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61.
Agathe Fourmont Jean-Jacques Macaire Jean-Gabriel Bréhéret 《Journal of Paleolimnology》2009,41(3):471-490
Since the end of the Last Glacial Maximum, hydrology in Europe has been influenced by both climate changes, and since Neolithic
times, an increase in human activity. Paleohydrological reconstructions, especially from lake studies, can help identify the
respective impact of these two factors. The present work focuses on a lacustrine geosystem, the Sarliève paleolake in the
Massif Central (France), in an unusually dry, temperate area. The lake sediment geometry (core drillings, geotechnical methods),
and the geochemical and mineralogical characterization of the catchment rocks and soils, and of the lacustrine deposits, indicate
major variations in paleohydrology during the last 12,000 years as dated by 14C, palynology and tephrochronology. In addition, a model quantifying detrital versus biochemical lacustrine components was
developed to identify hydrological trends. The data show that the Sarliève area was characterized mainly by remarkably dry
conditions, hence sharpening the climatic trends at middle latitudes in Western Europe. Three main hydrological phases are
distinguished since the Late Glacial: (1) 13.7–7.5 ka cal BP, a dominant dry climate, with a peak at ca. 8 ka cal BP, leading
to a lowstand in water level and unusual mineral authigenesis, zeolite then dolomite, constituting up to 60% of the lacustrine
sediments; (2) 7.5 to ca. 5.3 ka cal BP, repeated short-duration hydrological alternations that could have been climate-driven:
lowstands in water level with up to 60% biochemical minerals versus higher water levels with <10% biochemical minerals; (3)
5.3 ka cal BP to the Middle Ages (i.e. beginning in the 5th century AD), a hydrological trend towards perennial high water
level, with mainly detrital sediments, probably linked to climate evolution, except periods of obvious human-driven drying
during the last two millennia. 相似文献
62.
Jean-Jacques Schott Michel Roussignol Michel Menvielle & Flavien R. Nomenjahanary 《Geophysical Journal International》1999,138(3):769-783
In this paper, we will report on the application of Bayesian inference to DC resistivity inversion for 1-D multilayer models. The posterior probability distribution is explored through a Markov process based upon a Gibbs's sampler. The process would lead to unrealistic estimates without additional prior information, which takes the form of a second Markov chain where the transition kernel corresponds to a smoothness constraint. The outcomes are posterior marginal probabilites for each parameter, as well as, if required, joint probabilities for pairs of parameters. We will discuss the main properties of the method in the light of a theoretical example and illustrate its capabilities with some field examples taken from various contexts. 相似文献
63.
Aurélie Violette Jean Riotte Jean-Jacques Braun Priscia Oliva Jean-Christophe Marechal M. Sekhar Catherine Jeandel Jonathan Prunier Laurent Barbiero Bernard Dupre 《Geochimica et cosmochimica acta》2010,74(24):7059-7085
The influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the Western Ghats (Karnataka Plateau, South West India) has been studied. Along the climosequence, the current mean annual rainfall (MAR) varies within a 80 km transect from 6000 mm at the edge of the Plateau to 500 mm inland. Pedogenic carbonates occur in the MAR range of 500-1200 mm. In the semi-arid zone (MAR: 500-900 mm), carbonates occur (i) as thick hardpan calcretes on pediment slopes and (ii) as nodular horizons in polygenic black soils (i.e. vertisols). In the sub-humid zone (MAR: 900-1500 mm), pedogenic carbonates are disseminated in the black soil matrices either as loose, irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size. They also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate-cemented lumps of centimetric to decimetric size.Chemical and isotopic (87Sr/86Sr) compositions of the carbonate fraction were determined after leaching with 0.25 N HCl. The corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed. The trend defined by the 87Sr/86Sr signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition. Considering the residual fraction of each sample as the most likely lithogenic source of Ca in carbonates, it is estimated that from 24% to 82% (55% on average) of Ca is derived from local bedrock weathering, leading to a consumption of an equivalent proportion of atmospheric CO2. These values indicate that climatic conditions were humid enough to allow silicate weathering: MAR at the time of carbonate formation likely ranged from 400 to 700 mm, which is 2- to 3-fold less than the current MAR at these locations.The Sr, U and Mg contents and the (234U/238U) activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation. The relatively high concentrations of Sr, U and Mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite, possibly due to a better confinement of the pore waters which is supported by their high (234U/238U) signatures, and/or to higher content of dissolved carbonates in the pore waters. The occurrence of Ce, Mn and Fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation. The carbonate ages determined by the U-Th method range from 1.33 ± 0.84 kyr to 7.5 ± 2.7 kyr and to a cluster of five ages around 20 kyr, i.e. the Last Glacial Maximum period. The young occurrences are only located in the black soils, which therefore constitute sensitive environments for trapping and retaining atmospheric CO2 even on short time scales. The maximum age of carbonates depends on their location in the climatic gradient: from about 20 kyr for centimetric nodules at Mule Hole (MAR = 1100 mm/yr) to 200 kyr for the calcrete at Gundlupet (MAR = 700 mm/yr, Durand et al., 2007). The intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils. 相似文献
64.
