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The tectonic significance of dikes of irregular fold-like shape
Elena Druguet et al., Dept. de Geologia, Universitat Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Spain. Posted online 23 May 2012; doi: 10.1130/G32960.1.
Dikes and veins are common features in orogenic regions, and they may often deform subsequent to their emplacement. Deformation of dikes results in folded (shortened) or boudined (stretched) geometries, depending on the relative strengths of the dike and host rock and the relative orientation between the dikes and strain. The resultant geometries are often used to analyze the tectonic history, but analysis is typically predicated on the assumption that the dikes had planar shapes prior to deformation. This assumption is not always valid, particularly where dikes intrude layered or banded rocks with strength contrasts. In this study, Elena Druguet and colleagues test the effects of post-emplacement deformation on originally non-planar dikes experimentally and compare them to field data from the Rainy Lake zone (Ontario, Canada), where irregular fold-like structures are observed. The analyses reveal that initial geometrical irregularities can be amplified or subdued with deformation depending on the strength contrast between the dike and host rock, the layer geometries, and the orientation of the imposed strain, resulting in either over- or under-estimation of strain using standard procedures. Relevant criteria are given to distinguish between intrusive and deformation structures in intensely deformed areas.
Microbial nucleation of Mg-rich dolomite in exopolymeric substances under anoxic modern seawater salinity: New insight into an old enigma
Stefan Krause et al., Dept. of Marine Biogeochemistry, Helmholtz Centre for Ocean Research Kiel (GEOMAR), 24148 Kiel, Germany. Posted online 23 May 2012; doi: 10.1130/G32923.1.
The origin of the dolomite formations observed in the rock record has been debated for more than a 100 years. The present laboratory study revealed that biofilms of sulfate-reducing bacteria are causative for the formation of dolomite under modern anoxic seawater conditions. The findings support modern theories of microbially mediated dolomite formation, suggesting that microbial dolomite has been produced in the world's ocean in former geological periods. During culturing experiments with a pure bacteria strain, dolomite nano-crystals nucleated exclusively within extracellular polymeric substances (EPS) of the biofilm, subsequently intercalating to micrometer-scale crystals. With regards to the surrounding bulk liquid, EPS accumulated relatively more calcium than magnesium ions, causing a considerable decrease of the magnesium/calcium ratio within the biofilm. In addition, light calcium isotopes accumulated within the biofilm, creating a pre-fraction calcium reservoir, from which the dolomite crystals appeared to precipitate. The sequential increase of light calcium isotopes from the bulk liquid to the biofilm, and to the crystals suggests a two-step fractionation process during the sequestration of calcium from the bulk liquid into the crystal. The observed involvement of microbiologically-produced substances during isotopic fractionation might contribute to the interpretations of rock record calcium isotopy to further constrain paleo-environmental conditions.
Extreme change in sulfide concentrations in the Black Sea during the Little Ice Age reconstructed using molybdenum isotopes
G.L. Arnold et al., Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany. Posted online 23 May 2012; doi: 10.1130/G32932.1.
A novel approach allows G.L. Arnold and colleagues to use the molybdenum (Mo) isotope signature of sediments as a quantitative paleo-sulfide concentration proxy. Applying this technique in the Black Sea, they reconstruct historical sulfide concentrations in the area of the Bosporus Inlet, paving the way for a basin-wide reconstruction of the sulfide interface and gradient. The rapid shoaling of the sulfide interface in the Black Sea between 350 and 300 billion years before present corresponds with the end of the "Peak Bloom Period" of Emiliana huxleyi (Hay et al., 1991), coincident with the first cold interval of the Little Ice Age. Linked to this event, increased export of organic matter into anaerobic waters may have triggered increased production of hydrogen sulfide by bacterial sulfate reduction. Shoaling and steepening of the sulfide interface would have resulted in a positive feedback, as a smaller portion of the water column was available for aerobic degradation of organic matter, resulting in higher rates of sulfate reduction. This scenario may explain the rapid shift to a shallow sulfide interface, followed by a more gradual return to deeper levels in this area, as revealed by the sedimentary Mo isotope record.
Green rust formation controls nutrient availability in a ferruginous water column
Asfaw Zegeye et al., School of Civil Engineering and Geosciences, Newcastle University, Drummond Building, Newcastle upon Tyne NE1 7RU, UK. Posted online 23 May 2012; doi: 10.1130/G32959.1.
