AGU Journal highlights - 14 April 2005

04/14/05

Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

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., 2004GL987654). The doi is found at the end of each Highlight, below.

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1. A new take on Martian landslides

The 1976 Viking Orbiter was the first spacecraft to collect images of landslides in the Ganges Chasma, a large canyon on Mars. Landslides helped create Mars' present landscape and may be indicators of environmental conditions on the planet. From data collected on the Viking mission, researchers concluded that landslides are an order of magnitude larger than terrestrial ones and that the circular headscarps and large aprons were the result of a high-speed, catastrophic landslide. Using higher-resolution images from more recent Mars missions, Bulmer and Zimmerman propose an alternative explanation based on observations of large rock terraces, chutes, and talus fields. They compared the height, length, and volume of the Ganges Chasma landslide with landslides on Earth. Recalculating the dimensions using the newer data, they conclude that the Martian landslide may be the result of a slow gravity-driven process over the course of millions of years, not the catastrophic event previously suggested. The researchers propose that rock terraces can be explained by deep-seated gravitational creep, while the movement of the talus is similar to that of slow-moving rock glaciers on Earth, but they concede that this would require ice, which has not yet been found in Ganges Chasma.

Title: Reassessing landslide deformation in Ganges Chasma, Mars

Authors: Mark H. Bulmer and Brandy A. Zimmerman, University of Maryland, Baltimore, Maryland, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022021, 2005.

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2. From silk to satellite: A half century of ocean color

Phytoplankton play a major role in the absorption and storage of carbon dioxide in the oceans. Because phytoplankton contain chlorophyll, it is possible to estimate their abundance by measuring the color of ocean waters. Since 1931, researchers on ships have collected plankton on silk and estimated phytoplankton biomass based on the resulting color of the silk. Beginning in 1997, images from NASA's SeaWiFS satellite made it possible to measure the color of large regions of the ocean and estimate phytoplankton abundance. The usefulness of the SeaWiFS dataset is, however, limited by its short time span. Raitsos et al. compared the satellite data and the data collected from ships for a region of the Northeast Atlantic from 1997 to 2002. They found that the two datasets strongly agreed in their estimates of phytoplankton abundance. This allowed them to use the ship-collected data to extrapolate the SeaWiFS dataset back to 1948. They suggest that this extended dataset will improve models of marine ecosystems and our understanding of biogeochemical cycling and climate change.

Title: Extending the SeaWiFS chlorophyll data set back 50 years in the northeast Atlantic

Authors: Dionysios E. Raitsos, Philip C. Reid, Samantha J. Lavender, Martin Edwards, and Anthony J. Richardson, University of Plymouth, Plymouth, United Kingdom.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022484, 2005.

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3. Aerosols reduce solar radiation over China

Human activities, such as burning fossil fuels and disturbing natural land cover, increase the amount of aerosols suspended in the Earth's atmosphere. Changes in aerosol concentrations affect the balance of solar radiation entering and leaving the atmosphere, and are thus an important topic in climate change research. In contrast to greenhouse gases, an increase in aerosols tends to shift the balance towards radiation leaving the atmosphere. Using solar radiation measurements taken from 1961 to 2000 at stations throughout China, Che et al. characterize long-term trends in radiation levels. They found that since 1961, there have been significant decreases in China in daily surface solar radiation and monthly sunshine duration. The change was greatest in the eastern portion of the country, where the densest human populations and the majority of the country's industry are located. The researchers conclude that the best explanation for the changes in solar radiation and sunshine duration in much of China is rising aerosol concentrations in the atmosphere, due to increased burning of fossil fuels.

Title: Analysis of 40 years of solar radiation data from China, 1961 - 2000

Authors: H.Z. Che, G.Y. Shi, X.Y. Zhang, R. Arimoto, J.Q. Zhao, L. Xu, B. Wang, and Z.H. Chen, Institute of Earth Environment, Chinese Academy of Sciences, Xian, China

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022322, 2005.

