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Coeval stratigraphic units of similar petrology occur throughout the northern Bahamas islands. The petrographic composition of these limestones provides clues about regional sea level and climate changes during the late Quaternary. At least eight fossil shoreline units, which are linked to transgressive episodes between the middle Pleistocene and the late Holocene, are recognized in the Bahamas. The petrographic composition of these units is either dominated by ooids and peloids or by bioclasts. Sedimentological observations demonstrate that oolitic-peloidal units were formed when sea level was higher than today, whereas skeletal units were deposited at or below modern ordnance datum. Skeletal units may reflect times of partial, or modest platform flooding, when the bulk of sediments brought to islands originates from bank-margin reefs. In contrast, oolitic-peloidal units correspond to major flooding events and active water circulation on the bank top. Cement fabrics further show that the early diagenesis of oolitic units took place during warm and humid climatic conditions, whereas skeletal rock bodies underwent subaerial diagenesis during drier climatic conditions characterized by marked seasonal changes. This example from the Bahamas suggests that compositional analysis of limestone from fossil carbonate platforms could be used for resolving ancient climate and sea-level changes. 相似文献
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GWENAëLE JAN YVES MÉNARD MATHILDE FAILLOT FLORENT LYARD ERIC JEANSOU PASCAL BONNEFOND 《Marine Geodesy》2013,36(3-4):615-629
Absolute calibration of sea level measurements collected from space-borne radar altimeters is usually performed with respect to collocated sea level in situ records from tide gauges or GPS buoys (Ménard et al. 1994; Haines et al. 1996; Bonnefond et al. 2003; Haines et al. 2003; Schum et al. 2003; Watson et al. 2003; Watson et al. 2004). Such a method allows regular and long-term control of altimetric systems with independent records. However, this approach is based on a single, geographically dependent point. In order to obtain more significant and accurate bias and drift estimates, there is a strong interest in multiplying the number of calibration opportunities. This article describes a method, called the “offshore method” that was developed to extend the single-point approach to a wider regional scale. The principle is to compare altimeter and tide gauge sea level data not only at the point of closest approach of an overflying pass, but also at distant points along adjacent satellite passes. However, connecting sea level satellite measurements with more distant in situ data requires a more accurate determination of the geoid and mean ocean dynamic topography slopes, and also of the ocean dynamical changes. In this demonstration experiment, 10 years of averaged TOPEX/Poseidon mean sea level profiles are used to precisely determine the geoid and the mean ocean circulation slope. The Mog2d barotropic ocean model (Carerre et Lyard 2003) is used to improve our estimate of the ocean dynamics term. The method is first validated with Jason-1 data, off Corsica, where the dedicated calibration site of Senetosa provides independent reference data. The method is then applied to TOPEX/Poseidon on its new orbit and to Geosat Follow On. The results demonstrate that it is feasible to make altimeter calibrations a few tens to hundreds of kilometers away from a dedicated site, as long as accurate mean sea level altimeter profiles can be used to ensure the connection with reference tide gauges. 相似文献
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BRUCE HAINES YOAZ BAR-SEVER WILLY BERTIGER SHAILEN DESAI PASCAL WILLIS 《Marine Geodesy》2013,36(1-2):299-318
The U.S./French Jason-1 satellite is carrying a state-of-the-art GPS receiver to support precise orbit determination (POD) requirements. The performance of the Jason-1 “BlackJack” GPS receiver was strongly reflected in early POD results from the mission, enabling radial accuracies of 1–2 cm soon after the satellite's 2001 launch. We have made further advances in the GPS-based POD for Jason-1, most notably in describing the phase center variations of the on-board GPS antenna. We have also adopted new geopotential models from the Gravity Recovery and Climate Experiment (GRACE). The new strategies have enabled us to better exploit the unique contributions of the BlackJack GPS tracking data in the POD process. Results of both internal and external (e.g., laser ranging) comparisons indicate that orbit accuracies of 1 cm (radial RMS) are being achieved for Jason-1 using GPS data alone. 相似文献
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DIETMAR WAGENBACH PASCAL BOHLEBER SUSANNE PREUNKERT 《Geografiska Annaler: Series A, Physical Geography》2012,94(2):245-263
In the European Alps, ice core studies have been mainly performed in view of the recent man‐made influence on the atmospheric load of aerosol‐related species, while respective investigations on the pre‐industrial aerosol or on stable water isotope‐based climate records remained sparse. We address from a glaciological perspective the specific conditions of Alpine drilling sites and, in particular, the role of depositional noise. Thereby, we refer to two major drilling areas (located in the summit range of Monte Rosa and Mt Blanc massif, respectively) which largely differ in their snow accumulation rate and, hence, in their accessible time scale. A simple scheme considering the seasonality of both, the precipitation‐borne signal and the snow erosion‐controlled net accumulation rate is presented. It shows that water isotope trends are generally more sensitive to distortion by a seasonality effect than recent snow impurities trends, although the influence of a given seasonal accumulation rate cycle on the mean levels of water isotopes and impurities is similar. These findings are illustrated on the decadal and centennial time scale by the inter‐ and intra‐site variability of major ion and water isotope records. The intra‐site comparison includes the discussion of strong water isotope depletions seen some meters above bedrock at low accumulation drilling sites. 相似文献
