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AGU journal highlights -- August 1, 2006

American Geophysical Union

The following highlights summarize research papers in Geophysical Research Letters.

Glacier shrinkage and modeled uplift of the Alps

Glaciers put pressure on the crust, which flexes inward to bear their heavy loads. When the glaciers retreat, the load is released and the crust rebounds. Noting that the European Alps are currently experiencing rapid reductions in glacial loads, Barletta et al. sought to estimate the contribution of resulting rebound to the total observed uplift of the mountains. Using glacier inventories and induced viscoelastic responses from a stratified Earth model, they found that regions of the Alps experience a 0.4-0.5 millimeter [0.016-0.02 inch] per year uplift due to recent mass loss of largest ice complexes, which overprint observed uplift of 1.0-1.5 millimeter [0.039-0.059 inch] per year. Additionally, viscous stress relaxation in the lower crust due to glacier mass lost after the end of the Little Ice Age (c. 1850) is expected to produce uplift rates of 0.32 millimeter [0.013 inch] per year, leading to a combined uplift of about 0.8 millimeter [0.03 inch] per year, or half the total expected uplift signal. The authors note that such data will help constrain tectonic uplift values and uplift relating to unloading after the Last Glacial Maximum, 20,000 years ago.

Glacier shrinkage and modeled uplift of the Alps

V. R. Barletta, C. Ferrari, G. Diolaiuti, T. Carnielli, R. Sabadini, and C. Smiraglia: Section of Geophysics, Department of Earth Science, University of Milan, Milan, Italy.

Geophysical Research Letters (GL) paper 10.1029/2006GL026490, 2006

Mantle transition zone is not caused by high-pressure hydrous derivatives of olivine

Phase transitions among olivine and its high-pressure derivatives, wadsleyite and ringwoodite, are thought to be responsible for the seismic velocity discontinuities in the transition zone of the mantle, between 400 and 660 kilometers [250 and 410 miles]in depth. Recent studies of ringwoodite show that it can house vast quantities of water and that the incorporation of water results in a substantial decrease in rignwoodite's elastic moduli. Noting that past studies suggested that this could be responsible for trends in the mantle's seismic profile, Wang et al. investigated the elastic properties of hydrous ringwoodite at high-pressure room-temperature conditions. They found that at mantle pressures, the hydration of ringwoodite can increase pressure derivatives of elastic moduli up to seven percent for the bulk modulus and 30 percent for the shear modulus when compared with anhydrous ringwoodite. However, the velocity gradients as a function of pressure for hydrous ringwoodite are significantly less then the corresponding gradients in the Earth's transition zone. Thus, the authors conclude that the transition zone seismic velocity gradients are not due to "wet" ringwoodite, as previously speculated.

Elastic properties of hydrous ringwoodite at high pressure conditions

Jingyun Wang and Jay D. Bass: Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois, U.S.A.;

Stanislav V. Sinogeikin: Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois, U.S.A.; and Argonne National Laboratory, Argonne, Illinois, U.S.A.;

Toru Inoue: Department of Earth Science, Ehime University, Matsuyama, Japan.

Source: Geophysical Research Letters (GL) paper 10.1029/2006GL026441, 2006

Stratospheric oxygen/nitrogen ratios can help constrain carbon uptake budgets by the biosphere and ocean

Fossil fuel burning reduces the oxygen/nitrogen ratio in the troposphere [lower atmosphere] as oxygen is captured to form carbon dioxide. This initial carbon dioxide increase in the troposphere is tempered by oceanic carbon dioxide uptake, which does not affect atmospheric oxygen levels, and by land biotic uptake, which captures carbon and releases oxygen at a known rate that is smaller than the rate of carbon dioxide burning. The net result of these processes is that over time, carbon dioxide levels increase and oxygen/nitrogen ratios decrease in the troposphere. To study this further, Ishidoya et al. examined air samples collected from the stratosphere over Japan and Antarctica. They observed that the oxygen/nitrogen ratio decreased with height in this atmospheric layer, and through comparisons with diffusion models, suggested that this decrease is caused by a gravitational separation of heavy oxygen molecules from nitrogen molecules. The diffusion models, coupled with knowledge of tropospheric trends, allowed the authors to deduce the ages of their air samples and thus the amount of reduction in the tropospheric oxygen/nitrogen ratio. From this, the authors were able to calculate average terrestrial biospheric and oceanic carbon dioxide uptake for October 1993 through September 2001.

