AGU Journal Highlights -- Sept. 16, 2008

In arid and semiarid regions, the increasing demands upon groundwater resources due to expanding metropolitan and agricultural areas present a serious challenge. In Iran, decades of unrestrained groundwater extraction have resulted in a rapid depletion of this valuable resource. Motagh et al. show that associated declines in groundwater levels are linked to land surface deformation on local and regional scales. Combining water level data with satellite radar observations, the authors find that aquifers are compacting throughout the country. This compaction results from excessive groundwater withdrawal, which in turn causes subsidence and eventually the destruction of aquifers, as evidenced by observations of groundwater basins from central and northeastern Iran. Comparing ground deformation in Iran with other basins around the world revealed that Iran currently hosts some of the fastest sinking valleys and plain aquifers in the world. Because global warming and future climate change will likely affect arid and semiarid areas in the coming decades, further augmenting hazards associated with groundwater-induced land subsidence, the authors suggest that effective water resource management may help abate potential hazards.

Title: Land subsidence in Iran caused by widespread water reservoir overexploitation

Authors: Mahdi Motagh: Department of Earthquake Risk and Early Warning, GeoForschungsZentrum, Potsdam, Germany; also at Department of Geomatics and Surveying Engineering, University of Tehran, Tehran, Iran;

Thomas R. Walter and Jochen Zschau: Department of Earthquake Risk and Early Warning, GeoForschungsZentrum, Potsdam;

Mohammad Ali Sharifi: Department of Geomatics and Surveying Engineering, University of Tehran, Tehran, Iran;

Eric Fielding: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.;

Andreas Schenk: Geodetic Institute, Universität Karlsruhe, Karlsruhe, Germany;

Jan Anderssohn: Department of Remote Sensing, GeoForschungsZentrum, Potsdam, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL033814, 2008; http://dx.doi.org/10.1029/2008GL033814

2. Climate-related mechanism may explain strange Martian craters

Pedestal craters, a subclass of impact craters unique to Mars, are characterized by a crater perched near the center of a pedestal (a mesa or plateau) that is surrounded by a roughly circular scarp rising from tens to more than 100 meters (328 feet) above the surrounding terrain. Pedestals were originally thought to have formed by impacts that made the proximal surface resistant to eolian erosion, with surrounding unarmored material later blown away to form the scarps. Kadish et al. analyze 2696 pedestal craters on Mars at middle and low latitudes and find that several pedestal craters have pits in their marginal scarps similar to sublimation depressions seen on Earth and elsewhere on Mars. These pits suggest that pedestals formed from impacts into a volatile-rich substrate. Sublimation of the intervening volatiles then lowered the elevation of the surrounding terrain, except where inhibited by the impact-hardened surfaces, yielding perched pedestals. From this, the authors interpret that pedestal craters represent the armored remnants of an extensive, ice-rich climate-related deposit that existed over a substantial part of the recent past.

Title: Martian pedestal craters: Marginal sublimation pits implicate a climate-related formation mechanism

Authors: Seth J. Kadish and James W. Head: Department of Geological Sciences, Brown University, Providence, Rhode Island, U.S.A.;

Nadine G. Barlow: Department of Physics and Astronomy, Northern Arizona University, Flagstaff, Arizona, U.S.A.;

David R. Marchant: Department of Earth Sciences, Boston University, Boston, Massachusetts, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034990, 2008; http://dx.doi.org/10.1029/2008GL034990

3. Mantle plumes: Thin, fat, successful, or failing?

Volcanoes that exist far away from plate boundaries are fueled by mantle plumes, which are thought to be jets of magma rising from the mantle to the surface, independent of plate tectonics. Noting that density variations in the mantle influence mantle upwellings and likely modify plume characteristics, Kumagai et al. explore the dynamics of plumes containing both thermal and chemical anomalies through laboratory experiments designed to simulate the mantle. They find that because all hot plumes lose heat as they rise in a cooler mantle, plumes will eventually reach neutral buoyancy, at which point they will begin to lose their upward momentum. Separation within the plume will occur, with denser material sinking while the heated surrounding mantle keeps rising. This implies that mantle plumes are not necessarily narrow and continuous columns but can be fat and patchy. Additionally, scientists cannot assume that hot mantle regions are buoyant and rising—they may be sinking. These new insights lead the authors to believe that the evolution of a mantle plume through time is critical to understanding hot spot dynamics.

Title: Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space

Authors: Ichiro Kumagai: Institute de Physique du Globe de Paris, Université Paris Diderot, CNRS, Paris, France; also at Earthquake Research Institute, University of Tokyo, Tokyo, Japan;

Anne Davaille: Institute de Physique du Globe de Paris, Université Paris Diderot, CNRS, Paris, France; also at Laboratoire FAST, Université Pierre et Marie, Curie, Université Paris-Sud 11, Orsay, France;

Kei Kurita: Earthquake Research Institute, University of Tokyo, Tokyo, Japan;

Eléonore Stutzmann: Institute de Physique du Globe de Paris, Paris, France.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035079, 2008; http://dx.doi.org/10.1029/2008GL035079

4. Aurora pulses as the solar wind gusts

Ultralow-frequency (ULF ) radio waves, observed in the magnetosphere and used for secure military communications, can be disturbed and compressed by aurora and geomagnetic storms. To understand more about how ULF radio wave propagation changes over short timescales, Liou et al. analyze a sequence of auroral images acquired by the Ultraviolet Imager (UVI), an instrument on board NASA's Polar spacecraft. These data, captured during a time when streams of plasma ejected from the Sun's upper atmosphere (called the solar wind) hit the Earth at higher frequencies than usual, allow the authors to image the two-dimensional global pattern of ULF waves for that given time. Further, the enhanced solar wind is seen to have caused auroral intensity variations. These "auroral pulsations" were caused by compressions throughout the ULF spectrum. In particular, the Pc5 band (ranging from about 1.67 to 6.67 mHz) exhibited compressions that are directly correlated to solar wind variations.

Title: Polar Ultraviolet Imager Observations of solar wind–driven ULF auroral pulsations

Authors: K. Liou, K. Takahashi, and P.T. Newell: The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, U.S.A.;

K. Yumoto: Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034953, 2008; http://dx.doi.org/10.1029/2008GL034953

5. Assessing water-supply prospects of Upper Colorado River basin

Along the Colorado River watershed, increasingly dry conditions have caused scientists to speculate that current drought patterns may dominate conditions throughout the 21st century. For the Upper Colorado River basin (areas within Colorado, New Mexico, Utah, and Wyoming) the changing hydrologic regime complicates how resources managers assess water supply variations versus societal needs. Noting that such assessment requires knowledge of how runoff changes through time, Jain and Eischeid analyze runoff estimates from preexisting annually dated tree ring chronologies that extend to 762 A.D., in which ring width indicates moisture supply. Through this, they show that reservoir storage requirements are sensitive not only to the mean runoff but also to year-to-year variability and persistence. In fact, the authors find instances when modest changes in runoff characteristics have drastically changed reservoir storage requirements over the course of only a few decades. Thus, the authors stress that climate change assessments for the region must reproduce the mean, year-to-year variability, and persistence characteristics of runoff if they are to be useful to water resources managers.

Title: What a difference a century makes: Understanding the changing hydrologic regime and storage requirements in the Upper Colorado River basin

Authors: Shaleen Jain: Department of Civil and Environmental Engineering and Climate Change Institute, University of Maine, Orono, Maine, U.S.A.;

Jon K. Eischeid: NOAA Earth System Research Laboratory, Boulder, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL034715, 2008; http://dx.doi.org/10.1029/2008GL034715

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