U-Th chronology and paleoceanographic record in a Fe-Mn crust from the NE Atlantic over the last 700 ka   总被引：1，自引：0，他引：1  

Christelle Claude  Guenter Suhr  Albrecht W. Hofmann 《Geochimica et cosmochimica acta》2005,69(20):4845-4854

Hydrogenetic ferromanganese crusts (Fe-Mn crusts) provide a secular record of the variations of seawater composition responding to changes in ocean circulation and erosion processes. In this respect, the acquisition of an absolute and reliable chronology in Fe-Mn crusts is a prerequisite. Here we combine four different and complementary chronometers (¹⁰Be, ²³⁰Th_ex, ²³⁰Th_ex/²³²Th, ²³⁴U/²³⁸U) in a Fe-Mn crust dredged at ∼2000 m depth in the east Atlantic to first establish a reliable chronology over the Quaternary period. Then, we use EDS chemical analysis to look for correlation between major element chemistry and climate changes. (²³⁰Th_ex), (²³⁰Th_ex/²³²Th), and Be data give very consistent growth rates. In particular, the good match between (²³⁰Th_ex) and (²³⁰Th_ex/²³²Th) data indicates that at the location of crust 121DK, ²³⁰Th and ²³²Th fluxes in the water column change simultaneously and suggests that the normalization of ²³⁰Th_ex to ²³²Th makes (²³⁰Th_ex/²³²Th) a better chronometer. Our best-fit model suggests that crust 121DK experienced changes in growth rates at ∼122 and 312 ka and a growth with a constant ²³⁰Th initial flux. This chronology returns an age of 680 ka for the uppermost 1.5 mm. The (²³⁴U/²³⁸U) depth profile, however, was clearly affected by diffusion of ²³⁴U in the porous crust and can therefore not be used to derive a reliable chronology. One part of the crust seems isolated from pore water diffusion and can be physically recognized as a zone of very small porosity. On the basis of the (²³⁰Th_ex/²³²Th) chronology, major element chemistry is shown to be linked to climate change. Mn/Fe variations compare well with those in a Fe-Mn crust from the Pacific, showing systematic maxima during glacial stages 2 and 4. High Mn/Fe are tentatively interpreted to reflect expansion of the oxygen minimum zone during glacial periods, resulting from higher bioproductivity. In addition we note that the surface (²³⁰Th/²³²Th) activity ratio of crust 121DK is entirely consistent with advection of deep water from the western toward the eastern Atlantic basin.  相似文献   

Experimental sorption of Ni, Cs and Ln onto a montmorillonite up to 150°C     

Emmanuel Tertre  Gilles Berger  Michel Loubet 《Geochimica et cosmochimica acta》2005,69(21):4937-4948

The effect of temperature on the sorption of cations onto a dioctahedral smectite was investigated by running batch experiments at 25, 40, 80 and 150°C. We measured the distribution coefficient (Kd) of Cs⁺, Ni²⁺ and 14 lanthanides (Ln³⁺) between solutions and the montmorillonite fraction of the MX80 bentonite at various pH and ionic strengths. Up to 80°C we used a conventional experimental protocol derived from Coppin et al. (2002). At 150°C, the experiments were conducted in a PTFE reactor equipped with an internal filter allowing the sampling of clear aliquots of solution.The results show a weak but measurable influence of the temperature on the elements sorption. Kd’s for Ni²⁺ and Ln³⁺ increase by a factor 2 to 5 whereas temperature raises from 25 to 150°C. This effect seems higher at high ionic strength. The estimated apparent endothermic sorption enthalpies are 33 ± 10 kJ.mol⁻¹ and 39 ± 15 kJ.mol⁻¹ for Ni²⁺ and Eu³⁺, respectively. On the other hand, the temperature effect on Cs⁺ sorption is only evidenced at low ionic strength and under neutral conditions where the Kd decreases by a factor 3 between 25 and 150°C. Apparent exothermic sorption enthalpy for Cs⁺ on the montmorillonite is −19 ± 5 kJ.mol⁻¹.Experiments conducted at the four temperatures with the coexistence of all of the cations in the reacting solution (100 ppb of each element in the starting solution) or only one of them, produced similar values of Kd. This suggests the absence of competition between the sorbed cations, and consequently a low degree of saturation of the available sites. A fractionation of the lanthanides spectrum is also observed at high pH and high ionic strength whatever the temperature.The conclusion of this study is that the temperature dependence on sorption reflects, as the fractionation of REE or the pH and ionic strength effects, the chemical process which controls the overall reaction. In the case of an exchange dominated reaction (low pH and low ionic strength), the temperature effect is negligible. In the case of surface complexation (high pH and high ionic strength), the observed increase of Kd with temperature reflects either an increase of the sorption equilibrium constant with temperature or an endothermic property for reactions describing the montmorillonite surface chemistry.  相似文献   

