Large Source of Nitrate, a Nutrient and Potential Water Contaminant, Found in Near-Surface Desert Soils





Photo Caption: Desert pavement overlying soil. Image credit: Graham lab, UCR.
Photo Caption: Desert pavement overlying soil. Image credit: Graham lab, UCR.

A UC Riverside-led study in the Mojave Desert, Calif., has found that soils under “desert pavement” have an unusually high concentration of nitrate, a type of salt, close to the surface. Vulnerable to erosion by rain and wind if the desert pavement is disrupted, this vast source of nitrate could contaminate surface and groundwaters, posing an environmental risk.



Study results appear in the March issue of Geology.



Desert pavement is a naturally occurring, single layer of closely fitted rock fragments. A common land surface feature in arid regions, it has been estimated to cover nearly half of North America’s desert landscapes.



Nitrate, a water soluble nitrogen compound, is a nutrient essential to life. It is also, however, a contaminant. When present in excess in aquatic systems, it results in algal blooms. High levels of nitrate in drinking water have been associated with serious health issues, including methaemoglobinaemia (blue baby disease, marked by a reduction in the oxygen-carrying capacity of blood), miscarriages and non-Hodgkin’s lymphoma.



Salts, including nitrate, are formed in deserts as water evaporates on dry lake beds. These salts then get blown on to the desert pavement by winds. Other contributors of nitrate to desert pavement soils are atmospheric deposition (the gradual deposition of nutrient-rich particulate matter from the air), and soil bacteria, which convert atmospheric nitrogen into nitrate that is usable by plants and other organisms.



Ordinarily, in moist soils, plants and microbes readily take up nitrate, and water flushing through the soils leaches the soils of excess nitrate.



But desert pavement, formed over thousands of years, impedes the infiltration of water in desert soil, restricting plant development and resulting in desert pavement soils becoming nitrate-rich (and saltier) with time.


“After water, nitrogen is the most limiting factor in deserts, affecting net productivity in desert ecosystems,” said Robert Graham, a professor of soil mineralogy in the Department of Environmental Sciences and the lead author of the research paper. “The nitrate stored in soils under desert pavement is a previously unrecognized vast pool of nitrogen that is particularly susceptible to climate change and human disturbance. Moister climates, increased irrigation, wastewater disposal, or flooding may transport high nitrate levels to groundwater or surface waters, which is detrimental to water quality.”



In their study, Graham and his colleagues sampled three widely separated locations with well-developed desert pavement in the Mojave Desert. The locations were selected to represent a variety of landforms commonly found in the desert. The researchers found that the nitrate they observed in association with desert pavement was consistent across the landforms.



“Deserts account for about one-third of Earth’s land area,” Graham said. “If our findings in the Mojave can be extrapolated to deserts worldwide, the amount of nitrate – and nitrogen – stored in near-surface soils of warm deserts would need to be re-estimated.”



Graham and his team of researchers found that nitrate concentration in soils under desert pavement in the Mojave reached a maximum (up to 12,750 kilograms per hectare) within 0.1 to 0.6 meter depth. In contrast, at each location they studied, the soils without desert pavement had relatively low nitrate concentrations (80 to 1500 kilograms per hectare) throughout the upper meter. “In these nonpavement locations, water was able to infiltrate the soil and transport the nitrate to deeper within the soil,” Graham explained.



The researchers note in the paper that desert land use – road construction, off-road vehicle use, and military training – often disrupts fragile land surfaces, increasing surface erosion by rain and wind. According to them, nitrogen-laden dust transported by wind from disturbed desert pavement soils may impact distant nitrogen-limited ecosystems, such as alpine lakes.



Furthermore, the researchers note that increased soil moisture resulting from climate change increases the potential for “denitrification” – a naturally-occurring process in soil, where bacteria break down nitrates to return nitrogen gas to the atmosphere. “Denitrification also produces nitrous oxide, a major greenhouse gas,” Graham said.



Next in their research, Graham and his colleagues will examine the spatial distribution of desert pavement throughout the Mojave Desert to explore how different levels of nitrate are associated with different kinds of desert pavement. Together with UCR’s David Parker, a professor of soil chemistry, they will look in the desert also for perchlorate, which may be associated with nitrate.



Graham was joined in the study by Daniel Hirmas, a doctoral candidate in the Department of Environmental Sciences at UCR; Christopher Amrhein, a professor of soil chemistry at UCR; and Yvonne Wood of the University of California Cooperative Extension, Inyo-Mono Counties, Bishop, Calif. The research was funded by the University of California Kearney Foundation of Soil Science.

Studying Rivers for Clues to Carbon Cycle


In the science world, in the media, and recently, in our daily lives, the debate continues over how carbon in the atmosphere is affecting global climate change. Studying just how carbon cycles throughout the Earth is an enormous challenge, but one Northwestern University professor is doing his part by studying one important segment — rivers.



Aaron Packman, associate professor of civil and environmental engineering in Northwestern’s McCormick School of Engineering and Applied Science, is collaborating with ecologists and microbiologists from around the world to study how organic carbon is processed in rivers.



