AGU journal highlights -- 5 July 2006

Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL) and Paleoceanography (PA).

You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

1. GPS can aid with ocean-wide tsunami warning systems

The 26 December 2004 Sumatra earthquake (Mw 9.2-9.3) generated the most deadly tsunami in history. Yet within the first hour, the true danger of a major ocean-wide tsunami was not estimated by seismic magnitude estimates, which were far too low (Mw 8.0-8.5), due to the inherent saturation of seismic wave detection methods early in the process of the earthquake. Instead, Blewitt et al. propose a different method, where the earthquake's true size and tsunami potential is determined using Global Positioning System (GPS) data as soon as 15 minutes after earthquake initiation. The authors re-examined data collected during the Sumatra earthquake and found that GPS receivers as far away as India observed a change in relative position, corresponding to the movement of the plates, within minutes of the earthquake's initiation. By using the size and directions of these displacements and early seismographic estimates of the earthquake's location, they found that the best fitting model implied a moment magnitude of about 9.0, indicating a high tsunami potential. The authors suggest that GPS infrastructure could be developed into an effective component of tsunami warning systems.

[See also AGU press release 06-23: http://www.agu.org/sci_soc/prrl/prrl0623.html]

Title: Rapid determination of earthquake magnitude using GPS for tsunami warning systems

Authors: Geoffrey Blewitt, Corné Kreemer, William C. Hammond, and Hans-Peter Plag: Nevada Bureau of Mines and Geology, and Seismological Laboratory, University of Nevada, Reno, Nevada, USA; Seth Stein and Emile Okal: Department of Geological Sciences, Northwestern University, Evanston, Illinois, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026145, 2006

2. Unsolved problems in the lowermost mantle

Observed within the deepest several hundred kilometers [miles] of the mantle, large anomalies in seismic velocities, called the D" layer, have come under recent scrutiny since scientists announced the discovery of a possible new high-temperature, high-pressure, crystal packing structure transformed from the common mantle mineral perovskite. As new research emerges to synchronize this discovery with the current understanding of the lowermost mantle, Hirose et al. sought to list the important unsolved problems in mineral physics, seismology, and geodynamics that must first be addressed. Among other considerations, these questions involve the origin of the D" discontinuity; the determination of whether the D" layer is chemically heterogeneous, and if so, why; and the seismically inferred jump at this layer, which is too large in comparison to first-principles calculations. The authors expect that revealing the relative roles between phase transitions and chemical anomalies, though challenging, will prove critical to understanding the role of the D" layer in the thermal and chemical evolution of the Earth.

Title: Unsolved problems in the lowermost mantle

Authors: Kei Hirose: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan; Shun-ichiro Karato: Department of Geology and Geophysics, Yale University, New Haven, Connecticut, USA; Vernon F. Cormier: Physics Department, University of Connecticut, Storrs, Connecticut, USA; David A. Yuen: Department of Geology and Geophysics, and Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025691, 2006

3. Hurricanes, submarine groundwater discharge, and Florida's red tides

In January 2005, an intense red tide affected shallow waters off Florida's west-central coast, triggering a widespread hypoxic [low oxygen] zone that killed sea-bottom communities, fish, turtles, birds, and marine mammals. Over the past 120 years, numerous reports of red tides in the same area have puzzled scientists about the source of the nutrients that initiate and maintain such extensive toxic algal blooms. Noting that runoff alone could not likely have provided sufficient nitrogen to support the 2005 bloom, Hu et al. hypothesized that submarine groundwater discharge (SGD) may have supplied the missing nutrients. They found that SGD inputs of dissolved inorganic nitrogen in Tampa Bay alone make up a significant component--about 35 percent of that discharged by all central Florida rivers draining westward combined. Additionally, the unusual number of hurricanes in 2004 resulted in high runoff, and thus possibly in higher than normal SGD off Florida throughout 2005, which could have helped initiate and sustain the red tide. Similar SGD influx may also help initiate and sustain recurrent red tides elsewhere along the Gulf of Mexico, the authors suggest.

