Controls on iron-isotope fractionation in organic-rich sediments (Kimmeridge Clay, Upper Jurassic, Southern England)     

Alan Matthews  Helen S. Morgans-Bell  Hugh C. Jenkyns  Ludwik Halicz 《Geochimica et cosmochimica acta》2004,68(14):3107-3123

This study explores the fractionation of iron isotopes (⁵⁷Fe/⁵⁴Fe) in an organic-rich mudstone succession, focusing on core and outcrop material sampled from the Upper Jurassic Kimmeridge Clay Formation type locality in south Dorset, UK. The organic-rich environments recorded by the succession provide an excellent setting for an investigation of the mechanisms by which iron isotopes are partitioned among mineral phases during biogeochemical sedimentary processes.Two main types of iron-bearing assemblage are defined in the core material: mudstones with calcite ± pyrite ± siderite mineralogy, and ferroan dolomite (dolostone) bands. A cyclic data distribution is apparent, which reflects variations in isotopic composition from a lower range of δ⁵⁷Fe values associated with the pyrite/siderite mudstone samples to the generally higher values of the adjacent dolostone samples. Most pyrite/siderite mudstones vary between −0.4 and 0.1‰ while dolostones range between −0.1 and 0.5‰, although in very organic-rich shale samples below 360 m core depth higher δ⁵⁷Fe values are noted. Pyrite nodules and pyritized ammonites from the type exposure yield δ⁵⁷Fe values of −0.3 to −0.45‰. A fractionation model consistent with the δ⁵⁷Fe variations relates the lower δ⁵⁷Fe pyrite and siderite ± pyrite mudstones values to the production of isotopically depleted Fe(II) during biogenic reduction of the isotopically heavier lithogenic Fe(III) oxides. A consequence of this reductive dissolution is that a ⁵⁷Fe-enriched iron species must be produced that potentially becomes available for the formation of the higher δ⁵⁷Fe dolostones. An isotopic profile across a dolostone band reveals distinct zonal variations in δ⁵⁷Fe, characterized by two peaks, respectively located above and below the central part of the band, and decoupling of the isotopic composition from the iron content. This form of isotopic zoning is shown to be consistent with a one-dimensional model of diffusional-chromatographic Fe-isotope exchange between dolomite and isotopically enriched pore water. An alternative mechanism envisages the infiltration of dissolved ferrous iron from variable (high and low) δ⁵⁷Fe sources during coprecipitation of Fe(II) ion with dolomite. The study provides clear evidence that iron isotopes are cycled during the formation and diagenesis of organic carbon-rich sediments.  相似文献   

Minority women’s microenterprises in rural areas of the United States of America: African American,Hispanic American and Native American case studies     

Helen Ruth Aspaas 《GeoJournal》2004,61(3):281-289

Women in many rural areas of the US often engage in small-scale businesses as one of several avenues for contributing to household incomes. In those geographical areas that contain a significant minority population, many of the women's businesses display the cultural diversity of the regions in which they live. Likewise, women's roles as economic providers as well as wives and mothers are reflected in the manner in which they operate their businesses. Candida Brush suggests that women's strategies for operating their businesses are highly integrative because women's business decisions are intertwined with familial responsibilities, household economic demands and desires to contribute to their communities. Hispanic and Native American women in the Four Corners region of southwestern US and African American women in central Virginia participated in interviews in which they responded to numerous short answer and open-ended questions about their businesses and decision-making strategies. From these interviews an image emerges of rural minority business women who are operating their businesses at the nexus of family obligations, economic necessities, cultural ties and with a commitment for serving their communities. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Morphology and distribution of fossil soils in the Permo-Pennsylvanian Wichita and Bowie Groups, north-central Texas, USA: implications for western equatorial Pangean palaeoclimate during icehouse–greenhouse transition     

