996 resultados para 138-846A


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Strontium/calcium (Sr/Ca) ratios in bulk and foraminiferal calcite have been used to constrain the history of Sr/Ca in the oceans and to evaluate calcite diagenetic alteration. However bulk Sr/Ca records also may be influenced by differences in Sr uptake and/or in the diagenetic susceptibility of different calcium carbonate sedimentary components. We present data on the sediment size fraction and calcium carbonate distribution in bulk samples, Sr/Ca in a range of sedimentary size components, and Sr/Ca in bulk sediments. Ocean Drilling Program samples from sites on Ontong Java Plateau and Ceara Rise (in the western equatorial Pacific and Atlantic, respectively) and from sites in the eastern equatorial Pacific were selected to represent progressive stages in the diagenetic pathway from the sea floor through a range of burial depths equivalent to sediment ages of ~5.6, ~9.4, and ~37.1 Ma. Samples were subdivided by size to produce a unique data set of size-specific Sr/Ca ratios. Fine fraction (<45 ?m) Sr/Ca ratios are higher than those of all corresponding coarse fractions, indicating that fine nannofossil-dominated calcite has a Sr partition coefficient 1.3-1.5 times greater than that of coarse foraminifera-dominated calcite. Thus, absolute values of bulk Sr/Ca in contemporaneous samples reflect, in part, the ratio of fine to coarse calcite sedimentary components. Sr/Ca values in fine and coarse components also behave differently in their response to pre-burial dissolution and to recrystallization at depth. Coarse size components are sensitive to bottom water carbonate ion undersaturation, and they lose original Sr/Ca differences among contemporary samples over not, vert, similar10 my. In contrast, fine components recrystallize faster in more deeply buried samples. Interpretation of the historical Sr/Ca record is complicated by post-depositional diagenetic artifacts, and thus our data do not provide clear evidence of specific temporal changes in oceanic Sr/Ca ratios over the past 10 million years. This paper represents the first systematic attempt to examine trends in calcite Sr/Ca as a function of sediment size fraction and age.

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Interpretations of calcite strontium/calcium records in terms of ocean history and calcite diagenesis require distinguishing the effects on deep-sea calcite sediments of changes in ocean chemistry, of different mixes of calcite-depositing organisms as sediment contributors through time and space, and of the loss of Sr during diagenetic calcite recrystallization. In this paper Sr/Ca and d18O values of bulk calcium carbonate sediments are used to estimate the relative extent of calcite recrystallization in samples from four time points (core tops, 5.6, 9.4, and 37.1 Ma) at eight Ocean Drilling Program sites in the equatorial Atlantic (Ceara Rise) and equatorial Pacific (Ontong Java Plateau and two eastern equatorial Pacific sites). The possibility that site-to-site differences in calcite Sr/Ca at a given time point originated from temporal variations in ocean chemistry was eliminated by careful age control of samples for each time point, with sample ages differing by less than the oceanic residence times of Sr and Ca. The Sr/Ca and d18O values of 5.6- and 9.4-Ma samples from the less-carbonate-rich eastern equatorial Pacific sites and Ceara Rise Site 929 appear to be less diagenetically altered than the Sr/Ca and d18O values of contemporaneous samples from the more carbonate-rich sites. It is evident from these data that both Sr/Ca and d18O in bulk calcite have been diagenetically altered in some samples 5.6 Ma and older. These data indicate that noncarbonate sedimentary components, like clay and biogenic silica, have partially suppressed recrystallization at the lower carbonate sites. Sr/Ca data from the less altered, carbonate-poor sites indicate higher oceanic Sr/Ca relative to today at 5.6 and 9.4 Ma.

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In a recent paper, Gordon, Muratov, and Shvartsman studied a partial differential equation (PDE) model describing radially symmetric diffusion and degradation in two and three dimensions. They paid particular attention to the local accumulation time (LAT), also known in the literature as the mean action time, which is a spatially dependent timescale that can be used to provide an estimate of the time required for the transient solution to effectively reach steady state. They presented exact results for three-dimensional applications and gave approximate results for the two-dimensional analogue. Here we make two generalizations of Gordon, Muratov, and Shvartsman’s work: (i) we present an exact expression for the LAT in any dimension and (ii) we present an exact expression for the variance of the distribution. The variance provides useful information regarding the spread about the mean that is not captured by the LAT. We conclude by describing further extensions of the model that were not considered by Gordon,Muratov, and Shvartsman. We have found that exact expressions for the LAT can also be derived for these important extensions...

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A detailed single-crystal EPR study of phase IV of lithium potassium sulphate below -138 degrees C has been carried out using NH3+, which substitutes for K+, as the paramagnetic probe. The spin-Hamiltonian parameters have been evaluated at -140 degrees C and yield an isotropic g=2.0034; (AH)XX=(AH)YY=25.3 G and (AH)ZZ=23.8 G; (AN)XX=8.1 G, (AN)YY=21.2 G and (AN)ZZ=25.9 G. In this phase there are 12 magnetically inequivalent K+ sites and their occurrence is ascribed to the loss of a c glide.

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A discussion has been provided for the comments raised by the discusser (Clausen, 2015)1] on the article recently published by the authors (Chakraborty and Kumar, 2015). The effect of exponent alpha for values of GSI approximately smaller than 30 becomes more critical. On the other hand, for greater values of GSI, the results obtained by the authors earlier remain primarily independent of alpha and can be easily used. (C) 2015 Elsevier Ltd. All rights reserved.

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O Senador Mauro Benevides (PMDB-CE) assume a Presidência da Assembleia Nacional Constituinte (ANC) e preside a primeira sessão plenária do período de debates do novo anteprojeto. A Reforma Agrária terá dez artigos. A Deputada Raquel Capiberibe (PSB-AP) relata que o texto não reflete as aspirações do povo. O Deputado Vicente Bogo (PMDB-RS) relata que a reforma agrária atingirá o latifundiário e o pequeno proprietário. O Deputado Tadeu França (PMDB-PR) defende que as propriedades dos Estados do sul tenham, no máximo, mil hectares e, nas demais regiões, hum mil e quinhentos hectares. O Deputado Carlos Cardinal (PDT-RS) reivindica recursos para que hajam desapropriações rápidas. O Deputado Waldyr Pugliesi (PMDB-PR) esclarece que foram aprovados dispositivos que implicariam a derrogação de alguns artigos do Estatuto da Terra, uma lei criada no Governo Castello Branco, concluindo que a nova Constituição terá que avançar e jamais retroagir.