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GEOLOGY
How big was the Chesapeake Bay impact? Insight from numerical modeling
Gareth Collins, Imperial College London, Department of Earth Science and Engineering, London, NA SW7 2AZ, UK; and Kai Wünnemann, University of Arizona, Lunar and Planetary Laboratory, Tucson, AZ 85721, USA. Pages 925-928.
The Chesapeake Bay impact crater is the largest known crater in the United States. It has been studied extensively by seismology, gravity surveying and geological drill cores, yet it is still unclear how big the asteroid or comet was that collided 35 million years ago with the east coast of the United States. Through computer simulations of the impact event, Collins and Wünnemann find that formation of the Chesapeake Bay crater was dramatically influenced by strength differences between the hard bedrock and the weak overlying sediments at the impact site. They show that craters formed in such targets are much shallower and broader than their counterparts formed in uniformly hard rock, and they conclude that if the impact had occurred on dry land, the final crater diameter would have been closer to 40 km than the observed 80-90 km. The Chesapeake Bay impact was therefore much less energetic, and consequently less environmentally catastrophic, than previously assumed.
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Large-scale pseudotachylytes and fluidized cataclasites from an ancient subduction thrust fault
Christen Rowe, University of California-Santa Cruz, Earth Sciences, Santa Cruz, CA 95064, USA; et al. Pages 937-940.
Pseudotachylytes are, in effect, fossilized earthquakes. These rare rocks are formed during an earthquake when rocks are sheared in a fault zone at a high rate, creating enough frictional heat to melt the rock itself. This melt rapidly cools to form a natural glass, known as pseudotachylyte. A very unusual occurrence of pseudotachylyte has been discovered in the ancient subduction thrust faults of the Kodiak Complex, Kodiak Island, Alaska. The pseudotachylytes are exceedingly thick (>10 cm) and large in along-strike dimension (single horizon outcrops for >1 km). The pseudotachylyte occurs in association with cataclasite, rock that was finely ground in the fault zone but not melted. The association demonstrates that the associated cataclasite was also formed during the earthquakes that created the pseudotachylyte. Since melting conditions, and therefore pseudotachylyte, are so rare, it is possible that this type of cataclasite could be a useful proxy for "fossilized earthquakes" in similar faults.
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Catastrophic soil erosion during the end-Permian biotic crisis
Mark Sephton, Imperial College London, Earth, Science and Engineering, London, SW7 2AZ, UK; et al. Pages 941-944.
Around 250 million years ago, at the end of the Permian period, life on Earth had the rug pulled from under its feet as the supercontinent Pangea was poisoned by volcanic gas causing the loss of massive amounts of soil. Sephton et al. present new data that reveal that a unique set of molecules appear in rocks that chronicle the end-Permian extinction. The molecules are the remains of polysaccharides, large sugar-based structures common in plants and soil.
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The illusion of diffusion: Field evidence for depth-dependent sediment transport
Arjun Heimsath, Dartmouth College, Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA; et al. Pages 949-952.
Soil-covered upland landscapes are common for much of the habitable world, and understanding their evolution as a function of different climatic, tectonic, and geologic regimes is important across a wide range of disciplines. Erosion laws direct quantitative study of the processes shaping Earth's surface and form the basis of landscape evolution modeling, but they are based on limited field data. Heimsath et al. quantify an exponential decline in soil production with increasing soil thickness for a new field site in Point Reyes, California. Results are similar to soil production functions from two different, previously studied field sites and are used with extensive measurements of soil thickness to quantify depth-integrated sediment transport flux. These results provide the first field-based evidence that soil transport is a nonlinear, depth-dependent function and suggest that the widely used linear "diffusion" equation is not always appropriate. Quantifying both the mobile soil thickness and landscape morphology is therefore critical to understanding how landscapes evolve.
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Strong kinetic effects on Sr/Ca ratios in the calcitic bivalve Pecten maximus
Anne Lorrain, MRAC, Geology, Tervuren 3080, Belgium; et al. Pages 965-968.
Reconstructing climate change necessitates understanding the past, though instrumental climate records only extend back about 150 years. Therefore, researchers seek to find archives of climatic information, termed "proxies," in different substrates, such as corals, tree rings, ice cores, or sediments. In this context, the shells of some bivalves such as scallops are particularly suitable for paleoclimate reconstruction as they deposit a layer of calcium each day. The chemical or isotopic composition of calcareous skeletons has long been recognized as a record of past and present environmental conditions and thus allows reconstruction of the environmental history. However, because the composition of biogenic carbonates is also clearly influenced by biological factors, the correct interpretation of these chemical archives requires a precise understanding of the processes controlling the incorporation of elements. Lorrain et al. provide a detailed study of factors controlling Sr incorporation, a hypothetical temperature proxy, in the shells of the scallop Pecten maximus.
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Tendencies in paleontological practice when defining species, and consequences on biodiversity studies
Elisa Nardin, University of Burgundy, Biogeosciences, Dijon, Bourgogne 21000, France; et al. Pages 969-972.
The exploration of evolutionary patterns over geological time has recently received new impetus from the development of morphological disparity as a new biodiversity metric alongside taxonomic diversity. Clade dynamics can be analyzed by comparing and contrasting these two metrics. For a sample of Jurassic ammonites, morphological disparity increases faster than taxonomic diversity during paleontographical time (time between the first study of a given fossil and the last publication on this fossil). The results show that fluctuations in morphological disparity and taxonomic diversity are not synchronous: Disparity increases faster than diversity over paleontographical time. Compared with a random model, the results of Nardin et al. demonstrate there could be a trend in paleontological practice to recognize and name extreme forms before intermediate ones. The results imply that estimates of biodiversity are influenced by changes in knowledge over paleontographical time (an expected result), with different rates of variation for taxonomic diversity and morphological disparity (an unexpected result).
