AGU Journal Highlights -- April 16, 2007

Earthquakes can modify the Earth's gravity field by deforming the Earth's layers and by changing the density of rocks through dilation and compression of crust and mantle material. Such changes were detected after the 2004 Sumatra-Andaman earthquake by the Gravity Recovery and Climate Experiment (GRACE) satellite, which has previously been used to monitor dynamics such as changes in ground water storage on land masses, exchanges between ice sheets or glaciers and the oceans, and fluctuations in ocean currents. Ogawa and Heki studied GRACE data following this earthquake and found signatures of extensive post-seismic creep along the main rupture. The authors also saw evidence for slow recovery of gravity field changes, possibly induced by simultaneous diffusion of mantle water while post-seismic slip occurs. They expect that such a self-healing system of earthquake-induced gravity changes, which introduces a new role of water in the mantle, would significantly reduce the amount of permanent shifts of the Earth's rotation axis by earthquakes.

Title: Slow Postseismic Recovery of Geoid Depression Formed by the 2004 Sumatra-Andaman Earthquake by Mantle Water Diffusion

Authors: Ryoko Ogawa: Division of Earth and Planetary Science, Graduate School of Science, Hokkaido University, Sapporo, Japan;
Kosuke Heki: Department of Natural History Science, Faculty of Science, Hokkaido University, Sapporo, Japan

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029340, 2007

2. Recent trends in Arctic Ocean mass distribution

Satellite-detected declines in salinity and bottom pressure in parts of the Arctic Ocean indicate that a reversal of the polar ocean’s circulation may be under way. The spatial distribution and magnitude of these trends suggest a shift from a clockwise to a counterclockwise pattern prevalent prior to the 1990s. Changes in Arctic Ocean circulation are important to understanding seasonal weather patterns and decades-long trends in sea ice extent and thickness. Morison et al examined time-varying gravity field data produced by the Gravity Recovery and Climate Experiment (GRACE) satellite mission between 2002 and 2006. Changes in Arctic Ocean bottom pressure, and thus ocean circulation, result from variations in surface height and density of the sea. Comparisons of GRACE data with new direct measurements of Arctic Ocean bottom pressure not only confirm the accuracy and utility of GRACE measurements in the Arctic Ocean, but show a declining trend in bottom pressure corresponding to decreasing upper ocean salinities near the North Pole and in the Arctic's Makarov Basin.

Title: Recent trends in Arctic Ocean mass distribution revealed by GRACE

Authors: James Morison and Cecilia Peralta-Ferriz: Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington, U.S.A.;
John Wahr: Department of Physics and Cooperative Institute for Research in the Environmental Science, University of Colorado, Boulder, Colorado, U.S.A.;
Ron Kwok: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL029016, 2007

3. Ice-associated algal blooms and their impact on biological production in the southeastern Bering Sea

In the Pacific Ocean, water temperatures in the west cycle between warm and cold phases while eastern waters experience the opposite pattern. Called the Pacific Decadal Oscillation (PDO), the 20–30-year cycle is superimposed on global warming and associated northern ice cover reductions. Noting that the high biological productivity of the southeastern Bering Sea shelf is modulated by seasonal sea ice cover, Jin et al. developed an ecosystem model of the area which correctly reproduced blooms observed in 1997 and 1999. The model suggests that blooms, seeded by algae released from melting sea ice, can thrive if melting and resulting low-salinity stratification at the surface are not followed by wind-driven mixing. Changes in bloom patterns between the 1997 and 1999 events resulted from the PDO switching to a cold regime in the east during the early 21st century. Further, the authors noted that the bloom's timing and magnitude, as well as species shifts associated with fluctuating ice margins coupled with gradual ecosystem changes associated with global warming, can dramatically alter the Bering Sea ecosystem.

Title: Ice-associated phytoplankton blooms in the southeastern Bering Sea

Authors: Meibing Jin, Clara Deal, and Jia Wang: International Arctic Research Center, University of Alaska Fairbanks, Alaska, U.S.A.;
Vera Alexander and Rolf Gradinger: School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Alaska, U.S.A.;
Sei-ichi Saitoh and Takhiro Iida: Graduate School of Fisheries Sciences, Hokkaido University, Japan;
Zhenwen Wan: Xiamen University, China;
Phyllis Stabeno: Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington, U.S.A

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028849, 2007

4. Seismic studies could help identify areas saturated with toxic liquid contaminants

Seismic tests could help uncover the presence and extent of underground contamination by a class of toxic liquid contaminants, new experiments show. After injecting the chlorinated solvent trichloroethylene, or TCE, into water-saturated aquifer samples, Ajo-Franklin et al. used an ultrasonic measurement system to study the seismic signature of the chemical’s saturation in shallow soils. TCE is among the fluids known as dense non-aqueous phase liquids. By monitoring changes in ultrasonic signal characteristics, the researchers found reductions in P-wave velocity approaching 15 per cent. They expect that seismic detection, combined with measurements such as ground penetrating radar, will serve to delineate dense non-aqueous phase liquids in sites needing environmental remediation. However, similar velocity reductions could be generated by other subsurface processes, including biogenic gas production. Furthermore, spatial resolution may constrain the seismic tracing of contaminants, they noted.

