AGU journal highlights -- 23 April 2013

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

In this release:

1. Beachfront nourishment decisions: the "sucker-free rider" problem
2. Identifying the physical processes that control the stratigraphic record
3. Uplift of Zagros Mountains slows down convergence of two plates
4. Extensive Antarctic campaign finds cold bias in satellite records
5. Measuring tidal displacement using GPS
6. Hydrology affects carbon storage potential of prairie potholes

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1. Beachfront nourishment decisions: the "sucker-free rider" problem

Coastal communities and beachfront property owners often respond to erosion by adding sand to restore local beaches. But beach nourishment alters shoreline dynamics, not only at the replenishment site but also in adjacent coastal regions, as natural coastal processes shift sand from one location to another. The result is that "sucker" communities pay to build up their beaches, but that replenishment also helps protect the coastlines of "free rider" communities. The sucker-free rider situation is an example of a classic problem studied in economics and game theory frameworks.

Using model simulations of the dynamics of coastlines similar to those along much of the U.S. East Coast, coupled with a model of locally economically optimal nourishment decisions, Williams et al. find that interactions between the erosion processes and economic and physical feedbacks do tend to lead to the sucker-free rider situation. Large inequalities in property values can result, especially as the cost of sand for nourishment rises.

The results indicate that decentralized management of beach nourishment, in which individuals or communities make locally optimal decisions without taking into account the effects on others may lead to a use of resources that is not optimal over a wider area.

Source: Journal of Geophysical Research-Earth Surface, doi: 10.1002/jgrf.20066, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20066/abstract

Title: Coupled Economic-Coastline Modeling with Suckers and Free Riders

Authors: Zachary C. Williams and Dylan E. McNamara: Center for Marine Science, University of North Carolina Wilmington, Wilmington, North Carolina, USA, and Department of Physics and Physical Oceanography, University of North Carolina Wilmington, Wilmington, North Carolina, USA;

Martin D. Smith: Nicholas School of the Environment, Duke University, Durham, North Carolina, USA, and Department of Economics, Duke University, Durham, North Carolina, USA;

A Brad. Murray: Nicholas School of the Environment, Duke University, Durham, North Carolina, USA, and Center for Nonlinear and Complex Systems, Duke University, Durham, North Carolina, USA;

Sathya Gopalakrishnan: Department of Agricultural, Environmental and Development Economics, The Ohio State University, Columbus, Ohio, USA.

2. Identifying the physical processes that control the stratigraphic record

The stratigraphic record, the sequential layers of sediment that geologists use to reconstruct the history of a landscape, has been described as "more gaps than record." The record, laid down over time as sediment settles out from flowing water, does not grow consistently. Pauses in sediment deposition can leave gaps, and periods of heightened erosion can wipe sections out. Though attempts have been made to identify the processes that control the completeness of the stratigraphic record, early analyses relied on parameters (such as the long-term sediment accumulation rate) that are not first-order physical landscape processes.

Building on that earlier work, Straub and Esposito used a series of laboratory experiments to identify the physical processes that are relevant to controlling the accumulating stratigraphic record, and to determine the relationships between these processes. The authors built a series of artificial river deltas, each with varying water flow and sedimentation rates, and used time-lapse photography and topographic measurements to track how the stratigraphic record developed. They find that the stratigraphic record is most complete when the sediment deposition rate is high, when the water flow rate relative to the sediment flow rate is low, and when the river channel migrates slowly across the whole delta region.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1002/jgrf.20061, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20061/abstract

Title: Influence of water and sediment supply on the stratigraphic record of alluvial fans and deltas: Process controls on stratigraphic completeness

Authors: Kyle M. Straub and Christopher R. Esposito: Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana, U.S.A.

3. Uplift of Zagros Mountains slows down convergence of two plates

Research has indicated that mountain ranges can slow down the convergence between two tectonic plates on timescales as short as a few million years, as the growing mountains provide enough tectonic force to impact plate motions. Focusing on the convergence of the Arabian and Eurasian plates at the Zagros mountain range, which runs across Iran and Iraq, Austermann and Iaffaldano reconstructed the relative motion of the plates using published paleomagnetic data covering the past 13 million years, as well as current geodetic measurements. They show that the convergence of the two plates has decreased by about 30 percent over the past 5 million years. Looking at the geological record to infer past topography and using a computer model of the mantle-lithosphere system, the authors examined whether the recent uplift across the Zagros Mountains could have caused the observed slowdown. They also considered several other geological events that might have influenced the convergence rate, but were able to rule those out as dominant controls. The authors conclude that the uplift across the Zagros Mountains in the past 5 million years did indeed play a key role in slowing down the convergence between the Eurasian and Arabian plates.

Source: Tectonics, doi:10.1002/tect.20027, 2013 http://onlinelibrary.wiley.com/doi/10.1002/tect.20027/abstract

Title: The role of the Zagros orogeny in slowing down Arabia-Eurasia convergence since ~5 Ma

Authors: Jacqueline Austermann: Department of Earth and Planetary Sciences, Harvard University, USA;

Giampiero Iaffaldano: Research School of Earth Sciences, The Australian National University, Australia.

