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AGU Journal Highlights -- March 19, 2008

1. Higher CO2 boosts phytoplankton in Antarctic waters

Oceanographers and climatologists are intensely studying how the ocean acts as a source or a sink of carbon dioxide (CO2). Under normal conditions, highly productive areas of the ocean are carbon sinks: Biological activity traps atmospheric CO2 within organic particles that fall to the ocean floor when organisms die. The efficiency of this biological C pump is known to depend on the productivity and species composition of phytoplankton, yet almost no information is currently available on how CO2 concentrations may affect these parameters. The results of Tortell et al. show that elevated CO2 concentrations increase phytoplankton productivity in the Ross Sea, Antarctica, and promote the growth of larger diatom chains. These chains are prolific bloom formers with very high capacities to export organic carbon to sediments. The authors expect that as ocean CO2 levels rise, similar blooms may be found in regions subject to natural ion fertilization, through upwelling of deep waters, windblown input, island effects, or melting sea ice.

Title: CO2 sensitivity of Southern Ocean phytoplankton

Authors: Philippe D. Tortell: Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada; also at Department of Botany, University of British Columbia, Vancouver, British Columbia, Canada;

Christopher D. Payne and Yingyu Li: Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada;

Scarlett Trimborn and Bjrn Rost: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany;

Walker O. Smith: Virginia Institute of Marine Sciences, Gloucester Point, Virginia, U.S.A.;

Christina Riesselman and Robert B. Dunbar: Geological and Environmental Sciences, Stanford University, Soblem is sparse, but Stamps et al. find that geological data covering the past 3.2 million years along the SWIR are consistent with current geodetic data in east Africa. They invert the two data sets jointly to obtain the first complete kinematic model for the EAR. They show that the data are consistent with the existence of three subplates embedded within the rift, as previously suggested from earthquakes but left unquantified, illustrating the initial process by which a continent breaks apart.

Title: A kinematic model for the East African Rift

Authors: D. Sarah Stamps and Eric Calais: Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana, U.S.A.;

Elifuraha Saria: University College for Lands and Architectural Studies, University of Dar Es Salaam, Dar Es Salaam, Tanzania;

Chris Hartnady: Umvoto Africa Ltd., Cape Town, South Africa;

Jean-Mathieu Mocquet: CNRS Gosciences Azur, Valbonne, France;

Cynthia J. Ebinger: Department of Geology and Geophysics, University of Rochester, Rochester, New York, U.S.A.;

Rui M. Fernandes: CGUL, IDL, UBI, Covilha, Portugal

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032781, 2008;

11. Corals skeletons record nutrient changes

Physical and biogeochemical processes linked to climate variations control the concentration of nutrients within the low-latitude ocean. However, methods for reconstructing past nutrient concentrations in the surface ocean are few and indirect. LaVigne et al. hypothesize that massive coral skeletons, which grow in continuous annual bands, preserve a record of seawater nutrient concentrations present when the organism laid down each band. Noting that phosphorus is often a limiting nutrient in the ocean, the authors analyze a four-year record from a coral sample collected from the Gulf of Panam where deep waters upwell. Using a laser ablation technique, they found that the ratio of phosphorus to calcium within the coral skeleton tracks the ratio of strontium to calcium. Because high strontium-to-calcium ratios indicate cooler ocean temperatures, the authors are able to link the coral skeleton phosphorus content to documented seasonal upwelling of cold nutrient-rich water. The authors expect that further development of this new method will allow scientists to reconstruct changes in past nutrient availability in the tropics and subtropics with high precision over long timescales.

Title: Skeletal P/Ca tracks upwelling in Gulf of Panam coral: Evidence for a new seawater phosphate proxy

Authors: Michle LaVigne, M. Paul Field, and Eleni Anagnostou: Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey, U.S.A.;

Andra G. Grottoli: School of Earth and Planetary Sciences, Ohio State University, Columbus, Ohio, U.S.A.;

Gerard M. Wellington: Department of Biology and Biochemistry, University of Houston, Houston, Texas, U.S.A.;

Robert M. Sherrell: Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey, U.S.A.; also at Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031926, 2008;

12. Lead levels rose in North Pacific region atmosphere as declined around North Atlantic

Atmospheric lead pollution, primarily emitted by combustion of leaded gasoline and capable of being transported over long distances, can contribute to neurotoxic effects on children. Although lead pollution in regions surrounding the North Atlantic is known to have increased rapidly following the Industrial Revolution but declined after pollution controls phased out leaded gasoline, little is known about lead pollution trends in regions surrounding the North Pacific. Aerosol monitoring stations in western North America have only been operating since the late 1970s and thus cannot quantify lead levels prior to industrialization. Using an ice core collected from Mt. Logan, in Canadas Yukon Territory, Osterberg et al. study ice layers to develop the first continuous, 8000-year-long record of North Pacific atmospheric lead concentrations. They find that the largest rise in North Pacific lead pollution occurred after 1970, postdating the implementation of pollution controls in North America and Europe. The authors hypothesize that the elevated lead levels stem from the later industrialization and more relaxed pollution regulations in Asian countries compared to North America and Europe.