Packman, who specializes in studying how particles and sediment move around in rivers, is co-author of a paper on the topic published online in the journal Nature Geoscience.



The paper evaluates our current understanding of carbon dynamics in rivers and reaches two important conclusions: it argues that carbon processing in rivers is a bigger component of global carbon cycling than people previously thought, and it lays out a framework for how scientists should go about assessing those processes.



Much more is known about carbon cycling in the atmosphere and oceans than in rivers. Evaluating large-scale material cycling in a river provides a challenge — everything is constantly moving, and a lot of it moves in floods. As a result, much of what we know about carbon processing in rivers is based on what flows into the ocean.



“But that’s not really enough,” Packman said. “You miss all this internal cycling.”



In order to understand how carbon cycles around the globe — through the land, freshwater, oceans and atmosphere — scientists need to understand how it moves around, how it’s produced, how it’s retained in different places and how long it stays there.



In rivers, carbon is both transformed and consumed. Microorganisms like algae take carbon out of the atmosphere and incorporate it into their own cells, while bacteria eat dead organic matter and then release CO2 back into the atmosphere.


“It’s been known for a long time that global carbon models don’t really account for all the carbon,” Packman said. “There’s a loss of carbon, and one place that could be occurring is in river systems.” Even though river waters contain a small fraction of the total water on earth, they are such dynamic environments because microorganisms consume and transform carbon at rapid rates.



“We’re evaluating how the structure and transport conditions and the dynamics of rivers create a greater opportunity for microbial processing,” Packman said.



Packman is the first to admit that studying microorganisms, carbon and rivers sounds more like ecology than engineering. But such problems require work from all different areas, he said.



“We’re dealing with such interdisciplinary problems, tough problems, so we have to put fluid mechanics, transport, ecology and microbiology together to find this overall cycling of carbon,” he said. “People might say it’s a natural science paper, but to me it’s a modern engineering paper. To understand what’s going on with these large-scale processes, we have to analyze them quantitatively, and the tools for getting good estimates have been developed in engineering.”



Packman was introduced to the co-authors of the paper — ecologists who study how dead leaves and soil drive stream ecology and who come from as far away as Spain and Austria — about 10 years ago through the activity of the Stroud Water Research Center in Pennsylvania.



Since then, they have collaborated on many similar projects around river structure and transport dynamics. They are currently working on a project funded by the National Science Foundation on the dynamics of organic carbon in rivers and trying to understand how carbon delivered from upstream areas influence the ecology of downstream locations.



“The broadest idea is really part of global change efforts to understand carbon cycling over the whole Earth, which is an enormous challenge,” Packman said.



The lead author of the Nature Geoscience paper is Tom Battin of the Department of Freshwater Ecology at the University of Vienna in Austria. Other authors are Louis A. Kaplan, Stuart Findlay, Charles S. Hopkinson, Eugenia Marti, J. Denis Newbold and Francesc Sabater.

Cold conspirators: Ice crystals implicated in Arctic pollution





Field of frost flowers
Field of frost flowers

Frost flowers. Diamond dust. Hoarfrost. These poetically named ice crystal forms are part of the stark beauty of the Arctic. But they also play a role in its pollution, a new study by scientists at the University of Michigan, the Cold Regions Research & Engineering Laboratory and the University of Alaska has found.



After collecting and analyzing hundreds of samples from the Alaskan Arctic, the researchers determined that ice crystals that form from vapor clouds billowing up from cracks in sea ice help concentrate mercury from the atmosphere, and that certain types of crystals are more efficient than others. Their results appear in the cover article for the March 1 issue of Environmental Science & Technology.



“Previous measurements had shown that in polar springtime, the normally steady levels of mercury in the atmosphere drop to near zero, and scientists studying this atmospheric phenomenon had analyzed a few snow samples and found very high levels of mercury,” said Joel Blum, the John D. MacArthur Professor of Geological Sciences at U-M. “We wanted to understand what’s controlling this mercury deposition, where it’s occurring and whether mercury concentrations are related to the type and formation of snow and ice crystals.”



Mercury is a naturally occurring element, but some 150 tons of it enter the environment each year from human-generated sources in the United States, such as incinerators, chlorine-producing plants and coal-fired power plants. Precipitation is a major pathway through which mercury and other pollutants travel from the atmosphere to land and water, said lead author Thomas Douglas of the Cold Regions Research & Engineering Laboratory in Fort Wainwright, Alaska.



“Alaska receives air masses originating in Asia, and with China adding a new coal-fired power plant almost every week, it’s not surprising that we find significant amounts of mercury there,” Douglas said. “The concentrations we measured in some snow are far greater than would be found right next to a waste incinerator or power plant in an industrialized location.”



Once mercury from the atmosphere is deposited onto land or into water, micro-organisms convert some of it to methylmercury, a highly toxic form that builds up in fish and the animals that eat them. In wildlife, exposure to methylmercury can interfere with reproduction, growth, development and behavior and may even cause death. Effects on humans include damage to the central nervous system, heart and immune system. The developing brains of young and unborn children are especially vulnerable.



