AGU journal highlights -- July 29, 2011

(Press-News.org) The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL); Space Weather; Geochemistry, Geophysics, Geosystems (G-Cubed); Journal of Geophysical Research-Biogeosciences (JGR-G); Journal of Geophysical Research-Earth Surface (JGR-F); Journal of Geophysical Research-Space Physics (JGR-A) and Water Resources Research (WRR).

In this release:

What do sea measurements reveal about Earth's temperature trend? Japan's big earthquake shook the ionosphere Miniature detector measures deep space radiation New data refine the travels of ancient supercontinent Agricultural drainage affects export of dissolved organic carbon Magnetic behavior changes identified in natural rock formations The effect of sediment on mountain river erosion New watershed classification to make use of high-resolution data

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1. What do sea measurements reveal about Earth's temperature trend?

Despite the fact that average temperatures on land have been increasing from year to year, globally averaged surface temperatures from 2000 to 2010 have shown only moderate warming. This is because sea surface temperatures over the past decade have been flat, if not slightly decreasing. In light of this, scientists are curious about whether this reduced rate of surface warming indicates a reduction of the accumulation of heat in the Earth system over the same period.

Palmer et al. use multicentury climate model simulations to study the relationships among decadal trends in top-of-atmosphere radiation balance (which controls the heat content of the Earth system), ocean heat content, and surface temperature. Consistent with previous studies, they find that all models show large variability in sea surface temperature (SST). This large internal variability in SST could easily "mask the anthropogenic warming signal for a decade or more," the authors note. By contrast, ocean heat content more closely tracks the radiation budget at the top of the atmosphere, suggesting that measurements of ocean heat to deeper levels would help us monitor climate change more accurately.

Source: Geophysical Research Letters, doi:10.1029/2011GL047835, 2011 http://dx.doi.org/10.1029/2011GL047835

Title: Importance of the deep ocean for estimating decadal changes in Earth's radiation balance

Authors: Matthew D. Palmer, Douglas J. McNeall and Nick J. Dunstone: Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB, United Kingdom.

2. Japan's big earthquake shook the ionosphere

The giant 11 March 2011 magnitude 9 Tohoku earthquake not only shook the Earth and caused devastating tsunamis but also rattled the ionosphere, according to a new study. The surface seismic waves and tsunamis triggered waves in the atmosphere. These atmospheric waves propagated upward into the ionosphere, creating ripples in ionized gas nearly 350 kilometers above the Earth.

Liu et al. measured these disturbances, called seismotraveling ionospheric disturbances (STID), using GPS receivers in Japan. The first disturbance appeared as a disk-shaped increase in electron density in the ionosphere about 7 minutes after the earthquake. Sequences of concentric waves of increased electron density then traveled from the STID center. Similar ionospheric disturbances have been observed following other earthquakes, but these were the largest ever seen, the authors report.

Source: Journal of Geophysical Research-Space Physics, doi:10.1029/2011JA016761, 2011 http://dx.doi.org/10.1029/2011JA016761

Title: Ionospheric disturbances triggered by the 11 March 2011 M9.0 Tohoku earthquake

Authors: Jann-Yenq Liu: Institute of Space Science, National Central University, Chung-Li, Taiwan; Center for Space and Remote Sensing Research, National Central University, Chung-Li, Taiwan; and National Space Organization, Hsinchu, Taiwan;

Chia-Hung Chen: Department of Geophysics, Kyoto University, Kyoto, Japan;

Chien-Hung Lin: Department of Earth Science, National Cheng Kung University, Tainan, Taiwan;

Ho-Fang Tsai: Central Weather Bureau, Taipei, Taiwan;

Chieh-Hung Chen: Institute of Earth Science, Academia Sinica, Taipei, Taiwan;

Masashi Kamogawa: Department of Physics, Tokyo Gakugei University, Tokyo, Japan.