Title: The ultrasonic properties of granular media saturated with DNAPL/water mixtures

Authors: J. B. Ajo-Franklin: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.;
J. T. Geller: Earth Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, U.S.A.;
J. M. Harris: Department of Geophysics, Stanford University, Stanford, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL029200, 2007

5. Coralline alga gives first marine record of subarctic climate change in the northern Pacific Ocean

Recent changes in the subarctic climate of the northern Pacific Ocean had dramatic effects on ecosystems and fishery yields, but these climate dynamics are poorly understood due to the absence of century-long high-resolution marine records that far north. Noting that waters in the northern Pacific house species of long-lived coralline red algae, Halfar et al. used the calcitic skeleton of one alga to generate a 117-year record of marine climate history based on skeletal bands. Similar to tree rings, those bands represent yearly cycles of growth. The alga skeleton contained oxygen isotope patterns that indicate freshening and warming of surface waters after the middle of the 20th century. The time series developed by the authors correlates with large-scale climate patterns known as the Pacific Decadal Oscillation and the El Nino-Southern Oscillation (ENSO). Though the western Bering Sea/Aleutian Island region is thought to be outside the area of significant marine response to ENSO, the researchers propose that the Alaskan Stream transmits an ENSO signal from the Eastern North Pacific, an area of known ENSO teleconnections.

Title: Coralline alga reveals first marine record of subarctic North Pacific climate change

Authors: Jochen Halfar and G. W. K. Moore: Department of Chemical and Physical Sciences, University of Toronto at Mississauga, Ontario, Canada;
Robert Steneck: Darling Marine Center, University of Maine, Walpole, Maine, U.S.A;
Bernd Schöne: Institute for Geosciences, University of Mainz, Mainz, Germany;
Michael Joachimski: Institut für Geologie und Mineralogie, Universität Erlangen, Erlangen, Germany;
Andreas Kronz: Universität Göttingen, Geowissenchaftliches Zentrum, Göttingen, Germany;
Jan Fietzke: Leibniz Institute of Marine Sciences at the University of Kiel (IFM-GEOMAR), Kiel, Germany;
James Estes: U.S. Geological Survey, University of California at Santa Cruz, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028811, 2007

6. Air in the Northern Hemisphere cycles between the tropics and the poles

The Northern Annular Mode is a seesaw pattern of atmospheric anomalies between the subtropics and polar regions of the Northern Hemisphere. Ren and Cai investigated temperature anomalies associated with this pattern, focusing on the coupling between conditions at low and high latitudes and between conditions in the stratosphere and troposphere as revealed through established data sets. They found that temperature anomalies propagate poleward and downward above the boundary between the stratosphere and the troposphere, but propagate equatorward below this boundary. The timing between shifting from the meridional propagation of the pair of warm-stratospheric/cold-tropospheric anomalies to the meridional propagation of the pair of cold-stratospheric/warm-tropospheric anomalies was found to be about 55 days. Because this cycle originates in the deep tropics and has a long time scale, the authors stressed that understanding the tropical/extratropical coupling of the Northern Annular Mode will help improve long-term climate predictions.

Title: Meridional and vertical out-of-phase relationships of temperature anomalies associated with the Northern Annular Mode variability

Authors: R.-C. Ren: Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China;
Ming Cai: Department of Meteorology, Florida State University, Tallahassee, Florida, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL028729, 2007

7. Fractal topography and groundwater flow

The interaction of surface waters with groundwater influences the global cycling of both natural substances, such as nutrients, and human-made toxins that can affect aquatic ecosystems. However, it is often challenging to characterize subsurface water flow paths, which differ markedly in length, depth, and flow duration. Noting that fractal patterns are found in distributions of both watershed pathways and land surface topography, Woerman et al. studied such patterns in Scandinavia, North America, and other watersheds on continental shields. They found that the fractal nature of the land surface topography in riverine and glacial systems tends to produce fractal patterns in groundwater circulation over a wide range of spatial scales. This means that groundwater flow is influenced to some extent by the entire spectrum of surface topography, with smaller-scale features normally controlling the local flux but larger-scale features producing long flow paths that can substantially modify local behavior. The authors expect that similar processes operate in many different hydrologic systems over a very wide range of scales.

Title: Fractal topography and subsurface water flows from fluvial bedforms to the continental shield

Authors: Anders Wörman and Lars Marklund: Department of Land and Water Resources Engineering, The Royal Institute of Technology, Stockholm, Sweden;
Aaron I. Packman and Susa H. Stone: Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois, U.S.A.;

Judson W. Harvey: U.S. Geological Survey, Reston, Virginia, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029426, 2007

8. Stress before and after the 2002 Denali Fault Earthquake, Alaska

An analysis of the magnitude 7.9 Denali earthquake, which shook Alaska in 2002, has led to a new way to estimate stresses in the Earth’s crust associated with earthquakes. Before an earthquake occurs, estimating the amount of stress available deep in the crust to initiate fault rupture is difficult because of conflicting results from laboratory simulations, seismic observations, and heat flow calculations. The new method developed by Wesson and Boyd determines the spatially averaged stress field in three dimensions in the vicinity of the event, both before and after the earthquake, based on estimates of the orientations of principal stresses and the stress changes associated with the event. Although uncertainties are relatively large compared to the calculated magnitudes of the average shear stress, results argue strongly against estimates of average stress predicted from laboratory tests of static rock failure. The authors are optimistic that their method can be used to help inform calculations of seismic hazards.

Title: Stress before and after the 2002 Denali Fault Earthquake

Authors: Robert L. Wesson and Oliver S. Boyd: Geological Hazards Team, United States Geological Survey, Denver, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029189, 2007

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