Vertical profiles of the O2/N2 ratio in the stratosphere over Japan and Antarctica

Shigeyuki Ishidoya, Shuji Aoki, and Takakiyo Nakazawa: Center for Atmospheric and Oceanic Studies, Tohoku University, Sendai, Japan;

Satoshi Sugawara: Miyagi University of Education, Sendai, Japan;

Gen Hashida, Shinji Morimoto, and Takashi Yamanouchi: National Institute of Polar Research, Tokyo, Japan.

Geophysical Research Letters (GL) paper 10.1029/2006GL025886, 2006

Electric forces enhance the emission of mineral dust aerosols

Mineral dust particles influence Earth's climate by absorbing and scattering radiation and by serving as nuclei for cloud formation. These dusts are subject to relatively large interparticle forces when compared to their weight or typical wind stress forces, preventing them from being directly lifted by surface winds. Instead, larger sand particles undergo saltation, where they are bounced by wind over the surface, ejecting smaller dust particles into the air. Noting that large electric fields (sometimes up to around 160 kilovolts per meter [yard]) have been measured in all natural dust lifting phenomena, such as blowing dust, dust devils, and dust storms, Kok and Renno examined dust from Arizona's Sonoran desert. Using theoretical calculations and laboratory experiments, they showed that electric fields within dust lifting phenomena produce electric forces strong enough to directly lift dust and sand particles. Moreover, strong fields can intensify saltation, reducing the critical wind speed necessary to initiate dust lifting. The authors conclude that electrical forces enhance the natural lifting of mineral dust aerosols into the atmosphere.

Enhancement of the emission of mineral dust aerosols by electric forces

Jasper F. Kok: Applied Physics Program, University of Michigan, Ann Arbor, Michigan, U.S.A.;

Nilton O. Renno: Department of Atmospheric, Oceanic, and Space Science, University of Michigan, Ann Arbor, Michigan, U.S.A.

Geophysical Research Letters (GL) paper 10.1029/2006GL026284, 2006

Remote sensing of sea surface temperature is influenced by ocean slicks

Infrared measurements of sea surface temperature (SST) typically survey a very thin surface layer of the ocean, approximately 10 micrometers [0.0004 inch] thick. Previous ocean studies revealed the presence of natural filmed surfaces, or "slicks," which possess a surface-active agent that reduces fluidity. Marmorino and Smith sought to more rigorously understand the behavior of slicks. They collected airborne infrared imagery over the ocean and found that slicks are 0.1 to 0.4 degrees Celsius [0.2 to 0.7 degrees Fahrenheit] cooler than the surrounding water surface. By comparing this with data on sea surface roughness, an indicator of wind conditions and ocean turbulence, the authors found that the thermal contrast of slicks was higher during calmer sea conditions. They note that such studies can help determine errors in bulk SST retrievals, can act as a potential source of information about the ambient thermal boundary layer, and may help monitor plankton blooms, assess error in wind-retrieval algorithms, and refine studies of air-sea gas exchange.

Reduction of surface temperature in ocean slicks

G. O. Marmorino and G. B. Smith: Remote Sensing Division, Naval Research Laboratory, Washington D.C., U.S.A.

Geophysical Research Letters (GL) paper 10.1029/2006GL026502, 2006

Mountain-induced circulation on Mars vertically transports water to the high atmosphere

Observations show that all primary Tharsis volcanoes are associated with discrete, bright, and optically thick clouds that are "anchored" with respect to the underlying topography and are most prevalent during Martian summer. Using a three-dimensional mesoscale Mars atmospheric model that utilizes detailed aerosol and cloud microphysics, Michaels et al. simulated these observed phenomena and show that the formation of clouds over the volcanoes is due to upslope thermal flow working in concert with a lee mountain wave circulation. These two mountain-induced atmospheric circulations pump large quantities of water vapor, water ice aerosol, and dust upward from lower levels, injecting such material into the free atmosphere, where it can be further transported globally. These processes may also be a significant mechanism for supplying material to the upper atmosphere, above 40 kilometers [25 miles] in altitude. The authors conclude that these upslope currents are important parts of Mars' net atmospheric circulation, similar to terrestrial thunderstorm convection. Thus, they affect atmospheric water cycles, dust cycles, and Martian climate.

