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621.
Michel Magny Boris Vannière Camilla Calo Laurent Millet Aurélie Leroux Odile Peyron Gianni Zanchetta Tommaso La Mantia Willy Tinner 《Quaternary Science Reviews》2011,30(19-20):2459-2475
This paper presents a high-resolution lake-level record for the Holocene at Lago Preola (Sicily, southern Italy) based on a specific sedimentological approach, with a chronology derived from AMS radiocarbon dates. It gives evidence of three major successive palaeohydrological periods, with (1) a pronounced dryness during the early Holocene until ca 10300 cal BP, (2) a highstand from ca 10300 to 4500 cal BP, and (3) a marked lowstand from 4500 cal BP to present. Large amplitude lake-level fluctuations characterise two transition phases at ca 10300–9000 and 6400–4500 cal BP. Period 2 was interrupted between 8300 and 7000 cal BP by a dry phase that was punctuated to ca 7300 cal BP by the deposition of a tephra from neighbouring Pantelleria Island. Comparisons of the Preola record with other palaeohydrological records along north–south and west–east transects in the Mediterranean show contrasting patterns of hydrological changes: north (south) of around 40°N latitude, the records highlight a mid-Holocene period characterised by lake-level minima (maxima). Humid mid-Holocene conditions over the Mediterranean south of 40°N were probably linked to a strong weakening of the Hadley cell circulation and of monsoon winds. We suggest that the maximum of humidity in the Mediterranean during the mid-Holocene was characterised by humid winters to the north of 40°N and humid summers to the south. On a multi-centennial scale, the high-resolution palaeohydrological reconstructions in the central Mediterranean area reveal a strong climate reversal around 4500–4000 cal BP, with contrasting changes in the hydrological cycle. In addition to seasonal and inter-hemispherical changes related to orbital forcing, this major oscillation might be related to non-linear responses of the climatic system to the gradual decrease in summer insolation at northern latitudes. Another major climate oscillation around 7500–7000 cal BP may have resulted from the combined effects of (1) a strong rate of change in insolation, and (2) variations in solar activity. Finally, comparisons of the Preola lake-level record with Sicilian pollen records suggest a strong influence of moisture availability on vegetation development in Sicily. Very dry early Holocene conditions probably prevented the expansion of coastal evergreen forests, while decreasing moisture availability since the onset of the late Holocene may have exacerbated effects of intensive land-use. 相似文献
622.
Michel Magny Sébastien Joannin Didier Galop Boris Vannière Jean Nicolas Haas Michele Bassetti Paolo Bellintani Romana Scandolari Marc Desmet 《Quaternary Research》2012,77(3):382-396
A lake-level record of Lake Ledro (northern Italy) spans the entire Holocene with a chronology derived from 51 radiocarbon dates. It is based on a specific sedimentological approach that combines data from five sediment profiles sampled in distinct locations in the littoral zone. On a millennial scale, the lake-level record shows two successive periods from 11,700 to 4500 cal yr BP and from 4500 cal yr BP to the present, characterized by lower and higher average lake levels, respectively. In addition to key seasonal and inter-hemispherical changes in insolation, the major hydrological change around 4500 cal yr BP may be related to a non-linear response of the climate system to orbitally-driven gradual decrease in insolation. The Ledro record questions the notion of an accentuated summer rain regime in the northern Mediterranean borderlands during the boreal insolation maximum. Moreover, the Ledro record highlights that the Holocene was punctuated by successive centennial-scale highstands. Correlations with the Preboreal oscillation and the 8.2 ka event, and comparison with the atmospheric 14C residual record, suggest that short-lived lake-level fluctuations developed at Ledro in response to (1) final steps of the deglaciation in the North Atlantic area and (2) variations in solar activity. 相似文献
623.
Giovanni Zanchetta Carlo Giraudi Roberto Sulpizio Michel Magny Russell N. Drysdale Laura Sadori 《Quaternary Research》2012,78(2):236-247
A study of six tephra layers discovered in different deposits between 1600 and 2700 m a.s.l. in the Apennine chain in central Italy allowed precise stratigraphic constraints on environmental and climatic changes between ca. 4.5 and 3.8 cal ka BP. Chemical analyses allowed the correlation of these tephra layers with the eruptions of Agnano Mt Spina (AMST) from Phlegrean Field and Avellino (AVT) from Somma–Vesuvius. Major environmental changes in the high mountains of the Central Apennines occurred just after the deposition of the AMST and predate the deposition of the AVT. At this time, renewed growth of the Calderone Glacier occurred, marking the onset of the Apennine “Neoglacial”. The presence of the AMST and AVT enabled us to make a precise, physical correlation with other archives in central Italy. Synchronization of records between sites showed that the period intervening the deposition of the AMST and AVT layers coincided with environmental changes that were not always exactly in phase. This highlights the fact that stratigraphic correlations using only radiocarbon chronologies (the most common method used for dating archives during the Holocene) could produce erroneous correlation of events, giving rise to oversimplified paleoclimatic reconstructions. 相似文献
624.
