GEOLOGY HIGHLIGHTS (March 2011)
Boulder, CO, USA – Highlights of articles set for the March issue of GEOLOGY (posted for pre-issue publication on 3–4 February 2011) are provided below. Topics include marine ferromanganese crusts as potential mineral resources; CO2 emissions from volcanic lakes; deformation of the seemingly dormant Damavand volcano, northern Iran; a finding that relic chitin-protein complex plays critical role in the preservation of organic arthropod fossils; description and mapping of the 19 March 2008 Halema'uma'u crater eruption on Kilauea; and a study of the Cilaos deep-sea fan beneath La Reunion Island that raises the question as to which geological processes can be regarded as catastrophic and which are "normal."
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Keywords: Coso and Beowawe geothermal fields, Tibetan Plateau, megaclasts, Brazil, Imperial College Ocean Model, Bison Lake, Colorado, El Niño Southern Oscillation, Rocky Mountain snowpack, Jurassic, Toarcian, Oxfordian-Kimmeridgian, microbes, prokaryotes, Lower Saxony Basin, boreal Albian, SQUID microscopy, marine minerals, micromagnetostratigraphy, ferromanganese, Krafla volcano, Iceland, volcanic lakes, European Alps, San Andreas fault, Parkfield, braided rivers, Damavand volcano, Iran, chitin-protein complex, arthropods, marine habitats, fossils, Halema'uma'u crater, Kilauea, Cascade and Aleutian volcanic arcs, Northern Cordilleran slab window, La Reunion, Cilaos deep-sea fan, numerical modeling, ductile deformation, Chicxulub, Cretaceous-Paleogene boundary, Demerara Rise, Guiana craton, North Sea, tunnel valleys, northwest European ice sheets.
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Deep permeable fault- controlled helium transport and limited mantle flux in two extensional geothermal systems in the Great Basin, United States
Amlan Banerjee et al., Dept. of Earth and Environmental Sciences, New Mexico Tech, Socorro, New Mexico 87801, USA. Posted online 3 Feb. 2011; doi: 10.1130/G31557.1.
Amlan Banerjee of New Mexico Tech and colleagues assess the relative importance of deeply circulating meteoric water and direct mantle fluid inputs on near-surface 3He/4He anomalies reported at the Coso and Beowawe geothermal fields in the western United States. The depth of meteoric fluid circulation is a critical factor that controls temperature, extent of fluid-rock isotope exchange, and mixing with deeply sourced fluids containing mantle volatiles. The influence of mantle fluid flux on the reported helium anomalies appears to be negligible in both systems. Banerjee et al. illustrate the importance of deeply penetrating permeable fault zones in focusing groundwater and mantle volatiles with high 3He/4He ratios to shallow crustal levels. These continental geothermal systems are driven by free convection.
Increasing Nd isotopic ratio of Asian dust indicates progressive uplift of the north Tibetan Plateau since the middle Miocene
Gaojun Li et al., State Laboratory of Mineral Deposits Research, Institute of Surficial Geochemistry, Dept. of Earth Sciences, Nanjing University, Nanjing 210093, China. Pages 199-202. Posted online 3 Feb. 2011; doi: 10.1130/G31734.1.
The uplift history of the Tibetan Plateau is still greatly debated despite its importance on constraining the dynamics of giant plateau building and the influence of mountain uplift on climate. Much of the research has focused on southern Tibet, and revealed that the southern and central Tibetan Plateau was elevated to the modern level by at least the middle Miocene. While dynamic models predict a later uplift of the north Tibetan Plateau, geological clues suggest uplift scenarios that take place from the Early Miocene to as recent as a million years ago. Gaojun Li of the State Laboratory of Mineral Deposits Research, China, and colleagues propose a novel approach for paleoelevation reconstruction of the north Tibetan Plateau based on the neodymium isotope of Asian dust deposition in the central North Pacific Ocean. By finding that the source region of Asian dust receives material eroded from both the north Tibetan Plateau and Central Asia Orogen, and considering the nearly constant elevation of the orogen, the changing neodymium isotopic composition of Asian dust, which reflects increasing relative detritus contribution from north Tibetan Plateau, is interpreted as a monitor of the progressive surface uplift of the north Tibetan Plateau occurring since 15 million years ago.
