- Orbital variations and the rate of change in global ice volume
- Decomposing methane gas hydrates on the Arctic Shelf?
- Continental deformation in Asia using GPS arrays
- Rapid erosion of overridden soft sediments during glacial advance along the Alaskan coast
- Solar proton events may affect cloud formation in the upper mesosphere
- Saturn's satellite Rhea is a homogenous mix of rock and ice
- Precipitation rates in vertically sheared tropical cyclones
- Deformation in the Andaman Islands associated with the 2004 Sumatra-Andaman earthquake
The famous research of Milutin Milankovitch, completed in 1941, suggested that glacial advances in Europe coincided with periods in which the solar energy imparted to northern high latitudes was low because of variations in Earth's orbit. Gerard Roe studied previous research and model results to better define Milankovitch's hypotheses. Using basic physical reasoning, he challenges the recommendation of many previous studies that scientists look for correlations between the absolute global ice volume and the amount of solar energy received. Instead, he proposes, it is much more informative to investigate correlations between the rate of change in global ice volume and the solar energy received. This shift in perspective reveals that available records support inverse trends in the rate of change in global ice volume and the summertime influx of solar energy in the northern high latitudes. However, variations in atmospheric carbon dioxide appear to lag behind the rate of global ice volume changes, implying only a secondary role for carbon dioxide in driving changes in global ice volume.
Title: In defense of Milankovitch
Author: Gerard Roe: Department of Earth and Space Sciences, University of Washington, Seattle, Washington, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027817, 2006
More than 8,000 years ago, parts of the Arctic Shelf were an unglaciated coastal plain covered with thick permafrost. As sea levels rose, this permafrost became submerged and is currently inundated by relatively warm seawater. Permafrost is known to contain gas hydrates, a solid phase composed of water and gases that formed under low-temperature, high-pressure conditions. Because disturbances to permafrost may cause the release of methane, a potent greenhouse gas, from the hydrates, Paull et al. sought to determine whether such venting was occurring on the Arctic's submerged Beaufort Sea Shelf. They focused on underwater
features similar in shape to terrestrial pingos, which are conical, ice- cored hills. Data collected from eight pingo-like formations revealed systematically elevated methane concentrations and streams of methane-rich gas bubbles coming from the formations' crests. They hypothesize that pressure generated by methane gas hydrate decomposition within subsurface permafrost layers helped to push ice-rich sediment above the surrounding seafloor, forming pingo-like formations. They expect that after degassing, the seafloor subsides around the pingo-like formations, forming moat-like depressions that have also been observed in bathymetric surveys.
Title: Origin of pingo-like features on the Beaufort Sea Shelf and their possible relationship to decomposing methane gas hydrates
Authors:
Charles K. Paull and William Ussler III: Monterey Bay Aquarium Research Institute, Moss Landing, California, U.S.A.;
Scott R. Dallimore: Natural Resources Canada, Sidney, British Columbia, Canada;
Steve M. Blasco, Natural Resources Canada, Halifax, Nova Scotia, Canada;
Thomas D. Lorenson: United States Geological Survey, Menlo Park, California, U.S.A.;
Humfrey Melling and Fiona A. McLaughlin: Fisheries and Oceans Canada, Sidney, British Columbia, Canada;
Barbara E. Medioli and F. Mark Nixon: Natural Resources Canada, Ottowa, Ontario, Canada.
Source:
Geophysical Research Letters (GRL) paper
10.1029/2006GL027977, 2006
The mode of deformation of continents and the forces that drive this deformation are not well understood, despite detailed research on this topic. There are two opposing interpretations to explain actively deforming continents, such as Asia. One holds that the entire thickness of the lithosphere [solid Earth] is a mosaic of rigid blocks that deforms along fast slipping faults. The other hypothesizes malleable and viscous continents in which faults play a minor role. To help determine which interpretation better represents Asia, Calais et al. collected geodetic data from station arrays across the continent with relatively even spacing between stations. This wide coverage allowed them to construct a deformation velocity field that is model-independent. After analysis, the authors discovered that block- or plate-like motions appear to accurately describe deformation in most of Asia. They note that their results do not rule out continuous deformation models, provided that significant lateral variations in lithospheric strength exist.
