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11.
Frédéric Moynier Toshiyuki Fujii Gregory A. Brennecka Sune G. Nielsen 《Comptes Rendus Geoscience》2013,345(3):150-159
The nuclear field shift (NFS) is an isotope shift in atomic energy levels caused by a combination of differences in nuclear size and shape and electron densities at the nucleus. The effect of NFS in isotope fractionation was theoretically established by Bigeleisen in 1996 [Bigeleisen J. (1996) J. Am. Chem. Soc. 118:3676–3680] and has been analytically measured in laboratory chemical exchange reactions. More recently, some isotopic variations of heavy elements (Hg, Tl, U) measured in natural systems as well as isotopic anomalies measured for lower-mass elements in meteorites have been attributed to the NFS effect. These isotopic variations open up new and exciting fields of investigations in Earth sciences. In this paper, we review the different natural systems in which NFS has been proposed to be the origin of isotopic variations. 相似文献
12.
WANG Haobo Olesia V. BONDARENKO Frédéric M. B. JACQUES WANG Yuehu ZHOU Zhekun 《《地质学报》英文版》2015,89(5):1429-1439
A new species, Tsuga nanfengensis sp. nov. (Pinaceae), is described on the basis of lignified fossil wood from the late Miocene of the Xianfeng Basin, central Yunnan, southwestern China. Detailed observation of the fossil wood specimens show the following characteristics: distinct growth rings, absence of resin canals, uniseriate bordered pits in the radial wall of tracheids, ray tracheids and piceoid and cupressoid cross-field pits. These features indicate similarities to the wood of extant Tsuga canadensis, T. chinensis, and T. dumosa. According to the fossil record, Tsuga was present in Xundian County during the Miocene. Today Tsuga is drought intolerant, preferring wet conditions with no extant species growing naturally in Xundian County. The presence of Tsuga in the Miocene of Xundian County indicates a humid climate consistent with previous palaeoclimatic reconstructions showing a wetter and probably shorter dry season in the Miocene, relative to the present day. Therefore, the change in the local climate such as increasing aridity through the Miocene might explain the local extinction of Tsuga from central Yunnan. 相似文献
13.
Given the contrasting behaviour observed for geomaterials, for example, during landslides of the flow type, this contribution proposes an original constitutive model, which associates both an elasto‐plastic relation and a Bingham viscous law linked by a mechanical transition criterion. This last is defined as the second‐order work sign for each material point, which is a general criterion for divergence instabilities. Finite element method with Lagrangian integration points is chosen as a framework for implementing the new model because of its well‐known ability to deal with both solid and fluid behaviours in large deformation processes. A first boundary model considering a sample of initially stable soil, a slope and an obstacle is performed. The results show the power of the constitutive model because the consistent evolution of initiation, propagation and arrest of the mudflow is described. A parametric study is led on various plastic and viscous parameters to determine their influence on the flow development and arrest. Finally, forces against the obstacle are compared with good agreement with those of other authors for the same geometry and a pure viscous behaviour. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
14.
Sébastien Joannin Frédéric Quillévéré Christophe Lécuyer François Martineau 《Quaternary Research》2007,67(2):264-274
Vegetation inherited from a Pliocene subtropical climate evolved through obliquity oscillations and global cooling leading to modern conditions. An integrated, highly time-resolved record of pollen and stable isotopes (δ18O and δ13C of Globigerina bulloides) was obtained to understand vegetation responses to Early Pleistocene climate changes. Continental and marine responses are compared in the Central Mediterranean region with a particular consideration of environmental changes during anoxic events.Pollen data illustrate vegetation dynamics as follows: [1] development of mesothermic elements (warm and humid conditions); [2] expansion of mid- and high-altitude elements (cooler but still humid conditions); and [3] strengthening of steppe and herb elements (cooler and dry conditions). These successions correlate with precession. δ18O variations recorded by Globigerina bulloides define two cycles (MIS 43-40) related to obliquity. At northern low- to mid-latitudes, the pollen signal records temperature and wetness changes related to precession even during global climate changes induced by obliquity. This may result in unexpected increasing wetness during glacial periods, which has to be considered specific to the Central and Eastern Mediterranean region. Lastly, an analysis of anoxic events reveals that enhanced runoff is indicated by increasing frequency of the riparian trees Liquidambar and Zelkova. 相似文献
15.
16.