Apport de la modélisation géophysique pour la compréhension de la structuration du pas de Calais     

Bruno Minguely  Jean-Louis Mansy  Michel Everaerts  Geoffrey M. Manby  Olivier Averbuch 《Comptes Rendus Geoscience》2005,337(3):305-313

This study is based on an analysis of seismic, gravimetric and magnetic data to model the structure of the Dover Straits area. The Dover Straits are set in a transitional position within the Brabant para-autochthon, which lies between the Ardennes allochthon to the south and the Brabant Massif to the north. The seismic profiles show that this area is characterised by several superimposed Mesozoic basins, whose evolution has been controlled by the reactivation of Lower Palaeozoic faults. In common with the adjacent Weald–Wessex and southern North Sea basins, the Dover Straits area records a Jurassic extensional phase and a post-Cretaceous tectonic inversion. The geophysical modelling reveals the presence of a basement to these Mesozoic basins that exhibits Ordovician to Silurian structures, which cannot be detected on the seismic reflection profiles. In contrast to earlier interpretations, the modelling presented here suggests that the Brabant Massif continues southward beyond the Dover Straits and under the Brabant para-autochthon. Gravity data also provides evidence for the presence of granitic body at depth, in the vicinity of Km 27 on the seismic profile, which may have driven the uplift of the Silurian rocks in the area of the Straits. Similar granitic intrusions have been proposed to explain gravity anomalies along the Rhenohercynian Margin in Belgium, eastern and southern England. The geophysical modelling demonstrates the continuity of the basement and Mesozoic basin structures across the Dover Straits, suggesting a common geological evolution of the adjacent southern England–northern France regions. To cite this article: B. Minguely et al., C. R. Geoscience 337 (2005).  相似文献   

Sm–Nd isotopic compositions as a proxy for magmatic processes during the Neoproterozoic of the southern Brazilian shield     

Maria do Carmo P. Gastal  Jean Michel Lafon  Lo Afraneo Hartmann  Edinei Koester 《Journal of South American Earth Sciences》2005,18(3-4):255-276

Combined analyses of Nd isotopes from a wide range of Neoarchaean–Cretaceous igneous rocks provides a proxy to study magmatic processes and the evolution of the lithosphere. The main igneous associations include the Neoproterozoic granitoids from the southern Brazilian shield, which were formed during two tectonothermal events of the Brasiliano cycle: the São Gabriel accretionary orogeny (900–700 Ma) and the Dom Feliciano collisional orogeny (660–550 Ma). Rocks related to the formation of the São Gabriel arc (900–700 Ma) mainly have a depleted juvenile signature. For the Neoproterozoic collisional event, the petrogenetic discussion focuses on two old crustal segments and three types of mantle components. However, no depleted juvenile material was involved in the formation of the Dom Feliciano collisional belt (800–550 Ma), which implies an ensialic environment for the Dom Feliciano orogeny. In the western Neoproterozoic foreland, records of a Neoarchaean lower crust predominate, whereas a Paleoproterozoic crust does in the eastern Dom Feliciano belt. The western foreland includes two amalgamated geotectonic domains, the São Gabriel arc and Taquarembó block. In the collisional belt, the old crust was intensely reworked during the São Gabriel event. In addition to the Neoproterozoic subduction-processed subcontinental lithosphere (São Gariel arc), we recognize two old enriched mantle components, which also are identified in the Paleoproterozoic intraplate tholeiites from Uruguay and the Cretaceous potassic suites from eastern Paraguay. One end member displays the prominent influence of Trans-Amazonian (2.3–2.0 Ga) or older subduction events, whereas the other can be interpreted as a reenrichment of the first during the latest Trans-Amazonian collisional or younger events. This reenriched mantle is documented in late Neoproterozoic suites from the western foreland (605–550 Ma) and younger suites from the eastern collisional belt (600–580 Ma). The other enriched mantle component with an old subduction signature, however, appears only in older rocks of the collisional belt (800–600 Ma). The participation of the subduction-related Brasiliano mantle as an end member of binary mixing occurred in some early Neoproterozoic suites (605–580 Ma) from the western foreland, but the contribution of the Neoarchaean lower crust increased near the late igneous event (575–550 Ma).  相似文献   

