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MARK D. JOHNSON YLVA STÅHL 《Boreas: An International Journal of Quaternary Research》2010,39(2):199-214
Johnson, M. D. & Ståhl, Y. 2009: Stratigraphy, sedimentology, age and palaeoenvironment of marine varved clay in the Middle Swedish end‐moraine zone. Boreas, 10.1111/j.1502‐3885.2009.00124.x. ISSN 0300‐9483 Deglaciation of the Middle Swedish end‐moraine zone and age of the sediment in and between the moraines have been discussed for about a hundred years. The goal of this project was to determine the stratigraphy and age of the sediment in and between the moraines. Inter‐moraine flats are underlain by clay, 10–25 m thick, overlying thin sand and gravel or till on bedrock. The clay is overlain by a few metres of sand and gravel. Much of the clay beneath the flats consists of rhythmites that grade from grey to red and are 2–74 cm thick. Our interpretation of these rhythmites as being varves is supported by grain size and mineralogical and elemental variations. Foraminifera and ostracods show that the clay was deposited in an arctic marine environment, while radiocarbon dating of the microfossils indicates that the clay was deposited 12 150 cal. 14C years ago, during the Younger Dryas chronozone (YD). Most of the optical stimulated luminescence dates on the clay are much older, containing quartz sand that was insufficiently bleached. The stratigraphy indicates that the moraines are composed of YD clay pushed into ridge forms during ice‐front oscillations. It is not possible to determine how far north the Scandinavian Ice Sheet retreated prior to the YD advance. We neither support nor reject the suggestion that the ice margin retreated to the northern edge of Mt. Billingen during the Allerød, causing the Baltic Ice Lake to drain. 相似文献
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Cornelia MEYER Jörg FRITZ Martin MISGAISKI Dieter STÖFFLER Natalia A. ARTEMIEVA Ulrich HORNEMANN Ralf MOELLER Jean‐Pierre
De VERA Charles COCKELL Gerda HORNECK Sieglinde OTT Elke RABBOW 《Meteoritics & planetary science》2011,46(5):701-718
Abstract– Shock recovery experiments were performed with an explosive set‐up in which three types of microorganisms embedded in various types of host rocks were exposed to strong shock waves with pressure pulse lengths of lower than 0.5 μs: spores of the bacterium Bacillus subtilis, Xanthoria elegans lichens, and cells of the cyanobacterium Chroococcidiopsis sp. 029. In these experiments, three fundamental parameters were systematically varied (1) shock pressures ranging from 5 to 50 GPa, (2) preshock ambient temperature of 293, 233 and 193 K, and (3) the type of host rock, including nonporous igneous rocks (gabbro and dunite as analogs for the Martian shergottites and chassignites, respectively), porous sandstone, rock salt (halite), and a clay‐rich mineral mixture as porous analogs for dry and water‐saturated Martian regolith. The results show that the three parameters have a strong influence on the survival rates of the microorganisms. The most favorable conditions for the impact ejection from Mars for microorganisms would be (1) low porosity host rocks, (2) pressures <10–20 GPa, and (3) low ambient temperature of target rocks during impact. All tested microorganisms were capable of surviving to a certain extent impact ejection in different geological materials under distinct conditions. 相似文献
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LEÓ KRISTJÁNSSON ROBERT A. DUNCAN ÁGÚST GUÐMUNDSSON 《Boreas: An International Journal of Quaternary Research》1998,27(1):1-13
The 'Hreppar Formation' of southern Iceland is a succession mostly composed of basalt lavas and clastic material, with a slight anticlinal structure. It may represent a crustal fragment left in place by eastwards movement of the active spreading zone during the past 2–3 Ma. Evidence of several cycles of alternating glacial-interglacial conditions is seen in this succession. The first systematic stratigraphic study within the succession which is combined with K-Ar age determinations and laboratory palaeomagnetic measurements on oriented cores has yielded encouraging results. The age determinations confirm earlier estimates by Icelandic geologists on the age range spanned by the Hreppar succession. Units from the Jaramillo geomagnetic subchron are present in its upper part, and at least one polarity reversal event in the lower Matuyama chron is also recorded. 相似文献
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Nature, origin and classification of peritidal tepee structures and related breccias 总被引：4，自引：0，他引：4
Distinctive peritidal tepee antiform structures, buckled margins of saucer-like megapolygons are common in marine vadose fenestral and pisolitic limestones and/or dolomites of carbonate platform sequences and occur in intertidal and supratidal carbonates ranging in age from Silurian to Holocene. These megapolygons commonly form and are sometimes truncated before the deposition of the next sedimentary layer. The megapolygons result from the expansion of surface sediments by as much as 15%. The expansion is caused by the following continuously repeated sequence of processes: (1) Desiccation and thermal contraction causing small fractures; (2) phases of wetting causing enlargement of fractures; (3) phases of crystallization of calcium carbonate and other minerals causing the enlargement, fill and cementation of the fractures. Precipitation is from brines and meteoric waters; (4) hydration of minerals, thermal expansion, breaking waves and faulting may add to this disruption. The development of the tepee fabric can be traced from an initially cemented subaerial fenestral crust, exhibiting expansion and compressional structures, to a completely disrupted and brecciated sediment riddled by a labyrinth of fractures and solution cavities. These spaces are filled by numerous phases of internal marine and fresh-water cement and sediment, the latter containing penecontemporaneous or younger marine faunas. Peritidal tepees are useful tools for geologic reconstruction and provide evidence of subaerial exposure; a tropical to subtropical climate; and back-beach or back-barrier deposition. Proper identification of tepees is of economic importance, because they provide good early porosity and permeability for petroleum entrapment and a site for mineralization. Aesthetically, tepee rocks are a fine kaleidoscopic decorative stone. 相似文献
