Water table depth tied to droughts

Buried machinery in a barn lot; Dallas, South Dakota, May 1936.
Buried machinery in a barn lot; Dallas, South Dakota, May 1936.

Will there be another “dust bowl” in the Great Plains similar to the one that swept the region in the 1930s?

It depends on water storage underground. Groundwater depth has a significant effect on whether the Great Plains will have a drought or bountiful year.

Recent modeling results show that the depth of the water table, which results from lateral water flow at the surface and subsurface, determines the relative susceptibility of regions to changes in temperature and precipitation.

“Groundwater is critical to understand the processes of recharge and drought in a changing climate,” said Reed Maxwell, an atmospheric scientist at Lawrence Livermore National Laboratory, who along with a colleague at Bonn University analyzed the models that appear in the Sept. 28 edition of the journal Nature Geoscience.

Maxwell and Stefan Kollet studied the response of a watershed in the southern Great Plains in Oklahoma using a groundwater/surface-water/land-surface model.

The southern Great Plains are an important agricultural region that has experienced severe droughts during the past century including the “dust bowl” of the 1930s. This area is characterized by little winter snowpack, rolling terrain and seasonal precipitation.

While the onset of droughts in the region may depend on sea surface temperature, the length and depth of major droughts appear to depend on soil moisture conditions and land-atmosphere interactions.

That’s what the recent study takes into account. Maxwell and Kollet created three future climate simulations based on the observed meteorological conditions from 1999. All included an increase in air temperature of 2.5 degrees Celsius. One had no change in precipitation; one had an increase in precipitation by 20 percent; and one had a decrease in precipitation by 20 percent.

“These disturbances were meant to represent the variability and uncertainty in regional changes to central North America under global model simulations of future climate,” Maxwell said.

The models showed that groundwater storage acts as a moderator of watershed response and climate feedbacks. In areas with a shallow water table, changes in land conditions, such as how wet or dry the soil is and how much water is available for plant function, are related to an increase in atmospheric temperatures. In areas with deep water tables, changes at the land surface are directly related to amount of precipitation and plant type.

But in the critical zone, identified here between two and five meter’s depth, there is a very strong correlation between the water table depth and the land surface.

“These findings also have strong implications for drought and show a strong dependence on areas of convergent flow and water table depth,” Maxwell said. “The role of lateral subsurface flow should not be ignored in climate-change simulations and drought analysis.”

Lava flows reveal clues to magnetic field reversals

Ancient lava flows are guiding a better understanding of what generates and controls the Earth’s magnetic field – and what may drive it to occasionally reverse direction.

The main magnetic field, generated by turbulent currents within the deep mass of molten iron of the Earth’s outer core, periodically flips its direction, such that a compass needle would point south rather than north. Such polarity reversals have occurred hundreds of times at irregular intervals throughout the planet’s history – most recently about 780,000 years ago – but scientists are still trying to understand how and why.

A new study of ancient volcanic rocks, reported in the Sept. 26 issue of the journal Science, shows that a second magnetic field source may help determine how and whether the main field reverses direction. This second field, which may originate in the shallow core just below the rocky mantle layer of the Earth, becomes important when the main north-south field weakens, as it does prior to reversing, says Brad Singer, a geology professor at the University of Wisconsin-Madison.

Singer teamed up with paleomagnetist Kenneth Hoffman, who has been researching field reversals for over 30 years, to analyze ancient lava flows from Tahiti and western Germany in order to study past patterns of the Earth’s magnetic field. The magnetism of iron-rich minerals in molten lava orients along the prevailing field, then becomes locked into place as the lava cools and hardens.

“When the lava flows erupt and cool in the Earth’s magnetic field, they acquire a memory of the magnetic field at that time,” says Singer. “It’s very difficult to destroy that in a lava flow once it’s formed. You then have a recording of what the paleofield direction was like on Earth.”

Hoffman, of both California Polytechnic State University at San Luis Obispo and UW-Madison, and Singer are focusing on rocks that contain evidence of times that the main north-south field has weakened, which is one sign that the polarity may flip direction. By carefully determining the ages of these lava flows, they have mapped out the shallow core field during multiple “reversal attempts” when the main field has weakened during the past million years.

During those periods of time, weakening of the main field reveals “virtual poles,” regions of strong magnetism within the shallow core field. For example, Singer says, “If you were on Tahiti when those eruptions were taking place, your compass needle would point to not the North Pole, not the South Pole, but Australia.”