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Biomarker records of late Neogene changes in northeast African vegetation
Sarah Feakins, Columbia University, Lamont Doherty Earth Observatory, Palisades, NY 10964, USA; et al. Pages 977-980.
A new study of marine sediments off northeast Africa reveals repeated shifts between forest and grassland vegetation in East Africa. Previous studies have shown evidence for East African forest giving way to savanna grasslands over the past 10 million years. These new results use components of leaf wax material, blown off the continent and preserved in ocean sediments, to show the shift from forest to grassland was not gradual or continuous. Instead the proportion of grassland relative to forest grew and declined on tens of thousand-year cycles. Grassland expansions and retreats in these cycles were almost as dramatic as the overall trend, and provide strong evidence for significant environmental variability experienced by hominin species whose fossils are found in the East Africa Rift Valley.
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Holocene African droughts relate to eastern equatorial Atlantic cooling
Syee Weldeab, University of California-Santa Barbara, Department of Geological Sciences, Santa Barbara, CA 28357, USA; et al. Pages 981-984.
Weldeab et al. use temperature reconstruction from the chemistry of plankton shells deposited in marine sediments to demonstrate the pivotal role of surface ocean temperatures in the equatorial Atlantic in the strengthening or weakening African precipitation over the last 25,000 years. They show that, by modifying the land-ocean temperature gradient, the ocean strongly altered the moisture content in the monsoonal air masses transported onto the continent. These results draw attention to the important role of tropical ocean temperatures and land vegetation cover in such monsoon systems. Anthropogenically-caused reduction in land vegetation cover (deforesting and overgrazing) and changes in global temperatures through the emission of greenhouse gases will likely strongly affect the African monsoon system in the future.
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Basalt sills of the U reflector, Newfoundland Basin: A serendipitous dating technique
Garry Karner, Columbia University, Lamont-Doherty Earth Observatory, Palisades, NY 10964-8000, USA; and Donna Shillington, School of Earth and Ocean Science, National Oceanography Centre, University of Southampton, Southampton, England SO14 3ZH, UK. Pages 985-988.
Karner and Shillington describe a serendipitous situation in which they measure the present-day porosity of mudstones containing shortened calcite veins. They estimate the amount of shortening by integrating the length of the calcite folds and then estimating the porosity of the sediment during vein emplacement. Assuming the veining is synchronous with the magmatic activity responsible for emplacing the upper diabase, the age-porosity relationship can be used to infer the age of magma emplacement--that is, a dating approach completely independent of the usual radiometric techniques. The approach has been subsequently verified by radiometric dating techniques.
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Active strike-slip faulting in El Salvador, Central America
Giacomo Corti, Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, Sezione di Firenze, Firenze, 50121, Italy; et al. Pages 989-992.
El Salvador is a highly seismically active country, where at least 11 major earthquakes caused more than 3,000 casualties in the last 100 years. Seismic activity occurs as a consequence of deformation resulting from the collision between the Cocos and the Caribbean plates. Onland, the most destructive earthquakes are aligned parallel to the volcanic arc; their occurrence has been inferred to be related to slip along a major strike-slip fault system (El Salvador Fault Zone, ESFZ), which lacked a characterization of its architecture and current activity. Corti et al. fill this gap with new geological, structural, and geomorphic data that allow the constraint of the kinematics, rates of activity, and regional segmentation of this major fault system. The new data suggest a high Late Pleistocene-Holocene slip rate, with important implications for the evaluation of seismic hazard in the area.
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Global cooling initiated stony deserts in central Australia 2-4 Ma, dated by cosmogenic 21Ne-10Be
Toshiyuki Fujioka, The Australian National University, Research School of Earth Sciences, Canberra, ACT 0200, Australia; et al. Pages 993-996.
Fujioka et al. take a new approach to exposure-age dating, using cosmogenic neon-21 paired with beryllium-10, which reveals that Australian stony deserts developed 2 to 4 million years ago, when global cooling triggered Quaternary ice ages. This is the first direct determination of the age of stony deserts.
GSA TODAY Science Article
Continental-scale links between the mantle and groundwater systems of the western United States: Evidence from travertine springs and regional He isotope data
Dennis L. Newell, Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA, et al. Pages 4–10.
Western water linked to modern tectonics: Water quality has long been an issue in the western United States, particularly as standards for elements such as arsenic are developed through the National Water Quality Program Assessment. In the December issue of GSA Today, Dennis Newell, a Ph.D. student working with collaborators at University of New Mexico and Scripps Institution of Oceanography, proposes a direct link between contemporary tectonic processes and groundwater quality. They utilize a novel approach that combines the composition of helium isotopes and other elements in spring water with a map of seismic waves at a depth of nearly 100 km. The seismic map provides a geographic reference frame and images areas where hotter parts of the mantle are releasing fluids to the crust. Areas underlain by hotter mantle (much of the western U.S.) correlate with the occurrence of springs with unusual chemistry, including elevated concentrations of elements like arsenic. Their work raises the provocative perspective that water quality may owe its origin to tectonic processes and the thermal state of the mantle far below the Earth's surface.
To view the complete table of contents for the December issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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