Title: Hurricanes, submarine groundwater discharge, and Florida's red tides

Authors: Chuanmin Hu and Frank E. Muller-Karger: College of Marine Science, University of South Florida, St. Petersburg, Florida, USA; Peter W. Swarzenski: U.S. Geological Survey, St. Petersburg, Florida, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025449, 2006

4. Modeling Jupiter's x-ray aurorae

More than twenty-five years ago, x-ray emissions from Jupiter's atmosphere were first detected near the planet's north and south poles, defining what is now known as the Jovian x-ray aurora. Initial hypotheses on the aurora's origins lacked mechanisms to show how ions could be accelerated to high energies, a factor necessary to produce the observed emission spectrum. Later hypotheses suggested that heavy solar wind and outer magnetospheric ions can be accelerated along magnetic field lines to high energies, producing the necessary emission spectra. Kharchenko et al. sought to predict the exact x-ray spectra of the aurorae, which they modeled as a mixture of oxygen and sulfur ions precipitating into the Jovian atmosphere and undergoing charge-exchange collisions. They compared their simulations to observations from the Chandra and XMM-Newton observations, and found that the best agreements were reached when an equal population of sulfur and oxygen ions are present at high energies. Their calculations define the area of ion precipitation, the incoming ion flux, and Jupiter's upper atmospheric density, and their results support the view that precipitating ions are accelerated magnetospheric particles.

Title: Ion emission spectra in the Jovian x-ray aurora

Authors: V. Kharchenko and A. Dalgarno: Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA; D. R. Schultz: Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA; P.C. Stancil: Department of Physics and Astronomy, and Center for Simulational Physics, University of Georgia, Athens, Georgia, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026039, 2006

5. High-energy solitary waves propagate through the South China Sea

When oceanic tides encounter undersea topography, large wave-like disturbances are generated in the interior of the sea. If enough power is put into these so-called "internal tides," non-linear solitary waves packets can develop as the waves propagate, fueled by the energy of the underlying tide. Observed in satellite imagery by their surface expression, these solitons are among the most energetic phenomena in the ocean. Klymak et al. studied solitons in the South China Sea that appear to be the most energetic observed to date. The waves originated near the Straits of Luzon and propagated westward across the basin. The authors documented soliton evolution, tracking crest amplitudes, half-widths, and phase speeds. These speeds were noticeably faster than quasi-linear small amplitude waves. The authors also found that the solitons were associated with only moderate shears and triggered only weak turbulent dissipation, causing energy to be transported with relatively little loss. Based on models and observations, the larger (spring-tide) solitary wave packet supported around 4.5 gigawatts of energy flux per crest, with the energy dissipated on the Chinese continental slope and shelf.

Title: Prototypical solitons in the South China Sea

Authors: Jody M. Klymak and Robert Pinkel: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA; Cho-Teng Liu: Institute of Oceanography, National Taiwan University, Taipei, Taiwan; Antony K. Liu: Office of Naval Research Global-Asia, Tokyo, Japan; Laura David: Marine Science Institute, University of Philippines, Manila, Philippines.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025932, 2006

6. Are ice-rafted debris in cores indicative of past climate fluxes or ice sheet instability?

Ice-rafted debris (IRD) found within oceanic cores from mid latitudes of the North Atlantic has generally been thought to result from "Heinrich events" (HEs), intermittent instabilities in the Laurentide Ice Sheet, associated with a buildup of subglacial meltwater beneath the ice sheet. Several times during the last glacial cycle, the ice sheet surged over this lubricated base, transporting glacially-scoured continental detritus into the ocean. Marshall and Koutnik suggest that Dansgaard-Oeschger (D-O) cycles also generated ice-rafted debris, separate from ice sheet surges. D-O climate oscillations caused ice sheet extent to fluctuate. During cold periods of these cycles, ice sheets advanced onto the continent shelf, calved icebergs, and deposited glacially-scoured material into the oceans. Based on a model of ice sheet formation and advance, the authors note that peak iceberg fluxes associated with Dansgaard-Oeschger cycles occurred about 500 years after the inception of cooling. However, their model did not predict Heinrich events. HEs were infrequent, high impact events--only four to six ice-rafted debris layers during the last 60,000 years resulted from HEs--whereas IRD deposition from cold phases of the D-O cycles was more frequent and was derived from circum-Atlantic sources.

Title: Ice Sheet Action vs. Reaction: Distinguishing between Heinrich Events and Dansgaard-Oeschger Cycles in the North Atlantic

Authors: Shawn J. Marshall: Department of Geography, University of Calgary, Calgary, Alberta, Canada; Michelle R. Koutnik: Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA.