Douglas, Blum and co-workers discovered that certain types of ice crystals-frost flowers and rime ice-contained the highest concentrations of mercury. Because both types of crystal grow directly by water vapor accretion, the scientists reasoned that breaks in the sea ice, where water vapor rises in great clouds, contribute to Arctic mercury deposition.



“The vapor that rises through these openings in the ice brings with it bromine from the sea water. That gets into the atmosphere, where sunlight plus the bromine cause a catalytic reaction which converts mercury gas into a reactive form. If any ice crystals are present, the mercury sticks to them and comes out of the atmosphere,” Blum said.





Close-up of frost flowers
Close-up of frost flowers

The greater the surface area of the crystals, the more mercury they grab, which explains why frost flowers and rime ice, both delicate formations with high surface areas, end up with so much mercury. The mercury-tainted crystals aren’t, however, confined to the edges of breaks in the ice, the researchers determined. Bromine can travel great distances, resulting in mercury deposition in snow throughout the Arctic coastal region.



Collecting the samples was an undertaking that required a spirit of adventure as well as scientific savvy.



“It’s kind of a scary place to work,” Blum said. “It’s freezing cold, and you’re out on the sea ice as it’s breaking and shifting. You can very easily get stuck on the wrong side of the ice and get stranded. Our Inupiat guides would listen and watch, and when they told us things were shifting, we’d get out of there quickly.”



In one experiment the research team used Teflon containers filled with liquid nitrogen, attached to kites or long poles, to collect newly condensed frost over the open water. They also flew a remote-controlled airplane through the vapor cloud and collected ice from its wings.



Even getting out to the ice to do the work was a challenge. After flying to Barrow, Alaska, the northernmost settlement on the North American mainland, the team took off on snowmobiles, led by their Inupiat guides. That may sound like a lark, but traveling over sea ice was not exactly smooth sailing, Blum said. Though the ice freezes flat, it breaks up, smashes back together and refreezes, forming high ridges through which the team had to chip their way with ice-axes to make pathways for their snowmobiles.



But the results are worth the effort and the risks, Douglas said.



“Research like this will help to further the understanding of mercury deposition to a region that is generally considered pristine,” he said. “In the next phase of our work, we are expanding our knowledge by tracking the mercury during and following snow melt and studying its accumulation on the tundra.”



In addition to Blum and Douglas, the paper’s authors are Matthew Sturm of the Cold Regions Research & Engineering Laboratory in Fort Wainwright, Alaska; William R. Simpson and Laura Alvarez-Aviles of the University of Alaska, Fairbanks; Gerald Keeler, director of the U-M Air Quality Laboratory; Donald Perovich of the Cold Regions Research & Engineering Laboratory in Hanover, N.H.; U-M post-doctoral fellow Abir Biswas and U-M graduate student Kelsey Johnson.



The researchers received funding from the National Science Foundation’s Office of Polar Programs Arctic Science Section.

Past greenhouse warming events provide clues to what the future may hold





 James Zachos (foreground) inspects a sediment core drilled from the ocean floor. Photo courtesy of J. Zachos.
James Zachos (foreground) inspects a sediment core drilled from the ocean floor. Photo courtesy of J. Zachos.

If carbon dioxide emissions from the burning of fossil fuels continue on a “business-as-usual” trajectory, humans will have added about 5 trillion metric tons of carbon to the atmosphere by the year 2400. A similarly massive release of carbon accompanied an extreme period of global warming 55 million years ago known as the Paleocene-Eocene Thermal Maximum (PETM).



Scientists studying the PETM are piecing together an increasingly detailed picture of its causes and consequences. Their findings describe what may be the best analog in the geologic record for the global changes likely to result from continued carbon dioxide emissions from human activities, according to James Zachos, professor of Earth and planetary sciences at the University of California, Santa Cruz.



“All the evidence points to a massive release of carbon at the PETM, and if you compare it with the projections for anthropogenic carbon emissions, it’s roughly the same amount of carbon,” Zachos said. “The difference is the rate at which it was released–we’re on track to do in a few hundred years what may have taken a few thousand years back then.”



Zachos and his collaborators have been studying marine sediments deposited on the deep ocean floor during the PETM and recovered in sediment cores by the Integrated Ocean Drilling Program. He will discuss their findings, which reveal drastic changes in ocean chemistry during the PETM, in a presentation at the annual meeting of the Association for the Advancement of Science (AAAS) in Boston on Friday, February 15. His talk is part of a symposium entitled “Ocean Acidification and Carbon-Climate Connections: Lessons from the Geologic Past.”



The ocean has the capacity to absorb huge amounts of carbon dioxide from the atmosphere. But as carbon dioxide dissolves in the ocean, it makes the water more acidic. That, in turn, could make life more difficult for corals and other marine organisms that build shells and skeletons out of calcium carbonate.



Technically, the “acidification” is a lowering of the pH of ocean water, moving it closer to the acidic range of the pH scale, although it remains slightly alkaline. Lowering the pH affects the chemical equilibrium of the ocean with respect to calcium carbonate, reducing the concentration of carbonate ions and making it harder for organisms to build and maintain structures of calcium carbonate. Corals and some other marine organisms use a form of calcium carbonate called aragonite, which dissolves first, while many others build shells of a more resistant form called calcite.



