rays located in the magnetosphere, the intense magnetic field that surrounds the earth, "they don't usually enter the earth's atmosphere, and certainly not in big bursts like this," said Kirsten Lorentzen (University of Washington), one of three graduate students who made the discovery.

"The source [of the bursts] is simply not known," said Lorentzen, who, with codiscoverer Robin Millan (University of California at Berkeley), presented posters on the findings at the December meeting of the American Geophysical Union. The third team member is Jason Foat, also of UC Berkeley. The three made their discovery during an international campaign organized by the Université Paul Sabatier, Toulouse, France, to study the aurora from stratospheric balloons in conjunction with satellite measurements.

The aurora is visible more than 100 kilometers (60 miles) above the earth, in a region where electrons collide with atmospheric particles. The electrons come from near space around the earth and travel along the planet's magnetic field lines. Besides causing the aurora, the electrons also create a type of radiation known as bremsstrahlung, or X-rays. This radiation cannot penetrate the thickest layers of the earth's lower atmosphere, but can be observed from balloons at a height of about 32 km (20 mi).

What is new about this discovery, said Lorentzen, is that the X-rays were recorded during the day and there was no auroral activity overhead. Although high-energy X-rays have been observed in astrophysics before, she said, this is the first time that X-rays of such intense energy--mega-electron-volts--have been detected emerging from around the earth.

Lorentzen noted that it is known from satellite observations that high-energy electrons become trapped in the Van Allen radiation belt surrounding the earth, but it is not known how they could penetrate the planet's atmosphere to produce the type of energy bursts recorded.


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