AGU journal highlights – 17 September 2012

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

In this release:

1. Characterizing the surface composition of Mercury 2. African dust forms red soils in Bermuda 3. Sea level controls carbon accumulation in the Everglades 4. Climate change threatens permafrost in soil 5. China's Changbaishan volcano showing signs of increased activity 6. For first time, meandering river created in laboratory

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1. Characterizing the surface composition of Mercury

The MESSENGER spacecraft, which has been orbiting Mercury since March 2011, has been revealing new information about the surface chemistry and geological history of the innermost planet in the solar system. Weider et al. recently analyzed 205 measurements of the surface composition from MESSENGER's X-ray spectrometer, focusing on the large expanse of smooth volcanic plains at high northern latitudes and surrounding areas that are higher in crater density and therefore older than the northern plains.

In general, the measurements show that Mercury's surface composition is very different from that of other planets in the solar system. It is dominated by minerals high in magnesium and enriched in sulfur. This composition is similar to that expected from partial melts of enstatite chondrites, a rare type of meteorite that formed at high temperatures in highly reducing (low oxygen) conditions in the inner solar system.

In addition, the researchers find that the composition of Mercury's northern plains deposits differs from that of the surrounding older terrain. In particular, the older terrain has higher ratios of magnesium to silicon, sulfur to silicon, and calcium to silicon, but lower ratios of aluminum to silicon. These differences suggest that the smooth plains material erupted from a magma source that was chemically different from the source of the material in the older regions. Future studies will help constrain further the formation and geological history of Mercury.

Source: Journal of Geophysical Research-Planets, doi:10.1029/2012JE004153, 2012
http://dx.doi.org/10.1029/2012JE004153

Title: Chemical heterogeneity on Mercury's surface revealed by the MESSENGER X-Ray Spectrometer

Authors: Shoshana Zoe Weider, Larry Nittler, and Paul K. Byrne: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., USA;

Richard D Starr: Physics Department, The Catholic University of America, Washington, D.C., USA, and Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;

Timothy McCoy and Karen Stockstill Cahill: Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA;

Brett W Denevi: The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA;

James W. Head: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA;

Sean C. Solomon: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., USA, and Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA.

2. African dust forms red soils in Bermuda

In Bermuda, red iron-rich clayey soil horizons overlying gray carbonate rocks are visually stunning topographical features. These red soils, called terra rossa, are storehouses of information not only on past local processes that crafted the topography of the island but also on atmospheric circulation patterns that drove global climate during the Quaternary period (roughly 2.5 million years ago). The origin of the terra rossa, however, has remained a mystery for well over a century.

On one hand, dissolution features in the carbonate rocks suggest that local material, for example volcanic rocks, could be the source of these red beds. On the other hand, Bermuda is also uniquely located to receive airborne dust from western Africa as well as from loess deposits of the Mississippi Valley of the central United States — both of which could also be potential sources of the terra rossa soils in Bermuda.

For the first time, Muhs et al. analyzed trace element concentrations in terra rossa soils of Bermuda. Matching the terra rossa trace element profile to that of each potential source material, the authors suggest that airborne dust originating from a vast swath of western Africa may be the most likely parent material of the red beds of Bermuda. Until now, scientists had assumed that dust transport from western Africa was limited to the southern Caribbean, with maximum transport to Barbados, and in small amounts to Florida. The new finding suggests that dust from Africa not only reached more northern latitudes during the Quaternary but also must have occurred in significant quantities to account for the formation of red soils in Bermuda.

Source: Journal of Geophysical Research- Earth Surface, doi:10.1029/2012JF002366
http://dx.doi.org/10.1029/2012JF002366

Title: Soil genesis on the island of Bermuda in the Quaternary: The importance of African dust transport and deposition

Authors: Daniel R. Muhs, James R. Budahn and Gary Skipp: U.S. Geological Survey, Denver, Colorado, USA;

Joseph M. Prospero: Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida, USA;

Stanley R. Herwitz: UAV Collaborative, NASA Research Park, Moffett Field, California, USA.

3. Sea level controls carbon accumulation in the Everglades

How much carbon is stored in the organic soils of tropical wetlands is becoming an important question as erosion, agriculture, and global climate change slowly set into motion a series of processes that could potentially release carbon locked up in these wetlands. In a recent study, Glaser et al. reconstructed a complete, carbon-14 dated 4,000-year history of both organic and inorganic matter accumulation in the Everglades of south Florida.