Source: Paleoceanography (PA) paper 10.1029/2005PA001247, 2006

7. A new crystal packing system for high-pressure, high-temperature perovskite?

Perovskite, a densely-packed mineral form of pyroxine (MgSiO3), exists within high-pressure phases of the mantle. To understand the pressure-temperature dynamics of perovskite, Martin et al. studied neighborite (NaMgF3), a mineral with similar crystal lattice structure. Using x-ray diffraction techniques to analyze atomic packing within neighborite under various pressures and temperatures, the authors found that at high pressures, neighborite and perovskite behave similarly, transforming into a different high-pressure mineral form between 28 and 30 gigapascals. However, they noted that when heated with a laser, this new neighborite mineral form further transformed into a crystal packing system that has been hypothesized, but not observed in nature. When the pressure was released, this unknown mineral again transformed, this time into a crystal-barren amorphous structure, similar to glass. Though there are marked differences between perovskite and neighborite, the authors propose that if the analogy between their behaviors holds, then the unknown mineral phase in perovskite could explain the presence of a deep seismic reflection layer previously seen on the mantle side of the core-mantle boundary.

Title: Phase transitions and compressibility of NaMgF3 (neighborite) in perovskite- and post-persovskite-related structures

Authors: C. David Martin and Jiuhua Chen: Geosciences Department, Stony Brook University, Stony Brook, New York, USA; Wilson A. Crichton: European Synchotron Radiation Facility (ESRF), Grenoble, France; Haozhe Liu: High Pressure Collaborative Access Team (HP-CAT), Sector 16, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, USA; Vitali Prakapenka: Chemistry Department, and Geosciences Department, Stony Brook University, Stony Brook, New York, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026150, 2006

8. Oceanic deep water formation as a sink of persistent organic pollutants

Persistent organic pollutants (POPs), especially the group of polychlorinated biphenyls (PCBs), are potential human carcinogens that many organisms cannot metabolize, causing them to accumulate in tissue. Before their production was banned, PCBs were used as coolants in the electrical industry; now they are mainly found in old appliances and brownfields. Noting that global thermohaline circulation moves surface waters directly into the deep ocean due to gradients in salinity and temperature, Lohmann et al. hypothesized that during deep water formation, POPs such as PCBs are transported to the deep ocean, providing an important sink for these chemicals. For the four main deep water formation regions (the Norwegian, Labrador, Ross, and Weddell Seas), the authors calculated removal fluxes of PCBs, and found that more PCBs were removed from surface waters due to deep water formation than due to PCBs settling onto the ocean floor as organisms die. The authors conclude that several POPs could serve as tracers for oceanic deep water plumes, as these chemicals were produced for only a few decades.

Title: Oceanic deep water formation as a sink of persistent organic pollutants

Authors: Rainer Lohmann and Michael E. Q. Pilson: Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USA; Elena Jurado and Jordi Dachs: Department of Environmental Chemistry, Chemical and Environmental Research Institute of Barcelona; Spanish National Research Council (IIQAB-CSIC), Barcelona, Spain.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025953, 2006

9. Background stratospheric sulfate aerosol concentrations are dominated by pollution from the tropics

Stratospheric sulfate aerosols [fine particles] not only serve as catalysts in the chemical reactions that determine ozone concentrations, but their presence scatters sunlight, buffering against greenhouse warming. This layer is formed by natural volcanic sources and human industrial activities. To separate these contributions, Padma Kumari et al. used a twilight photometry technique to analyze atmospheric scattering patterns, in order to study stratospheric aerosol concentrations over tropical locations between January 2000 and December 2003, a period of volcanic quiescence. Using stations based in India, the authors found that annual variations occur in the background stratospheric aerosol concentrations, with maximum loading in winter, decreased levels in spring, and lowest levels during summer. This pattern is also seen at Mauna Loa Observatory in Hawaii, indicating that the variability is not local, but global. The authors note that the source for the background aerosol level appears to be in the tropics, and they indicate that as manmade emissions of sulfur over developing countries increases, the more aerosols may reach the stratosphere through strong monsoon convection and the systems of Inter-Tropical Convergence Zone.