Neil John Tabor  Isabel P. Montañez† 《Sedimentology》2004,51(4):851-884

Analysis of stacked Permo‐Pennsylvanian palaeosols from north‐central Texas documents the influence of palaeolandscape position on pedogenesis in aggradational depositional settings. Palaeosols of the Eastern shelf of the Midland basin exhibit stratigraphic trends in the distribution of soil horizons, structure, rooting density, clay mineralogy and colour that record long‐term changes in soil‐forming conditions driven by both local processes and regional climate. Palaeosols similar to modern histosols, ultisols, vertisols, inceptisols and entisols, all bearing morphological, mineralogical and chemical characteristics consistent with a tropical, humid climate, represent the Late Pennsylvanian suite of palaeosol orders. Palaeosols similar to modern alfisols, vertisols, inceptisols, aridisols and entisols preserve characteristics indicative of a drier and seasonal tropical climate throughout the Lower Permian strata. The changes in palaeosol morphology are interpreted as being a result of an overall climatic trend from relatively humid and tropical, moist conditions characterized by high rainfall in the Late Pennsylvanian to progressively drier, semi‐arid to arid tropical climate characterized by seasonal rainfall in Early Permian time. Based on known Late Palaeozoic palaeogeography and current hypotheses for atmospheric circulation over western equatorial Pangea, the Pennsylvanian palaeosols in this study may be recording a climate that is the result of an orographic control over regional‐scale atmospheric circulation. The trend towards a drier climate interpreted from the Permian palaeosols may be recording the breakdown of this pre‐existing orographic effect and the onset of a monsoonal atmospheric circulation system over this region.  相似文献   

The objectness of everyday life: disburdenment or engagement?     

Neil Maycroft 《Geoforum》2004,35(6):713-725

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The Gondwanan Carboniferous-Permian Boundary Revisited: New Data from Australia and Argentina     

Neil W. Archbold  Gabriela A. Cisterna  Tristan Simanauskas   《Gondwana Research》2004,7(1):125-133

The identification and correlation of the Carboniferous-Permian (Gzhelian-Asselian) boundary within the sedimentary sequences of Gondwana has always been a topic of debate. Type latest Carboniferous and earliest Permian marine sequences are characterised by warm tropical faunas and come from the Uralian Region of Russia and Kazakhstan. Faunas include conodonts and fusulinid foraminiferids which are prime tools for correlation. Such faunal groups are absent from most Gondwanan sequences where reliance for correlations must be placed primarily on brachiopods, bivalve molluscs and palynology. The Western Australian marine sequences, with their contained ammonoids, provide a pivotal link for the dating and correlating of Early Permian Gondwanan sequences with those of the type regions and their palynostratigraphical record is essential for trans-Australian correlations and correlations elsewhere throughout Gondwanaland.New data from the fully cored DM Tangorin DDHl bore hole, drilled in the Cranky Corner Basin, New South Wales, Australia, reveals a sequence of descending faunal zones. The stratigraphically highest zone with Eurydesrna cordaturn, encompasses the Late Sakmarian (Sterlitamakian). The middle zone with Torniopsis elongata, Sulciplica c r a m and Trigonotreta tangorini straddles the Sterlitamakian-Tastubian boundary, with the palynomorphs Pseudoreticulatispora pseudoreticulata high in the zone and Granulatisporites confluens low in the zone. An impoverished fauna with Trigonotreta nov., low in the Granulatisporites confluens Zone, is probably of latest Asselian or Tastubian age.Significant new data from Argentina has revealed marine faunas from below the occurrence of Granulatisporites confluens. These are considered to be of Asselian age. Outcrops of the Tupe Formation, with a marine fauna, at La Herradura Creek in the western Paganzo Basin, San Juan Province, are best regarded as being of mid to late Asselian age. The Tupe Fauna has been recognised as the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone. Previously, this fauna was considered to be of Late Carboniferous or Stephanian age. Three faunal associations are known from the Rio del Peii6n Formation, Rio Blanco Basin, La Rioja Province. The middle assemblage with Tivertonia, Costaturnulus, Kochiproductus and Trigonotreta, appears to correlate well with the Tupe Formation fauna. The lower assemblage, with Streptorhynchus, Etherilosia, Costaturnulus, Trigonotreta and a punctate spiriferid, as well as indeterminate productids, probably of Early Asselian age. The youngest assemblage includes a species ofRhynchopora that is close to Rhynchopora australasica from the latest Asselian-early Tastubian of Western Australia. The marine biostratigraphical data from Argentina has enabled a much greater understanding of the earliest Permian marine faunas to be achieved - a story that is apparently absent from the other cold and cool temperate regions of Gondwana.  相似文献   

London MIST 2004     

Neil Arnold 《Astronomy& Geophysics》2005,46(3):3.36-3.37

The annual one-day meeting of the MIST (Magnetosphere, Ionosphere and Solar–Terrestrial) community was held in November 2004 at Burlington House, with contributions on data and ideas ranging from Cassini to Cluster, with stops at many experiments in between.  相似文献   