Flavien Choulet Michel Faure Olivier Fabbri Patrick Monié 《International Journal of Earth Sciences》2012,101(2):393-413
The ENE–WSW Autun Shear Zone in the northeastern part of the French Massif Central has been interpreted previously as a dextral
wrench fault. New field observations and microstructural analyses document a NE–SW stretching lineation that indicates normal
dextral motions along this shear zone. Further east, similar structures are observed along the La Serre Shear Zone. In both
areas, a strain gradient from leucogranites with a weak preferred orientation to highly sheared mylonites supports a continuous
Autun–La Serre fault system. Microstructural observations, and shape and lattice-preferred orientation document high-temperature
deformation and magmatic fabrics in the Autun and La Serre granites, whereas low- to intermediate-temperature fabrics characterize
the mylonitic granite. Electron microprobe monazite geochronology of the Autun and La Serre granites yields a ca. 320 Ma age
for pluton emplacement, while mica 40Ar-39Ar datings of the Autun granite yield plateau ages from 305 to 300 Ma. The ca. 300 Ma 40Ar-39Ar ages, obtained on micas from Autun and La Serre mylonites, indicate the time of the mylonitization. The ca. 15-Ma time
gap between pluton emplacement and deformation along the Autun–La Serre fault system argue against a synkinematic pluton emplacement
during late orogenic to postorogenic extension of the Variscan Belt. A ductile to brittle continuum of deformation is observed
along the shear zone, with Lower Permian brittle faults controlling the development of sedimentary basins. These results suggest
a two-stage Late Carboniferous extension in the northeastern French Massif Central, with regional crustal melting and emplacement
of the Autun and La Serre leucogranites around 320 Ma, followed, at 305–295 Ma, by ductile shearing, normal brittle faulting,
and subsequent exhumation along the Autun–La Serre transtensional fault system. 相似文献
625.
The geochemical composition of the terrestrial surface (without soils) and comparison with the upper continental crust 总被引:1,自引:0,他引:1
Jens Hartmann Hans H. Dürr Nils Moosdorf Michel Meybeck Stephan Kempe 《International Journal of Earth Sciences》2012,101(1):365-376
The terrestrial surface, the “skin of the earth”, is an important interface for global (geochemical) material fluxes between
major reservoirs of the Earth system: continental and oceanic crust, ocean and atmosphere. Because of a lack in knowledge
of the geochemical composition of the terrestrial surface, it is not well understood how the geochemical evolution of the
Earth’s crust is impacted by its properties. Therefore, here a first estimate of the geochemical composition of the terrestrial
surface is provided, which can be used for further analysis. The geochemical average compositions of distinct lithological
classes are calculated based on a literature review and applied to a global lithological map. Comparison with the bulk composition
of the upper continental crust shows that the geochemical composition of the terrestrial surface (below the soil horizons)
is significantly different from the assumed average of the upper continental crust. Specifically, the elements Ca, S, C, Cl
and Mg are enriched at the terrestrial surface, while Na is depleted (and probably K). Analysis of these results provide further
evidence that chemical weathering, chemical alteration of minerals in marine settings, biogeochemical processes (e.g. sulphate
reduction in sediments and biomineralization) and evaporite deposition are important for the geochemical composition of the
terrestrial surface on geological time scales. The movement of significant amounts of carbonate to the terrestrial surface
is identified as the major process for observed Ca-differences. Because abrupt and significant changes of the carbonate abundance
on the terrestrial surface are likely influencing CO2-consumption rates by chemical weathering on geological time scales and thus the carbon cycle, refined, spatially resolved
analysis is suggested. This should include the recognition of the geochemical composition of the shelf areas, now being below
sea level. 相似文献
626.
