Public Release: 

AGU journal highlights -- April 9, 2013

American Geophysical Union

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Space Weather (SW), Journal of Geophysical Research-Earth Surface (JGR-F), and Journal of Geophysical Research-Biogeosciences, (JGR-G).

In this release:

  1. Characterizing the Moon's radiation environment
  2. Three-dimensional mapping of airflow over dunes
  3. Forest organic runoff breaks down faster than agricultural, urban runoff
  4. Examining CO2¬ concentrations and flow dynamics in streams
  5. Measuring the forces generated by erosive debris flows
  6. Agulhas Current leakage could stabilize Atlantic overturning circulation

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to and inserting into the search engine the full doi (digital object identifier), e.g. 10.1002/swe.20034. The doi is found at the end of each Highlight below.

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1. Characterizing the Moon's radiation environment

The radiation environment near the Moon could be damaging to humans and electronics on future missions. To characterize this potentially hazardous environment, the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on board the Lunar Reconnaissance Orbiter mission, which orbits at 50 kilometers (31 miles) above the Moon's surface, measures the radiation that would be absorbed by either electronic parts or human tissue behind the shielding of a spacecraft.

CRaTER has measured the lunar radiation environment since 2009, during the recent solar minimum. The solar wind is less turbulent during solar minima and thus presents less of a barrier to incoming galactic cosmic rays, so cosmic rays were at a high during that time period. Cosmic rays include various high energy particles, and they create a shower of secondary radiation upon impact with the Moon or a spacecraft's shielding.

Looper et al. ran simulations that model the response of the CRaTER sensor to various energetic particles. The simulations enabled the researchers to extract observations about the radiation environment from observations specific to the sensor. Looking at contributions from galactic cosmic rays, secondary particles, and sensor background, they were able to derive energy spectra for the radiation dose that humans or instruments would absorb in the lunar environment.

Source: Space Weather, doi: 10.1002/swe.20034, 2013

Title: Characterizing the Moon's radiation environment

Authors: M. D. Looper and J. B. Blake: The Aerospace Corporation, El Segundo, California, USA;

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

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

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

L. W. Townsend: Nuclear Engineering Department, University of Tennessee, Knoxville, Tennessee, USA.

2. Three-dimensional mapping of airflow over dunes

Similarly to the way a river, flowing across Earth's surface, influences sediment transport and shaping of the landscape, so coastal winds, flowing over dunes, affect how the dune shapes evolve and how sand is transported along the coast. Wind flow over dunes has been extensively studied, but in most cases studies have been two-dimensional and focused on straight dunes with smooth slopes and no vegetation or other features that might affect how airflow separates at the crest of the dune.

To get a more complete picture of the complex flows, Delgado-Fernandez et al. conducted what they believe is the first three-dimensional field study of airflow patterns on the leeward side of a coastal dune. Over the course of 10 days, the research team used an array of ultrasonic anemometers deployed over a beach-dune surface in Northern Ireland to map the complex wind patterns. The authors compare their findings with previous studies and discuss some possible similarities and differences between airflow patterns in the coastal setting and airflow and water-flow patterns in other environments. As the movement of sand through wind action can dramatically help beaches and dunes rebuild themselves after erosion by coastal storms, this study could help scientists understand how particular wind events can aid in post-storm recovery of sandy coastlines.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1002/jgrf.20036, 2013

Title: Field characterization of three-dimensional lee-side airflow patterns under offshore winds at a beach-dune system

Authors: Irene Delgado-Fernandez, Derek W.T. Jackson and J.Andrew G. Cooper: Centre for Coastal and Marine Research, School of Environmental Sciences, University of Ulster, Cromore Road, Coleraine, Northern Ireland, UK;

Andreas C.W. Baas: Department of Geography, King's College London, Strand, London, UK;

J.H.Meiring Beyers: Klimaat Consulting & Innovation Inc., Guelph, Canada;

Kevin Lynch: Department of Geography and Archaeology, National University of Ireland, Galway,University Road, Galway, Ireland.