The scientists believe the shallow core field may play a role in determining whether the main field polarity flips while weakened or whether it recovers its strength without reversing. “Mapping this field during transitional states may hold the key to understanding what happens in Earth’s core when the field weakens to a point where it can actually reverse,” Hoffman says.

Current evidence suggests we are now approaching one of these transitional states because the main magnetic field is relatively weak and rapidly decreasing, he says. While the last polarity reversal occurred several hundred thousand years ago, the next might come within only a few thousand years.

“Right now, historic records show that the strength of the magnetic field is declining very rapidly. From a quick back-of-the-envelope prediction, in 1,500 years the field will be as weak as it’s ever been and we could go into a state of polarity reversal,” says Singer. “One broad goal of our research is to provide some predictive capability for what could happen and what could be the signs of the next reversal.”

Hoffman and Singer’s research has been supported largely by grants from the National Science Foundation.

World’s Largest Tsunami Debris

Giant boulder was debris of ancient tsunami
Giant boulder was debris of ancient tsunami

A line of massive boulders on the western shore of Tonga may be evidence of the most powerful volcano-triggered tsunami found to date. Up to 9 meters (30 feet) high and weighing up to 1.6 million kilograms (3.5 million pounds), the seven coral boulders are located 100 to 400 meters (300 to 1,300 feet) from the coast. The house-sized boulders were likely flung ashore by a wave rivaling the 1883 Krakatau tsunami, which is estimated to have towered 35 meters (115 feet) high.

“These could be the largest boulders displaced by a tsunami, worldwide,” says Matthew Hornbach of the University of Texas Institute for Geophysics. “Krakatau’s tsunami was probably not a one-off event.” Hornbach and his colleagues will discuss these findings on Sunday, 5 October 2008, at the Joint Annual Meeting of the Geological Society of America (GSA), Soil Science Society of America (SSSA), American Society of Agronomy (ASA), Crop Science Society of America (CSSA), and the Gulf Coast Association of Geological Societies (GCAGS), in Houston, Texas, USA.

Called erratic boulders, these giant coral rocks did not form at their present location on Tongatapu, Tonga’s main island. Because the island is flat, the boulders could not have rolled downhill from elsewhere. The boulders are made of the same reef material found just offshore, which is quite distinct from the island’s volcanic soil. In fact, satellite photos show a clear break in the reef opposite one of the biggest boulders. And some of the boulders’ coral animals are oriented upside down or sideways instead of toward the sun, as they are on the reef.

Hornbach says the Tongatapu boulders may have reached dry land within the past few thousand years. Though their corals formed roughly 122,000 years ago, they are capped by a sparse layer of soil. And the thick volcanic soils that cover most of western Tongatapu are quite thin near the boulders. This suggests the area was scoured clean by waves in the recent past. Finally, there is no limestone pedestal at the base of the boulders, which should have formed as rain dissolved the coral if the boulders were much older.

Many tsunamis, like the one that struck the Indian Ocean in 2004, are caused by earthquakes. But the boulders’ location makes an underwater eruption or submarine slide a more likely culprit. A chain of sunken volcanoes lies just 30 kilometers (20 miles) west of Tongatapu. An explosion or the collapse of the side of a volcano such as that seen at the famous Krakatau eruption in 1883 could trigger a tremendous tsunami.

Another possibility is that a storm surge could have brought the boulders ashore. But that scenario isn’t likely. No storms on record have moved rocks this big. Another possibility is that a monster undersea landslide caused the tsunami. But Hornbach’s analyses of adjacent seafloor topography point to a volcanic flank collapse as the most probable source of such a wave.

“We think studying erratic boulders is one way of getting better statistics on mega-tsunamis,” Hornbach says. “There are a lot of places that have similar underwater volcanoes and people haven’t paid much attention to the threat.” The researchers have already received reports of more erratic boulders from islands around the Pacific. Future study could indicate how frequently these monster waves occur and which areas are at risk for future tsunamis.

The boulders are such an unusual part of the Tongan landscape that tales of their origins appear in local folklore. According to one legend, the god Maui hurled the boulders ashore in an attempt to kill a giant man-eating fowl.

And though many other Pacific islanders follow the custom of heading uphill after earthquakes, Tongans have no such teachings. Such lore may be useless for near-shore volcanically-generated tsunamis, which arrive too quickly for people to evacuate. Instead, most of Tongatapu’s settlements are huddled together on the northern side of the island-away from the brunt of the tsunami threat.