Yang Chu Michel Faure Wei Lin Qingchen Wang Wenbin Ji 《International Journal of Earth Sciences》2012,101(8):2125-2150
In orogenic belts, a basal décollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover. In the Early Mesozoic intracontinental Xuefengshan Belt of South China, such a décollement zone is exposed in the core of anticlines formed by the emplacement of the late-orogenic granitic plutons. Our detailed, multi-scale structural analysis documents a synmetamorphic ductile deformation. In the basal décollement, the Neoproterozoic pelite and sandstone, and the intruding Early Paleozoic granites were deformed and metamorphosed into mylonites and orthogneiss, respectively. The metamorphic foliation contains a NW–SE stretching lineation associated with top-to-the-NW kinematic indicators. The ductile shearing of these high-strained rocks can be correlated with NW-verging folds and thrusts recognized in the Neoproterozoic to Early Triassic sedimentary cover. Monazite U–Th–Pbtot chemical dating, and zircon SIMS U–Pb dating provide age constraints of the ductile shearing between 243 and 226?Ma, and late-orogenic granite emplacement around 235–215?Ma. In agreement with recent geochronological data, these new results show that the Xuefengshan Belt is an Early Mesozoic orogen dominated by the NW-directed shearing and thrusting. At the southeastern boundary of the Xuefengshan Belt, the Chenzhou-Linwu fault separates the Early Mesozoic domain to the NW from the Early Paleozoic domain to the SE. The tectonic architecture of this belt was possibly originated from the continental underthrusting to the SE of the South China block in response to northwest-directed subduction of the Paleo-Pacific plate. 相似文献
627.
Michel Brossard Gustavo Adolfo de Freitas Fregonezi Maria de Fátima Guimarães Cristiane de Conti Medina Amarildo Pasini Lourival Vilela Miguel Angel Ayarza 《Comptes Rendus Geoscience》2012,344(1):41-49
Ferralsols are characterized by poorly-defined horizons, weak macrostructure and strong development of a fine granular structure comprising subangular micro-aggregates. In this study, the morphological and physical modifications caused by earthworm activity in a clayey ferralsol were analysed. After describing soil structures, undisturbed samples were taken for evaluating aggregates and determining clod bulk density and particle density. Soil water retention properties were measured and an inventory of soil invertebrate macrofauna was created. The structural and porous transformations were due to aggregates created by earthworm activity. Changes in bulk density can be associated with pore modifications caused by a change in the proportions of aggregate types, and a notable reduction of total porosity was measured, tending to increase soil volume with dense aggregates. Structural modifications affected the topsoil down to 0.5 m and water retention between −1 and −33 kPa, the principal water compartment of these soils. 相似文献
628.
Beno?t Joseph Mbassa Emmanuel Njonfang Mathieu Benoit Pierre Kamgang Michel Grégoire Stephanie Duchene Pierre Brunet Bekoa Ateba Félix M. Tchoua 《Mineralogy and Petrology》2012,106(3-4):217-242
The Mbengwi recent magmatic formations consist of volcanics and syenites belonging to the same magmatic episode. Lavas form a bimodal basanite-rhyolite alkaline series with a gap between 50 and 62?wt.% SiO2. Mafic lavas (basanite-hawaiite) are sodic while felsic rocks (trachyte-rhyolite-syenites) are sodi-potassic, slightly metaluminous to peralkaline. The geochemical and isotopic characteristics (0.7031?<?(87Sr/86Sr)initial?<?0.7043; 1.03?<?εNdi?<?5.17) of these rocks are similar to those of other rocks from the CVL. The main differentiation process is fractional crystallization with two trends of fractionation. Their Rb/Sr isochron age of 28.2?Ma, almost similar to 27.40?±?0.6?Ma?K/Ar age obtained in a trachyte from neighboring Bamenda Mountains system, precludes any local age migration of an hypothetic hotspot. Mafic lavas have OIB features displaying an isotopic signature similar to that of HIMU mantle source different from FOZO known as source of most parental magmas along the CVL. 相似文献
629.
630.
Sylvain Fallourd Marc Poujol Philippe Boulvais Jean-Louis Paquette Michel de Saint Blanquat Philippe Rémy 《International Journal of Earth Sciences》2014,103(3):667-682
In the Pyrenees, in association with the rotation of the Iberian plate around Europe during the Mid-Cretaceous, a Na–Ca metasomatism is recognized as a complementary record of the hydrothermal activity that led to Na-metasomatism (albitization) and talc–chlorite mineralization. It affected metasedimentary rocks as well as Hercynian granites. In situ laser ablation ICP-MS U–Pb analyses of titanite grains formed in albitites during metasomatism date the Na–Ca metasomatism between 110 and 92 Ma. The temperature of the Na–Ca metasomatism is estimated to be approximately 550 °C. Both the time constraints and temperature estimates suggest that the Na–Ca metasomatism is related to the low-P high-T North Pyrenean metamorphism. 相似文献