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61.
Aurélia Mouret Pierre Anschutz Bruno Deflandre Gwénaëlle Chaillou Christelle Hyacinthe Jonathan Deborde Henri Etcheber Jean-Marie Jouanneau Antoine Grémare Pascal Lecroart 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(4):528-540
The relationship between particulate organic carbon (POC) concentrations measured in modern sediment and fluxes of exported POC to the sediment surface needs to be understood in order to use POC content as a proxy of paleo-environmental conditions. The objective of our study was to compare POC concentrations, POC mineralization rates calculated from O2 consumption and POC burial rates. Benthic O2 distributions were determined in 58 fine-grained sediment cores collected at different periods at 14 stations in the southeastern part of the Bay of Biscay with depths ranging from 140 to 2800 m. Depth-dependent volume-specific oxygen consumption rates were used to assess rates of aerobic oxidation of organic matter (OM), assuming that O2 consumption solely was related to heterotrophic activity at the sediment–water interface. Heterogeneity of benthic O2 fluxes denoted changes in time and space of fresh organic material sedimentation. The most labile fraction of exported POC engendered a steep decrease in concentration in the upper 5 mm of vertical O2 profiles. The rupture in the gradient of O2 microprofile may be related to the bioturbation-induced mixing depth of fast-decaying carbon. Average diffusive O2 fluxes showed that this fast-decaying OM flux was much higher than buried POC, although diffusive O2 fluxes underestimated the total sediment oxygen demand, and thus the fast-decaying OM flux to the sediment surface. Sedimentary POC burial was calculated from sediment mass accumulation rate and the organic carbon content measured at the top of the sediment. The proportion of buried POC relative to total exported POC ranged at the most between 50% and 10%, depending on station location. Therefore, for a narrow geographic area like the Bay of Biscay, burial efficiency of POC was variable. A fraction of buried POC consisted of slow-decaying OM that was mineralized within the upper decimetres of sediment through oxic and anoxic processes. This fraction was deduced from the decrease with depth in POC concentration. At sites located below 500 m water depth, where the fast-decaying carbon did not reach the anoxic sediment, the slow-decaying pool may control the O2 penetration depth. Only refractory organic material was fossilized in sedimentary records at locations where labile OM did not reach the anoxic portion of the sediment. 相似文献
62.
Olivier Merle Philippe Mairine Laurent Michon Patrick Bachèlery Magali Smietana 《Journal of Volcanology and Geothermal Research》2010,189(1-2):131-142
Based upon a re-interpretation of previous data and a new field campaign, a structural evolution is proposed for the early history of Piton de la Fournaise volcano from 500,000 to 50,000 years. Conceptually, it is shown that the formation of a caldera in which lava flows are contained inside the caldera depression, gives time for erosion to excavate deep canyons on the external slopes of the volcano, for example, the Rivière des Remparts, the Rivière Langevin and the Rivière de l'Est canyons on Piton de la Fournaise volcano. These canyons are infilled when lavas, filling the caldera and overflowing its rim, are able again to flow on the external slopes of the volcano. In the past, this excavating/infilling process has occurred twice following the formation of the Rivière des Remparts and Morne Langevin calderas. The formation of the third caldera, the Plaine des Sables caldera, was followed by the excavation of the current canyons. In addition to this process, two large landslides have been documented in the field. The first, which happened about 300,000 years ago, is apparently the first episode of the break up of Piton de la Fournaise volcano, predating the formation of the four large calderas. The second landslide, which occurred 150,000 years ago and is considered to be less extensive, has carried away the entire southern flank of the Rivière des Remparts caldera. 相似文献
63.
Separated gold grains from 94 samples of the Vaal Placer, Klerksdorp gold field, South Africa have been analyzed for Au, Ag and Hg. Average gold grain compositions in these samples range from 80 to 95 weight percent Au, 4 to 18 weight percent Ag and 0.5 to 4 weight percent Hg with an average composition around Au 90, Au 8, Hg 2. Individual grains are homogeneous but significant differences exist between gold grains from single small samples and also between average compositions in separate samples. The data do not fit any simple model of gold compositional control by provenance or by metamorphic homogenization. 相似文献
64.
Evolution of ESR spectra with thermal annealing at different temperatures is studied for a monocrystal calcite taken in the Orgnac III site. Up to 20 ESR lines are recognized from the angular dependence of these lines. These lines are compared with the ESR lines observed for powder and rotating monocrystal calcites.The h1 line (g = 2.0057) and h3 line (g = 2.0007) are isotropic or nearly isotropic and are due to the electrons trapped in the amorphous region formed by fission tracks or alpha-ray recoil nuclei. The h2 line (g = 2.0021) is a composite of several different lines. 相似文献
65.