Researchers find oldest rocks on Earth

Discovery of rocks as old as 4.28 billion years pushes back age of most ancient remnant of Earth’s crust by 300 million years

McGill University researchers have discovered the oldest rocks on Earth – a discovery which sheds more light on our planet’s mysterious beginnings. These rocks, known as “faux-amphibolites”, may be remnants of a portion of Earth’s primordial crust – the first crust that formed at the surface of our planet. The ancient rocks were found in Northern Quebec, along the Hudson’s Bay coast, 40 km south of Inukjuak in an area known as the Nuvvuagittuq greenstone belt. Their results will be published in the September 26 issue of the journal Science.

The discovery was made by Jonathan O’Neil, a Ph.D. candidate at McGill’s Department of Earth and Planetary Sciences, Richard W. Carlson, a researcher at the Carnegie Institution for Science in Washington, D.C., Don Francis, a McGill professor in the Department of Earth and Planetary Sciences, and Ross K. Stevenson, a professor at the Université du Québec à Montréal (UQAM).

O’Neil and colleagues estimated the age of the rocks using isotopic dating, which analyzes the decay of the radioactive element neodymium-142 contained within them. This technique can only be used to date rocks roughly 4.1 billion years old or older; this is the first time it has ever been used to date terrestrial rocks, because nothing this old has ever been discovered before.

The data from these findings will give researchers a new window on the early separation of Earth’s mantle from the crust in the Hadean Era, said O’Neil.

“Our discovery not only opens the door to further unlock the secrets of the Earth’s beginnings,” he continued. “Geologists now have a new playground to explore how and when life began, what the atmosphere may have looked like, and when the first continent formed.”

Stalagmites May Predict Next Big One along the New Madrid Seismic Zone

Small white stalagmites lining caves in the Midwest may help scientists chronicle the history of the New Madrid Seismic Zone – and even predict when the next big earthquake may strike
Small white stalagmites lining caves in the Midwest may help scientists chronicle the history of the New Madrid Seismic Zone – and even predict when the next big earthquake may strike

Small white stalagmites lining caves in the Midwest may help scientists chronicle the history of the New Madrid Seismic Zone (NMSZ) – and even predict when the next big earthquake may strike, say researchers at the Illinois State Geological Survey and the University of Illinois at Urbana-Champaign.

While the 1811-12, magnitude 8 New Madrid earthquakes altered the course of the Mississippi River and rung church bells in major cities along the East Coast, records of the seismic zone’s previous movements are scarce. Thick layers of sediment have buried the trace of the NMSZ and scientists must search for rare sand blows and liquefaction features, small mounds of liquefied sand that squirt to the surface through fractures during earthquakes, to record past events. That’s where the stalagmites come in.

The sand blows are few and far between, said Keith Hackley, an isotope geochemist with the Illinois State Geological Survey. In contrast, caves throughout the region are lined with abundant stalagmites, which could provide a better record of past quakes. “We’re trying to see if the initiation of these stalagmites might be fault-induced, recording very large earthquakes that have occurred along the NMSZ,” he said.

Hackley and co-workers used U-Th dating techniques to determine the age of stalagmites from Illinois Caverns and Fogelpole Cave in southwestern Illinois. They discovered that some of the young stalagmites began to form at the time of the 1811-12 earthquakes.

Hackley is scheduled to present preliminary results of the study in a poster on Sunday, 5 October, at the 2008 Joint Meeting of the Geological Society of America (GSA), Soil Science Society of America (SSSA), American Society of Agronomy (ASA), Crop Science Society of America (CSSA), and Gulf Coast Association of Geological Societies (GCAGS), in Houston, Texas, USA.

Water slowly trickles through crevices in the ceiling of a cave and drips onto the floor. Each calcium carbonate-loaded drip falls on the last, and a stalagmite slowly grows from the bottom up. Time is typically recorded in alternating light and dark layers – each pair represents a year.

When a large earthquake shakes the ground, old cracks may seal and new ones open. As a result, some groundwater seeping through the cave ceiling traces a new pattern of drips – and, eventually, stalagmites – on the cave floor. Thus it is possible that each new generation of stalagmites records the latest earthquake.

