Public Release:  AGU Journal Highlights -- Jan. 28, 2013

American Geophysical Union

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Water Resources Research, Journal of Geophysical Research - Planets (JGR-E), Journal of Geophysical Research - Oceans (JGR-C), and Journal of Geophysical Research-Biogeosciences (JGR-G).

In this release:

1. Io's volcanism controls Jupiter's magnetospheric activity

2. Projected U.S. water use likely to increase as climate warms

3. Mercury's crust likely made of magnesium-rich basalt

4. Assessing the Great Whirl, despite all the pirates

5. Tracing the origin of Arctic driftwood

6. Low density of Earth's core due to oxygen and silicon impurities

7. Understanding the structure of subducting plates

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 http://onlinelibrary.wiley.com/ and inserting into the search engine the full doi (digital object identifier), e.g. 10.1002/grl.50095. 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.

**Please note** AGU has recently partnered with Wiley, which will now publish AGU's journals. Registered reporters should have received an email from Wiley the week of 7 January with a new login and password, which will allow them to access journal articles for free through the Wiley Online Library at http://onlinelibrary.wiley.com/. If you are a reporter and have not yet registered for a complimentary press subscription, please fill out the form at http://sites.agu.org/sciencepolicy/agu-press-subscriptions/.


1. Io's volcanism controls Jupiter's magnetospheric activity

Jupiter's volcanic moon Io spews out volcanic gas, which reaches its atmosphere and becomes ionized, forming what is known as the Io plasma torus. This plasma torus can interact with Jupiter's magnetosphere, possibly affecting auroral activity there. To help determine whether Io's volcanic activity affects Jupiter's magnetosphere, Yoneda et al. analyzed ground-based observations of Jupiter's sodium nebula, which provides an indication of Io's volcanic activity and plasma content in the Io plasma torus, along with satellite-based measurements of radio emission called HOM emission, which is a sign of Jupiter's auroral activity.

They observe that Jupiter's sodium nebula was enhanced in late May through early June 2007, indicating that Io's volcanic activity increased during that period. The researchers observe that shortly after this enhancement began, Jupiter's HOM emission intensity decreased. As a result, the authors conclude that increased volcanic activity on Io lessens auroral activity in Jupiter's magnetosphere.

Source: Geophysical Research Letters, doi: 10.1002/grl.50095, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50095/abstract

Title: Io's volcanism controls Jupiter's radio emissions

Authors: M. Yoneda, F. Tsuchiya, H. Misawa, M. Kagitani, and S. Okano: Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, Sendai city, Miyagi prefecture, Japan;

B. Bonfond: Laboratoire de Physique Atmosphérique et Planétaire, Université de Liège, Liège, Belgium;

C. Tao: Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo ward, Sagamihara city, Kanagawa prefecture, Japan.


2. Projected U.S. water use likely to increase as climate warms

Despite increases in efficiency, water demand in the United States is likely to increase substantially in the future if climate continues to warm, new projections indicate. Brown et al. project future water use to 2090 based on past trends from U.S. Geological Survey water use data from 1960 to 2005 and trends in efficiency. They project U.S. water demand under climate change scenarios using three different global circulation models; they ran each model for three different global socioeconomic scenarios adapted from the scenarios used by the Intergovernmental Panel on Climate Change (IPCC).

They project that with no climate change, because of increasing efficiency, water demand in the United States over the next 50 years would stay within 3 percent of current demand, even with an expected 50 percent increase in population. The projections varied between the different climate models and emissions scenarios, but most show that if there is climate warming, projected water demand would rise substantially. This increased demand would be due mainly to increases in the need for water for irrigation as rising temperatures increase evapotranspiration. Electricity generation for additional air conditioning as temperatures rise would also contribute to increased water demand, though to a much lesser extent. The authors caution that projected increased demand under climate warming may lead to unsustainable water use even if available water supplies do not diminish as climate warms.

Source: Water Resources Research, doi: 10.1002/wrcr.20076, 2013 http://onlinelibrary.wiley.com/doi/10.1002/wrcr.20076/abstract

Title: Projected freshwater withdrawals in the United States under a changing climate

Authors: Thomas C. Brown: Rocky Mountain Research Station, U.S. Forest Service, Fort Collins, Colorado, USA;

Romano Foti: Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA;

Jorge A. Ramirez: Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, Colorado, USA.


3. Mercury's crust likely made of magnesium-rich basalt

With both x-ray and gamma-ray spectrometers, the MErcury Surface, Space ENvironment, GEochemistry and Ranging probe (MESSENGER), which entered orbit around Mercury in 2011, is well equipped for carrying out a detailed compositional analysis of Mercury's crust, the understanding of which could help determine the nature of the planet's formation, and of its volcanic past.

