Volatile ecosystems, a natural wind tunnel, volcanic lightning, and stress & strain on Venus

December 10, 2013

Boulder, Colo., USA - New Geology articles posted online ahead of print on 6 Dec. cover fossil estuarine fauna from the Austrian Miocene; a "natural wind tunnel" experiment on the Chinese Loess Plateau; frictional melting of a microgabbro in water; glacial moraines in Colorado; new topographic data from Venus; solving a major issue concerning melt extraction from Earth's mantle; and experimentally generated volcanic lightning.

Highlights are provided below. GEOLOGY articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary GEOLOGY articles by contacting Kea Giles at the address above.

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Long-term ecosystem stability in an Early Miocene estuary

Martin Zuschin et al., University of Vienna, Department of Palaeontology, Althanstrasse 14, A-1090 Vienna, Austria, martin.zuschin@univie.ac.at. Published online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/G34761.1.

In the face of the ongoing biodiversity crisis, ecosystem stability is one of the most important research topics in ecology and paleoecology. Coastal marine areas include the most endangered ecosystems of our planet, and estuaries are a case in point: They count as naturally stressed areas, affected for example by strong daily and seasonal changes in salinity, and they are among the most degraded ecosystems on the planet. The impacts involve global and local anthropogenic stressors, including global warming, ocean acidification, overfishing, and waste-water disposal. This study puts a fossil estuarine fauna from the Austrian Miocene into an environmental framework based on the sedimentary record. This approach demonstrates for the first time that these volatile ecosystems are resilient to the range of natural environmental perturbations and can persist over hundreds of thousands of years. The stability of such an endangered ecosystem over such a long time and across critical environmental boundaries, as reflected in the sedimentary record, supports the idea that the estuary organisms can adapt to various stressors and that the ecosystem can compensate for the typical range of natural changes in the environment -- a range that may be exceeded by human-related stressors.

Linking coarse silt production in Asian sand deserts and Quaternary accretion of the Chinese Loess Plateau

Rivka Amit et al., Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel. Published online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/G34857.1.

The sources of aeolian sediments comprising the vast Chinese Loess Plateau are old enigmas raised ~2000 years ago by Chinese scholars studying the relationship between wind-blown dust and loess. An international group of researchers from Israel, the USA, Netherlands, Mongolia, and China, funded by the Israel-U.S. Binational Foundation, identified new plausible sources, previously overlooked: the immense sandy dune fields of Mu Us, Tengger, and Badain Jaran deserts just north of the Loess Plateau. They base their conclusions on reinterpretation of earlier data pertaining to spatial depositional patterns of the loess and on a field-scale natural experiment in the Gobi desert. Sand streaks, 50-km-long, emanating from dunes on the windward side of a prominent inselberg show a systematic leeward decrease in grain-size attributed to sand abrasion in the source dunes and subsequent deposition of the abraded products down-wind. This "natural wind tunnel" experiment at the zone of tempest winds in the Mongolian Gobi supports aeolian abrasion of sand grains as an effective mechanism of grain-size communition and the formation of coarse silt. Abrasion of sand in proximal dune fields appears as the most likely persistent source for massive downwind accumulation of the coarse silt forming the Chinese Loess Plateau.

Effect of water on the frictional behavior of cohesive rocks during earthquakes

M. Violay et al., (Affiliation for paper) Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy; (Current address) ETH D-ERDW, Sonnegstrasse, 5 CH-8092 Zürich, Switzerland; marie.violay@ingv.it. Published online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/G34916.1.

Fluid-rock interactions can control earthquake nucleation and the evolution of earthquake sequences. Experimental studies of fault frictional properties in the presence of fluid can provide unique insights into these interactions. We report the first results from experiments performed on cohesive silicate-bearing rocks (microgabbro) in the presence of pressurized pore fluids (H2O, drained conditions) at realistic seismic deformation conditions. The experimental data are compared with those recently obtained from carbonate-bearing rocks (Carrara marble). Contrary to theoretical arguments, and consistent with the interpretation of some field observations, we show that frictional melting of a microgabbro develops in the presence of water. In microgabbro, the initial weakening mechanism (flash melting of the asperities) is delayed in the presence of water; conversely, in calcite marble the weakening mechanism (brittle failure of the asperities) is favored. This opposite behavior highlights the importance of host-rock composition in controlling dynamic (frictional) weakening in the presence of water: cohesive carbonate-bearing rocks are more prone to slip in the presence of water, whereas the presence of water might delay or inhibit the rupture nucleation and propagation in cohesive silicate-bearing rocks.

The effects of interannual climate variability on the moraine record

Leif S. Anderson et al., Institute of Arctic and Alpine Research, and Department of Geological Sciences, University of Colorado, Campus Box 450, Boulder, Colorado 80309, USA; leif@colorado.edu. Published online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/G34791.1.

