Late Pleistocene and Holocene Paleoenvironments of Chalco Lake, Central Mexico     

Maria Socorro Lozano-García  Beatriz Ortega-Guerrero  Margarita Caballero-Miranda  Jaime Urrutia-Fucugauchi 《Quaternary Research》1993,40(3)

In order to establish paleoenvironmental conditions during the late Quaternary, four cores from the Basin of Mexico (central Mexico) were drilled in Chalco Lake, located in the southeastern part of the basin. The upper 8 m of two parallel cores were studied, using paleomagnetic, loss-on-ignition, pollen, and diatom analyses. Based on 11 ¹⁴C ages, the analyzed record spans the last 19,000 ¹⁴C yr B.P. Volcanic activity has affected microfossil abundances, both directly and indirectly, resulting in absence or reduction of pollen and diatom assemblages. Important volcanic activity took place between 19,000 and 15,000 yr B.P. when the lake was a shallow alkaline marsh and an increase of grassland pollen suggests a dry, cold climate. During this interval, abrupt environmental changes with increasing moisture occurred. From 15,000 until 12,500 yr B.P. the lake level increased and the pollen indicates wetter conditions. The highest lake level is registered from 12,500 to ca. 9000 yr B.P. The end of the Pleistocene is characterized by an increase in humidity. From 9000 until ca. 3000 yr B.P. Chalco Lake was a saline marsh and the pollen record indicates warmer conditions. After 3000 yr B.P. the lake level increased and human disturbance dominates the lacustrine record.  相似文献   

Response of neutral boundary layers to changes of roughness     

Anna Maria Sempreviva  S. E. Larsen  N. G. Mortensen  I. Troen 《Boundary-Layer Meteorology》1990,50(1-4):205-225

When air blows across a change in surface roughness, an internal boundary layer (IBL) develops within which the wind adapts to the new surface. This process is well described for short fetches, > 1 km. However, few data exist for large fetches on how the IBL grows to become a new equilibrium boundary layer where again the drag laws can be used to estimate the surface wind.To study this problem, data have been sampled for two years from four 30-m meteorological masts placed from 0 to 30 km inland from the North Sea coast of Jutland in Denmark. The present analysis is limited to neutral stratification, and the surface roughness is the main parameter. The analysis of wind data and two simple models, a surface layer and a planetary boundary layer (PBL) model, are described.Results from both models are discussed and compared with data analysis. Model parameters have been evaluated and the model sensitivity to those parameters has been investigated. Using the model parameters, a large-scale roughness length has been estimated.Istituto Di Fisica dell' Atmosfera I.F.A. — CNR, Rome, Italy.  相似文献   

Temperature correction of electrical conductivity values     

Bengt Calles  Ulla Maria Calles 《地球表面变化过程与地形》1990,15(7):673-678

For a number of applications in geoscience, electrical conductivity of water is often used as a collective measure of the concentration of dissolved solids. A method for temperature correction of electrical conductivity data based on regression analysis of the actual temperature/conductivity relationship of a number of water samples from natural streams is presented. It is shown that the generally used formulae for temperature correction give corrected results that deviate considerably from the values determined by actual measurements. The error increases with decreasing water temperature and may result in data that deviate by as much as 20 per cent from the true values. The implications are thus especially important for measurements of solute loads and concentrations during periods of low stream water tetaperatures e.g. during snowmelt.  相似文献   

Rheology heterogeneities on fault surfaces inferred from the time history of afterslip events     

Maria Elina Belardinelli  Maurizio Bonafede 《Geophysical Journal International》1994,116(2):349-365

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Compositional and mineralogic constraints on the genesis of ophiolite hosted nickel mineralization in the Pevkos area,Limassol Forest,Cyprus     

Michael P. Foose  Maria Economou  A. Panayiotou 《Mineralium Deposita》1985,20(4):234-240

Mineralization composed dominantly of primary troilite, maucherite, pentlandite, and chalcopyrite, and secondary valleriite occurs in serpentinized transition zone rocks of the Limasol Forest segment of the Troodos ophiolite complex, Cyprus. Whole-rock and electron microprobe analyses of this mineralization gives ranges of Cu/(Cu+Ni)=0.16 to 0.47, Pt/(Pt+Pd)=0.66 to 0.51, Ni/Co=6.33 to 13.4, and chondrite normalized plots with low concentrations of Rh, Pt, and Pd, but relatively high Au. Estimated distribution coefficients of nickel and iron between olivine and ore range from 0.5 to 7.4. Most of these data are unlike values from magmatic sulfide deposits and indicate either a complete alteration of a preexisting magmatic sulfide concentration or, more likely, a nonmagmatic origin for this mineralization.  相似文献   

IR characteristics of hydrotalcite-like compounds     

Maria J. Hernandez-Moreno  María A. Ulibarri  J. L. Rendon  Carlos J. Serna 《Physics and Chemistry of Minerals》1985,12(1):34-38