3. Miniature detector measures deep space radiation

The 1972 journey of Apollo 17 marked not only the last time a human walked on the Moon but also the most recent manned venture beyond the outer reaches of the Earth's atmosphere. With preparations being made for humans to once again explore deep space, important steps are under way to quantify the hazards of leaving low-Earth orbit. One significant risk for long-distance missions is the increased exposure to ionizing radiation-energetic particles that can strip electrons off of otherwise neutral materials, affecting human health and the functioning of spacecraft equipment. The deep space probes that are being sent to measure the risks from ionizing radiation and other hazards can be costly, so maximizing the scientific value of each launch is important.

With this goal in mind, Mazur et al. designed and developed a miniature dosimeter that was sent into lunar orbit aboard NASA's Lunar Reconnaissance Orbiter (LRO) in 2009. Weighing only 20 grams (0.7 ounces), the detector is able to measure fluctuations in ionizing radiation as low as 1 microrad (equivalent to 10 billionths of a joule of energy deposited into 1 kilogram (2.2 pounds)) while requiring minimal power and computer processing. The postage stamp-sized detector tracked radiation dosages for the first year of LRO's mission, with the results being confirmed by other onboard and near-Earth detectors. The authors find that ionizing radiation levels were 30 percent lower in lunar orbit than when the spacecraft was in transit, with a total dosage roughly 22 times the annual background rate on Earth. They suggest that their detector, with its small footprint and low power demand, could be a staple for future deep space missions.

Source: Space Weather, doi:10.1029/2010SW000641, 2011 http://dx.doi.org/10.1029/2010SW000641

Title: New Measurements of Total Ionizing Dose in the Lunar Environment

Authors: J. E. Mazur: The Aerospace Corporation, Chantilly, Virginia;

W. R. Crain, M. D. Looper, D. J. Mabry and J. B. Blake: The Aerospace Corporation, El Segundo, California;

A. W. Case and J. C. Kasper: Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts;

M. J. Golightly and H. E. Spence: University of New Hampshire, Institute for the Study of Earth, Oceans, and Space, Durham, New Hampshire.

4. New data refine the travels of ancient landmass

The supercontinent Pangea, which roughly 300-200 million years ago existed as the only landmass on Earth, plays a special role in the history of geophysics. The geological, biological, and paleontological similarities between now-distant shores -- once connected in Pangea -- gave the first evidence for the theory of continental drift. That Pangea came apart to form the modern world is well established, but exactly how, when, and where the individual plates moved are still up for debate.

Drawing on new high-quality paleomagnetic data, Domeier et al. describe the movements of Gondwana that, until its separation from Laurasia 200 million years ago, formed the southern half of Pangea. The authors collected samples drawn from the Sierra Chica, a band of ancient volcanic rocks in central Argentina. Within the samples the magnetic minerals hematite and titanomagnetite were used to calculate the geographic location of the magnetic pole 263 million years ago. Because the Earth's magnetic poles drift only slightly over time and have well-known reversal episodes, deviations in the location of the calculated pole (paleopole) from the present location are an indication that the plate underlying the volcanic rock has moved since the lava solidified. Changes in the paleopole drawn from samples of different ages from the same plate give a map for the plate's movement.

Previous research had trouble reconciling the paths traveled by Gondwana and Laurasia, despite the fact that the two landmasses were still joined at the time. The authors' paleopole, whose location and age were determined more accurately than in most related research, falls in between those previously calculated from Gondwanan and Laurasian samples. They suggest that any apparent differences between the paleopoles for Gondwana and Laurasia are due to sampling bias and other sources of error.

Source: Geochemistry, Geophysics, Geosystems, doi:10.1029/2011GC003616, 2011 http://dx.doi.org/10.1029/2011GC003616

Title: New Late Permian paleomagnetic data from Argentina: Refinement of the apparent polar wander path of Gondwana

Authors: Mathew Domeier: Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, USA;

Rob Van der Voo; Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, United States and Center for Advanced Study, Norwegian Academy of Science and Letters, Oslo, Norway;

Eric Tohver: School of Earth and Geographical Sciences, University of Western Australia, Crawley, Western Australia, Australia;

Renata N. Tomezzoli and Haroldo Vizan: Departamento de Ciencias Geológicas, Universidad de Buenos Aires, Ciudad Universitaria, Buenos Aires, Argentina;

Trond H. Torsvik: Center for Advanced Study, Norwegian Academy of Science and Letters, Oslo, Norway; and Department of Physics, University of Oslo, Oslo, Norway; and School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa;

Jordan Kirshner: Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, United States.