Using spectrometric measurements and laboratory analyses of Mercury surface-analogue samples, Stockstill-Cahill et al. determine that the upper layers of Mercury's crust most closely resemble magnesian basalt terrestrial rocks, though with lower iron concentrations. To make their determination, the authors used a software package known as MELTS to simulate the cooling and crystallization of potential Mercurian lavas with different chemical compositions, estimating the temperatures at which minerals would crystallize out of the molten lava and the abundances of different mineral species. Similarly, the authors simulated the cooling of magnesium-rich terrestrial rocks and of meteoritic samples.

Based on their chemical compositional analysis, the authors infer a number of properties for an early lava on Mercury. They suggest that the lava would have had a very low viscosity, streaming across the surface in widespread but thin layers. Further, they calculate that the temperatures required to produce the magnesium-rich lava would have been much higher than for terrestrial rocks not enriched in magnesium. The authors say that the low-viscosity lava would leave tell-tale marks on the planet's surface that could be identified through further MESSENGER observations.

Source: Journal of Geophysical Research-Planets, doi: 10.1029/2012JE004140, 2012 http://onlinelibrary.wiley.com/doi/10.1029/2012JE004140/abstract

Title: Magnesium-rich crustal compositions on Mercury: Implications for magmatism from petrologic modeling

Authors: Karen R. Stockstill-Cahill and Timothy J. McCoy: Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, USA;

Larry R. Nittler and Shoshana Z. Weider: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, District of Columbia, USA;

Steven A. Hauck, II: Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, Ohio, USA.


4. Assessing the Great Whirl, despite all the pirates

Each year, the powerful southwest monsoon ramps up in midsummer, bringing life-giving rains to the Indian subcontinent. The monsoon winds also drive dramatic changes in the regional ocean currents, including a reversal in the circulation of the Arabian Sea, an energetic eddy field, and strong coastal upwelling. Off the east coast of Somalia, a large (300 to 550 kilometer wide, or 186 to 342 mile wide) anticyclone appears--known since 1876 as the Great Whirl--with surface currents as strong as 2.5 meters per second (8.2 feet per second). The Great Whirl, while associated with the seasonal arrival of the southwest monsoon, is not caused entirely by it; the circulation of the Great Whirl starts a month before, and persists for a month after, the monsoon.

Although the existence of the Great Whirl has been known for more than a century, rampant piracy in the waters off Somalia has prevented researchers from directly observing its behavior using modern oceanographic tools and techniques. To get around this limitation, Beal and Donohue used satellite observations of sea surface height to measure the intraseasonal evolution and interannual variation of the powerful anticyclone. The satellite altimetry measurements, collected from 1993 to 2010, supplemented measurements made during five research cruises conducted in 1995.

The authors find that the Great Whirl persists for roughly 166 days each year, initiating around May, strengthening and intensifying with the June arrival of the monsoon, and dissipating by November. They find that the Great Whirl is often ringed by smaller anticyclones that travel clockwise around its outside edge. Further, they find that rather than evolving gradually over the summer season, the anticyclone's size and shape can vary quickly.

Source: Journal of Geophysical Research-Oceans, doi: 10.1029/2012JC008198, 2013 http://onlinelibrary.wiley.com/doi/10.1029/2012JC008198/abstract

Title: The Great Whirl: Observations of its seasonal development and interannual variability

Authors: L. M. Beal: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, USA;

K. A. Donohue: Graduate School of Oceanography, University of Rhode Island, Narrangansett, USA.


5. Tracing the origin of Arctic driftwood

Tracking the origin of driftwood samples could help scientists to reconstruct past currents in the Arctic Ocean, a new study suggests. Arctic currents are likely to be affected by changing climate, but there are few observations that provide evidence on past current dynamics.

To evaluate the potential use of driftwood samples, Hellmann et al. analyzed 1445 driftwood remains collected in east Greenland and Svalbard, the largest compilation of Arctic driftwood samples so far compiled and analyzed. They were able to characterize four coniferous genera (Pinus, Larix, Picea, and Abies) and three deciduous genera (Populus, Salix, and Betula). At the species level, they distinguish two species of pine, which accounted for 40 percent of their samples. The pine originated mainly from western and central Siberia. Larch and spruce samples, which represented 26 percent and 18 percent, respectively, could have originated from either Siberia or North America, the authors report. They note that in addition to helping to reconstruct past currents, analysis of driftwood samples can help scientists to evaluate past environmental conditions during the sample tree's life span.