Title: Ice core record of rising lead pollution in the North Pacific atmosphere

Authors: E. Osterberg, Climate Change Institute, University of Maine, Orono, Maine, U.S.A.; also at Department of Earth Science, University of Maine, Orono, Maine, U.S.A.; now at Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire, U.S.A.;

P. Mayewski and K. Kreutz: Climate Change Institute, University of Maine, Orono, Maine, U.S.A.; also at Department of Earth Science, University of Maine, Orono, Maine, U.S.A.;

D. Fisher, C. Zdanowicz, J. Zheng, M. Demuth, and J. Bourgeois: Geological Survey of Canada, Ottawa, Ontario, Canada;

M. Handley and S. Sneed: Climate Change Institute, University of Maine, Orono, Maine, U.S.A.;

M. Waskiewicz: Datalogging North, Inc., Edmonton, Alberta, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032680, 2008;

13. Aurora on Jupiter: Strange effects from Io

Io, the innermost large moon of Jupiter, orbits within a plasma torus that forms when neutral atoms in Ios atmosphere are ionized and swept into Jupiters magnetosphere. This torus is tilted with respect to Jupiter's equator, so at times Io is below or above the toruss core. Ios orbit also crosses Jupiters magnetic field lines, coupling the moon and the planets upper atmosphere. This generates an electric current that ultimately produces ultraviolet light emissions in Jupiters northern and southern polar regions facing Io. Scientists term these aurorae the Io footprints. In each hemisphere, the footprint consists of a bright spot sometimes followed by secondary spots. To study the Io footprint, Bonfond et al. analyze measurements from the Hubble Space Telescope and discover the systematic appearance of a new faint spot, this time preceding the main bright spot. If Io is orbiting in the southern part of the plasma torus, this leading spot is seen in the northern polar regions, and vice versa. The authors hypothesize that current loops in Jupiters magnetosphere might accelerate electrons toward the other hemisphere, generating the footprints leading and secondary signatures.

See press release:

Title: UV Io footprint leading spot: A key feature for understanding the UV Io footprint multiplicity"

Authors: B. Bonfond, D. Grodent, J.-C. Grard, and A. Radioti: Laboratoire de Physique Atmosphrique et Plantaire, Universit de Lige, Lige, Belgium;

J. Saur and S. Jacobsen: Institute fr Geophysik und Meteorologie, Universitt zu Kln, Cologne, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032418, 2008;

tanford, California, U.S.A.;

Pete Sedwick: Bermuda Institute of Ocean Sciences, Inc., St. George's, Bermuda;

Giacomo R. DiTullio: Hollings Marine Laboratory, University of Charleston, Charleston, South Carolina, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032583, 2008;

2. Stabilizing climate requires near-zero carbon emissions

Current international climate mitigation efforts aim to cap levels of greenhouse gases in the atmosphere to avoid dangerous interference in the climate system. Nonetheless, stable greenhouse gas concentrations do not equate to stable global climate. Simulations show that human-induced climate warming will continue for many centuries, even after atmospheric carbon dioxide is kept level. To study how future warming could be avoided, Matthews and Caldeira use an Earth system model to assess emission requirements for global temperature stabilization within the next several centuries. They find that a single pulse of carbon released into the atmosphere increases globally averaged surface temperature by an amount that remains approximately constant for several centuries, even in the absence of additional emissions. Further, holding the climate constant at a given global temperature requires near-zero future carbon emissions. These results suggest that future greenhouse gas emissions by humans would need to be eliminated in order to hold global temperatures steady. As a consequence, any future emissions will commit the climate system to warming that is essentially irreversible on centennial timescales.

Title: Stabilizing climate requires near-zero emissions

Authors: H. Damon Matthews: Department of Geography, Planning and Environment, Concordia University, Montreal, Quebec, Canada;

Ken Caldeira: Department of Global Ecology, Carnegie Institution of Washington, Stanford, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032388, 2008;

3. A new method to trace ocean mixing

Before industries phased out chlorofluorocarbon (CFC) production in the 1990s due to its detrimental effect on stratospheric ozone, oceanographers used atmospheric ratios of different CFCs to learn more about ocean circulation rates and pathways. Because air-sea gas exchange processes imprint CFC ratios in the ocean's surface over time, when water mixes it will retain the CFC ratio it had when it was exposed to air at the surface. After CFC production halted, oceanographers began using the steadily increasing atmospheric concentrations of anthropogenic sulfur hexafluoride (SF6) gas, which is used as an insulator in the electric power industry, to trace ocean circulation in a similar manner. However, SF6 is also injected into the ocean to study short-term mixing, undermining its utility as an ocean circulation tracer. Searching for a different way to trace short-term mixing, Ho et al. injected trifluoromethyl sulfur pentafluoride (SF5CF3) and SF6 into the waters off southern California's coast in 2005. They find that the tracers' concentrations over about 2 years mirrored each other very closely, indicating that SF5CF3 can replace SF6 in ocean injection experiments and preserving SF6 as an ocean circulation tracer.

