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61.
Antoine Zazzo Christophe Lécuyer Simon M.F. Sheppard André Mariotti 《Geochimica et cosmochimica acta》2004,68(10):2245-2258
Intra-tooth δ18O variations within the carbonate (δ18Oc) and phosphate (δ18Op) components of tooth apatite were measured for Miocene and Pliocene hypsodont mammals from Afghanistan, Greece and Chad in order to evaluate the resistance of enamel to diagenetic alteration. Application of water-apatite interaction models suggest that the different kinetic behaviours of the phosphate-water and carbonate-water systems can be used to detect subtle oxygen isotope disequilibria in fossil enamel when intra-individual variations are considered. Selective alteration of the oxygen isotope composition from the carbonate component of Afghan and Greek enamels suggests inorganic isotopic exchange processes. Microbially-induced isotopic exchange for phosphate is demonstrated for the first time in enamel samples from Chad, in association with extensive recrystallization. In Chad, δ18Op values were derived from partial isotopic exchange with fossil groundwater during early diagenesis. Mass balance calculations using average carbonate content in enamel as a proxy for recrystallization, and the lowest δ18Op value of dentine as a proxy for the isotopic composition of the diagenetic fluid, indicate that diagenesis can alter δ18Op by as much as 3‰ in some enamel samples. This diagenetic alteration is also responsible for a decrease in intra-individual variations of up to 1‰ in affected specimens. The effects of diagenesis on δ18Op values of fossil enamel are not systematic, however, and can only be estimated if sequential δ18Op and δ18Oc analyses are performed on fossil enamel and dentine. Reconstruction of large temporal- or spatial-scale paleoclimates based on δ18Op analyses from mammalian teeth cannot be considered valid if enamel has been affected by bacterial activity or if the data cannot be corrected for diagenetic effects. 相似文献
62.
Pierre Antoine Albert Marchiol Marcel Brocandel Yves Gros 《Comptes Rendus Geoscience》2005,337(16):444
Since 1990, a dense network of subvertical cracks has been observed at the upper part of the Albian sands of the site of radioactive waste storage operated by ANDRA. New studies carried out in 1999 during enlargement of the site of Soulaines and in 2003 on the nearby TFA site highlighted an especially well-developed network of periglacial features that are mainly of the sand-wedge type (subvertical stratified infill). These observations demonstrate the periglacial origin of these cracks, which are mainly superimposed on a dense network of former tectonic extension joints, and that rapidly disappear at the top of the underlying clays. In addition, these observations currently represent the only example of a large sand-wedge network developed during the Last Glacial in northern France. To cite this article: P. Antoine et al., C. R. Geoscience 337 (2005). 相似文献
63.
The Fe3+/ΣFe ratio of 104 MORB glasses from the Pacific, the Atlantic, the Indian, and the Red Sea spreading centers have been determined using wet chemical Fe2+ analyses and electron microprobe FeOtotal measurements. The data provide a new estimate for the MORB oxygen fugacity (fO2) of 0.41 ± 0.43 (1sigma, N = 100) log units below the fayalite-magnetite-quartz buffer (FMQ), equivalent to a Fe3+/ΣFe = 0.12 ± 0.02 (1sigma, N = 104). This new fO2 estimate is 0.8 log units more oxidized than the average fO2 proposed by Christie et al. (1986) (FMQ-1.20 ± 0.44; Fe3+/ΣFe = 0.07 ± 0.01; N = 87). This slight difference may be related in part to the 3.5% underestimation of the Fe2+ concentration determined by Christie et al. (1986) compared with this study. MORB oxygen fugacity does not display any significant difference between the three main oceanic domains, or between enriched and depleted MORB. Yet, the iron red-ox state ratio shows a broad increase during fractional crystallization. Detailed study of magmatic suites highlights the lack of systematic Fe3+/ΣFe ratio fractionation during differentiation. Despite the large variations of inferred partial melting degrees (from 5 to 20%), the present data set does not provide any evidence of Fe3+/ΣFe relationships with partial melting proxies such as Na8.0.Based on the Fe3+ systematics during partial melting, it is suggested that the oxidation state of MORB reflects a “buffered mantle melting process” resulting in the apparent compatible behavior of Fe3+ during partial melting, and in the relatively constant Fe3+/ΣFe ratio irrespective of the extent of melting. This result implies that partial melting processes may be open relative to oxygen. We propose a model where the Fe3+/ΣFe ratio in the melt is buffered during partial melting. The MORB Fe2O3 systematics can be accounted for by using a fO2 of FMQ-1 that is equivalent to the average fO2 reported for abyssal peridotites. 相似文献
64.
65.
ESR chronology of the Somme River Terrace system and first human settlements in Northern France 总被引:1,自引:0,他引:1
Jean-Jacques Bahain Christophe Falgures Michel Laurent Pierre Voinchet Jean-Michel Dolo Pierre Antoine Alain Tuffreau 《Quaternary Geochronology》2007,2(1-4):356-362
The Somme Valley, Northern France, is famous for its archaeological sequence, where numerous rich Palaeolithic sites, such as Saint-Acheul, the type site of the Acheulian, have been discovered. The archaeological levels are often directly associated with fossil alluvial sediments of the River Somme or with slope deposits, including loess and palaeosols. In the middle reaches of the valley, near Amiens, the system of fossil-stepped fluvial terraces is particularly well developed and preserved, and occurs on 10 alluvial formations. These terraces, from +5 to +55 m above the present-day valley bedrock, allow the study of the environmental changes and the human settlement of this area through the Pleistocene.