Multiple origin and regional significance of bedding parallel veins in a fold and thrust belt: The example of a carbonate slice along the Appalachian structural front     

Stephan Sjourn  Michel Malo  Martine M. Savard  Donna Kirkwood 《Tectonophysics》2005,407(3-4):189-209

The development and evolution of bedding parallel veins (BPV) are investigated in the Saint-Dominique carbonate slice (southern Québec Appalachian structural front), in order to emphasize the regional significance of BPV in a fold and thrust belt and their importance in establishing its structural and diagenetic evolution. Structural and microstructural analyses reveal that most BPV display a laminated structure locally crosscut by a massive one. Both structures show mutual crosscutting relationships with bedding parallel or bedding perpendicular stylolites and with bedding perpendicular veinlets, indicating that BPV cementation and deformation are continuous processes. Opening both sub-parallel and sub-perpendicular to the BPV walls are documented. Early BPV are planes of weakness that focus later reactivation, and evidence of successive and sometimes opposite senses of opening are locally preserved within a single BPV. The bedding parallel orientation of these veins proved particularly helpful in establishing the structural evolution of an area mostly characterized by front parallel structures with little crosscutting relationships otherwise. Petrographic and geochemical (δ¹⁸O _VPDB and δ¹³C) analyses of structural cements suggest a common, locally derived source for the fluids that percolated through BPV, non-bedding parallel veins and faults. The isotopic ratios also reveal a significant enrichment in ¹⁸O and ¹³C through time. This evolution is tentatively correlated with tectonic, syn-convergence exhumation of the Saint-Dominique slice during its imbrication along the Appalachian structural front, after its burial under Taconian thrust sheets.  相似文献   

Discrete wavenumber solutions to numerical wave propagation in piecewise heterogeneous media – I. Theory of two-dimensional SH case     

Li-Yun Fu  Michel Bouchon 《Geophysical Journal International》2004,157(2):481-498

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The Kaiserslautern multiscale geopotential model SWITCH-03 from orbit perturbations of the satellite CHAMP and its comparison to the models EGM96, UCPH2002_02_0.5, EIGEN-1s and EIGEN-2   总被引：2，自引：0，他引：2  

M. J. Fengler  W. Freeden  V. Michel 《Geophysical Journal International》2004,157(2):499-514

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垄行作物玉米方向亮温野外测量中视场角影响的简单分析   总被引：3，自引：4，他引：3  

余涛  顾行发  田国良  Michel Legran  Jean-Francois Hanocq  Roland Bosseno 《遥感学报》2004,8(5):443-450

基于透视原理、地面试验中对于较高目标的观测存在着一定的偏差。这种偏差随传感器高度、观测角度、视场角大小、观测位置等多个因素改变。由于垄行作物空间结构和温度分布的复杂性 ,在采用较大视场角测量方向亮温的地面实验中 ,将不可避免地存在着误差。采用一个简化的三分量二维结构模型对这种误差进行初步的分析与估算。亮温三分量分别为植被、被阳光照到的亮土和植被阴影下的暗土。作物的结构简化为剖面为矩形的无限长平行体。通过对这三个分量在传感器视场中面积权重的计算来模拟目标结构、传感器高度、位置、视场角大小、观测角度等因素对测量结果产生的影响。模拟结果表明 ,在垂直观测中 ,视场中的植被权重往往被高估 ,偏差随传感器高度的降低急剧增加。在倾斜观测中 ,由于一种互补效应的产生 ,偏差被限制在一个较低的范围内。经过分析 ,减小误差的最有效办法是提高传感器高度。最后 ,实验数据与模拟结果进行了比较。恰当地选取模型输入 ,两种数据能非常好的吻合。  相似文献   

Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana)   总被引：2，自引：0，他引：2  

Ccile Gautheron  Pierre Cartigny  Manuel Moreira  Jeff. W. Harris  Claude J. Allgre 《Earth and Planetary Science Letters》2005,240(3-4):559-572

