AGU journal highlights -- Feb. 13, 2008

1. A forgotten source for planetary magnetic anomalies?

Anorthosites, igneous rocks rich in plagioclase feldspar, are common on the Earth, Moon, and possibly other planets. Though anorthosites are usually not considered to be strongly magnetic, Brown and McEnroe note that their magnetic properties could be useful in investigating mineral deposits in addition to magnetic anomalies on other planets, particularly Mars. Through investigations of three ancient anorthosite bodies in Norway, the authors find that two anorthosites have large natural remanent magnetization signatures, indicating that they contain strong signatures of the Earth's magnetic field direction that was present when the rocks crystallized roughly 1 billion years ago. These signatures are comparable in intensity to those found in freshly crystallized basalts. Microscopic observations reveal that, although only one body contains high levels of magnetite, all three anorthosites contain ilmenite and hematite, which are weakly magnetic minerals. Previous research suggests that submicroscopic plates of ilmenite with hematite intergrowths interact with each other to amplify magnetic anomalies, causing the authors to conclude that anorthosites can be important sources of magnetic anomalies on Earth and perhaps on other planets.

Title: Magnetic properties of anorthosites: A forgotten source for planetary magnetic anomalies?

Authors: Laurie L. Brown: Department of Geosciences, University of Massachusetts-Amherst, Amherst, Massachusetts, U.S.A.;

Suzanne A. McEnroe: Norwegian Geological Society, Trondheim, Norway.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032522, 2008; http://dx.doi.org/10.1029/2007GL032522

Ice core records from Antarctica and Greenland show greenhouse gas signatures consistent with climate inferences assumed from records of oxygen isotope fractions through time. Noting that atmospheric methane decays quickly, O'Hara seeks to understand what kind of methane source is capable of sustaining the levels and concentrations of methane seen in the ice core record. Through a kinematic model, the author finds that the methane source reservoir that created the ice core signals must have had high initial methane concentrations and long residence times. Because the glacial climate was dryer compared with today's climate, the author then assumes that the amount of methane currently escaping from wetlands is presently at a maximum. He finds, however, that the current storage capacity of the methane source for global wetlands is much too small to account for the signatures seen in the ice cores. Instead, the author proposes that gas hydrates in shallow marine sediments are the likely dominant source of the methane found in ice cores and thus the dominant source of methane in past environments.

Title: A model for late Quaternary methane ice core signals: Wetlands versus a shallow marine source

Authors: Kieran D. O'Hara: Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032317, 2008; http://dx.doi.org/10.1029/2007GL032317

Concerned that energy system transformations are proceeding too slowly to avoid risks from dangerous human-induced climate change, many scientists are wondering whether geoengineering (the deliberate change of the Earth's climate) may help counteract global warming. Sulfate aerosols, commonly released by volcanoes, serve to scatter incoming solar energy in the stratosphere, preventing it from reaching the surface. To investigate the feasibility of deliberately mimicking the effect of volcanic aerosols, Rasch et al. explore scenarios in which aerosol properties are varied to assess interactions with the climate system. Through model simulations, they discover that, because stratosphere-troposphere exchange processes change with increasing levels of aerosols, about 50 percent more aerosols would have to be injected into the atmosphere than in the scenario where such processes stayed constant. Further, almost double the level of aerosol loading is required to counteract greenhouse warming if aerosol particles are as large as those seen during volcanic eruptions. The authors caution that geoengineering methods to mask global warming may have serious environmental consequences that must be explored before any action is taken.

Title: Exploring the geoengineering of climate using stratospheric sulfate aerosols: The role of particle size

Authors: Philip J. Rasch and Danielle B. Coleman: National Center for Atmospheric Research, Boulder Colorado, U.S.A.;

Paul J. Crutzen: Max Plank Institute for Chemistry, Mainz, Germany; Also at Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032179, 2008; http://dx.doi.org/10.1029/2007GL032179

When atoms combine to form molecules in the upper atmosphere, energy may be released in the form of photons, causing the atmosphere to weakly glow. Called “dayglow” over the dayside and “nightglow” over the nightside, such emissions of light are seen over Earth, Mars, and Venus. Noting that nightglow emissions are important tracers of upper atmospheric transport, Gérard et al. study Venus’s nightglow using data from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the European Space Agency’s Venus Express spacecraft. Past research has shown that solar ultraviolet light splits CO₂ molecules on Venus’s dayside, releasing oxygen atoms; these atoms are transported to the nightside where they combine to form oxygen molecules, releasing energy and producing nightglow. Through VIRTIS images, the authors characterize the distribution of excited oxygen molecules over most of the southern hemisphere and find that the nightglow observed requires that approximately 50 percent of the oxygen atoms produced in the dayside be carried to the nightside by global circulation. This requirement will be important to refine circulation models of Venus’s atmosphere.

