Public Release:  AGU journal highlights -- Mar. 26, 2013

American Geophysical Union

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research-Planets (JGR-P), Space Weather (SW), and Journal of Geophysical Research-Biogeosciences, (JGR-G).

In this release:

1. Global fires after the asteroid impact probably caused the K-Pg extinction
2. Predicting fire activity using terrestrial water storage data
3. Monitoring subsidence and vent wall collapse on Kilauea Volcano, Hawaii
4. Italian all-sky imager tracks auroral red arcs over Europe
5. Nonnative salmon alter nitrification in Great Lakes tributaries
6. High rates of nitrogen fixation measured in equatorial upwelling region

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1. Global fires after the asteroid impact probably caused the K-Pg extinction

About 66 million years ago a mountain-sized asteroid hit what is now the Yucatan in Mexico at exactly the time of the Cretaceous-Paleogene (K-Pg) mass extinction. Evidence for the asteroid impact comes from sediments in the K-Pg boundary layer, but the details of the event, including what precisely caused the mass extinction, are still being debated.

Some scientists have hypothesized that since the ejecta from the impact would have heated up dramatically as it reentered the Earth's atmosphere, the resulting infrared radiation from the upper atmosphere would have ignited fires around the globe and killed everything except those animals and plants that were sheltered underground or underwater.

Other scientists have challenged the global fire hypothesis on the basis of several lines of evidence, including absence of charcoal-which would be a sign of widespread fires-in the K-Pg boundary sediments. They also suggested that the soot observed in the debris layer actually originated from the impact site itself, not from widespread fires caused by reentering ejecta.

Robertson et al. show that the apparent lack of charcoal in the K-Pg boundary layer resulted from changes in sedimentation rates: When the charcoal data are corrected for the known changes in sedimentation rates, they exhibit an excess of charcoal, not a deficiency. They also show that the mass of soot that could have been released from the impact site itself is far too small to account for the observed soot in the K-Pg layer. In addition, they argue that since the physical models show that the radiant energy reaching the ground from the reentering ejecta would be sufficient to ignite tinder, it would thereby spark widespread fires. The authors also review other evidence for and against the firestorm hypothesis and conclude that all of the data can be explained in ways that are consistent with widespread fires.

Source:
Journal of Geophysical Research-Planets, doi:10.1002/jgrg.20018, 2013
http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20018/abstract

Title:
K/Pg extinction: Reevaluation of the heat/fire hypothesis

Authors:
Douglas S. Robertson: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA; William M. Lewis: Department of Ecology and Evolutionary Biology and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA; Peter M. Sheehan: Department of Geology, Milwaukee Public Museum, Milwaukee, Wisconsin, USA; Owen B. Toon: Department of Atmospheric and Oceanic Sciences and Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA.


2. Predicting fire activity using terrestrial water storage data

High fire activity periods in the Amazon region can be predicted months in advance on the basis of water storage data, a new study shows. Chen et al. analyzed satellite observations of terrestrial water storage from the Gravity Recovery and Climate Experiment (GRACE) mission, along with satellite observations of fire activity from the Moderate Resolution Imaging Spectroradiometer (MODIS) mission. GRACE measures the Earth's gravity field by calculating the distance between two satellites as slight variations in density pull on one satellite more than the other. The gravity measurements provide information about the amount of groundwater or surface water in a given region.

The researchers contrasted high and low fire years in the period from 2002 to 2011 and find that in high fire years, terrestrial water storage during the months before the fire season was generally below average, while in low fire years, water storage in the months before the dry season was generally above average. This suggests that, at least qualitatively, water storage as measured by GRACE can provide information to help predict the severity of the fire season in the Amazon region several months in advance.

Source:
Journal of Geophysical Research-Biogeosciences, doi:10.1002/jgrg.20046, 2013
http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20046/abstract

Title:
Satellite observations of terrestrial water storage provide early warning information about drought and fire season severity in the Amazon

Authors:
Yang Chen, James S. Famiglietti and James T. Randerson: Department of Earth System Science, University of California, Irvine, California, USA; Isabella Velicogna: Department of Earth System Science, University of California, Irvine, CA, USA, and Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.


3. Monitoring subsidence and vent wall collapse on Kilauea Volcano, Hawaii

Kilauea Volcano in Hawaii experienced its first summit eruption in 26 years when a new vent along the east wall of Halema`uma`u Crater opened in March 2008. Since that time, the vent has become wider as parts of the wall around it became unstable and collapsed into the active lava lake within the vent, sometimes triggering small explosions. Richter et al. have monitored surface deformation in the area around the new vent since 2008 using interferometric synthetic aperture radar (InSAR) from the TerraSAR-X satellite along with a digital elevation model of the topography based on lidar data. They were able to generate interferograms (a type of image) with a pixel resolution of about 3 meters (10 feet), which revealed centimeter-scale subsidence in the area within 100 meters (328 feet) of the vent rim. They note that this deformation cannot be detected by other techniques.

In general, the authors find that subsidence and increasing vent area track each other: the vent was more stable at times when subsidence rates were lower, while periods when the subsidence rate increased tended to be followed by collapse of parts of the vent wall. They suggest that it may be possible to identify areas where the vent rim is likely to fail soon on the basis of subsidence rate monitoring. The study demonstrates the potential for using high-resolution satellite interferometry for monitoring potential hazards.

Source:
Geophysical Research Letters, doi:10.1002/grl.50286, 2013
http://onlinelibrary.wiley.com/doi/10.1002/grl.50286/abstract

Title:
TerraSAR-X interferometry reveals small-scale deformation associated with the summit eruption of Kilauea Volcano, Hawai'i

Authors:
Nicole Richter: Department of Earth Observation, Friedrich-Schiller-University, Jena, Germany; Michael P Poland: Hawaiian Volcano Observatory, U.S. Geological Survey, Hawai'i National Park, Hawaii, USA; Paul R Lundgren: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.