It is increasingly apparent that for much of Earth's history the oceans were devoid of oxygen and rich in dissolved iron. These conditions had major implications for the development of the planet as an environment able to support complex life, but the controls that this type of ocean chemistry exerted on the evolution of the biosphere are poorly understood. One major uncertainty concerns the nature of minerals that formed in the surface ocean under such conditions. These iron minerals scavenged bioessential elements from the water column and thus placed major constraints on key microbial processes that ultimately led to oxygenation of the oceans and atmosphere. In this study, Asfaw Zegeye and colleagues investigate iron mineralization in the largest oxygen-free and iron-rich basin on Earth: Lake Matano, Indonesia. An unusual mineral, Green Rust, was found forming in the water body. This Green Rust exerts a major control on the availability of nickel, which is an essential nutrient for microbes that produce methane. Nickel is considered to have been important in controlling methane production and atmospheric oxygen levels on the early Earth, but the control that Green Rust exerts on this suggests that nickel bioavailability through Earth's early history requires reevaluation.
New insights into the morphology and sedimentary processes along the western slope of Great Bahama Bank
T. Mulder et al., Université Bordeaux 1, UMR 5805 EPOC, 33405 Talence cedex, France . Posted online 23 May 2012; doi: 10.1130/G32972.1.
This paper by T. Mulder and colleagues presents new high-quality multibeam and seismic data image the western slope of the Great Bahama Bank and the adjacent floor of the Straits of Florida. The extensive survey reveals several unexpected large- and small-scale morphologies. These include bypass areas, channel-levee lobe systems, gullied slopes, and products of slope instabilities at various scales, including long slump scars at the lower slope and mass transport complexes that extend ~30 km into the adjacent basin floor.
Tidal signatures in an intracratonic playa lake
R. Bruce Ainsworth et al., Australian School of Petroleum, University of Adelaide, Adelaide, SA 5005, Australia. Posted online 23 May 2012; doi: 10.1130/G32993.1.
Lake Eyre is one of the largest ephemeral lakes in the world; it is also the depocenter for a river catchment that covers nearly one-sixth of the Australian continent. Shallow trenches dug into the floor of Lake Eyre during the dry season reveal structures in the sediment deposited in the land-locked lake that resemble sedimentary structures only previously described from marine environments. These structures have previously been attributed to astronomically forced marine tidal currents. R. Bruce Ainsworth and colleagues attribute the formation of these features in Lake Eyre to meteorological tides generated by daily changes in wind direction and velocity, along with weekly to monthly discharge variations in the feeder river system. This is the first time such sedimentary features have been documented together in a continental setting. A key implication of this is that the classic "tidal" sedimentary structures recognized in ancient rock successions should not automatically be attributed to astronomical or marine tidal currents, since similar structures can be generated by meteorological processes in continental environments. Some interpretations of ancient marine tidal deposits without other lines of evidence supporting a marine depositional environment may therefore require reevaluation.
The viscous-brittle transition of crystal-bearing silicic melt: Direct observation of magma rupture and healing
B. Cordonnier et al., Geological Institute, Swiss Federal Institute of Technology, Sonneggstrasse 5, 8092 Zürich, Switzerland. Posted online 23 May 2012; doi: 10.1130/G32914.1.
Whether a volcano creates lava flows or an explosive eruption depends on whether the magma flows or breaks during the eruption. Recent experimental work by B. Cordonnier and colleagues has identified how the transition between flowing and breaking is controlled by the fraction of crystals in the magma. Stresses are enhanced between the crystals, causing magma to break at lower stresses when crystals are present. The cracks that form can heal over short time scales at high pressures. Magma can thus break and heal many times as it rises to the surface.
Nile Delta vegetation response to Holocene climate variability
Christopher E. Bernhardt et al., U.S. Geological Survey, 926A National Center, Reston, Virginia 20192, USA. Posted online 23 May 2012; doi: 10.1130/G33012.1.