3. Forest organic runoff breaks down faster than agricultural, urban runoff

Dissolved organic matter in streams and rivers can be broken down by sunlight or bacteria, providing a fuel source for aquatic ecosystems and affecting carbon dioxide and carbon monoxide concentrations as the organic matter is mineralized. Researchers know that the amount of organic matter in streams fed by forest landscapes and those fed by watersheds affected by human activity, such as croplands, pasture, or urban environments, can differ greatly. What is less well known is how the organic matter from these various environments evolves as it flows downstream.

Taking water samples from the heads of seven Virginia rivers, Lu et al. studied how bacterial and photochemical reactions changed the concentration, isotopic signature, and fluorescent properties of dissolved organic compounds. The authors find that the organic matter stemming from forested environments is more susceptible to degradation by sunlight than that from landscapes affected by human activity. This differing rate of photochemical degradation means that for streams affected by farm and urban runoff the organic loads remain at higher levels longer, resulting in greater organic content at the river outlet and an increased potential for driving hypoxic conditions in downstream waterways.

The authors suggest that the higher persistence of anthropogenic dissolved organic compounds could help explain an observed long-term increase in river organic compound concentrations in Europe and North America. The authors also suggest that the forest-derived dissolved organic compounds may be more photoreactive because they haven't been exposed to as much light as those from landscapes affected by human activity, or because the organic compounds produced by plants rather than urban runoff bear varied chemical compositions. They note, however, that more research is needed to determine the exact cause.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1002/jgrg.20048, 2013

Title: Photochemical and Microbial Alteration of Dissolved Organic Matter in Temperate Headwater Streams Associated with Different Land Use

Authors: Yuehan Lu: Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama, USA;

James E. Bauer: Aquatic Ecology Laboratory, Department of Evolution, Ecology and Organismal Biology, Ohio State University, Columbus, Ohio;

Elizabeth A. Canuel: Department of Physical Sciences, Virginia Institute of Marine Sciences, Gloucester Point, Virginia;

Youhei Yamashita: Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido, Japan;

R. M. Chambers: Department of Biology, College of William and Mary, Williamsburg, Virginia;

Rudolf Jaffé: Southeast Environmental Research Center and Department of Chemistry & Biochemistry, Florida International University, Miami, Florida.

4. Examining CO2 concentrations and flow dynamics in streams

As part of the carbon cycle, carbon dioxide (CO2) is flushed from soils into stream water; this CO2 either escapes directly into the atmosphere from the water surface or gets transported downstream of the study site. To understand the amount and variability of both the carbon flushed from soils and the subsequent loss to the atmosphere, scientists first need to understand the variability in aquatic concentrations. Although previous studies have shown that CO2 concentrations vary considerably over time and are often linked to water discharge, measurements have primarily been based on low-frequency manual sampling rather than continuous monitoring, so much of the temporal pattern is lost.

To get a better understanding of the relationships between CO2 concentrations and discharge, Dinsmore et al. used CO2 sensors submerged in the water column to continuously monitor concentrations in five catchments across Canada, the United Kingdom, and Fennoscandinavia. The study sites covered a range of mean CO2 concentrations, flow regimes, and catchment characteristics, including flashy headwater streams, second-order streams, and lake outflows.

They find that the relationships between CO2 concentrations and discharge are not consistent across sites: in two of the sites, CO2 concentrations rose during high-flow periods, while in the other three, CO2 concentrations fell during high-flow periods. The differences in the CO2-discharge relationships between sites could be explained using measurable catchment characteristics providing a baseline set of sites that future studies can use for comparison. A more detailed analysis of the transport of CO2 showed that in three of the five catchments the highest 30 percent of flow had the greatest influence on the total amount of CO2 lost downstream annually. From this finding the authors suggest that an increase in precipitation extremes, which is predicted to occur with climate change, would have a greater effect on the flushing of CO2 from soils to surface waters than would be caused by an increase in mean precipitation.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1002/jgrg.20047

Title: Contrasting CO2 concentration discharge dynamics in headwater streams: A multicatchment comparison