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4. Poor estimates of GPS satellite offsets lead to large-scale errors

Data produced by the Global Positioning System (GPS) are used for a variety of purposes, from scientific research to aircraft navigation. Precise measurements from GPS networks require accounting for the offset between the geometric centers and the actual receiving locations, or phase centers, of the satellite antennas. In 2000, a large shift in the scale of GPS solutions corresponded with the launch of five new GPS satellites. Ge et al. suggest that the shift was caused by a large inconsistency in the International GPS Service standard offsets and by the incorrect assumption that all of the satellites belonging to the same block-type had the same offsets. They point out that each satellite is different, so assuming that an entire group has the same offset leads to inaccuracies that compound as the data from the satellites are combined. Using antenna offsets specific to individual satellites, they recalculated GPS data from 1997 to 2003. The results indicate that using satellite-specific offsets produces more accurate network scales. The researchers suggest that software used to process GPS data should use satellite-specific offsets, and that weekly GPS solutions should include offsets to the offset parameters for further improvement.

Title: Impact of GPS satellite antenna offsets on scale changes in global network solutions

Authors: M. Ge, G. Gendt, G. Dick, F. P. Zhang, and C. Reigber, GeoForschungsZentrum Potsdam, Potsdam, Germany.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022224, 2005.

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5. Role of gas-formed aerosols in cloud formation probed

Industrial emissions release aerosols directly into the atmosphere, where they affect climate and human health. These particles also form through nucleation, the condensation of gases in the atmosphere. Laaksonen et al. investigated the role of nucleation in the formation of aerosols in Italy's densely populated Po Valley region over two years, beginning in 2002. They wanted to know what part nucleation events played in the formation of cloud condensation nuclei (CCN), particles around which cloud droplets form and which affect the radiative properties of clouds. The researchers took continuous measurements just above the ground to determine the size distribution and growth rates of particles in the atmosphere and made microlight aircraft measurements of concentrations of nucleated particles at various altitudes. This allowed them to estimate the relative contribution of nucleation events to the concentration of aerosols larger than 100 nanometers in diameter, and thus to CCN production. The results suggest that nucleation events play a greater role than expected in producing CCN, and that these particles can form more quickly and are more durable than previously thought.

Title: Cloud condensation nucleus production from nucleation events in a highly polluted region

Authors: Ari Laaksonen, Amar Hamed, Jorma Joutsensaari, Laura Hiltunen, Fabrizia Cavalli, Wolfgang Junkermann, Ari Asmi, Sandro Fuzzi, and Maria Cristina Facchini, University of Kuopio, Kuopio, Finland.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022092, 2005.

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6. A better view of mantle magma formation

Volcanic island chains such as those in the Pacific Ocean result from "hot spots," where magma plumes form in Earth's mantle and erupt through the crust. The formation of voluminous magma beneath mid-ocean ridges is believed to result from adiabatic decompressions of rocks that do not involve significant loss of heat to the surroundings. Following up on a contribution by Waldbaum in 1971, where he emphasized the need to consider a result from the first law of thermodynamics that incorporates the effect of irreversible expansion of material during decompression, Ganguly formulated models of adiabatic decompression that include the effects of irreversibility, gravity, and acceleration. These results show that standard models of adiabatic decompression give inadequate portrayal of the ascent and melt productivity of the mantle rocks. Ganguly's reformulations provide better view of these processes.

Title: Adiabatic decompression and melting of mantle rocks: An irreversible thermodynamic analysis

Author: Jibamitra Ganguly, Department of Geosciences, University of Arizona, Tucson, Arizona, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022363, 2005

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7. Denmark Strait currents more dynamic than thought

Differences in the density of seawater help drive circulation of Earth's oceans and thus have a large influence on global climate. An important player in this circulation is the cold, dense water that flows from the Nordic Seas through the Denmark Strait and into the North Atlantic Ocean. Based on earlier studies it was thought that the density and temperature of the water flowing across the strait fluctuated little throughout the year. However, the studies may have suffered from short time frames and low-resolution data, preventing accurate characterization of the water flowing through the strait. Macrander et al. point out that the waters of the Nordic Seas fluctuate throughout the year, so it is probable that the strait waters fluctuate as well. Using data collected by sensors at various locations in the Strait from 1999 to 2003, they determined that the temperature and density of the water fluctuates much more than previously thought. They suggest that a long-term observing system be established to track the movement of water from the Nordic Seas to the Atlantic and to explore the effects of the observed variations on large-scale circulation in the Atlantic.

