Public Release: 

AGU Journal Highlights -- May 3, 2007

American Geophysical Union

1. Warming oceans may diminish length of day

Redistribution of ocean water on Earth's surface caused by a warmer climate will likely cause changes to the planet's rotation and incrementally affect the length of day, according to a new report. Landerer et al. analyze future ocean conditions predicted by the Intergovernmental Panel on Climate Change Fourth Assessment. Because increasing heat captured by the oceans may raise sea level, change the ocean's circulation, and affect the ocean-bottom pressure, a significant portion of ocean mass may transfer away from deep waters to shallower shelf areas. The researchers' model indicates that, by the end of the 22nd century as a result of expected warming, enough water mass could shift toward Earth's axis of rotation to shorten the length of day by approximately 0.12 milliseconds.

Title: Ocean bottom pressure changes lead to a decreasing length-of-day in a warming climate

Authors: Felix W. Landerer, Johann H. Jungclaus, Jochem Marotzke: Max Planck Institute for Meteorology, Hamburg, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL029106, 2007

2. Seasonal variations in the seismicity of the Himalayan Mountains

Cyclic variations in seismic patterns, which have previously been linked to snow loading, precipitation, and variations of the water table, can possibly provide insights into the physics of earthquake triggering. Bollinger et al. analyze data from Nepal's National Seismic Network, which records earthquakes in the Himalayas. They find that from 1995 to 2000, a dense set of seismic stations recorded roughly 37 percent more earthquakes in winter than in summer. Poorer detection because of enhanced seismic noise during monsoons explains only a part of the summer deficit of earthquakes, the researchers say. The remaining part is attributed to a physical mechanism able to modulate earthquake generation. The authors suggest an effect due to fluid infiltration, potentially lubricating faults at seismic depths. They also consider a reduction during summer of mechanical stresses associated with the weight of water filling northern India and the Himalayas.

Title: Seasonal modulation of seismicity in the Himalaya of Nepal

Authors: L. Bollinger: Laboratoire de Détection et de Géophysique, Commissariat à l'Énergie Atomique, Bruyères-le-Châtel, France;
F. Perrier: Institute de Physique du Globe, Laboratoire de Géomagnétisme, Paris, France;
J.-P. Avouac: Geology and Planetary Sciences Department, California Institute of Technology, Pasadena, California, U.S.A.;
S. Sapkota, U. Gautam, and D. R. Tiwari: Department of Mines and Geology, National Seismological Centre, Lainchaur, Kathmandu.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL029192, 2007

3. Lead in old Antarctic ice

Natural variation in lead isotopes allows lead to be used as a tracer of atmospheric pathways. Cores from peat bogs and polar ice show that, during pre-industrial times, lead and other rare trace metals were more abundant that can be explained solely by the silicate dusts that make up the bulk of transportable atmospheric material. To understand this enrichment of trace metals in atmospheric dust, Todd Hinkley analyzes published data from several Antarctic ice cores that listed the amounts and isotopic compositions of lead and mineral dust dating from more than 12,000 years ago to latest pre-industrial times. He finds that two distinct types of lead were provided by the atmosphere to Antarctica in varying proportions during this time. One type of lead was contained in the mineral lattices of dust and had non-radiogenic isotopic properties. The other type of lead was not associated with dust minerals, was more radiogenic, and was likely emitted from ocean island volcanoes or specific Antarctic volcanoes.

Title: Lead (Pb) in old Antarctic ice: Some from dust, some from other sources.

Authors: Todd Hinkley: National Ice Core Laboratory, U.S. Geological Survey, Denver, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028736, 2007

4. Reorientations of crystal lattice may explain deep Earth's seismic jumps

The major mineral phase in the lower mantle, perovskite, dominates the seismic properties and viscous deformation of the deep Earth. At high pressures and temperatures, perovskite transforms into an altered crystal-packing form called post-perovskite. The region, known as the D" layer, where the transformation occurs is directly above the core-mantle boundary and distinguished by large seismic velocity jumps. Using forms of the synthetic, solid compound CaIrO3 as analogs for perovskite and post-perovskite, Walte et al. conducted laboratory experiments in which both CaIrO3 forms were compressed and deformed. The authors found that CaIrO3 perovskite remained relatively unaffected by the experiments, but that crystal lattice orientations in CaIrO3 post-perovskite altered when deformed. If the analog between CaIrO3 and the perovskite crystal-packing system holds true, such lattice orientation changes may explain the observed seismic jumps.

Title: Texture development and TEM analysis of deformed CaIrO3: Implications for the D" layer at the core-mantle boundary

Authors: Nicolas Walte, Florian Heidelbach, Nobuyoshi Miyajima and Daniel Frost: Bayerisches Geoinstitut, Universität Bayreuth, Germany

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029407, 2007

5. Improved modeling of permafrost dynamics in global climate models

Across extensive areas in the Arctic and sub-Arctic, climate change will likely transform soil temperatures and degrade permafrost. Because permafrost covers about a quarter of the Northern Hemisphere's land surface, degradation will change hydrological and biological systems and feed back to affect climate. Global climate models (GCMs) are frequently used to understand and predict future climate change. However, most GCMs do not attempt to represent permafrost dynamics and their climate feedbacks. To improve modeling of permafrost and soil temperatures, Nicolsky et al. compare direct permafrost observations to a frequently used global climate model. From the comparison, the researchers are able to recommend specific modifications to the model. Those include increasing its total soil depth, incorporating a surface organic soil layer, and modifying the model's numerical scheme to more realistically simulate phase changes between ice and water.

