Boulder, CO, USA - Topics include: seismic threat to the Dalmation Islands; Caribbean coral tracks, long-term changes in hurricane activity, and the Atlantic Multidecadal Oscillation; fish DNA as a dating tool for topographic evolution; why terrestrial subduction is one-sided; evolution of atmospheric carbon dioxide on Snowball Earth; measuring magmatic water content and triggering of super-eruptions; modeling weathering profiles on Mars and implications for the planet's aqueous history; Barnes Ice Cap changes on Canada's Baffin Island; and blue diamond phosphorescence.
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Eocene to present subduction of southern Adria mantle lithosphere beneath the Dinarides
Richard Bennett et al., Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA. Pages 3-6.
Bennett et al. found that new geodetic data from the Dinarides mountains and Dalmatian islands in southern Croatia and Bosnia, indicate that the region is actively deforming. All indications are that the deformation is caused by seismically active thrust fault that will likely produce large earthquakes at some time in the future. Based on the long historical record of past earthquakes for the region, a future earthquake may be as large as M7 or more. A possible event of this magnitude poses a significant seismic threat to the Dalmatian islands (where the UNESCO heritage site Dubrovnik, also known as the "Pearl of the Adriatic," is located), as well as the possibility of a rare but dangerous tsunami in the Adriadic Sea.
Caribbean coral tracks Atlantic Multidecadal Oscillation and past hurricane activity
Steffen Hetzinger et al., Department of Chemical and Physical Sciences, University of Toronto at Mississauga, 3359 Mississauga Road N, South Building, Room 3007E, Mississauga, Ontario L5L 1C6, Canada. Pages 11-14.
Improved knowledge of long-term changes in hurricanes is important because the population of coastal areas affected directly by landfalls of major hurricanes is increasing.There is currently much debate about whether or not global warming has contributed to the strong hurricane activity observed during the last decade. The restricted length of the reliable instrumental record limits the detection of possible long-term changes in hurricane activity, which naturally exhibits strong multidecadal variations that are associated with the Atlantic Multidecadal Oscillation (AMO). Hetzinger et al. present the first marine record that clearly captures the AMO signal and multidecadal variations in North Atlantic hurricane activity. They obtained a climate record from a brain coral, found in the genesis region of Atlantic hurricanes, that sensitively records variations in seawater oxygen isotopes that are linked to precipitation and sea-surface temperature (SST) over the last century. As multidecadal SST variations in this region are closely related to the AMO, this study raises new possibilities to expand limited observations and gain new insights into the mechanisms of multidecadal climate variations and long-term changes in hurricane behavior.
Genetic ages for Quaternary topographic evolution: A new dating tool
Dave Craw et al., Geology Department, University of Otago, Leith Street, P.O. Box 56, Dunedin, New Zealand. Pages 19-22.
Craw et al. use the genetic makeup of freshwater fish to determine the age of river changes in the landscape. Where fish populations have become isolated from each other by glaciation or mountain-building, the fish DNA in the different populations changes in slightly different directions. The age of the isolation event can be determined from the amount of variation in DNA between the two isolated populations. Craw et al.’s study uses some geological events of known age in New Zealand to quantify the rate of change of fish DNA.
Meander cutoff and the controls on the production of oxbow lakes
José Constantine, Department of Earth Science, University of California–Santa Barbara, Santa Barbara, California 93106, USA; and Thomas Dunne, Department of Earth Science, and Donald Bren School of Environmental Science and Management, University of California-Santa Barbara, Santa Barbara, California 93106, USA. Pages 23-26.
Satellite images archived by Google Earth™ facilitate a global analysis of Earth’s surface. Using these images, Constantine and Dunne found that meandering rivers throughout the world similarly produce floodplain lakes. The size of a lake produced by a meandering river is determined by the sinuosity, or curviness, of the river. The more sinuous a river, the larger the lake it will produce. Using a simple model that describes how a meandering river shifts across its valley floor, it was also found that the number of lakes produced by the river is a function of how quickly it shifts position. The more quickly a river meanders, the more often floodplain lakes are produced. Floodplain lakes provide critical habitat for variety of riverine organisms and they serve as terrestrial sinks for sediment and pollutants in the environment. The images provided by Google Earth™ allowed, for the first time, the development of a model that predicts the production of these important landforms.
Using titanomagnetite textures to elucidate volcanic eruption histories
Shane Cronin et al., Institute of Natural Resources (PN432), Massey University, Private Bag 11 222, Palmerston North, New Zealand. Pages 31-34.
