Boulder, Colo., USA - Topics in the 26 March posting of Geology include anthropogenic impacts on the Indus River into the Arabian Sea; possible electrical conductivity beneath the Yellowstone hotspot track; mountain-forming volcanoes and deadly debris flows; melting beneath the Colorado Plateau; widespread weathered glass on Mars; and a new view into Mars' global aqueous history.
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Impact of Indus River discharge on productivity and preservation of organic carbon in the Arabian Sea over the twentieth century
Andreas Lückge et al., Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), 30655 Hannover, Germany. Posted online 26 March 2012; doi: 10.1130/G32608.1.
Coastal zones play an important role in the carbon cycle, because the amount of riverine materials and nutrients brought to the sea affects the high biological productivity nearshore. The fixation of atmospheric carbon and subsequent burial of marine organic matter in these ocean areas contributes to the global carbon cycle. But little direct evidence exists that can be used to clearly document the marine response to changes in the modern dynamics of large rivers. Anthropogenic activity during the past century, like changes in land use and the construction of river dams and irrigation facilities, has drastically decreased the discharge rate of suspensions and dissolved nutrients by the Indus River into the Arabian Sea off Pakistan. In this study, Andreas Lückge and colleagues found organic and inorganic indicators in the sediment that demonstrate that past changes in Indus River discharge have strongly impacted local productivity patterns. Between 1890 and 1998, the period over which this man-made reduction of Indus River discharge to the ocean occurred, primary productivity off the Pakistan coast seems to have decreased by more than a third. Over the same period, the regional oxygen minimum zone weakened, increasing the supply of oxygen to the sediments and reducing the burial of organic matter.
High-pressure granulites at the dawn of the Proterozoic
Jade R. Anderson et al., Centre for Tectonics, Resources and Exploration (TRaX), University of Adelaide, Adelaide SA 5005, Australia. Posted online 26 March 2012; doi: 10.1130/G32854.1.
The preservation of high-pressure metamorphism is rare in the ancient geological record. Jade Anderson and colleagues examine high grade metamorphic rocks from southern India and constrain that metamorphism occurred 2.49-2.47 billion years ago, at an estimated pressure of approx. 14-16 kbar and temperature of approx. 820-860 degrees Celsius. The estimated pressure and temperature of metamorphism indicates that the crust was capable of thickening to 45-50 km or thicker. Such crustal thickening provides support for a shift in the strength of the lithosphere at the Archean-Paleoproterozoic transition.
Disequilibrium melting during crustal anatexis and implications for modeling open magmatic systems
Claire L. McLeod et al., Northern Centre for Isotopic and Elemental Tracing (NCIET), Dept. of Earth Sciences, Durham University, South Road, Durham DH1 3LE, UK. Posted online 26 March 2012; doi: 10.1130/G33000.1.
Assessing the degree to which a crustal component has played a role during magmatic differentiation is often challenging, as is determining the nature of any potential crustal contaminant(s). Claire McLeod and colleagues assessed the assumption that the crustal contaminant in these open magmatic systems is a single composition through in-situ analysis of quenched anatectic melt trapped within its crustal source. Their results show significant chemical and Sr-isotopic disequilibrium between melt and source over submillimeter-length scales in a natural system. The isotopic disequilibrium between melt and source is understood to reflect the melting of minerals with different Rb/Sr (and therefore 87Sr/86Sr) more quickly than the isotopic composition can diffusively equilibrate between melt and minerals. McLeod and colleagues' results suggest that the mechanism of crustal anatexis produces contaminating melts that are geochemically heterogeneous both spatially and temporally. Furthermore, time scales of Sr diffusion and anatectic melt segregation promote the preservation of isotopic disequilibrium at the micro (submillimeter) and macro (crustal) scale. This highlights the need for detailed microscopic investigations coupled with petrogenetic modeling in order to develop more robust characterization and quantification of contamination in open magmatic systems.
Crust and upper mantle electrical conductivity beneath the Yellowstone hotspot track
A. Kelbert et al., College of Earth, Ocean and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin Building, Corvallis, Oregon 97331, USA. Posted online 26 March 2012; doi: 10.1130/G32655.1.