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Subsolidus phase relations have been determined in the systemsSiO2-Cr-0 and MgO-SiO2-Cr-O in equilibrium with metallic Cr,at 1100–1500C and 0–288 kbar. There are no ternaryphases in the SiO2-Cr-O system at these conditions, ie. onlythe assemblage eskolaite + Cr-metal + SiO2 is found. However,in the MgO-containing system, extensive substitution of Cr2+for Mg is observed in (Mg,Cr2+)2SiO4 olivine, (Mg,Cr2+)2Si2O6pyroxene, and (Mg,Cr2+)Cr3+204 spinel. Cr 3+ levels in olivineand pyroxene are below detection limits. The pyroxene is orthorhombicat monoclinic at higher . The structure of the spinels becomestetragonally distorted at is limited by the breakdown of olivine to pyroxene + spinel+ metal. This maximum amount increases strongly with increasingtemperature, reaching >0.2 at 1500C and 48 kbar. Increasing pressure reduces the maximum. Increasing temperature also increases the maximum amounts of Cr2+ which substitute intopyroxene and spinel, indicating that end-member Cr2Si206 andCr3O4 may become stable above 1650C if melting does not intervene.Powder X-ray diffraction analysis of selected runs has beenused to extract molar volumes of the Mg-Cr2+ solid solutionsas a function of composition, which may be extrapolated to predictmolar volumes for Cr2SiO4 (olivine), Cr2Si2O6 (ortho- and clino-pyroxene)and Cr2O4 (cubic spinel) of 477, 680 and 449 cm3, respectively.The experimental data have been fitted to a thermodynamic model,including free energies of formation for end member Cr2SiO4,Cr2Si2O6 and Cr3O4. This model is then used to predict the amountsof Cr2+ which can be expected in olivine in equilibrium withCr-bearing spinel as a function of T, P and fo2. This amountincreases strongly with temperature along standard T-fo2 buffercurves, and is sufficient to explain the observed high Cr contentsof olivine from komatiites and diamond inclusions at reasonableterrestrial fo, values. The lower fo2 of the lunar environmentresults in significant Cr2+ in olivine being stable to muchlower temperatures. The tendency for the oxidation state ofCr, and hence its geochemical properties, to change with temperaturerelative to other redox reactions makes it a potentially usefulmonitor of the temperatures of uppermantle processes, and isa significant factor in the differing styles of igneous differentiationin the Earth and Moon.
Corresponding author 相似文献
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This introductory paper lays the basis for this supplementary issue by briefly presenting the state of knowledge on the KwaZulu-Natal (KZN) Bight at the start of this multi-disciplinary, multi-institutional, ship-based research project that ran from 2009 to 2013. The rationale and aims of the project are also described. The project was a major component of the South African Department of Science and Technology’s African Coelacanth Ecosystem Programme (ACEP), which has been prominent in supporting research on the east coast of South Africa and the wider South-West Indian Ocean. Pivotal to this was the RS Algoa, which was made available for two 30-day surveys (winter and summer) in the KZN Bight by the Department of Environmental Affairs. Although some aspects of the bight ecology are known, much of the research is dated and fragmented, and required refreshing and consolidation in order to produce a platform upon which the understanding of the region’s ecosystem functioning could be established. Much of the oceanographic knowledge is also dated, with no dedicated surveys and significant measurements undertaken since 1989. The overarching theme of the KZN Bight project was to examine the relative importance of sources of nutrients to the central KZN coast and how these are taken up and recycled in the ecosystem, and to describe aspects of the benthic biodiversity, which is poorly described in much of this region. An ambitious project, its accessibility to a ship-based research platform and the diverse scientific skills of the participating scientists allowed considerable success, as reflected in the papers that follow. 相似文献
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Chromium as Cr3+ substitutes for octahedrally coordinated Alin upper-mantle minerals, thereby reducing the activity of Al2O3in the system and hence the concentration of Al2O3 in partialmelts. The effect of Cr2O3 on melt compositions multiply saturatedwith the spinel lherzolite phase assemblage has been quantifiedin the system CaO–MgO–Al2O3–SiO2–Cr2O3at 1·1 GPa as a function of 100 Cr/(Cr + Al) in the spinel(Cr#sp). The decrease of Al2O3 in the melt with increasing Cr#spis accompanied by increasing MgO and SiO2, whereas CaO remainsalmost constant. Consequently, the CaO/Al2O3 ratio of the meltincreases with Cr#sp, and the melt becomes richer in normativediopside, hypersthene and quartz. The effect may explain certainmantle melts with unusually high CaO/Al2O3 ratios. The concentrationof Cr2O3 in the melt remains low even at high Cr#sp, which meansthat the strong effect of Cr2O3 on partial melting equilibriais not readily apparent from its concentration in the melt itself.The existence of a highly refractory major component such asCr2O3 nullifies simplified conclusions from the ‘inverseapproach’ in the experimental study of basalt petrogenesis,as there is insufficient information in the composition of thepartial melt to reconstruct the conditions of melting. KEY WORDS: basalt petrogenesis; partial melting; reversal experiment; spinel lherzolite; system CMAS–Cr2O3; CaO/Al2O3 of melt; effect of Cr2O3 相似文献
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