The scientists use fine drills, much like those used by dentists, to burrow into the stalagmites to collect material for dating. In addition to the 1811-12 earthquakes, their investigation has recorded seven historic earthquakes dating as far back as almost 18,000 years before the present. Understanding the NMSZ’s past, including whether quakes recur with any regularity, will help the scientists predict the potential timing of future quakes.

In coming months, Hackley and his colleagues plan to expand the study, collecting stalagmites from caves across Indiana, Missouri and Kentucky. They hope that the new data will help to fill in more of the missing history of the NMSZ.

View abstract, paper 147-8, at “Paleo-Seismic Activity from the New Madrid Seismic Zone Recorded in Stalagmites. A New Tool for Paleo-Seismic History

Climate change, human activity and wildfires

2000-year fire track
2000-year fire track

Study of last 2,000 years of charcoal evidence suggests human impacts have curtailed fires in most areas

Climate has been implicated by a new study as a major driver of wildfires in the last 2,000 years. But human activities, such as land clearance and fire suppression during the industrial era (since 1750) have created large swings in burning, first increasing fires until the late 1800s, and then dramatically reducing burning in the 20th century.

The study by a nine-member team from seven institutions — led by Jennifer R. Marlon, a doctoral student in geography department at the University of Oregon — appears online ahead of regular publication in the journal Nature Geoscience. The team analyzed 406 sedimentary charcoal records from lake beds on six continents.

A 100-year decline in wildfires worldwide — from 1870 to 1970 — was recorded despite increasing temperatures and population growth, researchers found. “Based on the charcoal record,” Marlon said, “we believe the reduction in the amount of biomass burned during those 100 years can be attributed to a global expansion of agriculture and intensive grazing of livestock that reduced fuels plus general landscape fragmentation and fire-management efforts.”

Observations of increased burning associated with global warming and fuel build-up during the past 30 years, however, are not yet included in the sediment record.

Charcoal levels have drawn attention during the past 25 years because these data can track wildfire activity — both incidence and severity — over long time periods, providing information when similar data from satellites or fire-scarred trees do not exist. This study is among early efforts to analyze charcoal records for widespread patterns and trends over such a long period.

The importance of the data presented by Marlon’s team is put into perspective of overall information about the history of wildfires in a “News & Views” article, also appearing online, written by Andrew C. Scott, an earth sciences researcher at the University of London.

During the last 2,000 years, fire activity was highest between 1750 and 1870. “This was a period when several factors combined to generate conditions favorable to wildfires,” Marlon said. “Population growth and European colonization caused massive changes in land cover, and human-induced increases in atmospheric carbon dioxide concentrations may have started to increase biomass levels and fuels.”

From A.D. 1 to about 1750, wildfires worldwide declined from earlier years, probably resulting from a long-term global cooling trend that offset any possible influence of population growth and related land-use changes. Researchers pointed to charcoal evidence in western North America as an example of this trend. Similar records also were found in Central America and tropical areas of South America. In the western U.S. and in Asia, researchers noted, “initial colonization may have been marked by an increased use of fire for land clearance.”

Subsequently, expansion of intensive agriculture and grazing, as well as forest management activities, likely reduced wildfire activity, Marlon said. “Our results strongly suggest that climate change has been the main driver of global biomass burning for the past two millennia,” the researchers concluded. “The decline in biomass burning after A.D. 1870 is opposite to the expected effect of rising carbon dioxide and rapid warming, but contemporaneous with an unprecedentedly high rate of population increase.”

The eight co-authors with Marlon were: Patrick J. Bartlein and Daniel G. Gavin, professors in the geography department and members of the Environmental Change Research Group; C. Carcaillet of the Centre for Bio-Archaeology and Ecology in Montpellier, France; S.P. Harrison and I.C. Prentice, both of the University of Bristol in the United Kingdom; P.E. Higuera of Montana State University in Bozeman, Mont.; F. Joos of the Physics Institute and Oeschger Centre for Climate Change Research in Bern, Switzerland; and Mitchell .J. Power of the University of Utah in Salt Lake City and a curator at the Utah Museum of Natural History.

The National Science Foundation in the United States and Natural Environment Research Council in the United Kingdom funded the research.

Ocean floor geysers warm flowing sea water

An international team of earth scientists report movement of warmed sea water through the flat, Pacific Ocean floor off Costa Rica. The movement is greater than that off midocean volcanic ridges. The finding suggests possible marine life in a part of the ocean once considered barren.