Authors: K.J. Dinsmore and M.F. Billett: Centre for Ecology and Hydrology, Penicuik, UK;

M.B. Wallin: Department of Earth Sciences, Uppsala University, Uppsala, Sweden;

M.S. Johnson: Institute for Resources, Environment and Sustainability, University of British Columbia, Vancouver, BC, Canada, and Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC, Canada;

K. Bishop: Department of Earth Sciences, Uppsala University, Uppsala, Sweden, and Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden;

J. Pumpanen: Department of Forest Sciences, University of Helsinki, Helsinki, Finland;

A. Ojala: Department of Environmental Sciences, University of Helsinki, Lahti, Finland.

5. Measuring the forces generated by erosive debris flows

Like water flows, debris flows can carve out steep valleys and change landscapes. By studying the mechanics of bedrock incision caused by flowing debris, scientists are better able to understand patterns and rates of landscape evolution. Laboratory studies and models have shown how flowing granular materials can cut into rock, but field measurements are needed to confirm the findings and provide information about the more complex natural environment.

Monitoring a natural debris flow environment, McCoy et al. measured approximately 30-60 millimeters (1.2 to 2.4 inches) of bedrock lowering over a 4-year period. They observed the mechanisms by which the bedrock was removed by passing debris flows and analyzed the characteristics of several erosive debris flow events, focusing on the basal normal force--the downward force on the bedrock exerted by the flowing debris--which fluctuated substantially during the events due to particles entrained in the flow impacting the bed.

They also find that a thin layer of bed sediment can shield the bed from the impact of erosive particles. The findings, which allow the researchers to place constraints on the forces involved in the erosion process, show that debris flows are an important driver of landscape change.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1002/jgrf.20041, 2013

Title: Field measurement of basal forces generated by erosive debris flows

Authors: S. W. McCoy: Cooperative Institute for Research in Environmental Sciences (CIRES) & Department of Geological Sciences, University of Colorado, Boulder, Colorado, USA, and Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, Massachusetts, USA;

G. E. Tucker: Cooperative Institute for Research in Environmental Sciences (CIRES) & Department of Geological Sciences, University of Colorado, Boulder, Colorado, USA;

J. W. Kean: Now at: Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, Massachusetts, USA;

J. A. Coe: U.S. Geological Survey, Denver Federal Center, Denver, Colorado, USA.

6. Agulhas Current leakage could stabilize Atlantic overturning circulation

A major focus of concern for the coming century is understanding how the Atlantic Meridional Overturning Circulation (AMOC) will respond to the changing temperatures and wind patterns brought on by global climate change. Because the AMOC plays an important role in transporting heat to northern latitudes and modulating storm tracks, temperatures, and precipitation patterns in the surrounding continents, changes in its strength will have far-reaching consequences.

Scientists expect the AMOC's flows to weaken as Arctic ice melting reduces surface ocean salinities in the North Atlantic, hindering the production of cold deep water. The magnitude of that freshening depends, however, on the amount of warm, salty water brought northward by the Gulf Stream system, a northward flow predominantly fed by the warm salty waters that leak around the southern tip of Africa. After flowing down and around the southeastern coast of Africa, the Agulhas Current water turns and flows east into the Indian Ocean. However, some of this water streams, or leaks, into the South Atlantic, feeding the northward flow in the Atlantic Ocean.

Using modeled ocean circulation patterns, Biastoch and Böning find that if Southern Hemispheric westerly winds shift and strengthen, as projected in future climate scenarios, the Agulhas Current leakage would increase by one-third of the current rate. This elevated leakage, they find, would drive an increase in the temperature and salinity of South Atlantic waters. Within a few decades, the additional salt would flow toward the North Atlantic. The authors suggest that the corresponding increase in density could potentially stabilize the strength of the AMOC.

Source: Geophysical Research Letters, doi:10.1002/grl.50243, 2013

Title: Anthropogenic Impact on Agulhas Leakage

Authors: Arne Biastoch and Claus W. Böning: GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Germany.


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