[ Back to EurekAlert! ] Public release date: 29-Feb-2012
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Contact: Kate Ramsayer
kramsayer@agu.org
202-777-7524
American Geophysical Union

AGU Journal highlights - Feb. 29 2012

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research-Earth Surface (JGR-F), Journal of Geophysical Research-Atmospheres (JGR-D), Journal of Geophysical Research-Solid Earth (JGR-B), and Journal of Geophysical Research-Space Physics (JGR-A).

In this release:

1. Effect of vegetation die-off tested on tidal marshland

2. Capsizing icebergs release earthquake-sized energies

3. Asian emissions contribute to air pollution in western United States

4. Remote sensing of volcanic ash properties

5. In Japan, seismic waves slower after rain, large earthquakes

6. Molecular oxygen ions confirm exosphere at Saturn's moon Dione

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2011GL050502. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions who are registered with AGU also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at http://www.agu.org/news/press/papers.shtml.


1. Effect of vegetation die-off tested on tidal marshland

Consisting of densely vegetated platforms raised slightly above sea level, and interwoven by channels of water meandering inland from the coast, tidal marshlands help buffer against strong storm surges, protect against flooding, limit coastal erosion, and provide a valuable habitat for a vast array of coastal species. Continued global climate change, however, has researchers worried about the stability of coastal marshlands in light of rising temperatures, sea levels, and a declining ocean pH. Of particular concern over shorter timescales are the potential consequences for marsh dynamics should there be a mass die-off of marshland vegetation.

Investigations of the effects of mass vegetative death on marshland behavior have been conducted almost exclusively using computer simulations, but Temmerman et al. sought to bolster this previous research with empirical evidence. The authors measured water flow rates and directions in Kijkverdriet, a freshwater tidal marsh in northern Belgium, both before and after they clear-cut 0.04 square kilometers (10 acres) of vegetation. They find that flow rates increased over the previously vegetated land and decreased in the vegetation-free channels, essentially equalizing the flows over the whole area. They find that, following their intervention, the water flow direction over the freshly barren platforms became increasingly parallel to the nearby channel's flows.

Finding good agreement between their observations and the predictions of modeling efforts, the authors suggest that a large-scale plant die-off would lead to sediment infilling of marsh channels and reduced sedimentation to the previously vegetated platforms. They say that this would further reduce the survival of future marshland vegetation, triggering a runaway feedback cycle culminating in permanent marsh loss.

AGU's blog, GeoSpace, includes a story on the findings at http://bit.ly/Auy2wo.

Source: Geophysical Research Letters, doi:10.1029/2011GL050502, 2012 http://dx.doi.org/10.1029/2011GL050502

Title: Impact of vegetation die-off on spatial flow patterns over a tidal marsh

Authors: Stijn Temmerman and Jonas Schoelynck: Ecosystem Management Research Group, University of Antwerpen, Wilrijk, Belgium;

Gerard Govers and Pieter Moonen: Research Group for Physical and Regional Geography, Katholieke Universiteit Leuven, Heverlee, Belgium;

Tjeerd J. Bouma: Royal Netherlands Institute for Sea Research (NIOZ), Yerseke, Netherlands.


2. Capsizing icebergs release earthquake-sized energies

A large iceberg can carry a large amount of gravitational potential energy. While all icebergs float with the bulk of their mass submerged beneath the water's surface, some drift around with precarious orientations-they are temporarily stable, but an outside push would send them tumbling over. Large icebergs, like those that split from the Jakobshavn Isbrae glacier in Greenland, can release the energy equivalent to a magnitude 6 or 7 earthquake when they capsize. A 1995 event demonstrated the potential for destruction, as a tsunami spawned from a capsizing iceberg devastated a coastal Greenland community. Measuring how energy is dispersed during capsizing is crucial to understanding the risk associated with these events but is also key to determining their larger role in surface ocean dynamics.