In most mountainous regions, reconstructed glacial histories are the primary record of past climate. These histories are typically based on unsorted accumulations of debris (moraines) deposited at the terminus of paleoglaciers. Paleoglacier geometries preserved by moraines are the primary constraint for extracting paleoclimate estimates using equilibrium-line altitudes or glacier models. It is a common assumption that paleoglacier lengths defined by terminal moraines and cirque headwalls were formed by paleoglaciers responding to the mean value of summer temperature and winter precipitation. In reality, glacier termini fluctuate around a mean glacial length even in a constant climate. The furthest terminal moraines from the headwall during the time period of interest represent the maximum excursion of the glacier from the mean glacier length. The mean glacier length actually reflects the mean value of temperature and precipitation and is located some distance back from the furthest terminal moraine. We use a glacier model developed to determine the mean length of eleven Last Glacial Maximum (LGM) glaciers in the northern Front Range, Colorado, USA. We estimate that mean glacier lengths during the LGM were ~10% to 15% back from the LGM terminal moraines.

New constraints on volcano-tectonic evolution of large volcanic edifices on Venus from stereo topography-derived strain estimates

Patrick J. McGovern et al., Lunar and Planetary Institute, USRA (Universities Space Research Association), Houston, Texas 77062, USA; mcgovern@lpi.usra.edu. Published online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/G34919.1.

A number of large volcanoes on the planet Venus exhibit systems of fractures and faults on their surfaces, as seen in radar images of the planet's surface from the Magellan mission. Analysis of the heights of these faults can tell us the amount of stretching (strain) that the surface experienced. In turn, estimates of strain can be compared to predictions from mechanical models of inflation of sub-surface magma chambers, in order to constrain the size and depth of such chambers. Unfortunately, until recently the available topographic data could not resolve features of the scale of the faults. Here, Patrick McGovern and colleagues use new topographic data derived from applying stereo processing to the radar images. The new topography shows that fault systems on two prominent Venus volcanoes are significantly deeper than previously assumed. For the first time, McGovern and colleagues apply a model for a thin circular magma chamber, rather than the typical spherical chamber, to volcano strain analysis, yielding estimates for chamber depth and size at the volcanoes Anala Mons (depth less than 8 km, radius between 10 and 20 km) and Kunapipi Mons (depth 4-35 km, radius 18-36 km).

Melt migration and melt-rock reactions in the deforming Earth's upper mantle: Experiments at high pressure and temperature

Vincent Soustelle et al., Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany; vincent.soustelle@uni-bayreuth.de. Published online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/G34889.1.

This study reports a series of experiments that solve a major issue concerning melt extraction from Earth's mantle beneath mid-ocean ridges and other volcanic provinces. For many years, experimental and theoretical studies have examined how partial melt is squeezed out of the mantle as a result of ductile deformation. These studies have shown that melt segregates into bands inclined at approximately 20° to the principal deformation direction. However natural mantle rocks never record such inclined melt banding; on the contrary they almost always display evidence for melt segregation into bands that are parallel to the principal deformation direction. This has created problems in the interpretation, modeling, and geophysical analysis of melt extraction. In our experimental study, we show that although melt bands do form inclined to the deformation direction; as the melt crystallizes it precipitates minerals, which become aligned parallel to the deformation direction. As natural rocks only record evidence in the form of minerals crystallized from melts, their foliation is an erroneous indication for the direction of the original melt pathway. These results allow the reinterpretation of field evidence for one of the most important mantle processes to affect Earth's surface.

Experimental generation of volcanic lightning

C. Cimarelli et al., Department of Earth and Environmental Sciences, LMU München, Theresienstraße 41, 80333 Munich, Germany; cimarelli@min.uni-muenchen.de; corradocimarelli@gmail.com. Published online ahead of print on 6 Dec. 2013; http://dx.doi.org/10.1130/G34802.1.

Volcanic eruptions and lightning may have contributed significantly to sparking early life on our planet and are commonly presented in the media as a majestic wonder of nature. As a matter of fact, ash-rich explosive eruptions are associated with intense electrical activity, and volcanic lightning detection is becoming a regular technique to monitor volcanic eruptions. However, this phenomenon has been so far poorly investigated, preventing advancement of our understanding of electrification processes in volcanic plumes. We present the first evidence of experimentally generated volcanic lightning discharges under controlled laboratory conditions, providing a model for their occurrence in nature. Our results indicate that fluid dynamics of the particle-laden jet controls particle electrification by collision (triboelectrification) and the distribution of charge within the plume, which are necessary conditions for lightning generation. We also find that the number of discharges increases proportionally with increasing loading of fine particles, suggesting that lightning monitoring at active volcanoes can be implemented to detect emission of fine ash, thereby forecasting its transportability and residence time in the atmosphere. In this respect, volcanic lightning is not only a magnificent display of natural fireworks but can also help us understand the electrical properties of volcanic plumes by revealing their internal dynamics.http://www.geosociety.org

Geological Society of America

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