Title: Distribution of the O₂ infrared nightglow observed with VIRTIS on board Venus Express

Authors: J.-C. Gérard, A. Saglam, and C. Cox: Laboratoire de Physique Atmosphérique et Planétaire, Universite de Liège, Liege, Belgium;

G. Piccioni: Istituto Nazionale di Astrofisica and Istituto di Astrofisica Saziale e Fisica Cosmica, Rome, Italy;

P. Drossart and S. Erard: Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, Meudon, France;

R. Hueso and A. Sánchez-Lavega: Escuela Superior Ingeniería, Universidad del País Vasco, Bilbao, Spain.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032021, 2008; http://dx.doi.org/10.1029/2007GL032021

The strength and duration of an El Niño event are generally thought to depend on changes in subtropical overturning cells (STCs), in which water subducts into the subtropics, flows to the equator, upwells, and returns poleward along the surface. Important mechanisms by which heat and salt are transferred meridionally in the ocean, STCs are well studied in the Pacific Ocean north of the equator. Noting the paucity of data in the southern Pacific Ocean, Qu et al. analyze data collected from the recent deployments of the Argo observation system, a worldwide network of small, drifting oceanic probes. Through high-resolution conductivity-temperature-depth profiles, the authors calculate the rate at which water sinks in the southern Pacific Ocean. Further analyses show that the southern STCs contribute more significantly than previously recognized to the formation of water layers in the equatorial Pacific. The authors expect that future study of South Pacific STCs will reveal the degree to which eddies and larger oceanic vortices strengthen or weaken STC cycles, and how such strengthening or weakening influences El Niño events.

Title: Subduction of South Pacific waters

Authors: Tangdong Qu and Shan Gao: International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawai'i at Manoa, Honolulu, Hawaii, U.S.A.;

Ichiro Fukumori: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.;

Rana A. Fine: Rosentiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, U.S.A.;

Eric J. Lindstrom: Science Mission Directorate, NASA Headquarters, Washington D.C., U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032605, 2008; http://dx.doi.org/10.1029/2007GL032605

Aerosols influence global climate by scattering incoming solar radiation, causing a cooling effect. Much of this effect results from organic aerosols, which are classified as “primary” or “secondary.” Primary organic aerosols are emitted directly into the atmosphere and are thus relatively easy to monitor. Secondary organic aerosols (SOAs), those which form from reactions of precursor gases in the atmosphere, are more elusive. Recent research suggested that clouds are able to uptake water-soluble organics, which are then oxidized and form SOAs after cloud droplet evaporation. To better understand the dynamics of SOA formation through this pathway, Ervens et al. study isoprene, a volatile organic compound and a newly recognized source of atmospheric SOA. Through model studies based on laboratory experiments, the authors find that SOAs form through cloud-processing depend strongly on the initial ratio of isoprene to nitrogen oxides. In this way, combustion emissions (nitrogen oxides) contribute to SOA formation from biogenic hydrocarbons. Further, cloud-derived SOA concentrations increase with increasing cloud-contact time. The authors expect that such information can help improve climate and air quality models.

Title: Secondary organic aerosol yields from cloud-processing of isoprene oxidation products

Authors: Barbara Ervens: Atmospheric Science Department, Colorado State University, Fort Collins, Colorado, U.S.A.; also at Earth System Research Laboratory, U.S. National Oceanic and Atmospheric Administration, Boulder, Colorado, U.S.A.;

Annmarie G. Carlton: Atmospheric Sciences Modeling Division, Air Resources Laboratory, U.S. National Oceanic and Atmospheric Administration, Durham, North Carolina, U.S.A.;

Barbara J. Turpin: Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey, U.S.A.;

Katye E. Altieri: Institute of Marine and Coastal Sciences, Rutgers University, new Brunswick, New Jersey, U.S.A.;

Sonia M. Kreidenweis: Atmospheric Science Department, Colorado State University, Fort Collins, Colorado, U.S.A.;

Graham Feingold: Earth System Research Laboratory, U.S. National Oceanic and Atmospheric Administration, Boulder, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031828, 2008; http://dx.doi.org/10.1029/2007GL031828

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