Using a laboratory model fjord and 27 x 10 centimeter (10.6 x 4 inch) polyethylene iceberg analogues with varied widths, Burton et al. measured how energy is released to the surrounding water during capsizing. A camera tracking a floating buoy measured the height of any tsunami waves, and analysis of the iceberg's movement let them determine the kinetic energy involved in the rotation. Corroborating earlier research, the authors find that the size of any tsunami waves will be at most 1 percent of the iceberg's initial height. Further, they find that 84 percent of the iceberg's original potential energy would end up as turbulence or heat in the surface ocean waters. While such a large amount of turbulence would be important for surface dynamics in the open ocean, it would be particularly powerful in a semi-enclosed region like the fjord surrounding the Jakobshavn Isbrae glacier, where dozens of icebergs spawn each summer. In the fjord, trillions of megajoules worth of turbulence redistribute the water, destroying temperature and salinity stratifications.

Source: Journal of Geophysical Research-Earth Surface, doi: 10.1029/2011JF002055, 2011 http://dx.doi.org/10.1029/2011JF002055

Title: Laboratory investigations of iceberg capsize dynamics, energy dissipation and tsunamigenesis

Authors: J. C. Burton and N. Guttenberg: Department of Physics, University of Chicago, Chicago, Illinois, USA;

J. M. Amundson, D. S. Abbot, A. Boghosian, L. M. Cathles, K. N. Darnell, and D. R. MacAyeal: Department of Geophysical Sciences, University of Chicago, Chicago, Illinois, USA;

S. Correa-Legisos: Departamento de Física, Universidad de Santiago de Chile, Santiago, Chile;

D. M. Holland: Courant Institute of Mathematical Sciences, New York University, New York, New York, USA.


3. Asian emissions contribute to air pollution in western United States

As Asian countries develop, they are emitting more ozone precursors that pollute surface level air. Many studies have documented this pollution being carried by air currents to the western United States. To learn more about the mechanisms that transport air pollution across the ocean and determine the effects of Asian air pollution on air quality in the western United States, Lin et al. analyzed in situ and satellite measurements from May 2010 to June 2010 using a global high- resolution climate chemistry model.

They quantified the contribution of Asian pollution to surface ozone levels in both densely populated regions such as the Los Angeles area and rural areas such as national parks. They find that Asian pollution contributes as much as 20 percent of total ozone during springtime pollution episodes in western U.S. surface air.

Current guidelines from the Environmental Protection Agency dictate that, averaging over 8 hours, surface level air should have no more than 75 parts per billion per hour by volume of ozone. Although local pollution plays a large role on days when that standard is not met in Southern California, the authors estimate that 53 percent of the instances where that limit was exceeded would not have occurred without the contribution from Asian air pollution.

The researchers also find that an index based on satellite observations of Asian pollution plumes could serve as a qualitative early warning indicator, with a lead time of one to three days, of Asian pollution influence on western U.S. air quality.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2011JD016961, 2012 http://dx.doi.org/10.1029/2011JD016961

Title: Transport of Asian ozone pollution into surface air over the western United States in spring

Authors: Meiyun Lin: Atmospheric and Oceanic Sciences, Princeton University, Princeton, New Jersey, USA, and NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA;

Arlene M. Fiore: NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA, now at Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA;

Larry W. Horowitz: NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA;

Owen R. Cooper: Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado, USA, and NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

Vaishali Naik: NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA, and High Performance Technologies, Inc., NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA;

John Holloway, Bryan J. Johnson, and Ann M. Middlebrook: NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

Samuel J. Oltmans: Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado, USA;

Ilana B. Pollack: Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado, USA, and NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

Tomas B. Ryerson: NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

Juying X. Warner: Joint Center for Earth Systems Technology, University of Maryland, Baltimore, Maryland, USA;

Christine Wiedinmyer: National Center for Atmospheric Research, Boulder, Colorado, USA;

John Wilson and Bruce Wyman: NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA.