Nickel plays a central role as an enzyme co-factor in the metabolism of methanogenic Archaea. Methanogens can fractionate Ni isotopes during assimilation, opening the possibility of using the stable isotopic composition of Ni as a biomarker. However, other sources of Ni isotopic variations need to be evaluated before one can establish Ni isotopes as an unambiguous tracer of methanogenesis in the rock record. Equilibrium exchange of Ni between the different species present in the ocean is a potential source of isotopic fractionation. Through controlled laboratory experiments and theoretical calculations, we quantify equilibrium Ni isotope fractionation between different species relevant to the modern and ancient ocean: Ni(H2O)62+, Ni(H2O)182+, NiOH(H2O)5+, Ni(OH)2(H2O)4, NiCl(H2O)5+, cis-NiCl2(H2O)4, trans-NiCl2(H2O)4, NiHS(H2O)5+, Ni(HS)2(H2O)4, NiSO4(H2O)4, NiHCO3(H2O)4+, NiCO3(H2O)4, and organic ligands (crown ether and oxalic acid). The magnitude of ligand-controlled Ni isotopic fractionation, approximately 1.25‰/amu (2.5‰ for the 60Ni/58Ni ratio), is similar to that previously measured in cultures of methanogenic Archaea. 相似文献
17.
L. A. J. MARTIN M. BALLÈVRE P. BOULVAIS A. HALFPENNY O. VANDERHAEGHE S. DUCHÊNE E. DELOULE 《Journal of Metamorphic Geology》2011,29(2):213-231
The analysis of texture, major element and oxygen isotope compositions of cloudy garnet crystals from a metapelite sampled on Ikaria Island (Greece) is used to assess the model of growth and re‐equilibration of these garnet crystals and to reconstruct the pressure–temperature–fluid history of the sample. Garnet crystals show complex textural and chemical zoning. Garnet cores (100–200 μm) are devoid of fluid inclusions. They are characterized by growth zoning demonstrated by a bell‐shaped profile of spessartine component (7–3 mol.%), an increase in grossular from 14 to 22 mol.% and δ18O values between 9.5 ± 0.3‰ and 10.4 ± 0.2‰. Garnet inner rims (90–130 μm) are fluid inclusion‐rich and show a decreasing grossular component from 22 to 5 mol.%. The trend of the spessartine component observed in the inner rim allows two domains to be distinguished. In contrast to domain I, where the spessartine content shows the same trend as in the core, the spessartine content of domain II increases outwards from 2 to 14 mol.%. The δ18O values decrease towards the margins of the crystals to a lowest value of 7.4 ± 0.2‰. The outer rims (<10 μm) are devoid of fluid inclusions and have the same chemical composition as the outermost part of domain II of the inner rim. Garnet crystals underwent a four‐stage history. Stage 1: garnet growth during the prograde path in a closed system for oxygen. Garnet cores are remnants of this growth stage. Stage 2: garnet re‐equilibration by coupled dissolution–reprecipitation at the temperature peak (630 < T < 650 °C). This causes the creation of porosity as the coupled dissolution–reprecipitation process allows chemical (Ca) and isotopic (O) exchange between garnet inner rims and the matrix. The formation of the outer rim is related to the closure of porosity. Stage 3: garnet mode decreases during the early retrograde path, but garnet is still a stable phase. The resulting garnet composition is characterized by an increasing Mn content in the inner rim’s domain II caused by intracrystalline diffusion. Stage 4: dissolution of garnet during the late retrograde path as garnet is not a stable phase anymore. This last stage forms corroded garnet. This study shows that coupled dissolution–reprecipitation is a possible re‐equilibration process for garnet in metamorphic rocks and that intra‐mineral porosity is an efficient pathway for chemical and isotopic exchange between garnet and the matrix, even for otherwise slow diffusing elements. 相似文献
18.
E. Lloret C. Dessert J. Gaillardet P. Albéric O. Crispi C. Chaduteau M.F. Benedetti 《Chemical Geology》2011,280(1-2):65-78
Organic matter is an important factor that cannot be neglected when considering global carbon cycle. New data including organic matter geochemistry at the small watershed scale are needed to elaborate more constrained carbon cycle and climatic models. The objectives are to estimate the DOC and DIC yields exported from small tropical watersheds and to give strong constraints on the carbon hydrodynamic of these systems. To answer these questions, we have studied the geochemistry of eleven small watersheds around Basse-Terre volcanic Island in the French West Indies during different hydrological regimes from 2006 to 2008 (i.e. low water level versus floods). We propose a complete set of carbon measurements, including DOC and DIC concentrations, δ13C data, and less commonly, some spectroscopic indicators of the nature of organic matter. The DOC/DIC ratio varies between 0.07 and 0.30 in low water level and between 0.25 and 1.97 during floods, indicating that organic matter is mainly exported during flood events. On the light of the isotopic composition of DOC, ranging from ? 32.8 to ? 26.2‰ during low water level and from ? 30.1 to ? 27.2‰ during floods, we demonstrate that export of organic carbon is mainly controlled by perennial saprolite groundwaters, except for flood events during which rivers are also strongly influenced by soil erosion. The mean annual yields ranged from 2.5 to 5.7 t km? 2 year? 1 for the DOC and from 4.8 to 19.6 t km? 2 year? 1 for the DIC and exhibit a non-linear relationship with slopes of watersheds. The flash floods explain around 60% of the annual DOC flux and between 25 and 45% of the DIC flux, highlighting the important role of these extreme meteorological events on global carbon export in small tropical volcanic islands. From a carbon mass balance point of view the exports of dissolved carbon from small volcanic islands are important and should be included in global organic carbon budgets. 相似文献
19.