Three-dimensional seismic analysis of megaclast deformation within a mass transport deposit; implications for debris flow kinematics
Christopher A-L. Jackson, Dept. of Earth Science & Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK. Posted online 3 Feb. 2011; doi: 10.1130/G31767.1.
Christopher Jackson of Imperial College London describes the use of 3-D seismic reflection data to image megaclasts within a mass-transport deposit offshore of Brazil. These data permit detailed mapping of the internal deformation of the clasts, and this sheds new light on the kinematics associated with the initiation, motion, and arrest of large mass-transport deposits.
Tidal circulation in an ancient epicontinental sea: The Early Jurassic Laurasian Seaway
Andrew J. Mitchell et al., Dept. of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK. Posted online 3 Feb. 2011; doi: 10.1130/G31496.1.
Tidal currents are simulated in an expansive ancient sea using the numerical Imperial College Ocean Model (ICOM). Results show how, while the sea would have had a low tidal range (typically <2 m), tidal currents would have still been capable of influencing local sedimentation up to several thousand kilometers from the nearest ocean. This is due to the amplification of the flow by constriction between land masses and over submerged platforms. Areas where tidal currents were intensified to the point that they could influence sedimentation are shown to broadly correlate with known tidal deposits in the geological record. The results of this study by Andrew J. Mitchell of Imperial College London and colleagues show how, despite the apparent attenuation of the tidal range over expansive shallow water-continental shelves, tidal currents still had an important effect on sediment transport and deposition, with important implications for the formation of potential hydrocarbon reservoirs.
Holocene record of precipitation seasonality from lake calcite δ18O in the central Rocky Mountains, United States
Lesleigh Anderson, U.S. Geological Survey, Denver Federal Center Box 25046, MS-980, Denver, Colorado 80225, USA. Published online 3 Feb. 2011; doi: 10.1130/G31575.1.
USGS geoscientist Lesleigh Anderson presents geochemical analyses of alpine lake sediments in northwestern Colorado that provide a record of precipitation for the past 10,000 years. Bison Lake is a headwater of the Upper Colorado River; winter precipitation forms the primary water source for the watershed. The geochemical record documents past changes in the balance between rain and snow for 20- and 100-year periods and indicates that from about 10,000 to 4,000 years ago, there was more rain relative to snow compared to today. During the past 3,500 years, precipitation balances were shifted to more snowpack, but extreme wet and dry periods also occurred more frequently. Corresponding variations in the strength of solar radiation reaching Earth's surface, and the intensity and frequency of El Niño Southern Oscillation (ENSO) cycles, suggest that they are the major influences. The long-term perspective suggests that ENSO has been, and will likely continue to be, an important driver of Rocky Mountain snowpack and corresponding water availability.
Climatic ups and downs in a disturbed Jurassic world
Guillaume Dera et al., LSCE-IPSL (Laboratoire des Sciences du Climat et l'Environnement, Institut Pierre Simon Laplace), CNRS-UMR 8212, CEA, F-91191 Gif-sur-Yvette, France. Posted online 2 Feb. 2011; doi: 10.1130/G31579.1.
