AGU journal highlights -- Nov. 27, 2006
1. Melting the Snowball Earth
Scientists believe that during the Neoproterozoic era 750 million years ago, a severe ice age occurred that almost completely froze Earth's oceans. The factors that initiated this "Snowball Earth" have been the subject of much study. Lewis et al. Have instead focused on determining the factors that pulled Earth out of its snowball state. Noting that accepted values for both snow and ice albedo (ratio of incident and reflected solar radiation) cover a wide range, the authors sought to quantify the relative sensitivity of various surface albedos on the same climate model, as that model is pulled out of a snowball state. They found the range of ice, snow, and land albedos and the resulting minimum carbon dioxide greenhouse forcing required for deglaciation of the Neoproterozoic snowball Earth. They also found that greenhouse forcing can vary by nearly an order of magnitude within the accepted albedo ranges, suggesting that the physics of deglaciation in terms of radiation budgets, snow and ice dynamics, and atmospheric processes needs to be better modeled.
Title: Deglaciating the snowball Earth: Sensitivity to surface albedo
J. P. Lewis, A. J. Weaver, and M. Eby: School of Earth and Ocean Sciences, University of Victoria, British Columbia, Canada.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027774, 2006
2. Seismic wave speed in the lower mantle
Seismic wave speeds in Earth's lower mantle are governed by the elastic properties of the minerals ferropericlase and silicate perovskite. Therefore, knowing the velocity of sound though these minerals at high pressures and temperatures is essential to understanding properties of the lower mantle. However, experimental studies of ferropericlase have been restricted to pressures below about 20 gigapascals, which is insufficient to model the change in the electron spin configuration of iron known to occur within the lower mantle pressures. Past studies, using x-ray diffraction to produce the needed high pressures, showed increases in bulk modulus and bulk sound velocity for ferropericlase as pressure increased. In this study, Lin et al. used the nuclear resonance inelastic x-ray scattering technique to measure compressional and shear wave velocities and the shear modulus of ferropericlase at pressures above and below the threshold for iron electron spin pairing changes. The authors noted a dramatic jump in sound velocities across this transition as pressure increased. They suggest that models of seismic wave speed in the lower mantle be revised to reflect this result.
Title: Sound velocities of ferropericlase in the Earth's lower mantle
Jung-Fu Lin and Choong-Shik Yoo: Lawrence Livermore National Laboratory, Livermore, California, U.S.A.;
Steven D. Jacobsen: Geophysical Laboratory, Carnegie Institute of Washington, Washington D.C., U.S.A.; Department of Geological Sciences, Northwestern University, Evanston, Illinois, U.S.A.;
Wolfgang Sturhahn and Jiyong Zhao: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, U.S.A.;
Jennifer M. Jackson: Geophysical Laboratory, Carnegie Institute of Washington, Washington D.C., U.S.A.; now at Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028099, 2006
3. Water in Saturn's ionosphere
To learn more about Saturn's outer atmosphere, scientists use radio occultation, a method where radio signals are sent through Saturn's ionosphere to Earth from a nearby spacecraft. The characteristics of the received signal help to determine densities of ionospheric material. Past occultation data from Pioneer 11 and Voyager spacecraft revealed a highly variable Saturnian ionosphere. Moore et al. studied similar measurements recently reported by the Cassini radio science team. These new measurements not only reinforced the presence of ionospheric variability, but also provided enough data to clearly identify a dawn/dusk asymmetry in Saturn's equatorial ionosphere. Based on suggestions that water vapor in Saturn's upper atmosphere might originate from its satellites and rings, the authors used a new three-dimensional model that includes water diffusion to demonstrate the catalytic effect of water. They show that molecules of water can exchange charge with free protons to create short-lived molecular ions that combine quickly with electrons, thereby depleting Saturn's ionosphere. Such corrections better predict the dawn/dusk asymmetry in densities measured by Cassini.
Title: Cassini radio occultations of Saturn's ionosphere: I. model comparisons using a constant water flux
Luke Moore and Michael Mendillo: Center for Space Physics, Boston University, Boston, Massachusetts, U.S.A.;
Andrew F. Nagy: Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan, U.S.A.;
Arvydas J. Kliore: California Institute of Technology/Jet Propulsion Laboratory, Pasadena, California, U.S.A.;
Ingo Müller-Wodarg: Center for Space Physics, Boston University, Boston, Massachusetts, U.S.A.; Department of Physics and Astronomy, Imperial College, London, United Kingdom;
John D. Richardson: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.