Source: Journal of Geophysical Research-Biogeosciences, doi: 10.1002/jgrg.20022, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20022/abstract

Title: Tracing the origin of Arctic driftwood

Authors: Lena Hellmann: Swiss Federal Research Institute, WSL, Birmensdorf, Switzerland, and Oeschger Centre for Climate Change Research, Bern, Switzerland;

Willy Tegel: Institute for Forest Growth IWW, University of Freiburg, Freiburg, Germany;

Ólafur Eggertsson: Iceland Forest Service, Reykjavik, Iceland;

Fritz Hans Schweingruber: Swiss Federal Research Institute, WSL, Birmensdorf, Switzerland and Institute for Forest Growth IWW, University of Freiburg, Freiburg, Germany;

Robert Blanchette: University of Minnesota, Department of Plant Pathology, St. Paul, Minnesota, USA;

Alexander Kirdyanov: V.N.Sukachev Institute of Forest SB RAS, Krasnoyarsk, Russia;

Holger Gärtner: Swiss Federal Research Institute, WSL, Birmensdorf, Switzerland;

Ulf Büntgen: Swiss Federal Research Institute, WSL, Birmensdorf, Switzerland, and Oeschger Centre for Climate Change Research, Bern, Switzerland.


6. Low density of Earth's core due to oxygen and silicon impurities

During accretion and differentiation of the Earth, chemical interactions in a silicate magma ocean and liquid iron drove silicon and oxygen impurities into what went on to become the liquid outer core. Contrasting with previous research, which suggested that silicon and oxygen would only appear in very low concentrations (less than 1 percent by weight) in the liquid iron, Tsuno et al. find that at the base of a magma ocean 1,200 kilometers (750 miles) deep, these light elements could reach concentrations as high as 5 percent oxygen and 8 percent silicon by weight, simultaneously. Such impurity levels would decrease the density of the outer core, accounting for the so-called "density deficit" identified in previous research, whereby the outer core is roughly 10 percent less dense than a pure iron-nickel alloy.

The researchers also propose that at the present-day core-mantle boundary, high temperatures would drive additional silicon and oxygen into the core, creating a light, element-rich, buoyant layer on the top of the liquid outer core. They suggest that evidence for such a layer may have been observed in seismic studies.

Using a multianvil press, the authors drove a mixture of iron, magnesium silicate, silicon dioxide, and the iron oxide wüstite to 25 gigapascals (6.2 million pounds per square inch) of pressure and temperatures from 2,700 to 3,080 Kelvin (4,400 to 5,084 degrees Fahrenheit). They find that at temperatures below 3,000 Kelvin (4,940 degrees Fahrenheit), silicon and oxygen in the iron melt were mutually exclusive, with concentrations not rising above the low levels identified in previous research. Above 3,000 Kelvin (4,940 degrees Fahrenheit), however, they find that the presence of oxygen actually enhanced the partitioning of silicon into the iron, with the concentrations of both silicon and oxygen increasing.

Source: Geophysical Research Letters, doi: 10.1029/2012GL054116, 2013 http://onlinelibrary.wiley.com/doi/10.1029/2012GL054116/abstract

Title: Simultaneous partitioning of silicon and oxygen into the Earth's core during early Earth differentiation

Authors: Kyusei Tsuno: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany and Department of Earth Science, Rice University, Houston, Texas, USA;

Daniel J Frost and David C Rubie: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany.


7. Understanding the structure of subducting plates

Seismic studies are helping scientists learn more about the structure of subducting oceanic plates. Using an airgun array and 80 ocean bottom seismometers spaced along a 500 kilometer (310 mile) profile, Fujie et al. conducted a seismic reflection and refraction survey at the Kuril trench in the northwestern Pacific margin, where part of the Pacific plate is subducting beneath the Okhotsk plate. They estimate the water content of the subducting plate by measuring the velocity of seismic waves--both P waves and S waves--through the plate. The V sub p over V sub s ratio is an indicator of the lithology, porosity, and presence of fluid in the plate. Their findings show that the water content in the plate increased toward the trench, along with greater bending and fracturing, suggesting that water enters the plate through the fractures. The authors conclude that the bending and fracturing of the plate as it subducts plays an important role in the water cycle in subduction zones.

Source: Geophysical Research Letters, doi: 10.1029/2012GL054340, 2013 http://onlinelibrary.wiley.com/doi/10.1029/2012GL054340/abstract

Title: Systematic changes in the incoming plate structure at the Kuril trench

Authors: Gou Fujie, Shuichi Kodaira, Mikiya Yamashita, Takeshi Sato, Tsutomu Takahashi, and Narumi Takahashi: IFREE/JAMSTEC, Yokohama, Kanagawa, Japan.

###

Contact:
Kate Ramsayer
Phone (direct): +1 202 777 7524
E-mail: kramsayer@agu.org

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.