4. Extensive Antarctic campaign finds cold bias in satellite records

Advances in satellite sensing have now made it possible to track changes in the world's most remote locations. Over the Antarctic continental interior, the vast majority of profiles of atmospheric temperature are provided by satellite remote sensing, making proper calibration of the satellite observational equipment and analysis algorithms particularly important. However, the hostile environmental conditions in regions such as the Antarctic make it difficult or even dangerous to conduct the in-the-field observations needed to calibrate and validate the satellite observations.

From September through December 2010, Wang et al. carried out an extensive program to directly measure atmospheric temperature profiles over coastal and interior Antarctica and over the surrounding ocean. The researchers launched 13 Driftsondes, gondola-laden stratospheric balloons that carry up to 56 individual dropsondes-sensors designed to measure the pressure, temperature, humidity, and wind speed and direction as they fall from the stratosphere to the surface. Using the dropsondes, the authors collected 639 profiles of the Antarctic atmosphere. The campaign produced a collection of observations that spanned the southern continent and provided an opportunity to assess the skill of various remote-sensing satellites.

Comparing their observations against coincident measurements made with 10 different satellite sensors, the authors find that 9 of the 10 sensors consistently underestimated Antarctic atmospheric temperatures and that errors are generally larger over the continental interior than along the coast. The findings reveal that the cold biases can be from 0 to 4 degrees Celsius (0 to 7 degrees Fahrenheit), depending on the satellite sensor and the altitude of the atmosphere being observed. Despite the cold bias, however, the satellites do tend to properly represent the atmospheric temperature profile's shape.

Source: Geophysical Research Letters, doi:10.1002/grl.50246, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50246/abstract

Title: Unprecedented upper air dropsonde observations over Antarctica from the 2010 Concordiasi Experiment: Validation of satellite-retrieved temperature profiles

Authors: Junhong Wang: National Center for Atmospheric Research, Boulder, Colorado, and Department of Atmospheric & Environmental Sciences, State University of New York at Albany, New York;

Terry Hock, Stephen A. Cohn and Charlie Martin and Nick Potts: National Center for Atmospheric Research, Boulder, Colorado;

Tony Reale: NOAA/NESDIS, Camp Springs, Maryland;

Bomin Sun and Frank Tilley: IM System Group, Camp Springs, Maryland.

5. Measuring tidal displacement using GPS

GPS is making possible high-precision, high-resolution measurements of tidal displacement that could not be achieved with other methods. Earth's surface deforms due to both body tides-the deformation of the solid Earth due to the pull of the Sun and the Moon-and ocean tides-the redistribution of water mass loading over Earth's surface. Body tides and ocean tides both have components that vary on semidiurnal, diurnal, and longer periods.

Yuan et al. used data from 456 globally distributed continuous GPS stations covering the period from 1996 through 2011 to determine the 3-dimensional crustal displacements for semidiurnal and diurnal tides. They examined the accuracy and possible sources of error in the GPS-based estimates of tidal displacements, and were able to achieve sub-millimeter precision for the first time. They also compared the GPS-based estimates with model accuracies of both body tides and ocean tides and demonstrated that the GPS-based tidal displacements are more accurate than the modeled displacements, hence providing observations that could be useful for improving the models.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1002/jgrb.50159, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrb.50159/abstract

Title: The tidal displacement field at Earth's surface determined using global GPS observations

Authors: Linguo Yuan: Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu, China, and Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan;

Benjamin Fong Chao: Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan;

Xiaoli Ding: Department of Land Surveying and Geo-Informatics, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China;

Ping Zhong: Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu , China.

6. Hydrology affects carbon storage potential of prairie potholes

Prairie potholes, the small, dynamic, unconnected ponds that dot central Canada as well as parts of the north-central United States, can store significant amounts of soil nutrients that can be transformed to carbon dioxide and other greenhouse gases. Scientists would like to better understand how these regions could contribute to climate warming, but there are challenges, given the large heterogeneity in greenhouse gas emissions over the prairie pothole landscape.

To help gain a better understanding of the factors that influence these emissions, Creed et al. measured fluxes of carbon dioxide, methane, and nitrous oxide from the soils in the prairie pothole region of central Canada, along with hydrologic properties such as soil moisture. They find that soil moisture was an important driver of the differences in carbon dioxide emissions. Soil moisture controls occurred at multiple scales, from ridge to valley along individual potholes at the finest scale, and from the southern limit to the northern limit of potholes in Canada at the coarsest scale. By integrating these soil moisture controls across fine-to-coarse scales, the authors were able to show the potential contribution of prairie potholes to warming changes across the region. Greenhouse gas emission was smallest in the drier south, where the largest emissions came from the lowland area at the land-water interface, while in the north, large emissions came from a broader area of the hill slope.

The authors conclude that if hydrologic factors are not taken into account, studies could significantly underestimate or overestimate the potential effects of prairie pothole regions on warming.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1002/jgrg.20050, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20050/abstract

Title: Hydrologic profiling for greenhouse gas effluxes from natural grasslands in the prairie pothole region of Canada

Authors: Irena F. Creed, Johnston Miller, David Aldred, Jennifer K. Adams, and Salvatore Spitale: Department of Biology, Western University, London, Ontario, Canada;

Rick A. Bourbonniere: Water Science and Technology Directorate, Environment Canada, Burlington, Ontario, Canada.

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