“As the carbonate concentration starts to decrease, it becomes harder for some organisms to build their shells. They have to use more energy, and eventually it’s impossible–in laboratory experiments, they precipitate some shell during the day, and overnight it dissolves,” Zachos said. “If you lower the carbonate concentration enough, corals and eventually even calcite shells start to dissolve.”



The effect of ocean acidification on the chemistry of calcium carbonate is reflected in the sediment cores from the PETM. Marine sediments are typically rich in calcium carbonate from the shells of marine organisms that sink to the seafloor after they die. Sediments deposited at the start of the PETM, however, show an abrupt transition from carbonate-rich ooze to a dark-red clay layer in which the carbonate shells are completely gone.



Ocean acidification starts at the surface, where carbon dioxide is absorbed from the atmosphere, and spreads to the deep sea as surface waters mix with deeper layers. The calcium carbonate in marine sediments on the seafloor provides a buffer, neutralizing the increased acidity as the shells dissolve and enabling the ocean to absorb more carbon dioxide. But the mixing time required to bring acidified surface waters into the deep sea is long–500 to 1,000 years, according to Zachos.


“We are adding all this carbon dioxide in less than one mixing cycle. That’s important for how the ocean buffers itself, and it means the carbonate concentration in surface waters will get low enough to affect corals and other organisms, assuming emissions continue on the current trajectory,” he said.



In a recent article in Nature (January 17, 2008), Zachos and coauthors Gerald Dickens of Rice University and Richard Zeebe of the University of Hawaii provided an overview of the PETM and other episodes of greenhouse warming in the past 65 million years. These “natural experiments” can help scientists understand the complex interactions that link the carbon cycle and the climate.



Christina Ravelo, a professor of ocean sciences at UCSC and coorganizer of the symposium at which Zachos will speak, said climate records preserved in seafloor sediments provide a valuable test for the climate models scientists use to predict the future consequences of greenhouse gas emissions.



“There are no exact analogs in the past for what is happening now, but we can use past climates to test the models and improve them,” Ravelo said. “The ocean drilling program is the only way to get really good records of these past warm periods.”



Current climate models tend to have difficulty replicating the features of warm periods in the past, such as the PETM, she said. “Even though the models do a great job of simulating the climate over the past 150 years, the future probably holds many climatic surprises. As you run the models farther into the future, the uncertainties become greater.”



A particular concern over the long run is the potential for positive feedback that could amplify the initial warming caused by carbon dioxide emissions. For example, one possible cause of the PETM is the decomposition of methane deposits on the seafloor, which could have been triggered by an initial warming. Methane hydrates are frozen deposits found in the deep ocean near continental margins. Methane released from the deposits would react with oxygen to form carbon dioxide. Both compounds are potent greenhouse gases.



“We have some new evidence that there was a lag between the initial warming and the main carbon excursion of the PETM,” said Zachos, who is a coauthor of a paper describing these findings in the December 20/27, 2007, issue of Nature. “It’s consistent with the notion of a positive feedback, with an initial warming causing the hydrates to decompose,” he said.



Although this raises the possibility that the current global warming trend might trigger a similar release of methane from the ocean floor, that would not happen any time soon. It would take several centuries for the warming to reach the deeper parts of the ocean where the methane hydrate deposits are, Zachos said.



“By slowing the rate of carbon emissions and warming, we may be able to avoid triggering a strong, uncontrolled positive feedback,” he said.

Researchers find origin of ‘breathable’ atmosphere half a billion years ago


Ohio State University geologists and their colleagues have uncovered evidence of when Earth may have first supported an oxygen-rich atmosphere similar to the one we breathe today.



The study suggests that upheavals in the earth’s crust initiated a kind of reverse-greenhouse effect 500 million years ago that cooled the world’s oceans, spawned giant plankton blooms, and sent a burst of oxygen into the atmosphere.



That oxygen may have helped trigger one of the largest growths of biodiversity in Earth’s history.



Matthew Saltzman, associate professor of earth sciences at Ohio State, reported the findings Sunday at the meeting of the Geological Society of America in Denver .



For a decade, he and his team have been assembling evidence of climate change that occurred 500 million years ago, during the late Cambrian period. They measured the amounts of different chemicals in rock cores taken from around the world, to piece together a complex chain of events from the period.



Their latest measurements, taken in cores from the central United States and the Australian outback, revealed new evidence of a geologic event called the Steptoean Positive Carbon Isotope Excursion (SPICE).



Amounts of carbon and sulfur in the rocks suggest that the event dramatically cooled Earth’s climate over two million years — a very short time by geologic standards. Before the event, the Earth was a hothouse, with up to 20 times more carbon dioxide in the atmosphere compared to the present day. Afterward, the planet had cooled and the carbon dioxide had been replaced with oxygen. The climate and atmospheric composition would have been similar to today.



“If we could go back in time and walk around in the late Cambrian, this seems to be the first time we would have felt at home,” Saltzman said. “Of course, there was no life on land at the time, so it wouldn’t have been all that comfortable.”