The tropical, warm, and equable climate of the Jurassic world is regularly challenged by geoscientists, especially because geochemical techniques based on the oxygen isotope composition (δ18O) of marine fossils have been used to reconstruct the paleotemperature history of seawater. Here, Guillaume Dera of Institut Pierre Simon Laplace, France, and colleagues compile the first δ18O database covering the entire Jurassic (200 to 145 million years ago) and complete it with new data measured on mollusks from former European seas. By applying an innovative statistical approach to this updated database, Dera and colleagues demonstrate the occurrence of major and multiscale changes in geochemical data, mainly related to climate disturbances. For the first time, two long-term climatic anomalies are identified during two intervals of 5 million years (called Toarcian and Oxfordian-Kimmeridgian), in conjunction with intensive volcanism. These results support a strong influence of repeated volcanic pulses on the modulation of pCO2, temperatures, and polar ice cap volumes over protracted periods. At shorter time scales, 13 relatively rapid (0.5–1 million years) and significant warming and cooling events are identified, the causes of which include transient fluctuations in greenhouse gas concentrations related to still-debated mechanisms.
Prokaryotes stimulate mineral H2 formation for the deep biosphere and subsequent thermogenic activity
R. John Parkes et al., School of Earth and Ocean Sciences, Cardiff University, Main Building, Cardiff, Wales CF10 3AT, UK. Published online 3 Feb. 2011; doi: 10.1130/G31598.1.
There may be more life beneath our feet than on the whole Earth's surface. This life is microscopic (prokaryotes) and continuous down to kilometer depths, even though temperatures approach over 100 degrees Celsius. But where do these microbes get their energy from? R. John Parkes of Cardiff University and colleagues explore the potential for minerals in sediments to provide energy in the form of hydrogen gas. Hydrogen was formed in sediment slurries with common minerals, but only when microbes were present and not in sterile controls. This stimulated the development of anaerobic, microbial communities and activities typical of many previously studied deep sediments. Increasing the temperature of experiments to simulate temperature increases in deep sediments resulted in continuing microbial activity, especially above 70 degrees Celsius. Mechanochemistry may be responsible for this mineral-hydrogen formation, and this is enhanced by the presence of microbes (biomechanochemistry) and stress and fracturing in subsurface sediments and rocks. Such fracturing is widespread on Earth (e.g., earthquakes) and any tectonically active planet. Additional experiments showed that this microbial activity even stimulates geosphere reactions when temperatures were subsequently raised to above the upper temperature limit for microbes (above 120 degrees Celsius). This demonstrates that microbial reactions have a profound direct effect on the geosphere on Earth, including hydrocarbon generation.
Short-term warming events during the boreal Albian (mid-Cretaceous)
Jochen Erbacher et al., Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany. Published online 2 Feb. 2011; doi: 10.1130/G31606.1.
Pronounced paleotemperature variations associated with the carbon cycle perturbations of oceanic anoxic events are known to have occurred during the early and latest Albian. However, reliable paleotemperature data spanning the 12 million years between these two events are sparse. Here, Jochen Erbacher of the University of Southampton and colleagues present δ18O, δ13C, and Mg/Ca data for the middle to early-late Albian based on analysis of unusually well-preserved "glassy" foraminifers from a clay-rich sequence drilled in the boreal Lower Saxony Basin (northwestern Germany). Their records reveal pronounced variability in planktic δ18O on a 100-thousand-year time scale that they interpret to indicate substantial changes in sea-surface temperature and δ18O/salinity. These findings attest to the climatic sensitivity of marginal basins in these boreal latitudes during the mid-Cretaceous and suggest that the heat and salt budgets of the Lower Saxony Basin were orbitally conditioned during the Albian, presumably through atmospheric or oceanic heat transport from lower latitudes where surface waters were warmer than today, in response to strong greenhouse gas forcing.
Ultrafine-scale magnetostratigraphy of marine ferromanganese crust
Hirokuni Oda et al., Geological Survey of Japan, AIST (National Institute of Advanced Industrial Science and Technology), Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan. Published online 3 Feb. 2011; doi: 10.1130/G31610.1.