In an attempt to constrain the origin of polycrystalline diamond, combined analyses of rare gases and carbon and nitrogen isotopes were performed on six such diamonds from Orapa (Botswana). Helium shows radiogenic isotopic ratios of R/Ra = 0.14–1.29, while the neon ratios (²¹Ne/²²Ne of up to 0.0534) reflect a component from mantle, nucleogenic and atmospheric sources. ⁴⁰Ar/³⁶Ar ratios of between 477 and 6056 are consistent with this interpretation. The (¹²⁹Xe/¹³⁰Xe) isotopic ratios range between 6.54 and 6.91 and the lower values indicate an atmospheric component. The He, Ne, Ar and Xe isotopic compositions and the Xe isotopic pattern are clear evidence for a mantle component rather than a crustal one in the source of the polycrystalline diamonds from Orapa. The δ¹³C and δ¹⁵N isotopic values of − 1.04 to − 9.79‰ and + 4.5 to + 15.5‰ respectively, lie within the range of values obtained from the monocrystalline diamonds at that mine. Additionally, this work reveals that polycrystalline diamonds may not be the most appropriate samples to study if the aim is to consider the compositional evolution of rare gases through time. Our data shows that after crystallization, the polycrystalline diamonds undergo both gas loss (that is more significant for the lighter rare gases such as He and Ne) and secondary processes (such as radiogenic, nucleogenic and fissiogenic, as well as atmospheric contamination). Finally, if polycrystalline diamonds sampled an old mantle (1–3.2 Ga), the determined Xe isotopic signatures, which are similar to present MORB mantle – no fissiogenic Xe from fission of ²³⁸U being detectable – imply either that Xe isotopic ratios have not evolved within the convective mantle since diamond crystallization, or that these diamonds are actually much younger.  相似文献   

Ubinas: the evolution of the historically most active volcano in southern Peru     

Jean-Claude Thouret  Marco Rivera  Gerhard Wörner  Marie-Christine Gerbe  Anthony Finizola  Michel Fornari  Katherine Gonzales 《Bulletin of Volcanology》2005,67(6):557-589

Ubinas volcano has had 23 degassing and ashfall episodes since A.D. 1550, making it the historically most active volcano in southern Peru. Based on fieldwork, on interpretation of aerial photographs and satellite images, and on radiometric ages, the eruptive history of Ubinas is divided into two major periods. Ubinas I (Middle Pleistocene >376 ka) is characterized by lava flow activity that formed the lower part of the edifice. This edifice collapsed and resulted in a debris-avalanche deposit distributed as far as 12 km downstream the Rio Ubinas. Non-welded ignimbrites were erupted subsequently and ponded to a thickness of 150 m as far as 7 km south of the summit. These eruptions probably left a small collapse caldera on the summit of Ubinas I. A 100-m-thick sequence of ash-and-pumice flow deposits followed, filling paleo-valleys 6 km from the summit. Ubinas II, 376 ky to present comprises several stages. The summit cone was built by andesite and dacite flows between 376 and 142 ky. A series of domes grew on the southern flank and the largest one was dated at 250 ky; block-and-ash flow deposits from these domes filled the upper Rio Ubinas valley 10 km to the south. The summit caldera was formed between 25 and 9.7 ky. Ash-flow deposits and two Plinian deposits reflect explosive eruptions of more differentiated magmas. A debris-avalanche deposit (about 1.2 km³) formed hummocks at the base of the 1,000-m-high, fractured and unstable south flank before 3.6 ka. Countless explosive events took place inside the summit caldera during the last 9.7 ky. The last Plinian eruption, dated A.D.1000–1160, produced an andesitic pumice-fall deposit, which achieved a thickness of 25 cm 40 km SE of the summit. Minor eruptions since then show phreatomagmatic characteristics and a wide range in composition (mafic to rhyolitic): the events reported since A.D. 1550 include many degassing episodes, four moderate (VEI 2–3) eruptions, and one VEI 3 eruption in A.D. 1667. Ubinas erupted high-K, calc-alkaline magmas (SiO₂=56 to 71%). Magmatic processes include fractional crystallization and mixing of deeply derived mafic andesites in a shallow magma chamber. Parent magmas have been relatively homogeneous through time but reflect variable conditions of deep-crustal assimilation, as shown in the large variations in Sr/Y and LREE/HREE. Depleted HREE and Y values in some lavas, mostly late mafic rocks, suggest contamination of magmas near the base of the >60-km-thick continental crust. The most recently erupted products (mostly scoria) show a wide range in composition and a trend towards more mafic magmas.Recent eruptions indicate that Ubinas poses a severe threat to at least 5,000 people living in the valley of the Rio Ubinas, and within a 15-km radius of the summit. The threat includes thick tephra falls, phreatomagmatic ejecta, failure of the unstable south flank with subsequent debris avalanches, rain-triggered lahars, and pyroclastic flows. Should Plinian eruptions of the size of the Holocene events recur at Ubinas, tephra fall would affect about one million people living in the Arequipa area 60 km west of the summit.Editorial responsibility: D Dingwell  相似文献