Frédéric Girault Ananta Prasad Gajurel Frédéric Perrier Bishal Nath Upreti Patrick Richon 《Journal of Asian Earth Sciences》2011,40(2):595-610
Effective radium-226 concentration (ECRa) has been measured in soil samples from seven horizontal and vertical profiles of terrace scarps in the northern part of Kathmandu Valley, Nepal. The samples belong to the Thimi, Gokarna, and Tokha Formations, dated from 50 to 14 ky BP, and represent a diverse fluvio-deltaic sedimentary facies mainly consisting of gravelly to coarse sands, black, orange and brown clays. ECRa was measured in the laboratory by radon-222 emanation. The samples (n = 177) are placed in air-tight glass containers, from which, after an accumulation time varying from 3 to 18 days, the concentration of radon-222, radioactive decay product of radium-226 and radioactive gas with a half-life of 3.8 days, is measured using scintillation flasks. The ECRa values from the seven different profiles of the terrace deposits vary from 0.4 to 43 Bq kg?1, with profile averages ranging from 12 ± 1 to 27 ± 2 Bq kg?1. The values have a remarkable consistency along a particular horizon of sediment layers, clearly demonstrating that these values can be used for long distance correlations of the sediment horizons. Widely separated sediment profiles, representing similar stratigraphic positions, exhibit consistent ECRa values in corresponding stratigraphic sediment layers. ECRa measurements therefore appear particularly useful for lithologic and stratigraphic discriminations. For comparison, ECRa values of soils from different localities having various sources of origin were also obtained: 9.2 ± 0.4 Bq kg?1 in soils of Syabru–Bensi (Central Nepal), 23 ± 1 Bq kg?1 in red residual soils of the Bhattar-Trisuli Bazar terrace (North of Kathmandu), 17.1 ± 0.3 Bq kg?1 in red residual soils of terrace of Kalikasthan (North of Trisuli Bazar) and 10 ± 1 Bq kg?1 in red residual soils of a site near Nagarkot (East of Kathmandu). The knowledge of ECRa values for these various soils is important for modelling radon exhalation at the ground surface, in particular in the vicinity of active faults. Importantly, the study also reveals that, above numerous sediments of Kathmandu Valley, radon concentration in dwellings can potentially exceed the level of 300 Bq m?3 for residential areas; a fact that should be seriously taken into account by the governmental and non-governmental agencies as well as building authorities. 相似文献
20.
In South Africa attempts are being made to address the socio-spatial distortions of the apartheid era through a more equitable
distribution of resources, and the re-drawing of municipal geographical boundaries. However, boundaries are not neutral geographic
lines. Boundary changes are often associated with a redistribution of political power and resources. The aim of this paper
is to analyse the effects of the contemporary territorial and administrative restructuring on urban dynamics in South Africa.
More specifically, the focus is on how the process of territorial restructuring impacted on metropolitan areas as well as
on secondary cities and their hinterlands. Examining and elucidating the manner in which various social, economic and political
forces have manifested themselves in the process of boundary delimitation in a major metropolitan centre as well as adjacent
rural areas is a central theme of this paper. There were considerable contestations over the delimitation of new local government
boundaries. Affluent metro authorities like that in Durban were opposed to the spatial extension of their boundaries because
of the costs of the providing services and infrastructure in the deprived margins. Similarly, there was concern that incorporation
of rural areas will result in increased municipal service charges being imposed on these communities. Tensions were heightened
between urban and rural regions because traditional leaders believed that their territorial jurisdiction and authority were
being undermined. In other parts of the country, the merger of traditionally white and black fragments of secondary cities
often resulted in many black locations continuing to be marginalized. There appears to be neither the political will nor the
economic capacity to upgrade these zones of marginalized urban communities. While the Municipal Demarcation Board was largely
successful in eliminating the political geography of apartheid at a macro- scale, this paper suggests that the greater challenge
for government and policy makers is to reduce the socio-spatial and economic inequalities which appears to be still very high
and perhaps increasing.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献