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James Jackson Discusses How the Continents Deform: Interview for ScienceWatch
No full textSeismology is the discipline that interprets data from earthquakes in order to image structures, sources, and boundaries deep within the earth. The solid earth has three types of seismic waves: the longitudinal P-waves, the transverse S-waves, plus interface waves
Are egg shells a suitable DNA source for population genetic studies? = Eignen sich Eierschalen als DNA-Quelle für populationsgenetische Studien?
No full textRedstart: Dense meadows hamper catching the prey =Gartenrotschwanz: Dichte Wiesen erschweren den Beutefang
No full textDecarbonation efficiency in subduction zones: Implications for warm Cretaceous climates
No full textSubduction zones play a fundamental role in the geochemical cycle of carbon, and related arc volcanism is believed to exert primary control on atmospheric CO2 concentrations over geological time. Arc volcanism may have been particularly important in the most recent Greenhouse of the late Cretaceous, where it has been hypothesized that the subduction of the carbonate-rich Tethys contributed to overall higher volcanic CO2 outgassing rates and thus a warmer climate. To test this hypothesis, the decarbonation efficiencies of modern subduction zones were calculated through a geochemical database that compared subaerial arc CO2 fluxes with the subducting crust and sediment geochemistry. The modern data are used to postulate a CO2 recycling and degassing scenario for arc volcanism related to the closure of the Tethys. Our analysis indicates that the thermal structure of subduction zones controls the extent and depth of slab decarbonation, while the sediment geochemistry (e.g. the amount of carbonate sediment) may be of secondary importance. The calculated decarbonation efficiency of modern arcs ranges from 18 to 70%. Our calculations support recent models predicting carbon recycling through infiltration-driven decarbonation, and limited by water availability at sub-arc depths. This analysis allows us to make inferences about the potential volcanic CO2 flux from subduction of the Tethys during the Cretaceous, suggesting between an 8 and 222% increase over modern CO2 outgassing. We suggest that the primary reason for the increase in CO2 outgassing in the Cretaceous is contamination of arc magmas by platform carbonates in the overlying crust and increased decarbonation efficiency
Processes, rates and the role of water in metamorphism: a colloquium to commemorate the Alexander von Humbolt Award to Bruce Yardley
No full textIn previous decades, much effort has been devoted to working out how we can tell where the evolution of mineral composition and microstructure of an individual rock stalled on its progress round a P-T-t loop. Less attention has been paid to the actual processes by which metamorphic reactions take place: we know rather little about the parameters that control mechanism and rates. In recognition of this, attention was focused on these fundamental questions during a colloquium at the GeoForschungsZentrum in Potsdam on 2 July 2010
Eye and sheath folds in turbidite convolution lamination: Aberystwyth Grits Group, Wales.
No full textEye and sheath folds are described from the turbidites of the Aberystwyth Group, in the Silurian of west Wales. They have been studied at outcrop and on high resolution optical scans of cut surfaces. The folds are not tectonic in origin. They occur as part of the convolute-laminated interval of each sand-mud turbidite bed. The thickness of this interval is most commonly between 20 and 100 mm. Lamination patterns confirm previous interpretations that convolute lamination nucleated on ripples and grew during continued sedimentation of the bed. The folds amplified vertically and were sheared horizontally by continuing turbidity flow, but only to average values of about γ = 1. The strongly curvilinear fold hinges are due not to high shear strains, but to nucleation on sinuous or linguoid ripples. The Aberystwyth Group structures provide a warning that not all eye folds in sedimentary or metasedimentary rocks should be interpreted as sections through high shear strain sheath folds.
Highlights
► Eye folds occur in turbidite convolute lamination viewed down the palaeoflow direction. ► The convolutions formed during bed deposition and are overturned down-flow by current shear. ► The eye folds are sections through non-cylindrical sheath folds due to this shear. ► Average shear strains are less than 1.0, lower than generally necessary to generate sheath folds. ► Instead, sheath folds nucleate on already sinuous current ripples lower in the turbidite bed
Exceptionally preserved ostracodes from a Middle Miocene palaeolake, California, USA
No full textExceptionally well-preserved specimens of a new cypridid ostracode (Crustacea), Raepula ira sp. nov., are described from palaeolake sediments of the Middle Miocene Barstow Formation of the Mud Hills, southern California. This is only the second occurrence of exceptionally preserved ostracodes from the Miocene. Based on ages obtained from associated volcanic tuffs the palaeolake sediments were deposited between 16.3 and 15.8 Ma. The ostracodes form one element of a diverse lake community dominated by fairy shrimps, copepods, diatoms, larvae of diving beetles, flies and mosquitoes, and body fossils and ephippia of branchiopods and anomopods. The ostracodes are preserved three dimensionally with their soft anatomy replicated in microcrystalline silica. Submicron-scale details such as sensory setae are preserved, surpassing the resolution of most other ostracode-bearing lagerstatten and allowing their biology to be compared with extant taxa. Supplementary materialRed-blue anaglyphs of the exceptionally preserved ostracodes are available at http://www.geolsoc.org.uk/SUP18407
Silbury Hill – an Axis Mundi Connecting Underworld and Earth to Sky?