Title: Use of SF5CF3 for ocean tracer release experiments

Authors: David T. Ho and William M. Smethie, Jr.: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, U.S.A.;

James R. Ledwell: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032799, 2008;

4. Sound reflections reveal ocean temperature profiles

Reflection seismology, using an array of air guns and hydrophones towed from a ship, is commonly used to investigate geologic structures below the ocean floor. Recently, this technique has been applied to investigate oceanographic structures. Noting that layers in the ocean are nearly horizontal, and that sound speed profiles through layered media can be readily determined by reflection seismology inversion techniques, Wood et al. hypothesize that these sound speed profiles can be used to constrain ocean temperature profiles. The authors test their method using modeled seismic data and find good agreement. They then apply their method to actual seismic data acquired in the Norwegian Sea, corroborating the results with direct measurements of ocean temperature. They find that even with a seismic acquisition system not specifically designed or calibrated for seismic oceanography, temperature contrasts within the ocean can be recovered to within 1 degree Celsius. Because this method can be used remotely and rapidly, the authors expect that this new technique may prove useful in constraining models of ocean mixing and global heat transfer.

Title: Full waveform inversion of reflection seismic data for ocean temperature profiles

Authors: Warren T. Wood: Naval Research Laboratory, Stennis Space Center, Mississippi, U.S.A.;

W. Steven Holbrook: Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, U.S.A.;

Mrinal K. Sen and Paul L. Stoffa: Institute for Geophysics, University of Texas at Austin, Austin, Texas, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032359, 2008;

5. Tracking earthquake motion with GPS at finer timescales

After a large earthquake, a fault continues to creep aseismically, resulting in measurable ground deformation. Typically, such postseismic motions are measured with Global Positioning System (GPS) receivers. In order to reduce intrinsic GPS noise for these studies, most scientists average all GPS data collected within 24-hour spans. Miyazaki and Larson note that these 24-hour averages make it impossible to separate seismic deformations from the initial aseismic deformations. They study the 2003 Tokachi-Oki event, where the ground signal was further complicated by the rupture of a large aftershock (magnitude 7.4) about an hour after the main shock (magnitude 8). By reducing GPS noise, the authors estimate the displacements due to both earthquakes, along with aseismic motions in the first few hours. By inverting the measurements of surface deformation, they find that slip between the two earthquakes occurred between the two epicentral regions, possibly triggering the aftershock. The authors expect that higher sampling rates for GPS solutions will help scientists gain greater insight into studies of slip propagation and stress transfer.

Title: Coseismic and early postseismic slip for the 2003 Tokachi-Oki earthquake sequence inferred from GPS data

Authors: Shin'ichi Miyazaki: Earthquake Research Institute, University of Tokyo, Tokyo Japan;

Kristine M. Larson: Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032309, 2008;

6. Atlantic sea-surface temperatures affect Indian monsoon

The Indian monsoon is strongly influenced by El NioSouthern Oscillation (ENSO) patterns on interannual timescales, with a drier than normal monsoon season usually preceding peak El Nio conditions and a wetter than normal monsoon season preceding peak La Nia conditions. However, sea surface temperatures (SST) in the Pacific, which serve as an indicator of the phase of the ENSO cycle, are not the only factors affecting Indian monsoon patterns. Building on their recent discovery that atmospheric teleconnections between the tropical Atlantic Ocean and the Indian basin contributed to the weakening of the ENSO-monsoon link during the 1980s and 1990s, Kucharski et al. investigate the role of south equatorial Atlantic SSTs in forcing the volume and pattern of Indian monsoon rainfall. Using two observational data sets and two ensembles of models, the authors show that characteristics of Indian monsoon rainfall are significantly correlated with south equatorial Atlantic SSTs. The authors expect that their results might help with monsoon forecast efforts.