Title: Continental deformation in Asia from a combined GPS solution
Authors:
E. Calais and L. Dong: Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana, USA;
M. Wang: Institute of Earthquake Science, China Earthquake Administration, Beijing, China;
Z. Shen: State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China;
M. Vergnolle: Geosciences Azur, Centre National de la Recherche Scientifique (CNRS), University of Nice, Valbonne, France; Now at Laboratoire de Geophysique Interne et Tectonophysique, CNRS, Grenoble, France.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028433, 2006
Advancing glaciers often creep over loose sediments deposited during prior stages of glacial retreat. For glaciers located adjacent to the coast, erosion and subsequent redeposition of such sediments at the foot and sides of the advancing glacier can shoal deep fjords and introduce glacial deposits to the ocean floor. Understanding rates and mechanisms of such erosion will influence models of glacier and ice sheet expansion. Motyka et al. studied the maritime Taku Glacier in southeastern Alaska, which, unlike most modern glaciers,
has advanced 7.5 kilometers [4.7 miles] over the last 115 years. By comparing recent radio echo soundings at Taku with earlier radio echo soundings surveys and with bathymetric surveys of the underlying fjord, the authors documented exceptionally high basal erosion rates over a 100-year period. Erosion rates were found to increase as ice thickened. The authors argue that such amounts of sediment could only be reworked through subglacial hydrologic evacuation. They also suggest that once loosely consolidated sediments are totally eroded, bedrock will be reached, erosion will decrease, and the dynamics of basal sliding will change.
Title: Rapid erosion of soft sediments by tidewater glacier advance: Taku Glacier, Alaska, USA
Authors:
Roman J. Motyka and Martin Truffer: Geophysical Institute, University of Alaska, Fairbanks, Alaska, U.S.A.;
Elsbeth M. Kuriger: Geophysical Institute, University of Alaska, Fairbanks, Alaska, U.S.A.; now at Swiss Federal Institute of
Technology, Einsiedeln, Switzerland;
Adam K. Bucki: Geophysical Institute, University of Alaska, Fairbanks, Alaska, U.S.A.; now at ExxonMobil Exploration Company, Houston, Texas, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028467, 2006
Noctilucent clouds (NLCs) consist of water ice particles in the mesosphere, at altitudes of 80-85 kilometers [50-53 miles] and latitudes poleward of about 55 degrees, during summer in either hemisphere. NLCs are very sensitive to thermal conditions and water abundance in the upper boundary of the mesosphere and are considered to be possible early indicators of global climate change. Von Savigny et al. observe that little is known about space environment interactions with NLCs. They seek to determine how solar energetic particles, which precipitate into the Earth's polar cap areas, leading to ionization and subsequent chemical reactions, affect NLCs and their formation environment. Using satellite data on NLC occurrence and mesospheric temperature, the authors found that immediately after the onset of enhanced solar particle precipitation on 16 January 2005, a severe decrease in NLC occurrence rate was detected. Further, the decrease in the NLC occurrence rate was matched by temperature enhancements at NLC formation latitudes. The authors provide several mechanisms to explain this pattern, but suggest that detailed model simulations are needed to better understand causal mechanisms of temperature increase.
Title: On the disappearance of noctilucent clouds during the January 2005 solar proton events.