5. Agricultural drainage affects downstream dissolved organic carbon

Dissolved organic carbon (DOC) exported from agricultural lands can affect watershed carbon budgets and aquatic ecosystem functions. Agricultural practices, especially enhanced drainage, which is widespread in the midwestern United States and expanding, can influence how much DOC is exported from an area. Dalzell et al. compare fields that drained water at conventional rates (13 millimeters, or 0.51 inches, per day) with fields with intense drainage (51 mm, or 2.2 inches, per day). They find that intense drainage resulted in 55 percent more DOC exported per year. This is primarily due to increased water export but also due to increased DOC concentration. They observe that DOC export from drained fields depends strongly on precipitation rates and varies considerably from year to year. DOC concentrations increase in stream networks in the drained watershed and shift from microbial sources in farm field drainage to more terrestrially derived and higher molecular weight DOC at downstream sites. The results should help improve models of DOC export from agricultural areas and help researchers and planners understand how DOC export could change as more agricultural areas convert to intense drainage.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1029/2010JG001540, 2011 http://dx.doi.org/10.1029/2010JG001540

Title: Influence of subsurface drainage on quantity and quality of dissolved organic matter export from agricultural landscapes

Authors: Brent J. Dalzell: Department of Soil, Water, and Climate, University of Minnesota-Twin Cities, St. Paul, Minnesota, USA;

Jennifer Y. King: Department of Soil, Water, and Climate, University of Minnesota-Twin Cities, St. Paul, Minnesota, USA; now at Department of Geography, University of California, Santa Barbara, California, USA;

David J. Mulla: Department of Soil, Water, and Climate, University of Minnesota-Twin Cities, St. Paul, Minnesota, USA;

Jacques C. Finlay: Department of Ecology, Evolution, and Behavior, University of Minnesota-Twin Cities, St. Paul, Minnesota, USA;

Gary R. Sands: Department of Bioproducts and Biosystems Engineering, University of Minnesota-Twin Cities, St. Paul, Minnesota, USA.

6. Magnetic behavior changes identified in natural rock formations

How the magnetic properties of minerals such as magnetite change with grain size is of fundamental importance to understanding magnetic fields of the past and present, which are useful for tracking the movement of tectonic plates and in teasing out the histories of changing environmental factors. Materials ideal for experimental work on size-dependent properties are difficult to find, as natural materials tend to be messy mixtures of different magnetic minerals, with variable concentrations, chemical purities, and wide distributions of grain sizes, all of which serve to blur magnetic properties.

However, on the western base of Yucca Mountain lies the Tiva Canyon Tuff, a 12.7-million-year-old deposit of volcanic material that provides an ideal study site for investigations of grain size-dependent magnetic properties. The ash deposits of the Tiva Canyon Tuff contain abundant volcanic glass, within which the average particle size of magnetite nanoparticles increases systematically with height above the base of the tuff.

By drawing samples across a range of depths, Till et al. analyze how the magnetic properties of the rock change with height and thus with grain size. The authors identify two stratigraphic layers that mark distinct transitions in magnetic behavior. At 0.8 meters (2.6 feet) from the base of the tuff, where the grains are 40 nanometers (0.00000157 inches) long, they find a peak in magnetic susceptibility-a measure of how easy it is to change the magnetization of a population of grains. This indicates a transition from superparamagnetic to stable single-domain behavior. The magnetic moments of superparamagnetic grains are able to change direction with weak external forcing, while stable single-domain grain moments are static, each essentially pointing in one fixed direction. Above 3.25 meters (10.6 feet), with grain sizes between 250 and 500 nanometers (0.00000984 and 0.0000197 inches), the authors find a peak in the rock's remanence-how much magnetization the rock retains when any external fields are removed-marking the end of the single-domain regime. The fieldwork largely confirms previous theoretical and experimental findings by identifying regime boundaries, though some differences are found to be due to the idiosyncrasies of natural formations.