Since 1988, ESR dating was systematically applied on bleached quartz extracted from the fossil fluvial deposits, in order to better describe the geological evolution of the stepped system. More recently, U-series/ESR dating has also been performed on teeth collected from the different terrace deposits. Here we present a synthetic review of the main ESR results, and propose an ESR chronology for the geological evolution of the Somme fluvial system and for the Middle Pleistocene human settlement of northern France. 相似文献
66.
67.
IAP/LASG海洋环流模式对风应力的响应 总被引:21,自引:6,他引:15
为了检验第三代IAP/LASG全球海洋环流模式在模拟20世纪80年代热带太平洋年际变化方面特别是ElNino和LaNina事件的能力,作者使用了三种风应力资料强迫该模式.然后,将模拟的海温同NCEP海洋同化资料相比较.分析表明,在赤道附近,模式对风应力的响应在SST分布、温跃层的描绘以及突出E1Nino和LaNina事件方面同NCEP海洋同化资料有许多共同点.然而,在描绘l0°N以北和l0°S以南的SST异常以及描写季节循环方面,响应是不够精确的.另外,模拟的温度异常滞后于NCEP海洋同化数据两个月左右. 相似文献
68.
Antoine Godet Carl Guilmette Loic Labrousse Donald W. Davis Matthijs A. Smit Jamie A. Cutts Marc-Antoine Vanier Isabelle Lafrance Benoit Charette 《Journal of Metamorphic Geology》2020,38(3):235-264
Secular changes in the architecture, thermal state, and metamorphic style of global orogens are thought to have occurred since the Archean; however, despite widespread research, the driving mechanisms for such changes remain unclear. The Paleoproterozoic may prove to be a key era for investigating secular changes in global orogens, as it marks the earliest stage of an eon that saw the onset of modern-style global tectonics. The 2.1 Ga granulite-facies Mistinibi-Raude Domain (MRD), located in the Southeastern Churchill Province, Canada, offers a rare exposure of Paleoproterozoic high metamorphic grade supracrustal sequences (Mistinibi Complex, MC). Rocks from this domain were subjected to petrochronological investigations to establish P–T–t–X evolutions and to provide first order thermal state, burial and exhumation rates, and metamorphic gradients for the transient Paleoproterozoic times. To obtain comprehensive insight into the P–T–t–X evolution of the MRD, we used multi-method geochronology—Lu–Hf on garnet and U–Pb on zircon and monazite—integrated with detailed petrography, trace element chemistry, and phase equilibria modelling. Despite the extensive use of zircon and monazite as geochronometers, their behaviour in anatectic conditions is complex, leading to substantial ambiguity in interpreting the timing of prograde metamorphism. Our results indicate a clockwise metamorphic path involving significant melt extraction from the metasedimentary rocks, followed by cooling from >815°C to ~770°C at ~0.8 GPa. The timing of prograde burial and cooling from supra- to subsolidus conditions is constrained through garnet, monazite, and zircon petrochronology at 2,150–2,120 Ma and at 2,070–2,080 Ma, respectively. These results highlight long-lived residence of the rocks at mid-crustal supra-solidus conditions (55–70 Ma), with preserved prograde and retrograde supra-solidus monazite and zircon. The rocks record extremely slow burial rates (0.25–0.30 km/Ma) along a high metamorphic gradient (900–1,000°C/GPa), which appears symptomatic of Paleoproterozoic orogens. The MC did not record any significant metamorphism after 2,067 Ma, despite having collided with terranes that record high-grade metamorphism during the major 1.9–1.8 Ga Trans-Hudson orogeny. The MC would therefore represent a remnant of a local early Paleoproterozoic metamorphic infrastructure, later preserved as superstructure in the large hot Trans-Hudson orogen. 相似文献
69.
North-West Europe yields few traces of early human occupation, in particular for the Acheulean. In this context, the Somme Valley in northern France offers a route to Britain during various Pleistocene low sea levels, and has provided numerous evidence of Lower Palaeolithic human occupation through fieldwork initiated during the 19th century. These localities are associated with the original definition in the 1930s by the French prehistorian Abbé Henri Breuil of the ‘Abbevillien’ (Abbevillian facies), based on lithic pieces including crudely made bifaces recovered in particular in some famous key localities of Abbeville, Carpentier, Léon and Moulin Quignon quarries. The history of the term and its definition subsequently gave rise to debates concerning the chronocultural framework of Palaeolithic assemblages among the scientific community of prehistorians over time, from Jacques Boucher de Perthes, Gabriel de Mortillet, Geoffroy d'Ault du Mesnil, Victor Commont, Henri Breuil and François Bordes. New investigations at these three localities, all associated with the High Terrace of the Somme system, pushed back the age of the expansion of the Acheulean both in northern France and in Western Europe to c. 670–650 000 years. They imply that early hominins were able to settle in North-West Europe during both climatic temperate and cold phases. Our work, including new excavations and associated field observations of the three Abbeville localities involved at the onset of the controversy, allows a re-examination of the Abbevillian and contributes to the discussion of the history of Prehistoric Science and the Earliest ‘Acheuleans’ in North-West Europe. 相似文献
70.
Le développement d'un projet de recherche consacré à l'étude des nuages convectifs a amené les auteurs à concevoir et à élaborer un système complet de mesures adapté à cette étude. Les grandeurs thermodynamiques classiques (Température, Pression, Humidité) ainsi que les vitesses d'ascendance autour et à l'intérieur des nuages sont mesurées grâce à l'utilisation d'un avion Cessna 206 équipé de ce système. Une première étape consiste à mettre en évidence les phénomènes relatifs aux échanges entre le nuage et le milieu environnant. 相似文献