4. Remote sensing of volcanic ash properties

Remote sensing is an important tool to track hazardous volcanic plumes, providing an overview of the ash cloud. Francis et al. describe a method of using infrared data from instruments onboard the Meteosat satellite to detect volcanic ash and derive quantitative measurements of its physical properties. They demonstrate their technique using data from the 2010 Eyjafjallajokull eruption and validate their retrieved properties against spaceborne and airborne lidar data. They show that their retrieved properties are realistic, and the method should help improve ash cloud analysis and forecasting.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2011JD016788, 2012 http://dx.doi.org/10.1029/2011JD016788

Title: Retrieval of physical properties of volcanic ash using Meteosat: A case study from the 2010 Eyjafjallajökull eruption

Authors: Peter N. Francis, Michael C. Cooke, and Roger W. Saunders: Met Office, Exeter, UK.


5. In Japan, seismic waves slower after rain, large earthquakes

An earthquake is first detected by the abrupt side-to-side jolt of a passing primary wave. Lagging only slightly behind are shear waves, which radiate out from the earthquake's epicenter and are seen at the surface as a rolling wave of vertical motion. Also known as secondary or S waves, shear waves cause the lifting and twisting motions that are particularly effective at collapsing surface structures. With their capacity to cause damage, making sense of anything that can influence shear wave vertical velocities is important from both theoretical and engineering perspectives.

In Japan the Kiban-Kyoshin network (KiK-net) is made up of 700 seismic detection stations spread across the country. Each station has two separate seismic detectors, one at the surface and one buried in a borehole. Analyzing the KiK-net data for the nearly 112,000 earthquakes that hit Japan between 2000 and 2010, Nakata and Snieder identify a number of relationships that seem to affect shear wave vertical velocities in the near surface.

The authors find that in the months following a major earthquake, shear wave velocities at nearby stations were cut down by 3 percent to 4 percent. Further, the authors find that the shear waves that propagate fastest oscillate in the direction of motion of the underlying tectonic plate. Finally, they find that the shear wave velocity changed with the season. In the southern reaches of Japan the summer months are marked by heavy precipitation. Comparing rainfall records with the data derived from KiK-net's observations, the authors find that shear wave velocities were significantly reduced following periods of heavy rainfall. They suggest that groundwater infiltration would fill any cracks in the subsurface, increasing pore pressure and reducing seismic wave velocities.

Source: Journal of Geophysical Research-Solid Earth, doi:10.1029/2011JB008595, 2012 http://dx.doi.org/10.1029/2011JB008595

Title: Estimating near-surface shear wave velocities in Japan by applying seismic interferometry to KiK-net data

Authors: N. Nakata: Department of Urban Management, Kyoto University, Kyoto, Japan and Center for Wave Phenomena, Department of Geophysics, Colorado School of Mines, Golden, Colorado, USA;

R. Snieder: Center for Wave Phenomena, Department of Geophysics, Colorado School of Mines, Golden, Colorado, USA.


6. Molecular oxygen ions confirm exosphere at Saturn's moon Dione

The Cassini spacecraft flew by Dione, one of Saturn's icy moons, on 7 April 2010. During that flyby, instruments detected molecular oxygen ions around the moon. Tokar et al. used those measurements to estimate the density of the molecular oxygen ions to be in the range of 0.01 to 0.09 ions per cubic centimeter (or ions per 0.06 cubic inches). These molecular oxygen ions are produced when neutral molecules are ionized; the measurements confirm that a neutral exosphere surrounds Dione.

Source: Geophysical Research Letters, doi:10.1029/2011GL050452, 2012 http://dx.doi.org/10.1029/2011GL050452

Title: Detection of Exospheric O2+ at Saturn's Moon Dione

Authors: R. L. Tokar: Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA;

R. E. Johnson: Engineering Physics, University of Virginia, Charlottesville, Virginia, USA;

M. F. Thomsen: Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA;

E. C. Sittler: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;

A. J. Coates: Mullard Space Science Laboratory, University College London, Dorking, UK;

R. J. Wilson: Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Boulder, Colorado, USA;

F. J. Crary and D. T. Young: Southwest Research Institute, San Antonio, Texas, USA;

G. H. Jones: Mullard Space Science Laboratory, University College London, Dorking, UK, and Centre for Planetary Sciences at UCL/Birkbeck, London, UK.


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Contact:
Kate Ramsayer
Phone (direct): 202-777-7524
Email: kramsayer@agu.org



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