The significant vertical water transport by mountain-induced circulations on Mars.

T. I. Michaels and S. C. R. Rafkin: Department of Space Studies, Southwest Research Institute, Boulder Colorado, U.S.A.;

A. Colaprete: NASA Ames Research Center, Moffett Field, California, U.S.A.

Geophysical Research Letters (GL) paper 10.1029/2006GL026562, 2006

Smectite clays can contribute to slope instability on volcanic flanks: Evidence from the 1998 landslide at Casita volcano, Nicaragua

In October 1998, 1.6 million cubic meters [2.1 million cubic yards] of rock collapsed on the southern flank of the dormant Casita volcano in Nicaragua, resulting in a debris avalanche that destroyed two towns and killed more than 2,500 people. Previous studies have shown that saturation from prolonged seasonal rains and precipitation from Hurricane Mitch, along with clay layers that impeded drainage at depth, triggered the failure. To determine the mechanism by which these clay layers contributed to the landslide, Opfergelt et al. analyzed samples from the volcano's slopes, finding them rich in hydrothermally altered smectite clays. They determined that the clayey material, which swells with the presence of water, acted as a barrier to water infiltration. Noting that the clay shrinks during dry conditions, the authors hypothesize that this shrink-swell behavior progressively decreased the rock's shear strength and gradually destabilized the overlying rock mass in the months, years, decades, and centuries prior to the landslide. The authors suggeste that hazard assessments associated with unstable volcanic slopes, especially those with high hydrothermal activity, should include the potential impacts of swelling clays.

The 1998 debris avalanche at Casita volcano, Nicaragua--Investigation of the role of hydrothermal smectite in promoting slope instability

S. Opfergelt and B. Delvaux: Soil Science Unit, Université Catholique de Louvain, Louvain-la-Neuve, Belgium;

P. Delmelle: formerly at Soil Science Unit, Université Catholique de Louvain, Louvain-la-Neuve, Belgium; now at Environment Department, University of York, York, United Kingdom;

P. Boivin: Institute of Research for Development (IRD), Laboratoire d'étude des Transferts en Hydrologie et Environnement. Université J. Fourier, Grenoble, France.

Geophysical Research Letters (GL) paper 10.1029/2006GL026661, 2006

Ground truths of terrestrial water storage estimates generated from satellite gravity data

The Gravity Recovery and Climate Experiment (GRACE), jointly funded by the U.S. and Germany, consists of two identical satellites in identical Earth orbits, one following the other. As the satellites pass through gravity highs and lows, the distance between them changes; these changes are then used to solve for a new Earth gravity field every month. From these fields, changes in terrestrial water storage can be inferred. These estimates are significant because there exist few onsite networks of soil and groundwater observations at large scales. To ground-truth these estimates, Swenson et al. compared water storage estimates from GRACE satellite data with soil moisture and ground water estimates from a network of observing sites in Illinois. Using a filtering technique developed by one of the authors and designed to remove errors in GRACE's spectral data, the authors found that the application of this filter significantly improved the spatial resolution of GRACE water storage estimates, producing data that agree well with field measurements averaged over an area of about 280,000 square kilometers [108,100 square miles].

Comparison of terrestrial water storage variations from GRACE with in situ measurements from Illinois

Sean Swenson and John Wahr: Department of Physics, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, U.S.A;

Pat J.-F. Yeh and James Famiglietti: Department of Earth System Science, University of California, Irvine, California, U.S.A.

Geophysical Research Letters (GL) paper 10.1029/2006GL026962, 2006


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