With about 71 percent of the Earth’s surface being ocean, much remains unknown about what is under the sea, its geology, and the life it supports. A new finding reported by American, Canadian and German earth scientists suggests a rather unremarkable area off the Costa Rican Pacific coast holds clues to better understand sea floor ecosystems.

Carol Stein, professor of earth and environmental sciences at the University of Illinois at Chicago, is a member of the research team that has studied the region, located between 50 and 150 miles offshore and covering an area the size of Connecticut. The sea floor, some two miles below, is marked by a collection of about 10 widely separated outcrops or mounts, rising from sediment covering crust made of extinct volcanic rock some 20-25 million years old.

Stein and her colleagues found that seawater on this cold ocean floor is flowing through cracks and crevices faster and in greater quantity than what is typically found at mid-ocean ridges formed by rising lava. Water temperatures, while not as hot as by the ridge lava outcrops, are surprisingly warm as well.

Finding so much movement in a bland area of the ocean was surprising.

“It’s like finding Old Faithful in Illinois,” said Stein. “When we went out to try to get a feel for how much heat was coming from the ocean floor and how much sea water might be moving through it, we found that there was much more heat than we expected at the outcrops.”

The water gushing from sea floor protrusions warms as it moves through the insulated volcanic rock and picks up heat.

“It’s relatively warm and may have some of the nutrients needed to support some of the life forms we see on the sea floor,” said Stein. Her best guess as to why the water flows so rapidly is that it accelerates off nearby sea mounts and follows a well-connected network of cracks beneath the sea floor.

The earth scientists dropped probes from ships down to the pitch-dark ocean floor to collect temperature and heat-flow data to form images of what is happening in this area of the ocean, with water flowing down into rock, heating up and remixing below the floor sediment, and then escaping above the sea floor.

Only in recent decades have earth scientists discovered such life forms as bacteria, clams and tubeworm species living near the hot water discharges along the mid-ocean volcanic ridges. The rather flat undersea areas which Stein and her colleagues studied were thought to be lifeless, but the nutrient-enhanced warm water flows they discovered suggests this area too may be capable of supporting life.

“The sea floor may not be quite as much of a desert even as we thought maybe 20 or 10 years ago, but rather there may be a lot of locations similar to this well-studied area in terms of the water flow where there’s a lot more biological activity,” she said.

The earth scientists hope to do follow-up studies to add details to their findings, and see if they can find other regions comparable to the one off Costa Rica.

“We’re only beginning to really understand the interplay of the water flow and the nature of the ecosystem on the sea floor,” said Stein. “I think as we move away from the ridge crests, understand what’s going in the overall ocean, we’ll have a better understanding of how life is distributed and affects the oceans and our planet.”

The findings were reported in a letter printed in Nature Geoscience’s September 2008 issue. Other key authors of the letter include Andrew Fisher of the University of California, Santa Cruz, and Robert Harris of Oregon State University. The lead author is Michael Hutnak, now with the U.S. Geological Survey.

Arctic sea ice settles at second-lowest, underscores accelerating decline

Daily Arctic sea ice extent for September 12, 2008, was 4.52 million square kilometers (1.74 million square miles). The orange line shows the 1979 to 2000 average extent for that day. The black cross indicates the geographic North Pole. Sea Ice Index data. - Credit: National Snow and Ice Data Center
Daily Arctic sea ice extent for September 12, 2008, was 4.52 million square kilometers (1.74 million square miles). The orange line shows the 1979 to 2000 average extent for that day. The black cross indicates the geographic North Pole. Sea Ice Index data. – Credit: National Snow and Ice Data Center

The Arctic sea ice cover appears to have reached its minimum extent for the year, the second-lowest extent recorded since the dawn of the satellite era. While above the record minimum set on September 16, 2007, this year further reinforces the strong negative trend in summertime ice extent observed over the past thirty years. With the minimum behind us, we will continue to analyze ice conditions as we head into the crucial period of the ice growth season during the months to come.

Overview of conditions

On September 12, 2008 sea ice extent dropped to 4.52 million square kilometers (1.74 million square miles). This appears to have been the lowest point of the year, as sea has now begun its annual cycle of growth in response to autumn cooling.

The 2008 minimum is the second-lowest recorded since 1979, and is 2.24 million square kilometers (0.86 million square miles) below the 1979 to 2000 average minimum.