Source Geophysical Research Letters (GRL) paper 10.1029/2006GL027375, 2006
4. Mapping the boundaries between water currents using seismic reflection
Understanding the nature of the boundaries between water masses is critical to studies of large-scale oceanic circulation and global climate change. Termed finestructure, the detailed dynamics of these boundaries are usually investigated using vertical profiles of temperature and salinity, collected at discrete locations. Noting that the horizontal scale and continuity of finestructure cannot be well understood through these methods, Nakamura et al. used a new low-energy technique to map oceanic finestructure. In this method, seismic reflection profiles were used to determine where the warm Kuroshio current meets the cold Oyashio current in the ocean east of Japan. Because the boundary between the two water masses is characterized by different temperature and salinity profiles, the authors were able to use seismic reflection data to determine properties of the finestructure. Synthetic seismogram calculated from temperature and salinity data confirmed that the seismic reflections correlated well with physical oceanographic structures, suggesting that this new method will be useful for future studies of oceanic finestructure.
Title:Simultaneous seismic reflection and physical oceanographic observations of oceanic finestructure in the Kuroshio extension front
Yasuyuki Nakamura, Takashi Noguchi, Takeshi Tsuji, Sachihiko Itoh, and Hiroshi Niino: Ocean Research Institute, University of Tokyo, Tokyo, Japan;
Toshifumi Matsuoka: Graduate School of Engineering, Kyoto University, Kyoto, Japan.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027437, 2006
5. Landslide at Mt. Etna generated a large tsunami in the Mediterranean Sea nearly 8000 years ago
Geological evidence indicates that the eastern flanks of Mt. Etna volcano, located on Italy's island of Sicily, suffered at least one large collapse nearly 8,000 years ago. Pareschi et al. modeled this collapse and discovered that the volume of landslide material, combined with the force of the debris avalanche, would have generated a catastrophic tsunami, which would have impacted all of the Eastern Mediterranean. Simulations show that the resulting tsunami waves would have destabilized soft marine sediments across the floor of the Ionian Sea. The authors note that field evidence for this destabilization can be seen in other scientists' accounts of widespread large chaotic deposits of sediments in the Ionain and Sirte Abyssal Plains and tsunami-related deposits called homogenite on local depressions of the Ionian seafloor. They also speculate that this tsunami may have led to the abandonment of a Neolithic village in Israel.
Title: The lost tsunami
Maria Teresa Pareschi, Enzo Boschi, and Massimiliano Favalli: Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027790, 2006
6. Drill hole reveals what San Andreas Fault looks like at depth
The San Andreas Fault Observatory at Depth (SAFOD) main hole, drilled across the San Andreas Fault near Parkfield, California, in 2004 and 2005, provides project scientists with an opportunity to study active traces of the San Andreas Fault system that intersect the trajectory of the hole. Solum et al. analyzed four kilometers [two miles] of retrieved samples and described multiple faults that divide the SAFOD hole into mineralogical zones. In particular, they characterized six primary mineralogical zones, separated by five faults, providing the scientific community with a snapshot of what a major plate-bounding fault system looks like at depth. The authors note that the mineral assemblages and clay to non-clay ratio in fault rocks was highly variable, and they suggest that future work, planned for 2007, should focus on how mineralogy relates to active traces of the San Andreas Fault. They expect that such studies will help address long-standing issues, such as the causes of apparent weaknesses of the San Andreas Fault and the processes that control seismic motion and aseismic creep.
Title: Mineralogical characterization of protolith and fault rocks from the SAFOD main hole
John G. Solum, Stephen H. Hickman, David A. Lockner, and Diane E. Moore: Earthquake Hazards Team, U.S. Geological Survey, Menlo Park, California, U.S.A.;
Ben A. van der Pluijm and Anja M. Schleicher: Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, U.S.A.;
James P. Evans: Department of Geology, Utah State University, Logan, Utah, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027285, 2006
7. Nitrogen oxide pollutants have declined over the eastern United States since 1999
Nitrogen oxides (known as NOx) emitted by fossil fuel combustion play a crucial role in producing ground level ozone, a pollutant hazardous to human health that contributes to smog over urban areas. In 1999, coal-burning power plants represented about 25 percent of U.S. manmade NOx emissions, and recent pollution control measures by utility companies have sought to reduce NOx emissions. Kim et al. analyzed satellite data and air quality model simulations to document regional trends in emissions. They found a declining regional trend in NOx emissions in the eastern United States. Over the Ohio River Valley, where power plants dominate NOx emissions, NOx pollution has decreased by 40 percent since 1999. This decrease is larger than that seen in the northeast urban corridor. The researchers' model simulations predict lower ground-level ozone concentrations as a result of these NOx emission reductions. They suggest that further substantive reductions in eastern U.S. NOx levels will require decreases in mobile sources of Nox emissions, such as car exhaust.
Title: Satellite-observed US power plant NOx emission reductions and their impact on air quality
S.-W. Kim, S. A. McKeen, G. J. Frost, and E.-Y. Hsie: Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, U.S.A.; also at Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, U.S.A.;
A. Heckel, A. Richter, J. P. Burrows: Institute of Environmental Physics and Institute of Remote Sensing, University of Bremen, Germany;
M. K. Trainer: Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, U.S.A; S. E. Peckham and G. A. Grell: Global Systems Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, U.S.A.; also at Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027749, 2006
I. Highlights, including authors and their institutions
II. Ordering information for science writers and general public
I. Highlights, including authors and their institutions
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