This study uses high-quality electromagnetic data from the EarthScope USArray project to obtain detailed 3-D images of electrical resistivity in the crust and upper mantle beneath the Yellowstone-Snake River Plain volcanic province (Idaho and Wyoming, United States). A. Kelbert and colleagues note that the lowest resistivities in the area can only plausibly be explained by partial melt and/or fluids, providing valuable new information about the distribution of these phases deep within the Earth beneath the volcanic system. Unexpectedly, in light of the mantle plume models often used to explain Yellowstone volcanism, the electromagnetic data imply that there is no interconnected melt in the lower crust and uppermost mantle directly beneath the modern Yellowstone caldera. Instead, low resistivities consistent with 1%-3% melt in the uppermost mantle (depths of 40-80 km) extend at least 200 km southwest of Yellowstone. Shallower areas of reduced resistivity extend upward into the mid-crust around the edges of the seemingly impermeable Snake River Plain province, including beneath Yellowstone. Kelbert and colleagues suggest that the elevated temperatures beneath the active volcanic center have resulted in greater permeability, allowing magma to ascend to shallower depths and pool in the crust. Little melt is entering the system from below at present, perhaps due to intermittency of supply.
Short-term episodicity of Archaean plate tectonics
Jean-François Moyen, UMR 6524 CNRS and Université Jean-Monnet, 23 rue du Dr Michelon, 42023 Saint-Etienne, France., Durham DH1 3LE, UK; and Jeroen van Hunen Durham University. Posted online 26 March 2012; doi: 10.1130/G32894.1.
Plate tectonics, the dominant process shaping Earth as we know it today, may not have existed throughout Earth's history. Indeed, the interior of our planet (the mantle) cools progressively, by perhaps 300 degrees Celsius over the past 3.0 billion years. Numerical calculations reveal that in Archaean times (4.0-2.5 billion years ago), the mantle was too hot to support stable, long-lived plate tectonics. Rather, Jean-François Moyen and Jeroen van Hunen suggest that subduction -- a key component of plate tectonics, with cold, rigid plates sinking from the surface down into the mantle -- was an episodic process, stopping and starting frequently. Evidence for this episodicity is found in rocks from old geological units such as the Abitibi province of the Canadian Shield, where Moyen and van Hunen describe short, repeated, episodic bursts of subduction related lavas interlayered in non-subduction rocks. They propose that plate tectonics started progressively on Earth by more and more frequent, long-lived, and large-sized subduction events progressively evolving into the stable, large structures observed today.
Energy growth in laharic mass flows
Gert Lube et al., Institute of Natural Resources, Massey University, P.B. 11 222, Palmerston North, New Zealand. Posted online 26 March 2012; doi: 10.1130/G32818.1.
Lahars, debris flows, and sediment-rich floods are frequent and deadly hazards at all mountain-forming volcanoes. Their hazard potential is traditionally assessed through mass-conserving closed system models, where peak conversion rates of potential energy to mechanical energy and hence maximum destruction potential are predicted to occur on the steepest volcano flanks. This belies evidence of extremely high-energy and deadly catastrophes caused by such flows at large distances from volcanoes. Gert Lube and colleagues use the first high-resolution record of a moving lahar to develop a new model of the temporally and spatially variable mass-flow structure. They show that bulk flow energy can grow dramatically in such systems over tens to hundreds of kilometers via momentum transfers from the lahar into water and particles along its path. Lube and colleagues also demonstrate that dynamic transformations of such flows and their ultimate runout are primarily controlled by the mass flow front.
Melting under the Colorado Plateau, USA
Mary R. Reid et al., School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011-4099, USA. Posted online 30 Marc 2012; doi: 10.1130/G32619.1.
Relatively young volcanoes dot the landscape of the Colorado Plateau -- a 130,000-square-mile region that straddles Colorado, Utah, Arizona and New Mexico, USA. Compared to volcanism at tectonically active plate boundaries, the origin of volcanism within continental interiors like the Colorado Plateau is poorly understood. Mary R. Reid and colleagues assess recent models for generating molten rock under the Colorado Plateau using an integrated geophysical and geochemical approach. Chemical data for Colorado Plateau volcanic fields show that melts are derived from the uppermost mantle, at least 75 km below the surface. Some portions of this mantle layer may have been associated with the continent for approx. 1.7 billion years. Seismic data gathered by the massive USArray seismic observatory show that melts originate in relatively malleable mantle below the North American plate. Melts derived from shallower mantle conditions also represent proportionally larger degrees of partial melting, suggesting that mantle at depths of 75 to 100 km near the margins of the Colorado Plateau is apparently capable of rising plastically to cause decompression melting. To accommodate this upwelling, the North American plate may locally be extending, and cooler and denser portions of the lithosphere may be downwelling into the deeper mantle as drips or delaminations.
Widespread weathered glass on the surface of Mars
Briony Horgan and James F. Bell III, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85281, USA. Posted online 26 March 2012; doi: 10.1130/G32755.1.