Conditions in context

Despite overall cooler summer temperatures, the 2008 minimum extent is only 390,000 square kilometers (150,000 square miles), or 9.4%, more than the record-setting 2007 minimum. The 2008 minimum extent is 15.0% less than the next-lowest minimum extent set in 2005 and 33.1% less than the average minimum extent from 1979 to 2000.

This season further reinforces the long-term downward trend of sea ice extent.

Overlay of 2007 and 2008 at September minimum

The spatial pattern of the 2008 minimum extent was different than that of 2007. This year did not have the substantial ice loss in the central Arctic, north of the Chukchi and East Siberian Seas. However, 2008 showed greater loss in the Beaufort, Laptev, and Greenland Seas.

Unlike last year, this year saw the opening of the Northern Sea Route, the passage through the Arctic Ocean along the coast of Siberia. However, while the shallow Amundsen’s Northwest Passage opened in both years, the deeper Parry’s Channel of the Northwest Passage did not quite open in 2008.

A word of caution on calling the minimum

Determining with certainty when the minimum has occurred is difficult until the melt season has decisively ended. For example, in 2005, the time series began to level out in early September, prompting speculation that we had reached the minimum. However, the sea ice contracted later in the season, again reducing sea ice extent and causing a further drop in the absolute minimum.

We mention this now because the natural variability of the climate system has frequently been known to trick human efforts at forecasting the future. It is still possible that ice extent could fall again, slightly, because of either further melting or a contraction in the area of the pack due to the motion of the ice. However, we have now seen five days of gains in extent. Because of the variability of sea ice at this time of year, the National Snow and Ice Data Center determines the minimum using a five-day running mean value.

Ongoing analysis continues

We will continue to post analysis of sea ice conditions throughout the year, with frequency determined by sea ice conditions. Near-real-time images at upper right will continue to be updated every day.

In addition, NSIDC will issue a formal press release at the beginning of October with full analysis of the possible causes behind this year’s low ice conditions, particularly interesting aspects of the melt season, the set-up going into the important winter growth season ahead, and graphics comparing this year to the long-term record. At that time, we will also know what the monthly average September sea ice extent was in 2008-the measure scientists most often rely on for accurate analysis and comparison over the long-term.

Researchers Discover Unexpected Properties of Materials in Lowermost Mantle

Jung-Fu Lin and colleagues used a diamond anvil cell to recreate materials and conditions in Earth's lowermost mantle.
Jung-Fu Lin and colleagues used a diamond anvil cell to recreate materials and conditions in Earth’s lowermost mantle.

Materials deep inside Earth have unexpected atomic properties that might force earth scientists to revise their models of Earth’s internal processes, a team of researchers has discovered.

The researchers recreated in the lab the materials, crushing pressures and infernal temperatures they believe exist in the lowermost mantle, nearly 2,900 kilometers (1,800 miles) below Earth’s surface. They report in the journal Nature Geoscience the materials exhibit rare and unexpected atomic properties that might influence how heat is transferred within Earth’s mantle, how columns of hot rock called superplumes form, and how the magnetic field and heat generated in Earth’s core travel to the planet’s surface.

The planetary building blocks magnesium, silicon, oxygen and iron are the most abundant minerals in the lowermost mantle. A team of scientists led by Jung-Fu Lin at The University of Texas at Austin’s Jackson School of Geosciences synthesized materials from these building blocks in a diamond anvil cell, a device containing two interlocking diamond pieces that squeeze the sample like a vice. They subjected the sample to more than 1.3 million times standard atmospheric pressure. Shining a laser through the transparent diamonds, they then heated the sample to almost 3,000 degrees Celsius (5,400 degrees Fahrenheit) for several days.

The scientists used the nation’s most powerful source of X-rays, a facility at Argonne National Laboratory called a synchrotron light source, to reveal the sample’s electronic and atomic structure. They determined the high pressures had caused some of the electrons in the sample’s iron, which normally repel each other, to “pair up” or become bound to each other. Earlier experiments by Lin and others had found evidence for areas in the lower mantle in which electrons were either mostly paired up or were mostly unpaired. This was the first evidence of a broad region in the subsurface with what scientists describe as “intermediate-spin state,” or partially paired iron electrons.

“We were surprised to find partially paired electrons,” said Lin. “That doesn’t normally occur in other geological materials that we know about.”