Jean-Pierre Pagès Jean-Pierre Frangi Pierre Durand Claude Estournel Aimé Druilhet 《Boundary-Layer Meteorology》1988,43(1-2):183-203
This paper presents data concerning the energy budget in the surface layer in the Sahel region (a semi-desert area). The results
are drawn from a measurement campaign made in the Niamey region in the Niger, in April–May 1984 (the Yantala Campaign). The
sensible heat flux is computed with the profile method, the ground heat flux is deduced from measurement of the temperature
field, and the radiative net flux is measured directly with a balancemeter. The latent heat flux, which is deduced from the
energy budget balance is very weak and within the accuracy limit of the method. The diurnal variation of the net flux is symmetrical,
with a maximum at noon. On the other hand, the sensible heat flux variation is asymmetrical, with an afternoon decrease much
slower than the morning increase. After 3.30 pm, it becomes higher than the net flux. This is compensated for by the sign
change of the ground heat flux, whose maximum is found in the morning at 11 am.
The second part of this paper shows the importance of one term in the surface-layer energy budget: the long-wave radiative
divergence between the ground and the top of the surface layer in high superadiabatic conditions. We show, with a radiative
model on the one hand and direct measurement of the radiative divergence on the other hand, that this term reaches several
tens of W m-2 in the superadiabatic conditions found in the Sahel region.
相似文献
66.
A. Piqué G. Bossière J. -P. Bouillin A. Chalouan Ch. Hoepffner 《International Journal of Earth Sciences》1993,82(3):432-439
Stratigraphic and structural correlations between the Palaeozoic massifs of eastern Morocco and northern Algeria allow three tectonic domains to be distinguished: (1) The cratonic zone, i.e. the West African platform which remained outside the Variscan chain and its peripherical margin (Moroccan Anti-Atlas and Algerian Ougarta); (2) a WSW-ENE trending zone, over 1500 km from Marrakech to Kabylia and Calabria (in their assumed Palaeozoic location). — This zone was characterized during the Late Palaeozoic by a continuous instability indicated by the development of successive turbiditic basins and a major orogeny at the Devonian-Carboniferous boundary; and (3) central and western Morocco, which corresponds to the external zones of the European Hercynides.The Marrakech-Kabylia zone separates the Variscan domain from the stable and undeformed West African craton. During Early Palaeozoic times it began as an extensive or transtensive zone. It has been deformed by the Late Devonian orogeny and by Carboniferous and Permian reactivation. The zone represents the southern limit of the Hercynian chain and is distinguished by its transcurrent regime throughout the Late Palaeozoic.
Correspondence to: A. Piqué 相似文献
67.
Dominique Bonnin Georges Calas Hélène Suquet Henri Pezerat 《Physics and Chemistry of Minerals》1985,12(1):55-64
New data on the structure of Garfield nontronite have been produced by the use of different spectroscopic techniques: Mössbauer spectroscopy, optical spectroscopy, X-ray absorption edges and EXAFS and NMR. The tetrahedral iron content is found to be no higher than 1 percent. All iron atoms belong to the octahedral sheet excluding the possibility of the presence of non crystallized phases. Some ambiguities remain about the coherence of the octahedral sheet because of the presence of two doublets in the Mössbauer spectrum and at least two lineshapes in NMR spectra of OH which correspond to different environments. 相似文献
68.
69.
A statistical examination of isotopic distributions for MORB from various ocean ridges leads to the “blob cluster model”, in which the oceanic crust accreting at ridges results from the mixing of two components within the ascending mantle. These are (1) upper mantle material and (2) discrete rising blobs of more radiogenic material. The blobs are fractionated to a variable degree and are distributed in the upper mantle circulation in a manner that is related to the spreading rate.(1) Themean values of the isotopic distributions allow us to calculate the probabilities of the two types of material within the mantle. The results show that theproportion of asthenospheric material in the mixtureincreases with the spreading rate, in agreement with the hypothesis of blob dilution within the upper mantle convection.Mass fluxes can be estimated for the rising blobs from these probabilities, which depend on the respective concentrations in the sources of the two types of material. If the blobs originate in the lower mantle, this flux estimation would suggest that a significant part of the lower mantle has been injected into the upper mantle during earth history.(2) Thestandard deviations of the distributions depend on the “efficiency” of the mixing process:the more imbricated are the asthenospheric and blob materials in the mixture,the smaller is theisotopic spread. This efficiency parameter is shown to increase with the spreading rate, as already suggested by previous comparisons between the East Pacific Rise and the Mid-Atlantic Ridge. Moreover, this feature may also be correlated with other data such as ridge bathymetric variations. 相似文献
70.
J Jouzel R Vaikmae J R Petit M Martin Y Duclos M Stievenard C Lorius M Toots M A Mélières L H Burckle N I Barkov V M Kotlyakov 《Climate Dynamics》1995,11(3):151-161
The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record. 相似文献