Title: Seasonal variability in the stratospheric aerosol layer in the current volcanically quiescent period over two tropical stations in India using the twilight sounding method

Authors: B. Padma Kumari, A. L. Londhe, D. B. Jadhav, and H. K. Trimbake: Indian Institute of Tropical Meteorology, Pune, India.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026087, 2006

10. Induced seismicity from deep gold mines in South Africa can potentially shed light on natural earthquake processes

During laboratory experiments of earthquakes, seismologists have observed a stable rupture preceding an unstable rupture, a phenomenon called "earthquake nucleation process." Attempts have been made to record the nucleation process of natural earthquakes, but none has yet been observed. Noting that observing nucleation in natural earthquakes requires the observation of rock-mass behavior near the source and that mines are known to induce seismicity, Naoi et al. examined in high detail the continuous strain records within the Bambanani mine, a deep gold mine in South Africa. Using an Ishii strainmeter installed at a seismically active part of the mine, the authors found strain release events that occurred over various lengths of time. About 70 percent were normal earthquakes, while 24 percent were slow-step events, which release strain silently, without generating seismic shake. Some of the latter were preceded by nucleation. The authors anticipate that more spatially comprehensive work relating to mine induced seismicity will shed light on natural earthquake-generation processes.

Title: Small slow-strain steps and their forerunners observed in gold mine in South Africa

Authors: Makoto Naoi, Akihito Yamamoto, and Ken Morishita: Graduate School of Science and Engineering, Ritsumeikan University, Kusatsu, Japan; Hiroshi Ogasawara: Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Japan; Junichi Takeuchi: National Instrument Corporation, Tokyo, Japan; Naoyuki Shimoda: Japan Oil, Gas and Metals National Corporation, Kawasaki, Japan; Hiroshi Ishii: Tono Research Institute of Earthquake Science, Mizunami, Japan; Shigeru Nakao: Faculty of Science, Kagoshima University, Kagoshima, Japan; Gerrie van Aswegen, Aleksander J. Mendecki, and Patrick Lenegan: ISS International Ltd., Stellenbosch, South Africa; Rookshana Ebrahim-Trollope: Geohydroseis CC, Klerksdorp, South Africa; Yoshihisa Iio: Disaster Prevention Research Institute, Kyoto University, Uji, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026507, 2006

11. Monitoring the Atmospheric Boundary Layer through the occultation of GPS signals

The Atmospheric Boundary Layer (ABL) is the lowermost layer of the atmosphere and is characterized by turbulence. Knowing the height and thermodynamic properties of the ABL is important for understanding the vertical exchange of momentum, heat, and water vapor between Earth's surface and the atmosphere, and thus is important for weather prediction and climate science. The transition between the ABL and the overlying, more stably stratified troposphere is generally accompanied by a temperature inversion and a sharp decrease in water vapor. Sokolovskiy et al. examined the frequency and amplitude of radio signals as they passed through the atmosphere from the GPS satellites to the lower-orbiting Argentine SAC-C satellite, which carried a NASA GPS receiver developed by the Jet Propulsion Laboratory. The authors used abrupt changes in the slope of the vertical profile of refractivity to identify the top of the Atmospheric Boundary Layer. Obtaining accurate profiles of refractivity this low in the atmosphere through radio occultation was made possible by a new technique known as open-loop tracking. The authors expect that the newly launched COSMIC satellites will provide global observations of the ABL's height and structure.

Title: Monitoring the atmospheric boundary layer by GPS radio occultation signals recorded in the open-loop mode

Authors: S. Sokolovskiy, Y.-H. Kuo, C. Rocken, W. S. Schreiner, D. Hunt, and R. A. Anthes: University Corporation for Atmospheric Research, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL025955, 2006

12. Antarctic Bottom Water is produced and exported by tides in the Ross Sea

In the waters around Antarctica, scientists had previously identified the three water masses that contribute to the formation of Antarctic Bottom Water (AABW): cold, fresh Antarctic Surface Water; warm, salty Circumpolar Deep Water, and shelf water with temperatures near- to below-freezing at a broad range of salinities. It was assumed that AABW formation must take place near the edge of the continental shelf, where all three parent water masses are present, but the process had never been observed directly. Whitworth and Orsi analyzed data from an array of moored instruments deployed during the austral summer of 2003 near the shelf break of the Ross Sea as part of the multi-institutional AnSlope experiment. They found that AABW is produced by daily tidal stirring of the three parent water masses. Tides carry Circumpolar Deep Water from offshore to the shelf and stir the lower portion of the water column to a near-homogeneous mixture with Antarctic Bottom Water characteristics at the shelf break. The formation rate is estimated at about two million cubic meters [500 million gallons] per second.

Title: Antarctic Bottom Water production and export by tides in the Ross Sea

Authors: T. Whitworth III and A. H. Orsi: Department of Oceanography, Texas A&M University, College Station, Texas, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL026357, 2006

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