Marine ferromanganese crusts are iron-manganese oxide precipitated on the seafloor and growing over periods of tens of millions of years. They are considered to be potential marine mineral resources for cobalt, nickel, platinum, etc. They record paleoceanographic and geological environments as geochemical and mineralogical variations. However, reliable high-resolution age dating is crucial to reconstruct the paleoenvironment. Earlier isotopic chronology has limitations below a resolution of several millimeters. Here Hirokuni Oda of the Geological Survey of Japan and colleagues apply a new magnetostratigraphic method to a carefully selected ferromanganese crust of the northwest Pacific Ocean. Magnetostratigraphy is a technique that correlates the polarity of magnetization recorded in a sample with the known history of geomagnetic reversals in order to estimate the age. Using scanning SQUID (superconducting quantum interference device) microscopy, submillimeter-scale bedding-parallel stripes of oppositely magnetized regions can be identified on thin sections of the crust. Through this method, Oda and colleagues were able to determine an average growth rate of 5.1 plus or minus 0.2 mm/m.y., which is within 16% of beryllium-10 deduced rate (6.0 plus or minus 0.2 mm/m.y.). This is the finest-scale magnetostratigraphic study yet. The authors expect micromagnetostratigraphy to be uniquely powerful chronological tool for studying ferromanganese crusts. The highest possible accuracy of the method is promised by that of the astronomically calibrated magnetostratigraphic time scale (1000–40,000 years).
Origin of a rhyolite that intruded a geothermal well while drilling at the Krafla volcano, Iceland
Wilfred A. Elders et al., Dept. of Earth Sciences, University of California, Riverside, California 92521, USA. Posted online 3 Feb. 2011; doi: 10.1130/G31393.1.
An exploratory well designed to search for very deep (4-5 km) geothermal resources in the Krafla volcano, Iceland, was terminated when magma unexpectedly flowed into the well at only 2.1 km depth. Although the volcano is basaltic, the magma is a silica-rich rhyolite, apparently intruded at about 900 degrees Celsius. According to Wilfred Elders of the University of California at Riverside and colleagues, its hydrogen and oxygen isotopes indicate that this magma, rather than having fractionated from a mantle-derived basalt melt, formed by partial melting of hydrothermally altered basalts within the Krafla caldera. This shallow magma provides an opportunity to test a very hot geothermal system (greater than 500 degrees Celsius) as an energy source.
Global CO2 emission from volcanic lakes
Nemesio M. Perez et al., Environmental Research Division, Instituto Tecnologico y de Energias Renovables (ITER), 38611 Granadilla de Abona, Tenerife, Canary Islands, Spain. Posted online 2 Feb. 2011; doi: 10.1130/G31586.1.
One of the most important challenges the scientific community is facing is the attempt to estimate CO2 emissions in the atmosphere in an effort to understand and control them. Nemesio Perez of the Instituto Tecnologico y de Energias Renovables, Spain, and colleagues focus on one of the most unknown contributors to these emissions: volcanic lakes. Extensive research has been performed globally to gain a better understanding of how these systems work; from alkaline to acid lakes, and from small to giant volcanic lakes. Because of their work, the contribution from these systems to the atmosphere can be inferred: the estimated global CO2 emission from volcanic lakes is 117 plus or minus 19 Mt/yr, with 94 plus or 17 Mt/yr as deep-seated CO2. Perez and colleagues highlight the importance of a revision of the actual global CO2 discharge from subaerial volcanism, previously estimated on 300 Mt/yr.
Detrital thermochronology records changing source areas and steady exhumation in the Western European Alps
Christoph Glotzbach et al., Laboratoire de Géodynamique des Chaines Alpines, Universite Joseph Fourrier, Grenoble, France. Posted online 4 Feb. 2011; doi: 10.1130/G31757.1.
Spatial and temporal erosion patterns of mountain ranges are the result of changes in tectonics and climate. Christoph Glotzbach of Universite Joseph Fourrier, France, and colleagues use geochronological data to unravel the late erosional evolution of the European Alps. Despite major climate changes, erosion seems to be in steady state, and possibly balanced by decreasing tectonic convergence in the Alps.