The impact of epistemic uncertainty on an earthquake loss model     

Helen Crowley  Julian J. Bommer  Rui Pinho  Juliet Bird 《地震工程与结构动力学》2005,34(14):1653-1685

Models capable of estimating losses in future earthquakes are of fundamental importance for emergency planners, for the insurance and reinsurance industries, and for code drafters. Constructing a loss model for a city, region or country involves compiling databases of earthquake activity, ground conditions, attenuation equations, building stock and infrastructure exposure, and vulnerability characteristics of the exposed inventory, all of which have large associated uncertainties. Many of these uncertainties can be classified as epistemic, implying—at least in theory—that they can be reduced by acquiring additional data or improved understanding of the physical processes. The effort and cost involved in refining the definition of each component of a loss model can be very large, for which reason it is useful to identify the relative impact on the calculated losses due to variations in these components. A mechanically sound displacement‐based approach to loss estimation is applied to a test case of buildings along the northern side of the Sea of Marmara in Turkey. Systematic variations of the parameters defining the demand (ground motion) and the capacity (vulnerability) are used to identify the relative impacts on the resulting losses, from which it is found that the influence of the epistemic uncertainty in the capacity is larger than that of the demand for a single earthquake scenario. Thus, the importance of earthquake loss models which allow the capacity parameters to be customized to the study area under consideration is highlighted. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Sediment Distribution Around Glacially Abraded Bedrock Landforms (Whalebacks) at Lago Tranquilo, Chile     

Neil F. Glasser  Stephan Harrison 《Geografiska Annaler: Series A, Physical Geography》2005,87(3):421-430

Whalebacks are convex landforms created by the smoothing of bedrock by glacial processes. Their formation is attributed to glacial abrasion either by bodies of subglacial sediment sliding over bedrock or by individual clasts contained within ice. This paper reports field measurements of sediment depth around two whaleback landforms in order to investigate the relationship between glacigenic deposits and whaleback formation. The study site, at Lago Tranquilo in Chilean Patagonia, is situated within the Last Glacial Maximum (LGM) ice limits. The two whalebacks are separated by intervening depressions in which sediment depths are generally 0.2 to 0.3 m. Two facies occur on and around the whalebacks. These facies are: (1) angular gravel found only on the surface of the whalebacks, interpreted as bedrock fracturing in response to unloading of the rock following pressure release after ice recession, and (2) sandy boulder‐gravel in the sediment‐filled depressions between the two whalebacks, interpreted as an ice‐marginal deposit, with a mixture of sediment types including basal glacial and glaciofluvial sediment. Since the whalebacks have heavily abraded and striated surfaces but are surrounded by only a patchy and discontinuous layer of sediment, the implication is that surface abrasion of the whalebacks was achieved primarily by clasts entrained in basal ice, not by subglacial till sliding.  相似文献   

Employment mobility in high-technology agglomerations: the cases of Oxfordshire and Cambridgeshire   总被引：1，自引：0，他引：1  

Helen Lawton Smith  Rupert Waters 《Area》2005,37(2):189-198

This paper examines labour market behaviour of the highly skilled in high-tech local economies, taking the UK examples of Oxfordshire and Cambridgeshire as case studies. It reports on data from a survey of members of three scientific institutes to compare rates of employee mobility in the two locations and considers the likely explanations and implications of those patterns.  相似文献   

Uncertainty in adaptive capacity     

W. Neil Adger  Katharine Vincent 《Comptes Rendus Geoscience》2005,337(4):399-410

The capacity to adapt is a critical element of the process of adaptation: it is the vector of resources that represent the asset base from which adaptation actions can be made. Adaptive capacity can in theory be identified and measured at various scales, from the individual to the nation. The assessment of uncertainty within such measures comes from the contested knowledge domain and theories surrounding the nature of the determinants of adaptive capacity and the human action of adaptation. While generic adaptive capacity at the national level, for example, is often postulated as being dependent on health, governance and political rights, and literacy, and economic well-being, the determinants of these variables at national levels are not widely understood. We outline the nature of this uncertainty for the major elements of adaptive capacity and illustrate these issues with the example of a social vulnerability index for countries in Africa. To cite this article: W.N. Adger, K. Vincent, C. R. Geoscience 337 (2005).  相似文献