Title: Improved modeling of permafrost dynamics in a GCM land-surface scheme

Authors: D. J. Nicolsky and V. E. Romanovsky: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, U.S.A.;
V. A. Alexeev: International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, Alaska, U.S.A.;
D. M. Lawrence: National Center for Atmospheric Research, Boulder, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029525, 2007

6. New model shows how layering facilitates rock deformation

In the laboratory, rocks compressed at high temperatures and pressures can elongate and fold over a broad area without experiencing brittle cracks. However, field observations show that even at conditions that lead to widespread ductile deformation in the laboratory, natural deformation can be localized. Noting that rock structures tend to be layered in areas of the continental crust experiencing ductile deformation, Laurent Montési offers a theoretical model to estimate rock strength at different degrees of layer development. The model captures the fact that layered rocks are more prone to deformation than non-layered rocks when subjected to shear loading. Inspired by laboratory experiments in which ductility is facilitated by fracture in strong grains separating weaker minerals, the model also describes the progressive development of layers under specific loading scenarios. The author expects that through this model, a better understanding of rock deformation can be applied to tracing the structural evolution of geologic features and predicting the future behavior of rocks in fault zones.

Title: A constitutive model for layer development in shear zones near the brittle-ductile transition

Authors: Laurent G. J. Montési: Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029250, 2007

7. Hydrothermal systems may foment periodic unrest at caldera volcanoes

Several studies have suggested that magma emplacement at depth is the dominant source of volcanic unrest at calderas, causing distinct geophysical signals that can persist for months after the onset of unrest. However, very few unrest periods at calderas over the past decades have resulted in volcanic eruptions. Gottsmann et al. examine the role of hydrothermal processes during unrest, focusing on the restless Nisyros caldera in Greece. Through geodetic, seismic, gravimetric, and electromagnetic surveys from Nisyros, they find that periodic short-term signals associated with degassing within the caldera's hydrothermal conduits dominate their results. The authors note that theirs is the first quantitative multi-parametric study of the background dynamic processes at a caldera, and suggest that similar signals are seen at other periodically restless calderas. They conclude that shallow aqueous fluid migration can contribute significantly to periodic unrest and can explain the lack of eruptions in many cases of unrest.

Title: Oscillations in hydrothermal systems as a source of periodic unrest at caldera volcanoes: Multiparameter insights from Nisyros, Greece

Authors: Joachim Gottsmann and Stefanie Hautmann: Department of Earth Sciences, University of Bristol, Bristol, U.K.;
Roberto Carniel: Dipartimento di Georisorse e Territorio, Università di Udine, Udine, Italy;
Nicolas Coppo: Institute of Geology and Hydrogeology, University of Neuchâtel, Neuchâtel, Switzerland;
Luke Wooller and Hazel Rymer: Department of Earth Sciences, The Open University, Milton Keynes, United Kingdom.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029594, 2007

8. Fluid pore pressures in debris flows

Debris flows consist of rapidly moving soil and water that show behavior between landslides and sediment-transporting floods. Interactions between porous soil material and fluids enhance debris mobility by reducing cohesiveness of rock fragments. McArdell et al. set up a monitoring system in an area of Switzerland prone to storm-initiated debris flows. Using instrumentation that recorded flow velocity, pore fluid pressure, and shear stresses inside the flow, the authors monitored a debris flow in August 2005 that coursed four kilometers through a gently sloping channel. Their results show that excess pore fluid pressures are long lived in debris flows, illuminating the important role of pore fluid pressure in explaining the unusual mobility of debris flows. Though particle collisions enhance pore pressures and are likely present in rapidly sheared flows, the authors find that such mechanisms are not necessary to explain large pore pressure values. Instead, they expect that a mechanism exists to continually transfer the load from the solid phase to the fluid phase.

Title: Field observations of basal forces and fluid pore pressure in a debris flow

Authors: Brian W. McArdell: Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland;
Perry Bartelt and Julia Kowalski: Swiss Federal Institute for Snow and Avalanche Research, Davos, Switzerland.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL029183, 2007

9. Arctic sea ice vanishing faster than models forecast

From 1953 to 2006, Arctic sea ice extent at the end of the melt season in September has declined sharply. Although all models used in the Intergovernmental Panel on Climate Change's (IPCC) Fourth Assessment Report show declining Arctic ice cover over the observational record, none of the models individually show trends comparable to observations during this time period. Hypothesizing that the average of all model simulations provides an accurate representation of both natural and human-induced climate change feedbacks in the Arctic, Stroeve et al. suggest that 33 to 38 percent of the observed September trend from 1953 to 2006 is externally forced by greenhouse gas emissions. If only the past 27 years are considered, changes induced by greenhouse gas emissions increase to 47 to 57 percent of the observed September trend. Given that the models, as a group, still underestimate observed ice loss, the authors expect that the externally forced component of Arctic sea ice decline may be larger. This suggests that the Arctic could be seasonally free of sea ice earlier than the IPCC projections, which range from 2050 to well beyond 2100.

[See also AGU Press Release 07-11 of 30 April 2007:]

Title: Arctic Sea Ice Decline: Faster than Forecast?

Authors: Julienne Stroeve, Walt Meier, Ted Scambos, and Mark Serreze: National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, U.S.A.;
Marika M. Holland: National Center for Atmospheric Research, Boulder, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029703, 2007


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