Source: Geochemistry, Geophysics, Geosystems, doi:10.1029/2011GC003648, 2011 http://dx.doi.org/10.1029/2011GC003648

Title: Magnetic properties in an ash flow tuff with continuous grain size variation: A natural reference for magnetic particle granulometry

Authors: J. L. Till, M. J. Jackson and P. Solheid: Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, USA;

J. G. Rosenbaum: United States Geological Survey, Denver, Colorado, USA.

7. The effect of sediment on mountain river erosion

Mountain uplift and subsequent water-powered erosion are powerful and persistent processes shaping the landscape, and understanding the interactions between these two processes has been an area of active research for the past 2 decades. The rate of river erosion has long been considered to scale with stream power, the potential energy dissipated by the water as it flows downhill. While widely used, this model has been tested in only a limited number of situations, sometimes with little success. The stream power model, in which erosion rates scale with the area of the upstream drainage basin and the slope of the stream, makes no account for the amount of sediment in the river. Sediment can increase erosion when the overall sediment levels are low or prevent it when the sediment load is high and the sediment can form an armor-like cover on the river bed.

Using numerical simulations that implement four different river erosion processes, each of which deals with the effects of sediment transport on stream incision in a different way, Gasparini and Brandon find that the stream-power model works very well, but only as an empirical representation of the river erosion process. The researchers find that the models are sensitive to the spatial distribution of erosion in the landscape and to the downstream distribution of sediment load. Many mountains tend to grow fastest at their centers, producing a large amount of sediment that is sent downstream. Water- borne sediment in the upstream part of the mountain increases the ability of the flow to chip away at the riverbed, but downstream, away from the fast uplifting part of the mountain, this sediment load accumulates and forms a protective layer on the channel bed, reducing the amount of erosion. This natural feedback loop likely supports the erosion patterns observed in many natural mountain ranges and can affect the details of the empirical stream- power relationship.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1029/2009JF001655, 2011 http://dx.doi.org/10.1029/2009JF001655

Title: A generalized power law approximation for fluvial incision of bedrock channels

Authors: Nicole M. Gasparini: Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana, USA;

Mark T. Brandon: Department of Geology and Geophysics, Yale University, New Haven, Connecticut, USA.

8. New watershed classification to make use of high-resolution data

Models of watershed behavior are beginning to incorporate lidar (light detection and ranging) topography measurements with resolutions of the order of one square meter, but this increase in the availability of high-quality data is outpacing the ability to make the best use of them. At issue is the conceptual lens through which the data are analyzed, particularly when investigating hydrologic behavior at different scales. Scaling parameters are used to break down the masses of data into groupings that fit within certain constraints that can then be analyzed for trends, and watershed research has traditionally used either stream order-a measure based around how many tributaries a body of water has-or the upstream catchment area to make these divisions.

To best make use of the available high-resolution topography data, Gangodagamage et al. propose a new scaling parameter based on what the authors refer to as the "directed distance from the divide." The technique involves calculating, for each point in a watershed, the longest distance from that point to a drainage divide, taking into account variations in topography and recognizing that water only flows downhill. When two of these paths converge, the shorter is treated as a tributary to the longer, with this process being repeated until the entire watershed culminates in one major river. From these calculations the watershed can be broken down and analyzed using ensemble statistics based on flow paths of different lengths. In addition to their theoretical investigation of the technique, the authors analyzed a watershed in Mendocino County, California, at three different spatial scales, finding that directed distance from the divide provides new information, more clearly delineating transitions in geomorphic process regimes relative to other common scaling parameters.

Source: Water Resources Research, doi:10.1029/2010WR009252, 2011 http://dx.doi.org/10.1029/2010WR009252

Title: Revisiting scaling laws in river basins: new considerations across hillslope and fluvial regimes

Authors: Chandana Gangodagamage: Earth and Environmental Sciences & Space and Remote Sensing, Los Alamos National Laboratory, New Mexico, USA;

Patrick Belmont: Department of Watershed Sciences, Utah State University, Utah, USA;

Efi Foufoula-Georgiou: Department of Civil Engineering, National Center for Earth-Surface Dynamics and St. Anthony Falls Laboratory, University of Minnesota, Minnesota, USA.

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