Title: Atlantic forced component of the Indian monsoon interannual variability

Authors: F. Kucharski and J. H. Yoo: Earth System Physics Section, Abdus Salam International Center for Theoretical Physics, Trieste, Italy;

A. Bracco: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, U.S.A.;

F. Molteni: European Centre for Medium-Range Weather Forecasts, Reading, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL033037, 2008;

7. Improving measures of wildfire smoke heights from space

The elevation at which wildfire smoke is injected into the atmosphere has a strong influence on how the smoke is dispersed and is a key input into aerosol transport models. Currently, spaceborne lidar studies can constrain the height of smoke plumes with great precision, but horizontal sampling is very poor on large scales. Spaceborne stereo imaging has much greater coverage and captures the cores of major fires, but its ability to constrain the height of the smoke plume is limited to those with discernible features. Noting that data from both imaging techniques can be combined to yield a better picture of smoke transport during a wildfire, Kahn et al. study the fires that occurred in the Alaska-Yukon region during the summer of 2004. The authors find that at least 10 percent of wildfire smoke plumes reached the free troposphere. Because accurate knowledge of smoke transport patterns can help scientists understand the environmental effects of wildfires, the combined strengths of stereo and lidar observations will help scientists better monitor potentially harmful smoke plumes.

Title: Wildfire smoke injection heights: Two perspectives from space

Authors: Ralph A. Kahn: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.; now at NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.

Yang Chen, Qinbin Li; David J. Diner: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.;

David L. Nelson: Columbus Technologies and Services, Inc., Pasadena, California, U.S.A.;

Fok-Yan Leung and Jennifer A Logan: School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032165, 2008;

8. Examining a link between the Suns upper atmosphere and Earth

To help scientists understand the energy budget of the Earths mesosphere and lower thermosphere, NASA launched the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite mission in December 2001. Instruments on TIMED, one of which observes and quantifies the energy budget by measuring the thermospheres infrared emissions from radiative cooling and another of which measures ultraviolet energy radiated by the Sun, have now been measuring for more than 5 years. Mlynczak et al. analyze these data streams and find that the infrared and solar time series exhibit a decrease in radiated and absorbed power, consistent with the waning phase of the solar cycle. The infrared time series also exhibits high-frequency variations that are not evident in the solar power time series. Spectral analysis shows a statistically significant 9-day periodicity in the infrared but not the solar data. The authors link these 9-day periodicities to the recurrence of coronal holes on the Sun. These results demonstrate a direct coupling between the upper atmosphere of the Sun and the infrared energy budget of the thermosphere.

Title: Solar-terrestrial coupling evidenced by periodic behavior in geomagnetic indexes and the infrared energy budget of the thermosphere

Authors: Martin G. Mlynczak, Christopher J. Mertens, and Ellis E. Remsberg: NASA Langley Research Center, Hampton, Virginia, U.S.A.;

F. Javier Martin-Torres: Analytical Services and Materials, Inc., Hampton, Virginia, U.S.A.;

B. Thomas Marshall, R. Earl Thompson, and Larry L. Gordley: G & A Technical Software, Newport News, Virginia, U.S.A.;

Janet U. Kozyra: University of Michigan, Ann Arbor, Michigan, U.S.A.;

James M. Russell III: Hampton University, Hampton, Virginia, U.S.A.;

Thomas Woods: Laboratory for Atmospheric and Space Physics, Boulder Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032620, 2008;

9. North Atlantic Current spins off eddies as it broadens

The warm Gulf Stream flows east offshore of the North American coastline on its way north from Florida to Newfoundland. Southeast of Newfoundland, where it meets the cold Labrador Current, it turns and flows north for roughly 1 of latitude. This branch of the Gulf Stream is called the North Atlantic Current (NAC). At 51N, the NAC abruptly turns east from Newfoundland, broadens, and slows down. Noting that at this turning point the NAC sheds anticyclonic eddies every few months, Woityra and Rossby analyze 10 years of satellite data to measure the size and exact formation rate of these eddies. Using an energy budget approach, they find that eddy formation is a natural result of the slowing, broadening NAC. The authors hypothesize that as eddies form, severe heat loss due to their exposed position in the southern Labrador Sea causes the water within the eddies to sink. Once they are isolated from the surface, they may survive for years as indicated by their findings south and west in the Sargasso Sea.

Title: Current broadening as a mechanism for anticyclogenesis at the northwest corner of the North Atlantic Current

Authors: Willian Woityra: Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island, U.S.A.; now at U.S. Coast Guard, International Ice Patrol, Groton, Connecticut, U.S.A.;

T. Rossby: Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL033063, 2008;

10. Plate tectonics in the East African Rift

Although the East African Rift (EAR) is often cited as a modern archetype for rifting and continental breakup, it remains the least understood of all major plate boundaries. In particular, the rate of plate divergence across it, how this divergence is accommodated within the rift, and how the rift connects farther south with the Southwest Indian Ridge (SWIR) remain to be determined. Information needed to solve this pr

Contact: Peter Weiss
American Geophysical Union

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