Through a detailed study of a volcanic ash sequence at Mt. Taranaki (a New Zealand volcano very similar to Mt. Hood in the Cascade ranges), Cronin et al. revealed that the volcano’s long-term eruption rate had varied over time in a regular, repeated pattern. Periods of high-frequency explosive eruptions recurred on a 1,500-year cycle, interspersed by periods of less-frequent activity. This suggests that statistically based eruption forecasts for such volcanoes need to take into account the stage of this overall cycle the volcano is presently in. Cronin et al split record of volcanic eruption frequency over time at Mt. Taranaki into eruptions of explosive origin and eruptions involving magma slowly oozing out of magma in lava domes (like what is presently happening at Mt. St. Helens). By using a reflected-light microscope, Cronin et al. observed that volcanic ash from slow-oozing dome eruptions has very different features in its magnetite minerals (an iron+titanium-bearing oxide) than that of explosive origin.
Why is terrestrial subduction one-sided?
Taras Gerya et al., ETH-Hoenggerberg, HPP, L.8.2, 8093 Zurich, Switzerland. Pages 43-46.
Subduction of the lithosphere at convergent-plate boundaries takes place asymmetrically - the subducted slab sinks downward, while the overriding plate moves horizontally (one-sided subduction). In contrast, global mantle convection models generally predict downwelling of both plates at convergent margins (two-sided subduction). Gerya et al. carried out two-dimensional (2-D) numerical experiments with a mineralogical-thermomechanical viscoelastic-plastic model to elucidate the cause of one-sided subduction. Their experiments show that the stability, intensity, and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab. Two-sided subduction occurs at low slab strength (sin[?] < 0.15, where ? is effective internal friction angle), regardless of the extent of hydration. In contrast, steady-state one-sided subduction requires a weak hydrated slab interface and high slab strength (sin[?]?> 0.15). The weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism. The resulting weak interplate zone localizes deformation at the interface and decouples the strong plates, facilitating asymmetric plate movement. Gerya et al’s work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self-sustaining one-sided subduction processes.
Scenario for the evolution of atmospheric pCO2 during a snowball Earth
Guillaume Le Hir et al., LSCE, bat 701, Centre de l'Orme des Merisiers, Gif Sur Yvette 91191, France. Pages 47-50.
Snowball Earth describes a planet covered by glacial ice from pole to pole. The global mean temperature would be about -50 °C. The reduction of the water cycle and the growth of large ice sheets led to a collapse of CO2 consumption through continental weathering. In the absence of carbon sinks, after millions of years, the volcanic outgassing combined with reduced silicate weathering causes a CO2 accumulation into the atmosphere, providing an intense greenhouse warming that eventually drives deglaciation. Using a numerical model, Le Hir et al. suggest that during global glaciations, a very tiny area of open waters may allow an efficient atmospheric CO2 diffusion into the ocean. This exchange implies an intense acidification of the ocean, which increases the CO2 consumption through the low-temperature alteration of the oceanic crust. As a result, the atmospheric CO2 accumulation and the induced greenhouse effect is limited. Even after the maximum estimated duration of the glaciation (30 million years), atmospheric CO2 is far from reaching the minimum deglaciation threshold. Accounting for this previously neglected carbon sink, processes that decrease the CO2 deglaciation threshold must be further explored.
Yellowstone plume - continental lithosphere interaction beneath the Snake River Plain
Barry Hanan et al., Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA. Pages 51-54.
The Snake River Plain represents an 800 kilometer swath of volcanic centers stretching across southern Idaho to Wyoming (northwestern United States). The widespread Miocene to recent volcanism in the Pacific Northwest associated with the Columbia Plateau, Oregon High Lava Plains, Snake River Plain, Northern Nevada Rift, and Yellowstone Plateau may represent a single magmatic system related to a hotspot, where a plume of material from the deep mantle rises and melts to form lavas along the plain. The mantle plume is spatially fixed, and the Snake River Plain formed as the westward migrating North American continental plate rode over it. Hanan et al. discuss one problem with this interpretation—the radiogenic isotopic composition of the basaltic lavas is the same as that of the relatively shallow mantle attached to the overriding westward-migrating continent, instead of deep mantle sources as predicted. New mass-balance mixing models show that deep mantle plume melts inherit the radiogenic isotope composition of the sub-continental mantle as they passed through it on their way to erupt on the Snake River Plain. This happens because the concentration of radiogenic elements in the sub-continental mantle beneath the Snake River Plain is one to two orders of magnitude higher than in the mantle plume melts. A small amount of pollution (less than 4%) of the plume magmas by melt from sub-continental mantle is all that is required to wipe out the radiogenic isotope signature of the mantle plume source. The results support the idea that the Snake River Plain lavas and associated widespread Miocene to recent volcanism in the Pacific Northwest is likely related to a magmatic system controlled by a mantle plume.
Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas
Sharon Mosher et al., Department of Geological Sciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712-0254, USA. Pages 55-58.