The surface of Mars exhibits numerous lava flows and other signs of effusive volcanism. Although models suggest that explosive volcanism should also have produced extensive deposits, direct evidence for large-scale explosive volcanism on Mars has been scarce. A new investigation by Briony Horgan and James F. Bell III of the mineralogy of dark regions covering more than ten million square kilometers in the northern hemisphere of Mars has revealed that these regions are dominantly composed of glass. The glass is most likely volcanic glass produced during explosive eruptions, and potential sources include volcano-ice interactions in the northern lowlands as well as ash deposits from the large martian shield volcanoes. The glass deposits also exhibit signs of weathering, indicating widespread interactions with liquid water. Under the hyper-arid climatic conditions Mars has experienced over the past three billion years or more, the most likely source of this water is melting ice or snow. These results suggest that explosive volcanism may be a major source of sediments on Mars, and that limited liquid water has been present at the surface of Mars even under long-term hyper-arid conditions.
Lithologic and glacially conditioned controls on regional debris-flow sediment dynamics
Francesco Brardinoni et al., Dipartimento di Scienze Geologiche e Geotecnologie, Università degli Studi di Milano-Bicocca, Milan, Italy. Posted online 26 March 2012; doi: 10.1130/G33106.1.
Debris flow is an efficient process of sediment transfer from slope base to piedmont depositional fans in mountain drainage basins. To advance understanding of debris-flow sediment dynamics at the regional scale, Francesco Brardinoni and colleagues analyze a historical (1998-2009) database of debris flows from 77 basins of Alto Adige Province, northeastern Italy. By combining information on event volumetric deposition, high-resolution digital topography, and Quaternary sediment mapping, they are able to link debris-flow sediment flux to morphometry, lithologic variability, and sediment availability.
Picrites in central Hokkaido: Evidence of extremely high temperature magmatism in the Late Jurassic ocean recorded in an accreted oceanic plateau
Yuji Ichiyama et al., Data Research Center for Marine-Earth Sciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan. Posted online 26 March 2012; doi: 10.1130/G32752.1.
The Sorachi-Yezo belt, central Hokkaido, Japan, is composed of voluminous tholeiitic basaltic volcanics, and has been thought to be accreted fragments of an oceanic plateau formed in the Late Jurassic Pacific Ocean. Picrites have been reported as pillow lava and hyaloclastite from the Sorachi-Yezo belt. Authors Yuji Ichiyama of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and colleagues present a detailed study of these pricrites and show that they are divided into two groups that are chemically akin to the Neoarchean komatiites and Gorgona komatiites and Picrites. This indicates different melting regimes in an extremely hot mantle plume. The authors conclude that Sorachi-Yezo picrites provide evidence for extremely high temperature magmatism, like that of Archean komatiite caused by melting of the hottest mantle plume among the Phanerozoic oceanic large igneous provinces.
From crucible to graben in 2.3 Ma: A high-resolution geochronological study of porphyry life cycles, Boyongan Bayugo copper-gold deposits, Philippines
David P. Braxton et al., Centre for Ore Deposit Research, University of Tasmania, Private Bag 126, Hobart, 7001 Tasmania, Australia. Posted online 26 March 2012; doi: 10.1130/G33125.1.
The Boyongan and Bayugo porphyry copper-gold deposits are part of a belt of gold-rich copper deposits in the Surigao district of northeast Mindanao, Philippines. The detailed age relationships described in this study by David P. Braxton and colleagues provide insight into the geologically short life cycles that characterize porphyry formation in dynamic arc environments.
Atmospheric origin of Martian interior layered deposits: Links to climate change and the global sulfur cycle
Joseph Michalski, Planetary Science Institute, 1700 E. Ft. Lowell, Suite 106, Tucson, Arizona 85719, USA; and Paul B. Niles. Posted online 26 March 2012; doi: 10.1130/G32971.1.
Since the first photogeologic exploration of Mars, vast mounds of layered sediments found within the Valles Marineris troughs have remained unexplained. Recent spectroscopic results showing that these materials contain coarse-grained hematite and sulfate suggest that they are fundamentally similar to layered sulfate deposits seen elsewhere on Mars and are therefore a key piece of Mars' global aqueous history. Joseph Michalski and Paul Niles constrain the origin of these interior layered deposits (ILDs) by considering two models: (1) formation of the ILDs by groundwater upwelling, which requires that a significant fraction of the global Martian sulfur budget was concentrated in the Valles Marineris at the time the ILDs formed; (2) an alternate model in which the ILDs formed in a configuration similar to what is observed today through atmospherically driven deposition of ice, dust, and volcanogenic sulfuric acid. The first requires that a significant fraction of the global Martian sulfur budget was concentrated in the Valles Marineris at the time the ILDs formed. Michalski and Niles favor the second model, which they note is easily compatible with the global sulfur budget and does not require significant erosion rates or large volumes of liquid water. They propose that formation of sulfate-rich layered sediments on Mars was governed through time by volcanogenic SO2 and H2O emission rates and dust production against a backdrop of obliquity variation in a largely cold and dry climate.