Qingfeng Shao Wei Wang Chenglong Deng Pierre Voinchet Min Lin Antoine Zazzo Eric Douville Jean-Michel Dolo Christophe Falguères Jean-Jacques Bahain 《Quaternary Research》2014
Several Gigantopithecus faunas associated with taxonomically undetermined hominoid fossils and/or stone artifacts are known from southern China. These faunas are particularly important for the study of the evolution of humans and other mammals in Asia. However, the geochronology of the Gigantopithecus faunas remains uncertain. In order to solve this problem, a program of geochronological studies of Gigantopithecus faunas in Guangxi Province was recently initiated. Chuifeng Cave is the first studied site, which yielded 92 Gigantopithecus blacki teeth associated with numerous other mammalian fossils. We carried out combined ESR/U-series dating of fossil teeth and sediment paleomagnetic studies. Our ESR results suggest that the lower layers at this cave can be dated to 1.92 ± 0.14 Ma and the upper layers can be dated to older than 1.38 ± 0.17 Ma. Correlation of the recognized magnetozones to the geomagnetic polarity timescale was achieved by combining magnetostratigraphic, biostratigraphic and ESR data. The combined chronologies establish an Olduvai subchron (1.945–1.778 Ma) for the lowermost Chuifeng Cave sediments. We also analyzed the enamel δ13C values of the Gigantopithecus faunas. Our results show that southern China was dominated by C3 plants during the early Pleistocene and that the Gigantopithecus faunas lived in a woodland-forest ecosystem. 相似文献
65.
66.
Carl Martin Jean-Jacques Frenette Jean Morin 《Aquatic Sciences - Research Across Boundaries》2005,67(2):196-209
Large fluvial lakes, as part of river corridors, are recognized as spatially heterogeneous ecosystems. Due to their shallowness, the littoral zone of these lakes occupies a large proportion of their surface and is extensively covered by macrophytes that are known to affect their physical, chemical and biological environments in various ways. This study documents the small-scale (5 km) bio-optical variations associated with the longitudinal passage of a water mass through macrophyte beds during their maximal growth season. The utilization of a 2D hydrodynamic model allowed us to establish hydrological connectivity between stations within the beds, and thus to identify longitudinal heterogeneity along the macrophyte beds. Significant changes in the inherent properties of the water were observed along the upstream-downstream gradient. Due to their effects on hydrodynamics, macrophytes were responsible for a decrease in particles and dissolved organic carbon (DOC) resulting in an increasing penetration depth of ultraviolet (UV) and photosynthetically active radiation (PAR). Along the transect, chromophoric dissolved organic matter (CDOM) decreased more rapidly than DOC resulting in a decrease of the CDOM to DOC ratio. The observed changes in the DOC pool may be explained by the constant input of non-chromophoric DOC from macrophyte leachates and exudation within the bed and/or the photochemical and microbial degradation of CDOM. The implication of such small-scale heterogeneity on Lake Saint-Pierre productivity is discussed. 相似文献
67.
68.
