Public Release: 

AGU journal highlights -- Nov. 30, 2011

American Geophysical Union

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

In this release:

  1. Intensifying Pacific trade winds drive regional sea level trends
  2. Freshwater floods move salt into arid-region aquifers
  3. Forest carbon uptake recovers from modest tree losses
  4. Intensively farmed regions impact groundwater nitrate pollution
  5. Wetlands are dominant summer methane source in Arctic
  6. New complexities in ice stream flow behavior
  7. How did Martian polar gullies form?

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. doi:10.1029/2011GL049576. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions who are registered with AGU also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at

1. Intensifying Pacific trade winds drive regional sea level trends

Sea level in the western tropical Pacific Ocean has been increasing at a rate about three times the global average rate of sea level rise, as observed from satellite altimetry and tide gauges. Why are sea level trends so different in this region? Previous studies have suggested that the high rate of sea level change in the western tropical Pacific is associated with the natural variation of the El Niño-Southern Oscillation.

However, using a general circulation model, Merrifield and Maltrud show that western tropical Pacific sea level trends are likely due to a gradual intensification of the Pacific trade winds in the past 2 decades. They also highlight other aspects of ocean circulation that have been altered in response to the intensifying trade winds. Some previous research has suggested that the trade wind intensification is a result of global warming, although that has yet to be verified. If that is the case, the authors conclude the western tropical Pacific sea level trends will likely continue to be anomalously high.

Source: Geophysical Research Letters, doi:10.1029/2011GL049576, 2011

Title: Regional sea level trends due to a Pacific trade wind intensification

Authors: Mark A. Merrifield: Department of Oceanography, University of Hawai'i at Mānoa, Honolulu, Hawaii, USA;

Mathew E. Maltrud: Fluid Dynamics and Solid Mechanics Group, Los Alamos National Laboratory, Los Alamos, New Mexico, USA.

2. Freshwater floods move salt into arid-region aquifers

For people living in arid ecosystems, groundwater replenished during seasonal flooding is typically the most important source of freshwater. Yet these same floods may also be responsible for temporarily increasing the salinity of the vital freshwater stores, a relationship shown for the austral summer flooding of the Kuiseb River in Namibia. Between 2006 and 2008, Amiaz et al. recorded the effect of 12 floods on soil water storage, groundwater electrical conductivity (a measure of salinity), and solute movement throughout various layers of the subsurface along the river. The researchers' primary concern is understanding how soluble salts residing in the vadose zone, the subsurface layer that lies between the groundwater table and the surface, make their way into the aquifer.

Through in-field measurements and laboratory soil column experiments, the authors find that as floodwater travels along the surface it triggers a compression wave that moves down through the sandy sediments that compose the channel bed, destabilizing salts in the vadose zone and pushing them into the groundwater. As the floodwater itself percolates through the subsurface, additional salts dissolve into it, further increasing groundwater salinity. The authors notice that while the first flood of the season leads to the largest leap in groundwater salinity, subsequent flooding can draw even more salt out of the vadose zone due to the rising water table and the subsequent saturation of the vadose zone sediments. They note that it is only upon the arrival of substantial floodwater to the groundwater stores that dilution can compensate for the increased salinity.

Source: Water Resources Research, doi:10.1029/2011WR010747, 2011

Title: Solute transport in the vadose zone and groundwater during flash floods

Authors: Yanai Amiaz, Shaul Sorek and Ofer Dahan: Zuckerberg Institute for Water Research (ZIWR), J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Israel;

Yehouda Enzel: Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem, Israel.

3. Forest carbon uptake recovers from modest tree losses

By some accounts, forests are currently seeing the highest rates of disturbance since the recession of the Pleistocene glaciers over 10,000 years ago. In recent decades the global extent of forest-disrupting events has increased, yet their intensity has been on the decline. Significant tree losses upset carbon and nitrogen cycling, drastically extending recovery times. Throughout the northern temperate zone, forests that established a century ago on clear-cut or burned lands have been increasingly affected by subtle disturbances, like selective logging, pathogenic insects, or age-related mortality. The higher proportion of trees surviving these modest disturbances likely mitigates the effects on carbon and nitrogen cycling, but evidence is limited.

