AGU journal highlights -- Feb. 4, 2014

(Press-News.org) Contact information: Nanci Bompey
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American Geophysical Union
AGU journal highlights -- Feb. 4, 2014 The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research-Solid Earth (JGR-B), and Paleoceanography.

In this release:

1. Canada's subarctic lakes could face widespread desiccation 2. Seafloor sites could stably store centuries' worth of carbon emissions 3. Detection of supershear rupture in 2013 Craig, Alaska, earthquake 4. Acoustic emissions unveil internal motion in granular materials 5. New high-resolution record of middle to late Miocene climate evolution 6. Systematic shifts in subducting slab behavior with depth

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1. Canada's subarctic lakes could face widespread desiccation

In Canada's subarctic—the boreal ecosystem that spans most of mainland Canada—the temperature is climbing and the snowpack is thinning. Previous research has shown that snow is disappearing even faster than sea ice.

Researchers are concerned that the decline in snow cover will spell the end of many of the country's abundant subarctic lakes, and the unique ecosystems they support. These worries are supported by recent observations that show subarctic lakes drying out. To assess the susceptibility of subarctic lakes to widespread desiccation, Bouchard et al. spent multiple years monitoring changes in subarctic lakes. They find that many subarctic lakes are sensitive to changes in snowmelt, and that recent bouts of drying may be unprecedented in the past 200 years.

The Old Crow Flats and the Hudson Bay Lowlands, the sites of the authors' investigation, are two of the largest subarctic lake-rich ecosystems in North America. Permafrost in the ground, and silt- and clay-rich soils prevent water from seeping through the ground, creating landscapes covered with thousands of shallow thermokarst lakes.

With little water moving through the ecosystem other than that which flows overland, the authors find that the stability of these subarctic lakes depends crucially on winter snowfall and spring snowmelt. They find that lakes in regions with flat terrain and sparse vegetation are most susceptible to evaporative lake-level drawdown at times when snowmelt runoff is low. If current trends continue, the researchers say, many of these small snow-fed tundra lakes could disappear within the next few years to decades.

Source: Geophysical Research Letters, doi: 10.1002/2013GL058635, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013GL058635/abstract

Title: Vulnerability of shallow subarctic lakes to evaporate and desiccate when snowmelt runoff is low

Authors: F. Bouchard: Centre d'études nordiques and Département de Géographie, Université Laval, Québec, Canada and Department of Geography and Environmental Studies, Wilfrid Laurier University, Waterloo, Canada;

K. W. Turner: Department of Geography and Environmental Studies, Wilfrid Laurier University, Waterloo, Canada and Department of Geography, Brock University, St. Catharines, Canada;

L. A. MacDonald: Department of Biology, University of Waterloo, Canada;

C. Deakin, H. White, N. Farquharson , A. S. Medeiros, and B. B. Wolfe: Department of Geography and Environmental Studies, Wilfrid Laurier University, Waterloo, Canada;

R. I. Hall: Department of Biology, University of Waterloo, Canada;

R. Pienitz: Centre d'études nordiques and Département de Géographie, Université Laval, Québec, Canada;

T. W. D. Edwards: Department of Earth and Environmental Sciences, University of Waterloo, Canada.

Press release: http://www.relationsmedias.ulaval.ca/comm/2013/novembre/les-lacs-des-regions-subarctiques-assechent-3372.html?an=1

2. Seafloor sites could stably store centuries' worth of carbon emissions

If the world does not make a concerted effort to actively remove carbon dioxide from the atmosphere, we will be locked into 10,000 years of elevated temperatures. The implementation of a large-scale carbon capture and storage (CCS) program, however, could stave off the bulk of this warming.

However, any plan to actually design and operate an industrial-scale CCS program faces serious technological challenges. One major barrier is in finding a way to stably store large volumes of carbon dioxide for centuries or longer. In a new modeling study, Marieni et al. find that in at least five locations worldwide, liquid carbon dioxide can be stored deep beneath the seafloor. Each of these five potential storage sites, they suggest, could conservatively hold several centuries' worth of anthropogenic carbon dioxide emissions.

As its pressure increases, or as temperature decreases, the density of carbon dioxide rises dramatically. The density of seawater, however, is relatively immutable. With the right mix of temperature and pressure, then, carbon dioxide becomes denser than seawater. If the gas were injected into the seafloor at a site with the right conditions—a depth of at least 2,500 meters (8,202 feet) and a temperature between 0 degrees Celsius (32 degrees Fahrenheit) and 30 degrees Celsius (86 degrees Fahrenheit)—the carbon dioxide would be gravitationally stable.