The degree of electron pairing, also known as electronic spin state, can affect how well the materials conduct heat and electricity. Lin said modelers who make computer simulations of mantle dynamics will now have to go back and try to determine how this intermediate-spin state might affect the way heat is transferred within Earth, how superplumes form, how convection occurs in the mantle and how Earth’s magnetic field might radiate from the core.

The electronic spin state can also affect the speed of seismic waves traveling through material in the deep mantle. As a result, seismic images of the lowermost mantle-collected when earthquake vibrations travel through and reflect off of material in the subsurface-may have to be reinterpreted.

Nature Geoscience will publish the paper, “Intermediate-Spin Ferrous Iron in Lowermost Mantle Post-Perovskite and Perovskite,” in its October 2008 edition and online Sept. 14.

Lin’s co-authors include Heather Watson and William J. Evans at Lawrence Livermore National Laboratory; György Vankó at KFKI Research Institute for Particle and Nuclear Physics in Budapest, Hungary; Esen E. Alp and Jiyong Zhao at Argonne National Laboratory; Vitali B. Prakapenka, Przemek Dera and Atsushi Kubo at the University of Chicago; Viktor V. Struzhkin at Carnegie Institution of Washington; and Catherine McCammon at Universität Bayreuth in Germany.

Small glaciers – not large – account for most of Greenland’s recent loss of ice, study shows

The recent dramatic melting and breakup of a few huge Greenland glaciers have fueled public concerns over the impact of global climate change, but that isn’t the island’s biggest problem.

A new study shows that the dozens of much smaller outflow glaciers dotting Greenland’s coast together account for three times more loss from the island’s ice sheet than the amount coming from their huge relatives.

In a study just published in the journal Geophysical Research Letters, scientists at Ohio State University reported that nearly 75 percent of the loss of Greenland ice can be traced back to small coastal glaciers.

Ian Howat, an assistant professor of earth sciences and researcher with Ohio State’s Byrd Polar Research Center, said their discovery came through combining the best from two remote sensing techniques. It provides perhaps the best estimate so far of the loss to Greenland’s ice cap, he says.

Aside from Antarctica, Greenland has more ice than anywhere else on earth. The ice cap covers four-fifths of the island’s surface, is 1,491 miles (2,400 kilometers) long and 683 miles (1,100 kilometers) wide, and can reach 1.8 miles (3 kilometers) deep at its thickest point.

As global temperatures rise, coastal glaciers flow more quickly to the sea, with massive chunks breaking off at the margins and forming icebergs. And while some of the largest Greenland glaciers – such as the Jakobshavn and Petermann glaciers on the northern coast – are being closely monitored, most others are not.

Howat and his colleagues concentrated on the southeastern region of Greenland, an area covering about one-fifth of the island’s 656,373 square miles (1.7 million square kilometers). They found that while two of the largest glaciers in that area – Kangerdlugssuaq and Helheim – contribute more to the total ice loss than any other single glaciers, the 30 or so smaller glaciers there contributed 72 percent of the total ice lost.

“We were able to see for the first time that there is widespread thinning at the margin of the Greenland ice sheet throughout this region.

“We’re talking about the region that is within 62 miles (100 kilometers) from the ice edge. That whole area is thinning rapidly,” he said.

Howat says that all of the glaciers are changing within just a few years and that the accelerated loss just spreads up deeper into the ice sheet.

To reach their conclusions, the researchers turned to two ground-observing satellites. One of them, ICESAT (Ice, Cloud, and land Elevation Satellite), does a good job of gauging the ice over vast expanses which were mostly flat.

On the other hand, ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) does a better job at seeing changes at the steeper, less-flat margins of the ice sheet, Howat said.

“We simply merged those data sets to give us for the first time a picture of ice elevation change – the rate at which the ice is either going up or down – at a very high (656-foot or 200-meter) resolution.

“They are a perfect match for each other,” Howat said.

“What we found is the entire strip of ice over the southeast margin, all of these glaciers, accelerated and they are just pulling the entire ice sheet with it,” he said.

Howat said that their results show that such new findings don’t necessarily require new types of satellites. “These aren’t very advanced techniques or satellites. Our work shows that by combining satellite data in the right way, we can get a much better picture of what’s going on,” Howat said.

Along with Howat, B.E. Smith and I Joughin, both of the University of Washington, and T.A. Scambos from the National Snow and Ice Data Center at the University of Colorado worked on the project.