Authors:
C. von Savigny, M. Sinnhuber, H. Bovensmann, and J. P. Burrows: Institute of Environmental Physics, University of Bremen, Bremen, Germany;
M.-B. Kallenrode: Department of Physics, University of Osnabrueck, Osnabrück, Germany;
M. Schwartz: Jet Propulsion Laboratory, Pasadena, California, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028106, 2006
Rhea is the second largest satellite after Titan in the Saturnian system with a radius of nearly 765 kilometers [475 miles]. On 26 November 2005, the Cassini spacecraft flew close to Rhea, providing scientists with coherent radio Doppler and ranging data. Using this information, Anderson and Schubert inferred Rhea's mass and its gravitational attraction. From this, they determined that Rhea's mean density is about 1233 kilograms per cubic meter [3,556 pounds per cubic yard] and that the satellite contains an undifferentiated interior. The low density implies an interior made up of about 25 percent rock-metal and 75 percent water ice by mass. Further, the rotational dynamics of Rhea imply that its composition is homogenous, with some compression of the ice and the transition to different ice packing structures at depth.
Title: Saturn's satellite Rhea is a homogenous mix of rock and ice
Authors:
J. D. Anderson: Global Aerospace Corporation, Altadena, California, U.S.A.;
G. Schubert: Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los
Angeles, California, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028100, 2006
When tropical cyclones interact with environmental vertical wind shear, they often take on an asymmetric structure. Prior observations and model studies have identified a preference for rainfall enhancement in the directions downward of the shear and to the left of the shear vector in the Northern Hemisphere., Daniel Cecil used satellite-borne passive microwave radiometer estimates from 1988 through 2004 to compile hundreds of snapshots of rain fields for Atlantic tropical cyclones. By grouping data from each hurricane into quadrants (northeast, northwest, southeast, and southwest), he found that mean rain rates were fairly symmetrical among the quadrants of weakly sheared hurricanes. However, for cases with shear greater than 10 meters [30 feet] per second, the downshear and left-of-shear quadrants had a factor of two greater rain rates than the other quadrants, when looking at data within 100 kilometers [60 miles] of the center. Beyond that distance, the asymmetry increased to a factor of four or more.
Title: Satellite derived rain rates in vertically sheared tropical cyclones
Authors:
Daniel J. Cecil: Earth System Science Center, University of Alabama in Huntsville, Huntsville, Alabama, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027942, 2006
Analysis of the 26 December 2004 Sumatra-Andaman earthquake and resulting tsunamis indicate that fault slip was as large as 30 meters [100 feet] on the southern segment near the epicenter, off the Sumatran coast. However, little is known about the how the rupture terminated near the northern end. Although little slip was estimated on the fault beneath the Andaman Islands from seismic waves, global positioning system data indicate that fault slip also occurred in this region. Kayanne et al. conducted field campaigns in 2005 and 2006 on the Andaman Islands to study this phenomenon. By measuring disturbances to coral beds and recording eyewitness accounts, they confirmed that large surface deformation occurred over the islands during and after the earthquake. The uplift during the quake did not generate large seismic waves or tsunamis, indicating that slip was much slower in the Andaman Islands than off Sumatra. Further, the uplift was followed by subsidence over the following two months, as evidenced by accounts of local residents. The authors propose a simple model to explain the deformation pattern observed.
Title: Coseismic and postseismic creep in the Andaman Islands associated with the 2004 Sumatra-Andaman Earthquake.
Authors:
Hajime Kayanne and Yasukata Ikeda: Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan;
Tomoo Echigo: Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan; now at Geo-Research Institute, Osaka, Japan;
Masanobu Shishikura and Kenji Satake: Active Fault Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan;
Takanobu Kamataki: Active Fault Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan; now at Tono Geoscience Center, Japan Atomic Energy Agency, Toki, Gifu, Japan;
Javed N. Malik: Department of Civil Engineering, Indian Institute of Technology, Kanpur, India;
Shaikh R. Basir and Gautam K. Chakrabortty: Geological Survey of India, Kolkata, India;
Ashish K. Gosh Roy: Geological Survey of India, Kolkata, India; now deceased.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028200, 2006
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