The land was devoid of plants and animals, but there was life in the ocean, mainly in the form of plankton, sea sponges, and trilobites. Most of the early ancestors of the plants and animals we know today existed during the Cambrian, but life wasn’t very diverse.



Then, during the Ordovician period, which began around 490 million years ago, many new species sprang into being. The first coral reefs formed during that time, and the first true fish swam among them. New plants evolved and began colonizing land.



“If you picture the evolutionary “tree of life,’ most of the main branches existed during the Cambrian, but most of the smaller branches didn’t get filled in until the Ordovician,” Saltzman said. “That’s when animal life really began to develop at the family and genus level.” Researchers call this diversification the “Ordovician radiation.”



The composition of the atmosphere has changed many times since, but the pace of change during the Cambrian is remarkable. That’s why Saltzman and his colleagues refer to this sudden influx of oxygen during the SPICE event as a “pulse” or “burst.”



“After this pulse of oxygen, the world remained in an essentially stable, warm climate, until late in the Ordovician,” Saltzman said.


He stopped short of saying that the oxygen-rich atmosphere caused the Ordovician radiation.



“We know that oxygen was released during the SPICE event, and we know that it persisted in the atmosphere for millions of years — during the time of the Ordovician radiation — so the timelines appear to match up. But to say that the SPICE event triggered the diversification is tricky, because it’s hard to tell exactly when the diversification started,” he said.



“We would need to work with paleobiologists who understand how increased oxygen levels could have led to a diversification. Linking the two events precisely in time is always going to be difficult, but if we could link them conceptually, then it would become a more convincing story.”



Researchers have been trying to understand the sudden climate change during the Cambrian period ever since Saltzman found the first evidence of the SPICE event in rock in the American west in 1998. Later, rock from a site in Europe bolstered his hypothesis, but these latest finds in central Iowa and Queensland, Australia, prove that the SPICE event occurred worldwide.



During the Cambrian period, most of the continents as we know them today were either underwater or part of the Gondwana supercontinent, Saltzman explained. Tectonic activity was pushing new rock to the surface, where it was immediately eaten away by acid rain. Such chemical weathering pulls carbon dioxide from the air, traps the carbon in sediments, and releases oxygen — a kind of greenhouse effect in reverse.



“From our previous work, we knew that carbon was captured and oxygen was released during the SPICE event, but we didn’t know for sure that the oxygen stayed in the atmosphere,” Saltzman said.



They compared measurements of inorganic carbon — captured during weathering — with organic carbon — produced by plankton during photosynthesis. And because plankton contain different ratios of the isotopes of carbon depending on the amount of oxygen in the air, the geologists were able to double-check their estimates of how much oxygen was released during the period, and how long it stayed in the atmosphere.



They also studied isotopes of sulfur, to determine whether much of the oxygen being produced was re-captured by sediments.



It wasn’t.



Saltzman explained the chain of events this way: Tectonic activity led to increased weathering, which pulled carbon dioxide from the air and cooled the climate. Then, as the oceans cooled to more hospitable temperatures, the plankton prospered — and in turn created more oxygen through photosynthesis.



“It was a double whammy,” he said. “There’s really no way around it when we combine the carbon and sulfur isotope data — oxygen levels dramatically rose during that time.”



What can this event tell us about climate change today? “Oxygen levels have been stable for the last 50 million years, but they have fluctuated over the last 500 million,” Saltzman said. “We showed that the oxygen burst in the late Cambrian happened over only two million years, so that is an indication of the sensitivity of the carbon cycle and how fast things can change.”



Global cooling may have boosted life early in the Ordovician period, but around 450 million years ago, more tectonic activity — most likely, the rise of the Appalachian Mountains — brought on a deadly ice age. So while most of the world’s plant and animal species were born during the Ordovician period, by the end of it, more than half of them had gone extinct.



Coauthors on this study included Seth Young, a graduate student in earth sciences at Ohio State; Ben Gill, a graduate student, and Tim Lyons, professor of earth sciences, both at the University of California, Riverside; Lee Kump, professor of geosciences at Penn State University; and Bruce Runnegar, professor of paleontology at the University of California, Los Angeles.

Extinction Theory Falls From Favor





Doctoral student Catherine Powers traveled to fossil sites around the world, including this one in Greece, to study ancient bryozoan marine communities.
Doctoral student Catherine Powers traveled to fossil sites around the world, including this one in Greece, to study ancient bryozoan marine communities.

The greatest mass extinction in Earth’s history also may have been one of the slowest, according to a study that casts further doubt on the extinction-by-meteor theory.



Creeping environmental stress fueled by volcanic eruptions and global warming was the likely cause of the Great Dying 250 million years ago, said USC doctoral student Catherine Powers.



Writing in the November issue of the journal Geology, Powers and her adviser David Bottjer, professor of earth sciences at USC College, describe a slow decline in the diversity of some common marine organisms.



The decline began millions of years before the disappearance of 90 percent of Earth’s species at the end of the Permian era, Powers shows in her study.



More damaging to the meteor theory, the study finds that organisms in the deep ocean started dying first, followed by those on ocean shelves and reefs, and finally those living near shore.