No full textSilbury Hill is the largest man - made Neolithic mound in Europe. Built around 2400-2300BC at Avebury in Wiltshire UK, it predates any primary literary sources describing its intended purpose(s). It remains an enigma from a contemporary perspective, with differing disciplines proposing differing functions corresponding to the methodological-dependent findings made. The aim of this presentation is to explore the possible reason(s) why Silbury Hill was built by using four contemporary methodological approaches: modern archaeological findings, post-modern arguments based on archaeological evidence, consideration of Silbury Hill as a sacred space as delineated by Mircea Eliade and a phenomenological survey of Silbury Hill and the surrounding area as explicated by Christopher Tilley. While a range of possible functions was found when collating the findings from the four approaches, it appears that most of the findings can be placed in three general sub-headings: first the heavens or sky, second the landscape or earth and third the underworld. These three classes can be symbolically linked together to form what Eliade calls an Axis Mundi, allowing a communication between these three cosmic levels, creating a sacred space with a distinct centre and cosmology. The presentation discusses these findings and concludes that first a multi – disciplinary approach may represent a more robust way forward in understanding Neolithic monuments to the contemporary mind and second that Silbury Hill appears to symbolize a link between the underworld, earth and sky.
Rapid formation and exhumation of the youngest Alpine eclogites: A thermal conundrum to Barrovian metamorphism
No full textEclogite facies metamorphic rocks provide critical information pertaining to the timing of continental collision in zones of plate convergence. Despite being amongst Earth's best studied orogens, little is understood about the rates of Alpine metamorphism within the Eastern Alps. We present LA-MC-ICPMS and ID-TIMS U-Pb ages of metamorphic allanite from the Eclogite Zone, Tauern Window, which when coupled with rare earth element analysis and thermobarometric modelling, demonstrate that the European continental margin was subducted to between 8 and 13 kbar (30-45 km) by 34.2 ± 3.6 Ma. These data define: (i.) an upper limit on the timing of eclogite facies metamorphism at 26.2 ± 1.8 kbar (70-80 km) and 553 ± 12 °C, (ii.) plate velocity (1-6 cm·a-1) exhumation of the Eclogite Zone from mantle to mid-crustal depths, and (iii.) a maximum duration of 10 Ma (28-38 Ma) for juxtaposition of Alpine upper-plate and European basement units and subsequent conductive heating thought to have driven regional Barrovian (re)crystallisation at ca. 30 Ma. One-dimensional thermal modelling of tectonically thickened crust shows that conductive heating is too slow to account for Tauern Barrovian conditions (550 °C at 9-13 kbar) within the maximum 10 Ma interval between eclogite exhumation and the thermal peak (28-32 Ma). Given that the Tauern Window is a classic locality for understanding rates of conductive thermal relaxation in tectonically thickened crust, this work raises questions of fundamental importance concerning the length scales of the mechanisms responsible for heat transfer within orogenic crust
Nonstretch moveout correction of long-offset multichannel seismic data for subbasalt imaging: Example from the North Atlantic
No full textWe have developed a pragmatic new processing strategy to enhance seismic information obtained from long-offset multichannel seismic data. The conventional processing approach, which treats data on a sample-by-sample basis, is applied at a coarser scale on groups of samples. Using this approach, a reflected event and its vicinity remain unstretched during the normal moveout correction. Isomoveout curves (lines of equal moveout) in the time-velocity panel are employed to apply a constant moveout correction to selected individual events, leading to a nonstretch stack. A zigzag stacking-velocity function is introduced as a combination of segments of appropriate isomoveout curves. By employing a zigzag velocity function, stretching of key events is avoided and thus information at far offset is preserved in the stack. The method is also computationally cost-effective. However, the zigzag stacking-velocity field must be consistent with target horizons. This method of horizon-consistent nonstretch moveout has been applied to a wide-angle data set from the North Atlantic margin, providing improved images of the basement interface, which was previously poorly imaged