The authors find that despite the fact that erosion, fires, and similar processes may have removed as much as 2 meters (6.56 feet) of soil from the Everglades, there is a remarkable consistency in the accumulation rates of both organic and inorganic matter in the Everglades over the past 4,000 years. They speculate that processes such as sea level rise that operate on time scales of centuries or even millennia may be ultimately controlling the rates of formation and accumulation of organic matter in the Everglades.

They further show that the rate of organic matter accumulation in the southern Everglades is two to four times lower than its counterparts in colder and high-latitude environments. The authors attribute the low accumulation rates mostly to the slow rise in sea level since the mid-Holocene, but also to low supply of nutrients and high temperatures; all of these factors favor low rates of organic matter production but faster rates of decomposition. They note that compared to the northern peatlands, tropical wetlands store relatively small amounts of carbon.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1029/2011JG001821
http://dx.doi.org/10.1029/2011JG001821

Title: Carbon and sediment accumulation in the Everglades (USA) during the past 4000 years: Rates, drivers, and sources of error

Authors: Paul H. Glaser and Barbara C. S. Hansen: Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota, USA;

John C. Volin: Department of Natural Resources and the Environment, University of Connecticut, Storrs, Connecticut, USA;

Thomas J. Givnish: Department of Botany, University of Wisconsin–Madison, Madison, Wisconsin, USA;

Craig A. Stricker: U. S. Geological Survey Stable Isotope Lab, Denver, Colorado, USA.

4. Climate change threatens permafrost in soil

In the coming century, permafrost in polar regions and alpine forests in the Northern Hemisphere may thaw rapidly, potentially releasing carbon and nitrogen that could cause additional regional warming. Permafrost occurs in soils where ground temperatures remain below freezing for at least two consecutive years. These special types of soil, called Gelisols, are large reservoirs of organic carbon and nitrogen. Thawing is likely to release the carbon and nitrogen in these soils to rivers and lakes, ecosystems, and the atmosphere; different soil types are vulnerable to different thawing processes.

There is field evidence that permafrost cover has been moving poleward since around 1900. Scientists predict that of the many ways permafrost can thaw, "top-down" and "lateral thawing" will be the most dominant modes of degrading permafrost. Harden et al. compiled a database of published and unpublished carbon and nitrogen content from a variety of Gelisols. Using predictions of soil temperature in the climate model CCSM4, the authors studied the role of top-down thawing processes in degrading several types of Gelisols under future climate scenarios.

They find that forest fires and thawing-related decomposition of different types of Gelisols would take place over the next century, which could potentially release up to 850 billion tons of carbon and up to 44 billion tons of nitrogen into atmosphere-water and high-latitude ecosystems. The authors recommend combining extensive field and model studies, such as theirs, to understand the impact of permafrost thawing on global and regional climate by the middle of this century.

Source: Geophysical Research Letters, doi:10.1029/2012GL051958, 2012
http://dx.doi.org/10.1029/2012GL051958

Title: Field information links permafrost carbon to physical vulnerabilities of thawing

Authors: Jennifer W. Harden: U.S. Geological Survey, Menlo Park, California, USA;

Charles D. Koven: Lawrence Berkeley National Laboratory, Berkeley, California, USA;

Chien-Lu Ping and Gary J. Michaelson: Palmer Research Center, University of Alaska Fairbanks, Fairbanks, Alaska, USA;

Gustaf Hugelius and Peter Kuhry: Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden;

A. David McGuire: Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska Fairbanks, Fairbanks, Alaska, USA;

Phillip Camill: Environmental Studies Program and Department of Earth and Oceanographic Science, Bowdoin College, Brunswick, Maine, USA;

Torre Jorgenson: Alaska Ecoscience, Fairbanks, Alaska, USA;

Jonathan A. O'Donnell: U.S. Geological Survey, Boulder, Colorado, USA;

Edward A. G. Schuur: Department of Biology, University of Florida, Gainesville, Florida, USA;

Charles Tarnocai: Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada;

Kristopher Johnson: USDA Forest Service, Newtown Square, Pennsylvania, USA;

Guido Grosse: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA.