By manipulating an experimental forest at the University of Michigan Biological Station, Nave et al. determine how forests' carbon and nitrogen cycles respond to subtle disruptions. In 2008, the authors and their team culled 39 percent of the forest's tree population using stem girdling--the process of removing a ring of the tree's bark and starving it of its nutrient and water supplies. The aim was to accelerate the loss of short-lived tree species, driving the forest into a more complex state, commonly found in older forests, rather than the homogeneous conditions that result from clear-cutting or a forest-clearing fire. Initially, the authors find that the forest's carbon uptake declined and soil nitrogen availability and leaching increased, reminiscent of severe disturbances. However, most forest ecosystems are inherently limited by the availability of nitrogen, and the newly liberated stores were drawn up by the remaining longer-lived trees, producing new leaf area and mitigating the decrease in carbon uptake. The authors suggest that, as the forest structure continues to change in the wake of subtle disturbances, the forest's carbon uptake will continue to increase.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1029/2011JG001758, 2011

Title: Disturbance and the resilience of coupled carbon and nitrogen cycling in a north temperate forest

Authors: L. E. Nave and C. S. Vogel: University of Michigan Biological Station, Pellston, Michigan, USA:

C. M. Gough: Department of Biology, Virginia Commonwealth University, Richmond, Virginia, USA;

K. D. Maurer and G. Bohrer: Department of Civil and Environmental Engineering and Geodetic Science, Ohio State University, Columbus, Ohio, USA;

B. S. Hardiman and P. S. Curtis: Department of Evolution, Ecology, and Organismal Biology, Ohio State University, Columbus, Ohio, USA;

J. Le Moine and K. J. Nadelhoffer: University of Michigan Biological Station, Pellston, Michigan, USA, and University of Michigan, Department of Ecology and Evolutionary Biology, Ann Arbor, Michigan, USA;

A. B. Muñoz: Columbia University, New York, New York, USA;

J. P. Sparks: Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, USA;

B. D. Strahm: Department of Forest Resources and Environmental Conservation, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA.

4. Intensively farmed regions impact groundwater nitrate pollution

Intensified agricultural practices that have developed during the past century have helped improve food security for many people but have also added to nitrate pollution in water supply. Balancing the needs for agriculture and clean groundwater for drinking requires understanding factors such as the routes by which nitrate enters the water supply and how long nitrate remains in the water.

The Thames River catchment provides a good study example because the water quality in the river, which supplies drinking water to millions of people, has been monitored for the past 140 years, and the region has undergone significant agricultural development over the past century. Howden et al. study nitrate transport from agricultural land to water in the Thames basin using a simple model that considers an estimate of the amount of nitrate that could leach the groundwater based on land use practices along with an algorithm that determines the route nitrate would take to reach surface or groundwater from agricultural areas.

They find that nitrate concentrations in the Thames rose significantly during and after World War II to about double their previous level, then increased again in the early 1970s. Nitrite concentrations have remained at that high level even though nitrate from inputs from agriculture declined from the late 1970s to early 2000s. The authors note it takes some time for nitrate to reach the river, and their analysis suggests that the jump in nitrate concentrations from 1968 to 1972 is actually a response to plowing during World War II. The study could help water and land management planners identify practices that best preserve both agricultural production and water quality.

See blog post about this paper in AGU's GeoSpace blog at:

Source: Water Resources Research, doi:10.1029/2011WR010843, 2011

Title: Nitrate pollution in intensively farmed regions: what are the prospects for sustaining high-quality groundwater?

Authors: Nicholas J.K. John Kenneth Howden: Faculty of Engineering, University of Bristol, Bristol, UK;

Tim P. P Burt: Department of Geography, Durham University, Durham, UK;

Fred Worrall: Department of Earth Science, Durham University, Durham, UK;

Simon A. Mathias: Department of Earth Science, Durham University, Durham, UK;

Mick J. Whelan: School of Applied Sciences, Cranfield University, Cranfield, UK.

5. Wetlands are dominant summer methane source in Arctic

Methane, a potent greenhouse gas that contributes to climate change, enters the atmosphere from a variety of sources--it can leak from industrial gas fields or pipelines, escape from submarine hydrates that decompose with warming temperatures, or be released from decaying organic matter. Methanes from different sources have different isotopic compositions, allowing researchers to identify the source of methane in the air, as Fisher et al. do in a new study. The researchers analyzed the isotopic composition of methane in the air off Spitsbergen, Norway, in 2008 and 2009. They find that in the summer, wetlands were the dominant methane source. Methane is being released to the water column from gas hydrates on the seabed, but the study indicates that so far this methane has not reached the atmosphere. Wetlands are likely to release more methane as temperatures warm, feeding further climate change.