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塔里木板块西南缘托云、奥依塔格晚侏罗世古地磁研究,获得了托云地区特征磁化方向:Ds=1.5°,Is=50.8°;古地磁极:λ=81.0°,ψ=247.0°,古纬度31.5°N。奥依塔格地区特征磁化方向:Ds=118.4°,Is=55.9°;古地磁极:λ=4.9°,ψ=120.6°,古纬度=36.4°N。从古地磁结果认为:①托云和奥依塔格相对托云地区产生顺时针旋转;②这两个地区古纬度为31°～36°左右,与塔里木平均古纬度30°N是基本一致的,但与现在纬度差达8°～10°左右,造成纬度差主要原因是构造运动造成的;③塔里木晚侏罗世古地磁亦与华北、华南基本一致,说明晚侏罗世塔里木、华北、华南三板块已联合成一个整体,拼合成统一的中国大陆,其位于热带—亚热带古气候环境。 相似文献
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PASCAL VENIER 《The Geographical journal》2004,170(4):330-336
This article offers a re-interpretation of the geographical pivot of history by setting it in the context of the international relations and strategic debates of the early twentieth century, and by engaging with the historiography of British foreign policy during the Edwardian period. The overall argument is that the prospective effort of Mackinder does not appear as merely speculative but is in fact rather well grounded in the fundamentals of British foreign policy and more directly relevant than hitherto assumed. Identifying a trend towards a shift in the equilibrium between land power and sea power, Mackinder emphasized – and perhaps over-emphasized – a Russian threat to British interests but considered that Germany alone only posed a minor threat. Such analysis seems to be remarkably in tune with British geopolitical culture around 1904, and with the ideas of the British Prime Minister Arthur Balfour. The way in which Mackinder further stressed that a combination of the two main continental powers would represent a major peril for Britain has hitherto been treated rather dismissively. This article however shows how twice during the Russo-Japanese War of 1904–5 Wilhelm II attempted to bring such an alliance about. This article attempts to reassess the actual impact of the geographical pivot of history and more broadly of Mackinder's thought, usually considered minimal, on British foreign policy before 1914. Finally, a number of issues that seem to deserve further attention are suggested, in particular that of the influence exerted by Mackinder before 1914, and that of the genesis of Mackinder's conception of the 'pivot zone' or 'heartland'. 相似文献
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Fabric characteristics of subaerial slope deposits 总被引：1，自引：0，他引：1
The clast fabrics of certain types of terrestrial slope deposits are reviewed and compared, including the deposits of rockfalls, solifluction, debris flows, dry grain flows, frost-coated clast flows and run-off. The analysis is based on modern deposits in active environments. The study shows that fabric characteristics allow discrimination between ‘collective’and individual movement of rock particles. The individual particle movement generally results in a random clast orientation, whereas the processes of ‘collective’movement typically create distinct preferred orientations. The highest fabric strengths together with low values of spherical variance are found in solifluction deposits. A survey of Pleistocene slope deposits indicates, however, that clast fabric has to be used with caution in the identification of past slope dynamics, because significant post-depositional changes may occur during ageing and burial of deposits. The diagnostic significance of fabric characteristics may also be low due to the overlap of the statistical values that typify different processes. 相似文献
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Properties of debris flow deposits and source materials compared: implications for debris flow characterization 总被引：1，自引：0，他引：1
In the Alps, debris flow deposits generally contain <5% clay‐size particles, and the role of the surface‐charged <2 μm particles is often neglected, although these particles may have a significant impact on the rheological properties of the interstitial fluid. The objective of this study was to compare debris flow deposits and parent materials from two neighbouring catchments of the Swiss Alps, with special emphasis on the colloidal constituents. The catchments are small in area (4 km2), 2·5 km long, similar in morphology, but different in geology. The average slopes are 35–40%. The catchments were monitored for debris flow events and mapped for surface aspect and erosion activity. Debris flow deposits and parent materials were sampled, the clay and silt fractions extracted and the bulk density, <2 mm fraction bulk density, particle size distribution, chemical composition, cation exchange capacity (CEC) and mineralogy analysed. The results show that the deposits are similar to the parent screes in terms of chemical composition, but differ in terms of: (i) particle size distribution; and (ii) mineralogy, reactivity and density of the <2 mm fraction. In this fraction, compared with the parent materials the deposits show dense materials enriched in coarse monocrystalline particles, of which the smallest and more reactive particles were leached. The results suggest that deposit samples should not be considered as representative of source or flow materials, particularly with respect to their physical properties. 相似文献
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Résumé Cet article décrit la façon dont ont été calculés les débits journaliers de l'Amazone à Óbidos. La relation hauteur-débit est univoque en basses et moyennes eaux. En période de hautes eaux la dispersion des jaugeages par rapport à la courbe de tarage univoque amène à corriger les débits en fonction des niveaux à Santarém puis à Parintins. La méthode de la “dénivelée normale” n'étant pas bien adaptée pour Óbidos: elle a été remplacée par une “dénivelée à géométrie variable” fonction de la hauteur d'eau à Óbidos. En utilisant seulement la station de Santarém, l'application de cette nouvelle méthode ramène la dispersion à 3.0% sur la totalité de la courbe de tarage. 相似文献