5. China's Changbaishan volcano showing signs of increased activity

Roughly 1,100 years ago, the Changbaishan volcano that lies along the border between northeastern China and North Korea erupted, sending pyroclastic flows dozens of kilometers and blasting a 5-kilometer (3-mile) wide chunk off of the tip of the stratovolcano. The eruption, known as the Millennium eruption because of its proximity to the turn of the first millennium, was one of the largest volcanic events in the Common Era. In the subsequent period, there have been three smaller eruptions, the most recent of which took place in 1903. Starting in 1999, spurred by signs of resumed activity, scientists established the Changbaishan Volcano Observatory, a network to track changing gas compositions, seismic activity, and ground deformation. Reporting on the data collected over the past 12 years, Xu et al. find that these volcanic indices each leapt during a period of heightened activity from 2002 to 2006.

The authors find that during this brief active period, earthquake occurrences increased dramatically. From 1999 to 2002, and from 2006 to 2011, they registered 7 earthquakes per month using 11 seismometers. From 2002 to 2006, this rate increased to 72 earthquakes per month, peaking in November 2003 with 243 events. Further, tracking the source of the earthquakes, the authors tie the bulk of the events to a region located 5 kilometers (3 miles) beneath the volcanic caldera, a source that slowly crept upward throughout the study period, suggestive of an ongoing magmatic intrusion. Gas composition measurements collected from hot springs near the volcano showed spikes in carbon dioxide, hydrogen, helium, and nitrogen gases, which the authors suggest could be related to magmatic outgassing. Ground deformation studies, too, show a brief period of rapid expansion. The authors suggest that though Changbaishan is likely not gearing up for an imminent eruption, one could be expected in the next couple of decades.

Source: Geophysical Research Letters, doi:10.1029/2012GL052600, 2012
http://dx.doi.org/10.1029/2012GL052600

Title: Recent unrest of Changbaishan volcano, northeast China: A precursor of a future eruption?

Authors: Jiandong Xu and Bo Pan: Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing, China and Changbaishan Volcano Observatory, Antu, China;

Guoming Liu and Junqing Liu: Changbaishan Volcano Observatory, Antu, China;

Jianping Wu and Yuehong Ming: Changbaishan Volcano Observatory, Antu, China and Institute of Geophysics, CEA, Beijing, China;

Qingliang Wang: Changbaishan Volcano Observatory, Antu, China and Second Monitoring Center, CEA, Xi'an, China;

Duxin Cui: Second Monitoring Center, CEA, Xi'an, China;

Zhiguan Shangguan and Xudong Lin: Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing, China.

6. For first time, meandering river created in laboratory

Natural rivers are not straight, and they are rarely idle. Instead, they bend and curve and sometimes appear to wriggle across the surface over time. That rivers can meander is obvious but how and why they do so is less well known. These questions are complicated by the fact that researchers have for the most part been unable to realistically create a meandering river in a laboratory. Scientists have previously created simulated streams that bend and branch, but they were not able to limit the river to only a single main flow path or maintain such dynamic motion past the initial bend formation. Working with a 6-by-11 meter (20-by-36 foot) river simulator called the Eurotank, van Dijk et al. created a dynamically meandering river. In so doing, the authors identify two conditions necessary to induce meandering: the availability of mixed sediment and a continuously varying upstream water source.

For 260 hours the authors pumped a steady stream of water and mixed sediment onto a sediment-filled basin. First, they held the inflow point steady, which resulted in a straight channel. Then, they moved the inflow point horizontally, which caused the downstream flows to bend. Finally, the authors reversed the horizontal motion of the input point, which further increased the downstream complexity. Photographs taken every 10 minutes and high-resolution laser topography scans captured every 7 hours captured the details of the river's evolution.

The authors suggest that the drifting inflow point caused the channel to meander, while the presence of mixed sediments sealed off defunct paths, preventing the single channel from turning into a multithreaded braided system. The finding suggests that meandering at any point in a river depends on lateral drift in upstream reaches, such that an immobile bottleneck at any one site will decrease downstream complexity.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1029/2011JF002314, 2012
http://dx.doi.org/10.1029/2011JF002314

Title: Experimental meandering river with chute cutoffs

Authors: W. M. van Dijk, W. I. van de Lageweg, and M. G. Kleinhans: Faculty of Geosciences, Department of Physical Geography, Utrecht University, Utrecht, Netherlands.

Contact: Mary Catherine Adams
Phone (direct): +1 202 777 7530
Email: mcadams@agu.org ### END