The question of when plate tectonics began has remained controversial. Until recently, most geoscientists agreed that modern plate tectonic processes were active for about the last 2.5 billion years, or perhaps even longer but with a hotter Earth. Recently, however, several geologists have suggested that key evidence for modern plate tectonics is lacking in rocks older than 1 billion years old. Mosher et al.’s paper discusses the 1.15 to 1.12 billion-year-old Llano Uplift exposed in central Texas that contains key evidence and demonstrates that subduction of continental crust and continent-continent collision occurred during that time span. Moreover, their paper shows that the evolution of this orogenic belt is very similar to that of the modern Alps, and reflects modern plate tectonic processes.
Magmatic degassing histories from apatite volatile stratigraphy
Jeremy W. Boyce and Richard L. Hervig (corresponding author), School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, Arizona 85287-1404, USA. Pages 63-66.
Water content is the single strongest compositional control on the explosivity of volcanoes, and also plays a significant role in the generation and evolution of magmas. Boyce and Hervig present the initial results in the development of a new technique for measuring magmatic water content and how it varies with time, using the common mineral apatite. Because apatite incorporates water (as well as chlorine, fluorine, and other important volatile elements such as sulfur) into its structure during growth, it can record variations in magmatic water through time, much like growth rings in trees record climate through time. Unlike other techniques that require recent, fresh material, apatite hygrometry can be applied to many volcanic rocks of any age, potentially opening up ancient volcanoes to this study. The example in Boyce and Hervig’s paper is the super-eruption at the volcano Cerro Galan (Argentina, approximately 2 million years before present). The chemistry of apatite crystals from this massive, 1000-km3-volume eruption suggests that influxes of water-rich magma in the months prior to eruption may have triggered the cataclysmic event. As dozens of similar super-eruptions have taken place in North America in the past 50 million years (including at Yellowstone caldera), this work has significant implications for how super-eruptions are triggered in North America.
Basalt weathering rates on Earth and the duration of liquid water on the plains of Gusev Crater, Mars
Elisabeth Hausrath et al., Johnson Space Center, 2101 NASA Parkway, Mail Code KX, Houston, Texas 77058-3696, USA. Pages 67-70.
The possibility of life on Mars is closely linked to the duration of liquid water on Mars. Hausrath et al. use reactive transport modeling to predict depth profiles through weathered material on Earth, in locations where the duration of contact with liquid water is known. They then use the same model to predict the duration of weathering in a location on Mars, sampled by one of the Mars Exploration rovers. This interpretation of weathering profiles shows promise for interpreting future depth profiles on Mars, to better understand the aqueous history and the potential for life on that planet.
Thinning of the south dome of Barnes Ice Cap, Arctic Canada, over the past two decades
William Sneed et al., Climate Change Institute, University of Maine, 316 Bryand Global Science Center, Orono, Maine 04469-5790, USA. Pages 71-74.
Sneed et al. undertook to document changes to the south dome of Barnes Ice Cap, Baffin Island, Nunavut, Canada, using satellite images, satellite altimetry data, and climate data. They found that the rate of elevation change on a portion of the dome has increased significantly during the last 35 years and they related this elevation loss to increasing summer temperatures during the same period.
Using phosphorescence as a fingerprint for the Hope and other blue diamonds
Sally C. Magaña et al., Gemological Institute of America, 5345 Armada Drive, MS# 38, Carlsbad, California 92008, USA. Pages 83-86.
The 45.52 carat blue Hope Diamond is the centerpiece of the National Gem Collection at the Smithsonian’s National Museum of Natural History Washington, D.C., and it attracts the attention of millions of visitors each year. One aspect of this famous diamond that most viewers do not get to see is its fiery red phosphorescence that results from exposure to ultraviolet light and continues for more than a minute. This dramatic red glow was prominently featured in a Discovery Channel television special about the Hope Diamond ("Unsolved History: The Hope Diamond."). The mysterious red phosphorescence, rarely seen in other blue diamonds, added to the Hope Diamond’s mystique and allure. However, the mystery has now been solved. Researchers at the Smithsonian Institution and the U.S. Naval Research Laboratory measured the phosphorescence spectra of the Hope Diamond and of 66 other natural blue diamonds, including the 30.82 carat Blue Heart of the National Gem Collection. Phosphorescence spectroscopy reveals the wavelengths of light that contribute to the phosphorescence observed by the eye. Magaña et al. found that almost all natural blue diamonds show phosphorescence peaks at 500 nm (this would appear blue) and at 660 nm (which would appear red). In some cases, such as for the Hope Diamond, the red peak dominates, and the phosphorescence appears red to the eye, but for others blue is dominant. The phosphorescence peaks are linked to the impurity boron, the same element that gives blue diamonds their color. The specific parameters of the phosphorescence, such as the peak intensity and rate of decay, were specific to each of the 67 diamonds, and help create a “fingerprint” that can be used to individually identify natural blue diamonds. Lab-created blue diamonds were also tested and proved to show different phosphorescence spectra than natural blue diamonds; this provides an additional means to determine the origin of a diamond.
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