Jean-Len Léticée Auran Randrianasolo Jean-Jacques Cornée Philippe Münch Jean-Frédéric Lebrun Jean-Paul Saint-Martin Michel Villeneuve 《Comptes Rendus Geoscience》2005,337(6):617-624
A major emersion surface (S1) reveals at least a 60-m relative sea-level fall within the ‘Calcaires supérieurs’ of Grande-Terre (Lesser Antilles fore-arc), an Upper Pliocene–Lower Pleistocene reefal unit. We thus divide the ‘Calcaires supérieurs’ into 2 units. S1, together with the transgresssive calcarenitic and the Acropora-rich reefal units on top of it, corresponding to the record of a complete eustatic cycle by the carbonate platform of this active margin. A comparison with the carbonate platform of the Bahama Bank stable margin, allows us to relate S1 to the major emersive event of 1.66 Ma and to infer that its origin is glacio-eustatic. To cite this article: J.-L. Léticée et al., C. R. Geoscience 337 (2005). 相似文献
69.
Michel Villeneuve Jean-Jacques Cornée Agus Harsolumakso Rossana Martini et Louisette Zaninetti 《Eclogae Geologicae Helvetiae》2005,98(2):297-310
Resume. De nombreuses échelles stratigraphiques de l’?le de Timor ont vu le jour car les corrélations stratigraphiques étaient rendues
difficiles par la complexité tectonique de l’?le et par la variété des faciès. Dès les premiers travaux, plusieurs séries
ont été distinguées, regroupées en trois unités litho-structurales principales: unité allochtone, sub-autochtone et autochtone.
Les données stratigraphiques et géodynamiques récentes montrent que l’?le de Timor est en fait constituée par six unités litho-structurales,
chacune témoignant d’un épisode majeur de l’histoire géologique de l’Ile. Cette histoire consiste d’abord à la séparation,
au Jurassique, d’un bloc issu du continent de Gondwana (unité para-allochtone) qui est entré en collision avec l’arc volcanique
asiatique à l’Oligo-Miocène (unités allochtone et sub-allochtone). Cet ensemble a ensuite été séparé de l’Asie lors de l’ouverture
de la mer de Banda sud, au Miocène supérieur (unité sub-autochtone) pour finalement entrer en collision avec la marge nord
australienne au cours du Pliocène moyen (plateforme australienne et groupe de Kolbano). Timor est depuis incorporée à la marge
nord australienne (unité autochtone).
Ce travail présente les différentes échelles stratigraphiques de chaque nouvelle unité, des propositions de corrélations et
une comparaison avec les ?les voisines de Sumba et de Sulawesi pour aboutir à une proposition d’évolution géodynamique.
Manuscrit re?u le 13 février 2004 Révision acceptée le 14 février 2005 相似文献
70.
Antoine Thill Stphane Moustier Jean-Marie Garnier Claude Estournel Jean-Jacques Naudin Jean-Yves Bottero 《Continental Shelf Research》2001,21(18-19)
The salt induced flocculation phenomenon is often proposed as a major mechanism influencing the deposition of the suspended matter near the mouth of major rivers. However, depending on the particular mixing conditions between fresh water and marine water and on suspended solids (SS) reactivity, salt induced aggregation may be a minor factor controlling fast deposition of SS. This work combines field studies and laboratory experiments in order to assess the importance of salt induced flocculation in the case of the saltwedge estuary of the Rhône river. Sampling of the mixing zone has been performed in contrasted hydrodynamic conditions from a low water discharge period (500 m3 s−1) to a small flood event (2400 m3 s−1) for particle counting and salinity measurements. Through laboratory experiments, it is shown that the Rhône river particulate matter has a poor average reactivity regarding salt induced flocculation. Considering the hydrodynamics of the estuary, we show that the evolution of the concentration of larger particles (>5 μm) can be explained by settling and dilution. In contrast, the smallest measured fraction (2–5 μm) shows a more complex behavior and settling processes alone cannot explain the observed particle concentrations during the field studies. Four hypotheses are discussed for explaining the 2–5 μm particle concentration evolutions. 相似文献