“Something has to be coming from the deep ocean,” Powers said. “Something has to be coming up the water column and killing these organisms.”



That something probably was hydrogen sulfide, according to Powers, who cited studies from the University of Washington, Pennsylvania State University, the University of Arizona and the Bottjer laboratory at USC.



Those studies, combined with the new data from Powers and Bottjer, support a model that attributes the extinction to enormous volcanic eruptions that released carbon dioxide and methane, triggering rapid global warming.


The warmer ocean water would have lost some of its ability to retain oxygen, allowing water rich in hydrogen sulfide to well up from the deep (the gas comes from anaerobic bacteria at the bottom of the ocean).



If large amounts of hydrogen sulfide escaped into the atmosphere, the gas would have killed most forms of life and also damaged the ozone shield, increasing the level of harmful ultraviolet radiation reaching the planet’s surface.



Powers and others believe that the same deadly sequence repeated itself for another major extinction 200 million years ago, at the end of the Triassic era.



“There are very few people that hang on to the idea that it was a meteorite impact,” she said. Even if an impact did occur, she added, it could not have been the primary cause of an extinction already in progress.



In her study, Powers analyzed the distribution and diversity of bryozoans, a family of marine invertebrates.



Based on the types of rocks in which the fossils were found, Powers was able to classify the organisms according to age and approximate depth of their habitat.



She found that bryozoan diversity in the deep ocean started to decrease about 270 million years ago and fell sharply in the 10 million years before the mass extinction that marked the end of the Permian era.



But diversity at middle depths and near shore fell off later and gradually, with shoreline bryozoans being affected last, Powers said.



She observed the same pattern before the end-Triassic extinction, 50 million years after the end-Permian.



Powers’ work was funded by the Geological Society of America, the Paleontological Society, the American Museum of Natural History and the Yale Peabody Museum, and supplemented by a grant from USC’s Women in Science and Engineering program.



Geology is published by the Geological Society of America.

Methane Bubbling From Arctic Lakes, Now And At End Of Last Ice Age





UAF researcher Katey Walter lights a pocket of methane on a thermokarst lake in Siberia in March of 2007. Igniting the gas is a way to demonstrate, in the field, that it contains methane. (Credit: Photo by Sergey Zimov)
UAF researcher Katey Walter lights a pocket of methane on a thermokarst lake in Siberia in March of 2007. Igniting the gas is a way to demonstrate, in the field, that it contains methane. (Credit: Photo by Sergey Zimov)

A team of scientists led by a researcher at the University of Alaska Fairbanks has identified a new likely source of a spike in atmospheric methane coming out of the North during the end of the last ice age.



Methane bubbling from arctic lakes could have been responsible for up to 87 percent of that methane spike, said UAF researcher Katey Walter, lead author of a report printed in the Oct. 26 issue of Science. The findings could help scientists understand how current warming might affect atmospheric levels of methane, a gas that is thought to contribute to climate change.



“It tells us that this isn’t just something that is ongoing now. It would have been a positive feedback to climate warming then, as it is today,” said Walter. “We estimate that as much as 10 times the amount of methane that is currently in the atmosphere will come out of these lakes as permafrost thaws in the future. The timing of this emission is uncertain, but likely we are talking about a time frame of hundreds to thousands of years, if climate warming continues as projected.”



Ice cores from Greenland and Antarctica have shown that during the early Holocene Period–about 14,000 to 11,500 years ago–the levels of methane in the atmosphere rose significantly, Walter said. “They found that an unidentified northern source (of methane) appeared during that time.”



Previous hypotheses suggested that the increase came from gas hydrates or wetlands. This study’s findings indicate that methane bubbling from thermokarst lakes, which are formed when permafrost thaws rapidly, is likely a third and major source.



Walter’s research focused on areas of Siberia and Alaska that, during the last ice age, were dry grasslands atop ice-rich permafrost. As the climate warmed, Walter said, that permafrost thawed, forming thermokarst lakes.



“Lakes really flared up on this icy permafrost landscape, emitting huge amounts of methane,” she said.


As the permafrost around and under the lakes thaws, the organic material in it–dead plants and animals–can enter the lake bottom and become food for the bacteria that produce methane.



“All that carbon that had been locked up in the ground for thousands of years is converted to potent greenhouse gases: methane and carbon dioxide,” Walter said. Walter’s paper hypothesizes that methane from the lakes contributed 33 to 87 percent of the early Holocene methane increase.



To arrive at the hypothesis, Walter and her colleagues traveled to Siberia and northern Alaska to examine lakes that currently release methane. In addition, they gathered samples of permafrost and thawed them in the laboratory to study how much methane permafrost soil can produce immediately after thawing.



“We found that it produced a lot very quickly,” she said.



Finally, she and other researchers studied when existing lakes and lakes in the past formed and found that their formation coincided with the early Holocene Period northern methane spike.



“We came up with a new hypothesis,” she said. “Thermokarst lake formation is a source of atmospheric methane today, but it was even more important during early Holocene warming. This suggests that large releases from lakes may occur again in the future with global warming.”