To prevent the carbon dioxide from seeping into the surrounding ocean and to increase the safety of this strategy, the authors also suggest physically trapping the gas. If carbon dioxide were injected into the porous upper basement, beneath several hundred meters of sediments, it would be not only gravitationally stable but also trapped by a low-permeability sediment cap. Using existing measurements, the authors combine the pressure, temperature, and sediment requirements to identify several locations where vast volumes of carbon dioxide could potentially be stored for centuries or longer.

Source: Geophysical Research Letters, doi: 10.1002/2013GL058220, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013GL058220/abstract

Title: Geological storage of CO2 within the oceanic crust by gravitational trapping

Authors: Chiara Marieni, Timothy J. Henstock, and Damon A.H. Teagle: Ocean & Earth Science, National Oceanography Centre Southampton, University of Southampton, United Kingdom.

Press release: http://www.southampton.ac.uk/mediacentre/news/2013/dec/13_216.shtml#.UvENzfuUVI0

3. Detection of supershear rupture in 2013 Craig, Alaska, earthquake

Seismic ruptures are akin to unzipping a zipper—a gap in the crust starts in one location and travels along the fault in a particular direction. When a strained fault ruptures in an earthquake, seismic waves also spread out from the epicenter. In some cases, the waves' passage can trigger the initiation of a new rupture ahead of the initial expanding rupture in locked portions of the fault. If the triggered rupture grows successfully, the overall rupture front can then outpace the passage of the shear waves, secondary seismic waves that travel slowly after the earthquake begins and are responsible for the bulk of violent shaking. These earthquakes display what is known as supershear rupture, and only seven such earthquakes have previously been recorded.

Based on new observations, Yue et al. describe the occurrence of supershear rupture in a magnitude 7.5 earthquake that hit near Craig, Alaska, in January 2013. The observations mark the first time supershear rupture has ever been detected on an offshore boundary between two plates.

By inverting and modeling observed seismic waves, the authors describe the earthquake's rupture mechanism. They find that from the earthquake's epicenter the supershear rupture front moves northward at 5.5 to 6 kilometers per second (3.4 to 3.7 miles per second) down a length of the fault system roughly 100 kilometers (62 miles) long. This supershear rupture moves faster than the motion of secondary seismic waves and nearly as quickly as the primary waves, pressure waves that travel quickly from an epicenter when an earthquake starts. The earthquake also shows additional rupture to the south, but that was caused by regular subshear rupture.

Aside from having an unusual mechanism, supershear rupture affects the potential damage caused by an earthquake. Supershear rupture increases the amount of ground shaking in the propagation direction. According to the authors, the clearest detectable sign of a supershear earthquake is the detection of secondary seismic waves that are produced by the rupture front that arrive before the secondary waves that are produced at the initiation point of the earthquake.

Source: Journal of Geophysical Research-Solid Earth, doi: 10.1002/2013JB010594, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013JB010594/abstract

Title: Supershear Rupture of the 5 January 2013 Craig, Alaska (Mw7.5) Earthquake

Authors: Han Yue and Thorne Lay: Department of Earth and Planetary Sciences, University of California—Santa Cruz, USA;

Jeffrey T. Freymueller and Natalia A. Ruppert: Geophysical Institute, University of Alaska Fairbanks, USA;

Kaihua Ding: China University of Geosciences, Wuhan, China;

Luis Rivera: Institut de Physique du Globe de Strasbourg, Université de Strasbourg/CNRS, France;

Keith D. Koper: Department of Geology and Geophysics, University of Utah, Salt Lake City, USA.

4. Acoustic emissions unveil internal motion in granular materials

When confronted with a heavy load or deformed by stress, the individual particles in a granular material will sometimes reorganize to a more stable arrangement. At small scales these reorganizations are little more than the redistribution of grains in the pile. In some cases, though, a reorganization is the first step of a critical failure, the trigger for an avalanche or landslide.

Understanding how the motion of individual grains translates into mass movement requires having a way to peer inside the pile without interfering with its behavior. Through a series of experiments, Michlmayr et al. find that specially tuned vibration sensors could be used to listen in on grain-scale dynamics. They find that elastic waves of different frequencies can be used to track and measure different types of motion within a granular material.