Source: Geophysical Research Letters, doi:10.1029/2011GL049319, 2011

Title: Arctic methane sources: Isotopic evidence for atmospheric inputs

Authors: R. E. Fisher, S. Sriskantharajah, D. Lowry, M. Lanoisellé, and C. M. R. Fowler: Department of Earth Sciences, Royal Holloway, University of London, Egham, UK;

R. H. James: National Oceanography Centre, University of Southampton, Southampton, UK;

O. Hermansen, C. Lund Myhre, and A. Stohl: Norwegian Institute for Air Research, Kjeller, Norway;

J. Greinert: Department of Marine Geology, Royal Netherlands Institute for Sea Research, Den Burg, Netherlands;

P. B. R. Nisbet-Jones: Department of Physics, Clarendon Laboratory, University of Oxford, Oxford, UK;

J. Mienert: Department of Geology, University of Tromsø, Tromsø, Norway;

E. G. Nisbet: Department of Earth Sciences, Royal Holloway, University of London, Egham, UK.

6. New complexities in ice stream flow behavior

Ice flow speed within an ice sheet can vary from a few meters to thousands of meters per year. Fast flowing ice streams can affect sea level, and their flow variation is one factor that determines whether an ice sheet is gaining or losing mass. But different ice streams exhibit different behaviors, and these spatial and temporal variations are not well understood, causing uncertainty about sea level change when ice streams stop or accelerate.

Bougamont et al. use a three-dimensional ice sheet model coupled with a model of the underlying subglacial environment to investigate the relationship between ice flow and basal till properties. This new coupled model reveals new and significant complexities in ice stream behavior, which are related to the spatial and temporal evolution of bed properties. Furthermore, they find that even small differences in the subglacial hydrology can significantly influence ice stream flow. Their results also reveal that ice streams may well have a memory insofar as their current flow is conditioned by their previous behavioral history and physical condition. The study is a step toward understanding the complex behavior of Antarctic ice streams.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1029/2011JF002025, 2011

Title: Dynamic patterns of ice stream flow in a 3-D higher-order ice sheet model with plastic bed and simplified hydrology

Authors: M. Bougamont: Scott Polar Research Institute,Cambridge, UK;

S. Price: Los Alamos National Laboratory, Los Alamos, New Mexico, USA; and Bristol Glaciology Centre, Bristol, UK;

P. Christoffersen: Scott Polar Research Institute, Cambridge, UK;

A. J. Payne: Bristol Glaciology Centre, Bristol, UK.

7. How did Martian polar gullies form?

Gullies on Mars have been pointed to as evidence for the presence of flowing liquid water. However, gullies also exist in Mars' polar regions, where temperatures are too low to support liquid water. Other processes have been proposed to explain the origin of gullies but have not been confirmed. For instance, sediment lying on top of a seasonal accumulation of carbon dioxide frost could flow like a fluid if the frost sublimes (turns to gas directly from the solid stage) sufficiently quickly. This fluidized sediment could form gullies.

To determine whether conditions are suitable for such fluidization to occur in Mars' polar regions, Cedillo-Flores et al. calculated the carbon dioxide sublimation rate needed to fluidize sand and dust lying on top of the carbon dioxide frost. They then used a thermal model of Mars' surface and subsurface to determine whether buried carbon dioxide frost could potentially sublimate at that rate. The researchers confirm that sediment fluidization could indeed occur in Mars' polar regions, and thus, Martian gullies can form without the presence of liquid water.

Source: Geophysical Research Letters, doi:10.1029/2011GL049403, 2011

Title: CO2 gas fluidization in the initiation and formation of Martian polar gullies

Authors: Yolanda Cedillo-Flores: Facultad de Filosofía y Letras, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico and Lunar and Planetary Institute, Houston, Texas, USA;

Allan H. Treiman: Lunar and Planetary Institute, Houston, Texas, USA;

Jeremie Lasue: Lunar and Planetary Institute, Houston, Texas, USA and Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA;

Stephen M. Clifford: Lunar and Planetary Institute, Houston, Texas, USA.


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