Late Holocene slip rate of the San Andreas fault and its accommodation by creep and moderate-magnitude earthquakes at Parkfield, California
Nathan A. Toké et al., School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA. Published online 3 Feb. 2011; doi: 10.1130/G31498.1.
Utilizing a sedimentary record extending several thousand years into the past, Nathan Toké of Arizona State University provide the first documented prehistoric slip rate on the Parkfield section of the San Andreas fault. Their geologic measurements are consistent with rates that have been observed historically by geophysicists at Parkfield. However, these rates are slower than the rates measured to the southeast along the part of the fault that ruptured in the great 1857 Fort Tejón earthquake. They did not find evidence for earthquakes larger than those observed historically at Parkfield. These results indicate that the intriguing fault behavior observed at Parkfield over the past century, with both aseismic fault creep and periodic magnitude 6 earthquakes, has persisted for more than a millennium and it raises questions about how slip along the San Andreas fault is distributed between different sections of the fault.
Geometry and grain-size characteristics of the basal surface of a braided river deposit
J. Tobias Gardner and Peter E. Ashmore, Dept. of Geography, University of Western Ontario, London, Ontario N6A 5C2, Canada. Posted online 3 Feb. 2011; doi: 10.1130/G31639.1.
The deposits of multi-channel braided rivers, and the rocks formed from them, are important reservoirs for water and hydrocarbons. Describing and explaining the characteristics and origins of these deposits in relation to the processes of river deposition has proved difficult because of the need to observe processes over long time periods or to infer processes from the resulting deposits. J. Tobias Gardner and Peter Ashmore of the University of Western Ontario take a completely new approach of using small-scale models of braided rivers in a flume to capture successive detailed maps of river topography derived from stereo photogrammetry in order to build up a three-dimensional view of the river deposit that also includes the details of the local variations in grain size of the deposits. Using this novel combination of techniques, Gardner and Ashmore show the temporal development and characteristics of braided river deposit geometry. For the first time, they are able to map the characteristics and development of the basal erosion surface of braided river deposits, which is much more variable in age, topography, and grain size than had previously been assumed.
Gravity-driven deformation of Damavand volcano, Iran, detected through InSAR time series
M. Shirzaei et al., Dept. of Physics of the Earth, Section 2.1, GFZ (GeoForschungsZentrum) Telegrafenberg, D-14473 Potsdam, Germany. Published online 3 Feb. 2011; doi: 10.1130/G31779.1.
Damavand volcano in northern Iran seems to be dormant, but M. Shirzaei of GeoForschungsZentrum, Germany, and colleagues reveal that it is deforming. This hitherto unrecognized deformation was discovered from analysis of a large set of high-resolution space-borne radar data. The data show a lateral extension of the volcano at the relative rate of up to 6 mm/yr accompanied by a subsidence at the rate of up to 5 mm/yr at the volcano summit. Lateral motion of the east flank is more significant than that of the west flank, which is interpreted as a result of slow and asymmetric gravitational spreading. Damavand may therefore still pose a major hazard to the local population in terms of landslides, lateral blasts, and debris avalanches. Moreover, Shirzaei and colleagues highlight the importance of identifying long-lived gravity-driven deformation for hazard assessment at dormant or inactive volcanoes.
Molecular signature of chitin-protein complex in Paleozoic arthropods
George D. Cody et al., Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, USA. Published online 3 Feb. 2011; doi: 10.1130/G31648.1.
Vestige chitin-protein complex is preserved in Paleozoic arthropod cuticle. According to Cody et al., it is likely that relic chitin-protein complex plays critical role in the preservation of the organic arthropod fossil.
Stratigraphic distribution of marine fossils in North America
Shanan E. Peters and Noel A. Heim, Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. Published online 3 Feb. 2011; doi: 10.1130/G31442.1.