In their experiments, the authors stressed granular materials with varying grain sizes. They find that when subjected to a constant deformation, stresses in the materials oscillate in a sawtooth pattern—increasing steadily before dropping suddenly. The drops in shear stress—the sign of a reorganization—correlate with observations of low-frequency acoustic emissions. Materials with smaller grain sizes experience more frequent but less powerful stress drops than those with larger grain sizes. Observations of high-frequency acoustic emissions, the authors find, were associated with grain-on-grain interactions.

Source: Journal of Geophysical Research-Solid Earth, doi: 10.1002/2012JB009987, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2012JB009987/abstract

Title: Shear induced force fluctuations and acoustic emissions in granular material

Authors: Gernot Michlmayr and Dani Or: Institute of Terrestrial Ecosystems, ETH Zurich, Zurich, Switzerland;

Denis Cohen: Institute for Environmental Sciences, University of Geneva, Carouge, Switzerland.

5. New high-resolution record of middle to late Miocene climate evolution

After the fairly warm Miocene climate optimum about 17-15 million years ago, Earth's climate began to cool. Holbourn et al. present a new high-resolution record of climate evolution over the middle to late Miocene from 12.9 to 8.4 million years ago based on stable isotopes in sedimentary benthic foraminifera in the western Pacific Ocean. They also combine their data with previously published data going back to 16 million years ago from the same location to study the transition from a warmer climate to a cooler one. They focus on the relationship between climate and changes in the eccentricity and obliquity of Earth's orbit around the Sun.

They find that changes in carbon isotope ratios track long-period (400,000 year) variations in the eccentricity cycle, and changes in oxygen isotope ratios track shorter-term (100,000 year) variations in eccentricity and 41,000 year variations in obliquity. From the oxygen isotope record, the authors observe that Earth’s climate cooled in a series of incremental steps at about 14.6, 13.9, 13.1, 10.6, 9.9, and 9.0 million years ago. In general, climate variability decreases after about 13 million years ago, except for a warming episode about 10.8–10.7 million years ago, which the authors associate with a maximum in the eccentricity of Earth’s orbit.

Source: Paleoceanography, doi: 10.1002/2013PA002538, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013PA002538/abstract

Title: Middle to late Miocene stepwise climate cooling: Evidence from a high-resolution deep-water isotope curve spanning 8 million years

Authors: Ann Holbourn and Wolfgang Kuhnt: Institute of Geosciences, Christian-Albrechts-University, Kiel, Germany;

Steven Clemens and Warren Prell: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA;

Nils Andersen: Leibniz Laboratory for Radiometric Dating and Stable Isotope Research, Christian-Albrechts-University, Kiel, Germany.

6. Systematic shifts in subducting slab behavior with depth

When tectonic plates collide, the less buoyant plate will, in some cases, be forced beneath the other. At such subduction zones the sinking tectonic plate, known as a slab, does not follow a simple path from the surface to the deeper mantle. Instead, new research by Fukao and Obayashi suggests that subducting slabs pass through four largely distinct stages as they penetrate toward the core. To systematically catalog the stages of slab subduction, the authors analyzed roughly 10 million observations of the subsurface that were part of a tomographic study that used primary seismic waves to detect the structure of slabs in subduction zones around the Pacific.

The authors find that for a given subduction zone, subducting slab properties vary progressively along the subduction arc. Slab dynamics are broken down into four major stages with differing behavior organized around a discontinuity centered at a depth of 660 kilometers (410 miles) —the barrier between the upper and lower mantle. In some depth range down to around 660 kilometers many slabs get stuck and are temporarily prevented from penetrating deeper. Some slabs in this stuck zone, however, penetrate right through the 660-kilometer-deep discontinuity, moving at a steep angle. From a depth of 660 to 1000 kilometers (410 to 621 miles) the slabs often get stuck once more. Some slabs make it farther down, below 1000 kilometers, where the slab is free to sink into the lower mantle.

The authors suggest that the lower stuck zone, from a depth of 660 to 1000 kilometers, serves as an important stable reservoir of slab material and may play a unique role in mantle convection processes.

Source: Journal of Geophysical Research-Solid Earth, doi: 10.1002/2013JB010466, 2013 http://onlinelibrary.wiley.com/doi/10.1002/2013JB010466/abstract

Title: Subducted slabs stagnant above, penetrating through, and trapped below the 660-km discontinuity

Authors: Yoshio Fukao: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Kanagawa, Japan;

Masayuki Obayashi: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Kanagawa, Japan.

### Contact: Nanci Bompey
Phone (direct): +1 202 777 7524
E-mail: nbompey@agu.org

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