4. Italian all-sky imager tracks auroral red arcs over Europe

During geomagnetic storms, stable auroral red (SAR) arcs reach down from polar latitudes, their faint glow stretching equatorward of the traditional auroral oval. Invisible to the naked eye, SAR arcs are an upper atmospheric occurrence produced by the emission of light from oxygen atoms in the thermosphere. The excitation of the ionospheric oxygen that produces SAR arcs is caused, in turn, by the conduction of heat from the magnetospheric ring current. Advances in camera optics, including more sensitive sensors and highly specific filters, have allowed researchers to track the occurrence of SAR arcs, opening a window into the dynamics of the inner magnetosphere.

In northern Italy a new all-sky imaging system, described by Baumgardner et al., uses highly sensitive sensors and a fish-eye lens to simultaneously observe SAR arc and faint auroral activity over the majority of Europe. The authors report on the all-sky SAR arc observations made during a geomagnetic storm that took place from 26 to 27 September 2011. Comparing their observations with coincident satellite- and ground-based observations, the authors find that their all- sky imager was able to identify the lowest latitudes where magnetospheric sources can create a SAR arc. They suggest that the detection of a SAR arc, separated from the diffuse ionospheric aurorae, can indicate the region of maximum electron heating from the inner magnetosphere to the ionosphere. They also suggest that the new all-sky imager could be used to help interpret in real time the effect of space weather on radio communications or to help validate space weather modeling efforts.

Source:
Space Weather, doi:10.1002/swe.20027, 2013
http://onlinelibrary.wiley.com/doi/10.1002/swe.20027/abstract

Title:
Imaging space weather over Europe

Authors:
Jeffrey Baumgardner, Joei Wroten, Michael Mendillo and Carlos Martinis: Center for Space Physics, Boston University, Boston, Massachusetts, USA; Cesare Barbieri and Gabriele Umbriaco: Department of Physics and Astronomy, University of Padova, Padova, Italy; Cathryn Mitchell and Joe Kinrade: Department of Electronic and Electrical Engineering, University of Bath, United Kingdom; Massimo Materassi and Luigi Ciraolo: Institute for Complex Systems, ISC-CNR, 50019 Sesto Fiorentino-Firenze, Italy; Marc Hairston: William B. Hanson Center for Space Science, University of Texas, Dallas, Texas, USA.


5. Nonnative salmon alter nitrification in Great Lakes tributaries

Nonnative species can affect the biogeochemistry of an ecosystem. For instance, Pacific salmon have been introduced as a sport fishery in many streams and lakes beyond their native range, but their introduction may be altering nitrogen cycling in those ecosystems.

Salmon excrete ammonium, which can be transformed into nitrate by bacteria in a process known as nitrification. Nitrate can be used by plants as an inorganic nitrogen source, but in excess it can also cause potentially harmful algal blooms to grow and, at high concentrations, is considered a pollutant in drinking water.

Levi and Tank measured sediment nitrification rates before, during, and after the salmon run in 2009 to study the effects of nonnative Pacific salmon in five tributaries to the Great Lakes in Michigan and Ontario. Though the variation in nitrification rates was habitat-specific, the researchers observe increases in sediment nitrification rates in these streams. These changes in the form of dissolved inorganic nitrogen can affect nutrient dynamics not only where the salmon are but also in ecosystems located downstream. Fisheries managers may need to monitor the quantity and type of inorganic nitrogen export to avoid possible unintended consequences for ecosystems associated with introduced salmon populations.

Source:
Journal of Geophysical Research-Biogeosciences, doi:10.1002/jgrg.20044, 2013
http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20044/abstract

Title:
Nonnative Pacific salmon alter hot spots of sediment nitrification in Great Lakes tributaries

Authors:
Peter S. Levi1 and Jennifer L. Tank: Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana, USA.


6. High rates of nitrogen fixation measured in equatorial upwelling region

Surface waters in upwelling regions of the ocean are generally rich in nutrients. Scientists had thought that these areas would have low rates of nitrogen fixation because diazotrophs-microbes that convert nitrogen gas from the atmosphere into usable forms, such as ammonia-could use the nutrients in the water directly instead of having to fix nitrogen gas. However, researchers recently recorded high rates of nitrogen fixation in an upwelling region in the equatorial Atlantic.

Subramaniam et al. studied the extent of diazotrophic activity in the equatorial Atlantic during the upwelling period in May and June 2009. They measured rates of nitrogen fixation as well as nutrient concentrations and the structure of the phytoplankton community. The researchers observe rates of nitrogen fixation 2- to-7 times higher during the upwelling period than had been reported during non- upwelling periods.

They suggest that as waters rich in iron but with a low nitrate-to-phosphate ratio upwell, a bloom of non-diazotrophic phytoplankton grows and removes the upwelled nitrate. Diazotrophs then use the residual phosphate and iron, and nitrogen fixation increases. The study could help improve scientists' understanding of nitrogen and carbon dynamics in upwelling regions.

Source:
Geophysical Research Letters, doi:10.1002/grl.50250, 2013
http://onlinelibrary.wiley.com/doi/10.1002/grl.50250/abstract

Title:
Equatorial Upwelling Enhances Nitrogen Fixation in the Atlantic Ocean

Authors:
Ajit Subramaniam: Lamont Doherty Earth Observatory, Columbia University, Palisades, New York, USA; Claire Mahaffey: Department of Earth, Ocean and Ecological Sciences, School of Environmental Sciences, University of Liverpool, Liverpool, UK; William Johns: Division of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA; Natalie Mahowald: Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA.

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