Co-authors on the paper include Mary Edwards of the University of Southampton and the UAF College of Natural Science and Mathematics; Guido Grosse, an International Polar Year postdoctoral fellow with the UAF Geophysical Institute; Sergey Zimov of the Russian Academy of Sciences; and Terry Chapin of the UAF Institute of Arctic Biology. Funding was provided by the National Science Foundation, the Environmental Protection Agency and the National Aeronautics and Space Administration.

Oxygen on Earth: 50 to 100 Million Years Earlier Than Scientists Thought





New findings reveal the importance of oxygen in the environment shortly before the deposition of this massive formation of iron oxide--rust--in the Hamersley Basin in Western Australia. - Photo Credit: A. D. Anbar, ASU
New findings reveal the importance of oxygen in the environment shortly before the deposition of this massive formation of iron oxide–rust–in the Hamersley Basin in Western Australia. – Photo Credit: A. D. Anbar, ASU

Scientists have found that traces of oxygen appeared in Earth’s atmosphere 50 to 100 million years earlier than previously thought–before what geologists call the “Great Oxidation Event.”



This event happened between 2.3 and 2.4 billion years ago, when most geoscientists think atmospheric oxygen rose sharply from very low levels. The amount of oxygen before that time has been uncertain.



Analyzing layers of sedimentary rock in a kilometer-long core sample from the Hamersley Basin in Western Australia, the researchers report finding evidence that a small but significant amount of oxygen–a whiff–was present in the oceans and possibly Earth’s atmosphere 2.5 billion years ago.



The data also suggest that oxygen was nearly undetectable just before that time. Their findings appear in a pair of papers in the Sept. 28 issue of the scientific journal Science. The National Science Foundation (NSF) funded the research.



“We seem to have captured a piece of time before the Great Oxidation Event during which the amount of oxygen was actually changing–caught in the act, as it were,” said Ariel Anbar, a biogeochemist at Arizona State University in Tempe.



Anbar led one of the teams of investigators and participated in another team led by Alan Jay Kaufman, a geochemist at the University of Maryland in College Park. The collaborators analyzed a drill core for geochemical and biological tracers representing the time just before the rise of atmospheric oxygen.



“We have compelling evidence for a shift in the oxidation state of the surface ocean 50 million years before the Great Oxidation Event,” said Kaufman. “These findings are a significant step in our understanding of the oxygenation of Earth. They link changes in the environment with changes in the biosphere.”



The project also brought together scientists from the University of Washington, University of California in Riverside and University of Alberta.



“These results are the culmination of a successful effort to recover suitable rock material, and to test hypotheses regarding the evolution of biogeochemical cycles on Earth in the time period after the appearance of life and prior to the Great Oxidation Event,” said Enriqueta Barrera, program director in NSF’s Division of Earth Sciences, which funded the research.



The work also received support from the Astrobiology Drilling Program (ADP) of the NASA Astrobiology Institute (NAI) and the Geological Survey of Western Australia.



In the summer of 2004, the scientists bored into the geologically-famous Hamersley Basin in Western Australia, extracting a core of sedimentary rock 908 meters (about 3,000 feet) long.



“The core provides a continuous record of environmental conditions, analogous to a tape recording,” explains Anbar. Because it was recovered from deep underground, it contains materials untouched by the atmosphere for billions of years.


Anbar and his research group began an analysis of selected bands of the late Archean Mt. McRae Shale found in the upper 200 meters of the drill core. They analyzed amounts of the trace metals molybdenum, rhenium and uranium. The amounts of these metals in oceans and sediments depends on the amount of oxygen in the environment.



The goal was to characterize the nature of the environment and life in the oceans leading up to the Great Oxidation Event.



“The Maryland group began seeing funny variations in the chemistry of sulfur along this stretch of the drill core,” said Anbar. “We sped up our research to see if we found variations in metal abundances in the same places – and we did.”



Finding evidence of oxygen some 50 to 100 million years earlier than what was previously known was completely unexpected, say the scientists.



For the first half of Earth’s 4.56-billion-year history, the environment held almost no oxygen, other than that bound to hydrogen in water or to silicon and other elements in rocks. “Then, some time between 2.3 and 2.4 billion years ago, oxygen rose sharply in the Earth’s atmosphere and oceans, during the ‘Great Oxidation Event,'” said Anbar.



The event was a major step in Earth’s history, said Anbar, but its cause remains unexplained. How did Earth’s atmosphere go from being oxygen-poor to oxygen-rich, why did it change so quickly, and why did its oxygen content stabilize at the present 21 percent?



“Studying the dynamics that gave rise to the presence of oxygen in Earth’s atmosphere deepens our appreciation of the complex interaction between biology and geochemistry,” said Carl Pilcher, director of the NAI. “The results support the idea that our planet and the life on it evolved together.”



One possibility for explaining the Great Oxidation Event is that the ancient ancestors of today’s plants first began to produce oxygen by photosynthesis at this time.



“What we have now are new lines of evidence for oxygen in the environment 50 to 100 million years before its big rise,” Anbar said. This discovery strengthens the notion that organisms produced oxygen long before the Great Oxidation Event, creating features in the geologic record such as banded iron formations, and that the rise of oxygen in the atmosphere was ultimately controlled by geological processes.