Fossils are not uniformly distributed in the rock record. Here, Shanan E. Peters and Noel A. Heim of the University of Madison show that fossils of marine organisms are more abundant than expected in sediments deposited in the last 40% of the lifespans of shallow marine sedimentary environments, and less abundant than expected in the first 25%. Peters and Heim also show that the extinctions of marine genera in the fossil record commonly co-occur in time with the elimination of shallow marine habitats, whereas the formation of shallow marine habitats does not coincide with the origination of marine genera. These results indicate that environmental changes associated with the destruction or restriction of shallow marine habitats, such as those caused by long-term sea level fall, result in biological extinction, and that the maintenance of biodiversity is promoted by the temporal persistence of widespread shallow marine habitats.
Pigeonholing pyroclasts: Insights from the 19 March 2008 explosive eruption of Kilauea volcano
B.F. Houghton et al., Dept. of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA. Published online 3 Feb. 2011; doi: 10.1130/G31509.1.
The eruption of Halema'uma'u crater on Kilauea that began in March 2008 has been an unprecedented opportunity to observe in fine detail the evolution of a basaltic explosive eruption. B.F. Houghton of the University of Hawaii and colleagues describe unique features of the opening phase of the eruption on 19 March 2008 and describe features of the explosion that are not compatible with any of the classical styles of basaltic eruptions. Mapping the 19 March 2008 deposit a day after the eruption highlighted how difficult it is to constrain the true edges of deposits from unobserved or prehistoric eruptions of all magnitudes. Houghton and colleagues suggest that the dimensions of the "footprint" of such eruptions could be miscalculated by up to 30% of the total, an issue when trying to set realistic hazard zones on a volcano. Eruptions of this type challenge existing classification schemes for eruptions and, in the past, were probably interpreted as steam explosions, linked to rapid heating of groundwater by magmatic heat.
Mantle flow through the Northern Cordilleran slab window revealed by volcanic geochemistry
Derek J. Thorkelson et al., Dept. of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. Published online 3 Feb. 2011; doi: 10.1130/G31522.1.
Earth's rigid outer layer, the lithosphere, is divided into tectonic plates. Where one plate dives down below another, a chain of volcanoes forms. Such an environment exists along the west coast of North America, and the volcanic chain is made up of the Cascade and Aleutian volcanic "arcs" separated by a field of smaller volcanoes that extends from central British Columbia to eastern Alaska. This intervening field is situated above a tectonic feature termed a slab window. Thorkelson et al. show that the chemical compositions of the lavas in this intervening field are markedly different from those in the flanking Cascade and Aleutian volcanic arcs. Derek J. Thorkelson of Simon Fraser University and colleagues demonstrate that the difference in lava compositions was caused by upward flow of Earth's mantle through the slab window starting at about 45 million years ago and continuing to the present day.
A volcaniclastic deep-sea fan off La Reunion Island (Indian Ocean): Gradualism versus catastrophism
Francky Saint-Ange et al., Bedford Institute of Oceanography, Geological Survey of Canada (Atlantic), P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada. Published online 3 Feb. 2011; doi: 10.1130/G31479.1.
La Reunion is a volcanic island about 220 km in diameter, rising 7000 m above the Indian Ocean floor. The island is deeply dissected, which is related to a high erosion rate, controlled by the intense precipitation occurring during the storm season. Sediments are transported only during flash floods. Multibeam bathymetry data from the seafloor around La Reunion reveal the existence of several complex deep-sea fans, with similarities to those found seaward of rivers draining continents, each directly connected to a major torrential river. Sediment storage at the coast is limited, suggesting that the volcanic detritus is carried directly to the ocean floor by direct flow of the sediment-laden river into the sea. Normally, volcanic detritus is deposited in the ocean by catastrophic events such as giant volcano-flank collapses or flows of hot erupted ash. Here, Francky Saint-Ange of the Geological Survey of Canada and colleagues show that the Cilaos deep-sea fan is the result of flood supply of sediment by rivers. It raises the question as to which geological processes can be regarded as catastrophic and which are "normal."