Title: Interannual changes in the overflow from the Nordic Seas into the Atlantic Ocean through Denmark Strait

Authors: A. Macrander, U. Send, H. Valdimarsson, S. Jonsson, and R. H. Kaese, Leibniz-Institut fuer Meereswissenschaften, IFM-GEOMAR, Kiel, Germany.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021463, 2005.

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8. Accurately modeling the shelf sea pump

A new study shows that the mechanisms driving the "shelf sea pump" that helps transfer carbon dioxide from the atmosphere to the deep ocean are not as simple as previously thought. Rippeth et al. analyzed the mixing zones at several locations in the continental shelf seas--the often-shallow coastal waters surrounding the continents-- in northwest Europe and found a surprising amount of mixing between the warm, carbon dioxide-rich surface waters and the cold, deeper water far from the continental shelf. The authors propose that the mixing was likely caused by local instabilities initiated by shear flow from, for example, currents or wind energy. Earlier studies had posited that the largest contributor to the turbulence that mixes the carbon dioxide was from energy stirred up close to the seabed and where the continental shelf ends. The researchers suggest that continental shelf models should better reflect the mixing linking the warm surface waters with the cold deepwater in shelf seas.

Title: Thermocline mixing in summer stratified continental shelf seas

Authors: Tom P. Rippeth, Matthew R. Palmer, John H. Simpson, Neil R. Fisher, University of Wales Bangor, Anglesey, United Kingdom; Jonathan Sharples, University of Liverpool, Liverpool, United Kingdom.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022104, 2005.

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9. Improving accuracy of ozone depletion models

Measurements of bromine monoxide recorded from aircraft, balloon, and space-borne instruments reveal much higher amounts of the ozone-depleting radical than estimates used in current models. Salawitch et al. present data indicating that inorganic bromine levels throughout the lowermost stratosphere could be as high as 4-8 parts per trillion, compared to the negligible amounts assumed in existing ozone assessment models. Such models underestimate the amount of ozone that has been lost over the past 25 years at northern hemisphere mid-latitudes due to human activities. The authors suggest that the additional bromine could resolve the discrepancy between the actual and calculated ozone loss, as the bromine compound reacts with other man-made atmospheric chemicals. The researchers suggest that the additional bromine likely comes from natural emissions from Earth's oceans and note that the emissions may be sensitive to changes in the ocean circulation and temperature.

Title: Sensitivity of ozone to bromine in the lower stratosphere

Authors: Ross J. Salawitch, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA;
D. K. Weisenstein, Atmospheric and Environmental Research, Inc., Lexington, Massachusetts, USA;
L. J. Kovalenko, P. O. Wennberg, California Institute of Technology, Pasadena, California, USA;
C. E. Sioris, K. Chance, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA;
M. K. W. Ko, NASA Langley Research Center, Hampton, Virginia, USA;
C. A. McLinden, Meteorological Service of Canada, Toronto, Ontario, Canada.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021504, 2005.

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10. Similarities in early Mars, Earth evolution

Volcanic plumes and thunderstorms billions of years ago on Mars may have led to lightning strikes on the planet and created conditions favorable for biological life similar to those that preceded molecular life on Earth. Segura and Navarro-Gonzalez estimate the amount of nitrogen compounds on early Mars, when the planet is believed to have been significantly warmer and contained active volcanoes and flowing groundwater on its surface. The authors propose a scenario where lightning that formed in explosive volcanic clouds created an energy source for key molecules needed for chemical evolution and the origins of life. The researchers simulated such volcanic lightning in a laboratory, deriving the chemical mixture from Martian meteorites. They propose that although lightning likely produced the only source of hydrogen cyanide in the Martian atmosphere, the amount of reactive nitrogen on early Mars was similar to that estimated to exist on Earth in its prehistoric past.

Title: Nitrogen fixation on early Mars by volcanic lightning and other sources

Authors: Antigona Segura, Rafael Navarro-Gonzalez, Institute of Nuclear Science, National Autonomous University of Mexico, Mexico City, Mexico.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021910, 2005.

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