“This knowledge is relevant to today’s studies of environmental and climate issues because it helps us understand the interactions between biology, geology and the composition of the atmosphere,” Anbar said.



“It also has implications for the search for life on planets outside our solar system. In the near future the only way we can look for evidence of life in such far-off places is to look for the fingerprints of biology in the compositions of their atmospheres. We are not far off from being able to detect Earth-like planets elsewhere in the galaxy, and eventually, we will be able to use telescopes to measure the oxygen content of their atmospheres.”



Questions Anbar hopes to investigate include: if scientists find that no Earth-like planets have undergone Great Oxidation Events, what will that mean about life on Earth? Is it inevitable that the evolution of oxygen-producing organisms results in an oxygen-rich atmosphere?



And, he said: “Can we find evidence that oxygen was produced even earlier?”

Researchers Reassess Theories on Formation of Earth’s Atmosphere





Scientists propose that argon in our atmosphere came from the weathering of the upper crust and not the melting of the mantle
Scientists propose that argon in our atmosphere came from the weathering of the upper crust and not the melting of the mantle

Geochemists at Rensselaer Polytechnic Institute are challenging commonly held ideas about how gases are expelled from the Earth. Their theory, which is described in the Sept. 20 issue of the journal Nature, could change the way scientists view the formation of Earth’s atmosphere and those of our distant neighbors, Mars and Venus. Their data throw into doubt the timing and mechanism of atmospheric formation on terrestrial plants.



Lead by E. Bruce Watson, Institute Professor of Science at Rensselaer, the team has found strong evidence that argon atoms are tenaciously bound in the minerals of Earth’s mantle and move through these minerals at a much slower rate than previously thought. In fact, they found that even volcanic activity is unlikely to dislodge argon atoms from their resting places within the mantle. This is in direct contrast to widely held theories on how gases moved through early Earth to form our atmosphere and oceans, according to Watson.



Scientists believe that shortly after Earth was formed, it had a glowing surface of molten rock extending down hundreds of miles. As that surface cooled, a rigid crust was produced near the surface and solidified slowly downward to complete the now-solid planet. Some scientists have suggested that Earth lost all of its initial gases, either during the molten stage or as a consequence of a massive collision, and that the catastrophically expelled gases formed our early atmosphere and oceans. Others contend that this early “degassing” was incomplete, and that primordial gases still remain sequestered at great depth to this day. Watson’s new results support this latter theory.



“For the ‘deep-sequestration’ theory to be correct, certain gases would have to avoid escape to the atmosphere in the face of mantle convection and volcanism,” Watson said. “Our data suggest that argon does indeed stay trapped in the mantle even at extremely high temperatures, making it difficult for the Earth to continuously purge itself of argon produced by radioactive decay of potassium.”



Argon and other noble gases are tracer elements for scientists because they are very stable and do not change over time, although certain isotopes accumulate through radioactive decay. Unlike more promiscuous elements such as carbon and oxygen, which are constantly bonding and reacting with other elements, reliable argon and her sister noble gases (helium, neon, krypton, and xenon) remain virtually unchanged through the ages. Its steady personality makes argon an ideal marker for understanding the dynamics of Earth’s interior.



“By measuring the behavior of argon in minerals, we can begin to retrace the formation of Earth’s atmosphere and understand how and if complete degassing has occurred,” Watson explained.



Watson’s team, which includes Rensselaer postdoctoral researcher Jay B. Thomas and research professor Daniele J. Cherniak, developed reams of data in support of their emerging belief that argon resides stably in crystals and migrates slowly. “We realized from our initial results that these ideas might cause a stir,” Watson said. “So we wanted to make sure that we had substantial data supporting our case.”


The team heated magnesium silicate minerals found in Earth’s mantle, which is the region of Earth sandwiched between the upper crust and the central core, in an argon atmosphere. They used high temperature to simulate the intense heat deep within the Earth to see whether and how fast the argon atoms moved into the minerals. Argon was taken up by the minerals in unexpectedly large quantities, but at a slow rate.



“The results show that argon could stay in the mantle even after being exposed to extreme temperatures,” Watson said. “We can no longer assume that a partly melted region of the mantle will be stripped of all argon and, by extension, other noble gases.”



But there is some argon in our atmosphere – slightly less than 1 percent. If it didn’t shoot through the rocky mantle, how did it get into the atmosphere?



“We proposed that argon’s release to the atmosphere is through the weathering of the upper crust and not the melting of the mantle,” Watson said. “The oceanic crust is constantly being weathered by ocean water and the continental crust is rich in potassium, which decays to form argon.”



And what about the primordial argon that was trapped in the Earth billions of years ago? “Some of it is probably still down there,” Watson said.



Because Mars and Venus have mantle materials similar to those found on Earth, the theory could be key for understanding their atmospheres as well.



Watson and his team have already begun to test their theories on other noble gases, and they foresee similar results. “We may need to start reassessing our basic thinking on how the atmosphere and other large-scale systems were formed,” he said.



The research was funded by the National Science Foundation.