Strain localization and porphyroclast rotation
Albert Griera et al., Departament de Geologia, Universitat Autonoma de Barcelona, E-08193 Bellaterra (Cerdanyola del Valles), Spain. Published online 3 Feb. 2011; doi: 10.1130/G31549.1.
It has been debated for decades whether rigid inclusions, such as porphyroclasts and porphyroblasts, do or do not rotate in a softer matrix during ductile deformation. A correct understanding of this problem is therefore of crucial importance in unraveling the deformation histories of deformed rocks and ancient orogens. Albert Griera of Universitat Autonoma de Barcelona, Spain, and colleagues focus on the problem using a novel numerical approach based on crystal plasticity, with rocks simulated as polycrystalline aggregates. This is a significant advance in numerical modeling because it overcomes the problem of using viscous continuum approaches to simulate crystalline rocks. Griera et al. show that the inclusion’s rotation strongly depends on the mechanical anisotropy of the matrix minerals. Strongly anisotropic minerals will develop shear bands that reduce the rotation of inclusions. Their results help to explain why geologically rigid objects often show evidence of rotation, but not necessarily in accordance with the viscous theory that is usually applied to these systems.
The Chicxulub ejecta deposit at Demerara Rise (western Atlantic): Dissecting the geochemical anomaly using laser ablation- mass spectrometry
Jasper Berndt et al., Institut fur Mineralogie, Westfalische Wilhelms-Universitat Munster, Corrensstrasse 24, D-48149 Munster, Germany. Published online 3 Feb. 2011; doi: 10.1130/G31599.1.
Chicxulub and the Cretaceous-Paleogene (K-Pg) boundary - a never ending story? Using high-resolution laser ablation-mass spectrometry to analyze 33 trace elements in successive 235-µm-sized spots across the undisturbed 2-cm-thick K-Pg boundary layer at the Demerara Rise (western Atlantic, Ocean Drilling Program Leg 207), Jasper Berndt of Westfalische Wilhelms-Universitat Munster, Germany, and colleagues were able to demonstrate that this Chicxulub ejecta bed records in detail a succession of, in part surprising, events: At the base of the layer, impact spherules ejected from the target site at Yucatan with low trace element concentrations yield the dominant signal; in the middle part, materials with exotic low Zr/Hf and Nb/Ta ratios washed off the nearby Guiana craton are most prominent, whereas projectile matter is only, but particularly, highly concentrated in the uppermost 1000 µm of the ejecta layer concomitant to the presence of ejected shocked quartz and carbonate clasts. Immediately above the top of the event bed, trace element characteristics match those of the uppermost Cretaceous pelagic sediments below this layer ("reset to pre-impact conditions"). Sedimentological features restrict the time span comprised by the event bed to about one year.
Seven glacial cycles in the middle-late Pleistocene of northwest Europe: Geomorphic evidence from buried tunnel valleys
Margaret A. Stewart and Lidia Lonergan, Dept. of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 1AZ, UK. Published online 3 Feb. 2011; doi: 10.1130/G31631.1.
Evidence from glacial landforms and deposits suggests that during the past 500,000 years or so, major ice sheets advanced over much of continental northwest Europe and the North Sea three different times. This is at odds with the data from deep ocean sediments and Antarctic and Greenland ice cores, which show that there were at least six major global glaciations in the same time period. New mapping of tunnel valleys in Quaternary sediments in the North Sea by Margaret Stewart and Lidia Lonergan of Imperial College London shows that ice advanced seven times across the central North Sea over an area covering about 150,000 square kilometers. Tunnel valleys are broad valleys several hundred meters wide and 10 to 100 m deep, cut by pressurized meltwater under the base of an ice sheet. 180 crosscutting tunnel valleys were mapped and correlated on three-dimensional seismic data. Stewart and Lonergan's analysis of the valley infill and crosscutting relationships shows that each set formed as a result of an individual ice sheet advance and retreat cycle. These new data provide the most complete documentary evidence for repeated advance and retreat of the northwest European ice sheets in the past 500,000 years in accord with global climate change records.