The infrared spectra of well characterized hydrotalcite-like compounds with M ²⁺/M ³⁺= 2/1 and M ²⁺/M ³⁺=3/1 chemical ratios indicate that octahedral cation ordering is only present for the 2/1 composition. For M ²⁺/M ³⁺ ratios greater than 2/1, the octahedral sheets are disordered although local cation order can be present through cation segregation. The order-disorder characteristics of these compounds can be explained by the total cation charge around any hydroxyl which cannot exceed seven units nor be lower than six. In addition, the infrared spectra suggest that the interlayer anions are symmetrically perturbed, the electrostatic interaction being greater for the 2/1 compositions.  相似文献   

A study for improving precipitation occurrences modelling with a Markov chain     

Maria Mimikou 《Journal of Hydrology》1984,70(1-4):25-33

Daily precipitation occurrences and their monthly wet-days' sums of precipitation-measuring stations in Greece are modelled with a Markov chain. The order of the chain is taken to be seasonally varying in accordance with the precipitation station's meteorological conditions and geographical location. The modelling efficiency of the Markov chain is significantly improved when it is conjunctively used with a second-order autoregressive stochastic model fitted on the monthly wet-days' sums.  相似文献   

Sub-solidus reactions and corona structures in the Niquelândia layered complex (Central Goiás,Brazil)     

Dr. Maria Angela Fornoni Candia  Dr. Maurizio Mazzucchelli  Dr. Franca Siena 《Mineralogy and Petrology》1989,40(1):17-37

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Electron microprobe dating of monazite from high-T shear zones in the São José de Campestre Massif, NE Brazil     

Zorano Srgio de Souza  Jean-Marc Montel  Simone Maria Lima Costa Gioia  Maria Helena Bezerra Maia de Hollanda  Marcos Antonio Leite do Nascimento  Emanuel Ferraz Jardim de S  Venerando Eustquio Amaro  Mrcio Martins Pimentel  Jean-Marc Lardeaux  Michelle Veschambre 《Gondwana Research》2006,9(4):441-455

The easternmost domain of the Borborema Province, northeastern Brazil, presents widespread, extensional-related high-temperature metamorphism during the Brasiliano (=Pan-African) orogeny. This event reached the upper amphibolite to granulite facies and provoked generalized migmatization of Proterozoic metapelitic rocks of the Seridó Group and tonalitic to granodioritic orthogneisses of the Archean to Paleoproterozoic basement. We report new geochronological data based on electron microprobe dating of monazite from metapelitic migmatite and leuconorite within the high-T shear zones that make up the eastern continuation of the huge E–W Patos shear belt. These data were also constrained by using the Sm–Nd isotopic systematic on garnet from a syntectonic alkaline granite and two garnet-bearing leucosomes. The results suggest an age of about 578 to 574 Ma for the peak of the widespread high-T metamorphism. This event is best recorded by Sm–Nd garnet-whole rock ages. The U–Th–Pb isotopes on monazite of the metapelitic migmatite show a younger thermal event at 553 ± 10 Ma. When compared to the Sm–Nd garnet-whole rock ages, the U–Th–Pb electron probe monazite ages seem to record an event of slightly lower temperatures after the peak of the high-T metamorphism. This may reflect the difference in the isotopic behavior of the geochronological methods employed. Otherwise, the U–Th–Pb ages on monazites could indicate an event not yet very well defined. In anyway, this paper reveals the partial or even complete re-opening and resetting of the U–Th–Pb isotopic system produced by the action of low-T Ca-rich fluid.  相似文献   

High-temperature single-crystal neutron diffraction study of natural chondrodite     

Martin Kunz  George A. Lager  Hans-Beat Bürgi  Maria Teresa Fernandez-Diaz 《Physics and Chemistry of Minerals》2006,33(1):17-27

The H-atom environment in a Tilly Foster chondrodite was analyzed using single-crystal neutron-diffraction data collected at 500, 700 and 900 K and previously published low temperature data collected at 10, 100 and 300 K on the same crystal (Mg_4.64Fe_0.28Mn_0.014Ti_0.023(Si_1.01O₄)₂F_1.16(OH)_0.84; Friedrich et al. in Am Mineral 86:981–989, 2001). The full mean square displacement matrix Σ of the O–H pair was determined from the temperature dependence of the anisotropic displacement parameters, enabling a proper correction of the O–H bond for thermal vibration without assumptions about the correlation of O and H movements. The results show that the perpendicular O–H motions in chondrodite are intermediate between the riding and the independent motion models. The corrected O–H bond lengths do not change with temperature whereas the corrected H···F distances show an increase of ~0.02 Å with temperature, as do the Mg–O distances. This result shows that spectroscopic observations on the strength of the covalent O–H bond cannot be interpreted unambiguously in terms of a corresponding behaviour of the associated H···O/F hydrogen bond.  相似文献