This study provides evidence of changes to vegetation in the Nile Delta region in response to several marked regional drought events that affected civilizations in ancient Egypt and the Middle East. Using the palynologic record from a well-dated core, aged ~7,000 cal. yr B.P. (calibrated years before present) to present, collected in the Burullus Lagoon of the Nile Delta, Egypt, Christopher Bernhardt of the U.S. Geological Survey and colleagues from the University of Pennsylvania and Smithsonian Institution recorded decreases to Cyperaceae pollen, which they put forth as a sensitive marker of precipitation over the Nile headwaters and Nile flow. The pollen data indicate decreases in precipitation and Nile flow along with an increase in relative abundance of microscopic charcoal at four separate time periods between ~6,000 and ~3,000 cal. yr B.P., including the event associated with the demise of Egypt's Old Kingdom more than 4,200 years ago, all of which correspond to extreme regional and global aridity events. Supported by other proxy studies, the research shows a strong correlation between terrestrial vegetation, Nile flow, and precipitation over the Nile headwaters. The study illustrates the importance of understanding natural climate variability and its role on human societies.
Estimating rates of decompression from textures of erupted ash particles produced by 1999-2006 eruptions of Tungurahua volcano, Ecuador
Heather M.N. Wright et al., U.S. Geological Survey, MS 910, 345 Middlefield Road, Menlo Park, California 94025, USA. Posted online 23 May 2012; doi: 10.1130/G32948.1.
Living with persistently active volcanoes can be challenging for local populations, particularly when the activity ranges from light ash fall to large explosions. Tungurahua volcano, Ecuador, which has been erupting since 1999, is just such a volcano. Volcanic activity over this time period has ranged from large bubble bursts that scatter bombs around the volcano's summit, to small explosions that dust the volcano's flanks with volcanic ash, to large explosions that blanket the countryside with fragmental material and send mudflows down major river drainages. The challenge for volcanologists, then, is to provide timely information about the nature of current and future volcanic activity. Traditional monitoring signals, such as earthquakes, ground deformation, and volcanic gases, provide crucial information on movement of magma below the volcano. However, predicting variations in eruptive styles requires monitoring of the magma itself, particularly physical changes in magma beneath the surface. Here, Heather M.N. Wright and colleagues present a new technique to track changes in magma by analyzing the crystal content of ash emitted from the volcano. Importantly, the crystal content of ash decreases as magma rises more rapidly. This information can then be used to improve models of magma ascent and eruption and to interpret past eruptive histories of the volcano.
Seawater oxygenation during the Paleocene-Eocene Thermal Maximum
Alexander J. Dickson et al., Dept. of Environment, Earth and Ecosystems, Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Walton Hall, Milton Keynes, Buckinghamshire MK7 6AA, UK. Posted online 23 May 2012; doi: 10.1130/G32977.1.
Recent studies have suggested that the concentration of oxygen in the world's oceans will become lower as global warming progresses. This hypothesis is supported by observations of apparently enhanced deoxygenating in several regions of the oceans, but the magnitude and severity of deoxygenating in the long term is difficult to assess with direct observational datasets. In this study by Alexander J. Dickson and colleagues, a technique based on isotope ratios of the element molybdenum (Mo) is used to reconstruct the global extent of seawater deoxygenating during the Paleocene-Eocene Thermal Maximum (PETM), a period of rapid CO2-driven global warming 56 million years ago. The new data suggest that compared with the present day, the extent of poorly oxygenated ocean seafloor was expanded by a small but significant amount. This finding supports a link between higher global temperatures and lower seawater oxygen concentrations, reinforcing the notion that higher temperatures are likely to have a significant adverse effect on marine biota in the future.
P-wave velocity differences between surface-derived and core samples from the Sulu ultrahigh-pressure terrane: Implications for in situ velocities at great depths
Shengsi Sun et al., Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Quebec H3C 3A7, Canada. Posted online 23 May 2012; doi: 10.1130/G33045.1.
The interpretation of seismic data in terms of lithology and chemical composition in the crust and upper mantle has been made essentially by comparison of in-situ seismic velocities with laboratory-measured properties of rock samples. However, the vast majority of the laboratory measurements of seismic velocities and anisotropy have been performed on rock samples collected from surface exposures that have experienced weathering and low temperature alteration, particularly along microcracks and grain boundaries. Comparison between the samples from the Chinese Continental Scientific Drilling borehole at depths up to 5118 m and their equivalents from surface exposures suggests that extrapolation of the surface rock physical data to depth may result in substantial underestimates in seismic velocities, and that the intrinsic pressure derivatives obtained from the Vp-pressure relations for the core samples are more suitable for the determination of the in-situ velocities at great depth than those derived from measurements on rocks exposed at the surface.
Provided by Geological Society of America
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