Results of magnetotelluric and gravimetric measurements in western Nicaragua, Central America

Magnetotelluric and gravity data have been collected within a ca. 170 km long traverse running from the Pacific coast of Nicaragua in the west to the Nicaraguan Highland in the east. This part of Nicaragua is characterized by sedimentary rocks of the Pacific Coastal Plain, separated from the Tertiary volcanic rocks of the Highland by the NW-SE-trending Nicaraguan Depression. 2-D interpretation of the magnetotelluric (MT) data, collected at 13 stations, indicates four regions of high electrical conductivity in addition to the conductive coastal region. Two of these are associated with conducting sediments and pyroclastics in the upper part of the crust. Two other conductive structures have been defined at depth around 20 km and the one best defined is located below the depression. From the distribution of seismic events, volcanic activity in the depression and the similarity in geophysical characteristics with areas such as the Rio Grande Rift, this conductor is interpreted as a melt layer or a complex of magma chambers. Models of the upper lithosphere, constrained by the MT model, vertical electrical sounding (VES) data, seismic data and densities, have been tested using gravity data. A model that passes this test shows a gradual thickening of the crust eastwards from the Pacific coast. An anomaly centred over the depression is interpreted to have its origin in a thinning of the crust. In this model the melt layer is situated on top of the bulge of the lower lithosphere. A change in the composition of the crust, from the Pacific Coastal Plain to the Highland, is indicated from the change in character of the MT response and from the density distribution in the gravity model. This may support the hypothesis that the Pacific region is an accreted terrane. MT and gravity data indicate a depth to a resistive and high-density basement in the depression of ca. 2 km. On the basis of this, the vertical setting in the depression is estimated to be of the order of 2.5 km.     

The Global Stratotype Section and Point （GSSP） for the base of the Holocene Series/Epoch （Quaternary System/Period） in the NGRIP ice core

The Greenland ice core from NorthGRIP （NGRIP） contains a proxy climate record across the Pleistocene-Holocene boundary of unprecedented clarity and resolution. Analysis of an array of physical and chemical parameters within the ice enables the base of the Holocene, as reflected in the first signs of climatic warming at the end of the Younger Dryas/Greenland Stadial 1 cold phase, to be located with a high degree of precision.     

Long-range atmospheric transport of trace metals: the need for geoscience perspectives

 The scientific literature of the past 25 years indicates a lack of consensus over the relative significance of anthropogenic and natural sources of metals in rural and remote ecosystems, meaning areas located several hundred to several thousand kilometers from industrial point sources. Geoscience perspectives are needed to address the critical information gaps associated with this issue, particularly where information on the significance and variability of naturally occurring metals is required. Uncertainties in source apportionment need to be addressed by improving methods of fingerprinting sources of airborne particles, and establishing identities of individual particles on the basis of characteristic morphologies and chemical composition. Further research is required to obtain robust estimates of total metal emissions to the atmosphere and to the oceans in order to determine the relative contribution from anthropogenic sources. Such research must involve a systematic attempt to obtain spatially and temporally representative data on the natural release of metals at the global scale. Moreover, a clear understanding of all local natural and anthropogenic loadings to an ecosystem is required before firm conclusions can be drawn about the relative importance of long-range atmospheric transport. Received: 31 October 1996 · Accepted: 27 May 1997     

The single-cell transport problem for two-phase flow with polymer

Polymer injection is a widespread strategy in enhanced oil recovery. Polymer increases the water viscosity and creates a more favorable mobility ratio between the injected water and the displaced oil. The computational cost of simulating polymer injection can be significantly reduced if one splits the governing system of two-phase equations into a pressure equation and a set of saturation/component equations and use a Gauss–Seidel algorithm with optimal cell ordering to solve the nonlinear systems arising from an implicit discretization of the saturation/component equations. This approach relies on a robust single-cell solver that computes the saturation and polymer concentration of a cell, given the total flux and the saturation and polymer concentration of the neighboring cells. In this paper, we consider a relatively comprehensive polymer model used in an industry-standard simulator, and show that, in the case of a discretization using a two-point flux approximation, the single-cell problem always admits a solution that is also unique.     

NORTHERN AND SOUTHERN HEMISPHERIC POLAR LOWS-A COMPARATIVE STUDY

Northern Hemispheric "polar lows",i.e.maritime,small,but fairly intense cyclonic systems have been studied for more than two decades.Recently researchers have turned their attention towards the occurrence of polar lows and other small-scale cyclonic disturbances on the Southern Hemisphere.In this paper a short account of our present knowledge of Northern Hemispheric polar lows will be given,followed by some preliminary results of an investigation of the nature and occurrence of polar lows on the Southern Hemisphere.     

The cooling rates of iron meteorites—A new approach

Measurements of Ni concentration profiles of a large number of neighboring kamacite and taenite lamellae in the iron meteorite Cape York (IIIA) have revealed that the kamacite plates have nucleated in a taenite of varying Ni concentration, equal to or above the bulk Ni concentration of the meteorite. This variation indicates that the kamacite plates nucleated stepwise (i.e., independently) during cooling through a certain temperature interval, rather than simultaneously after more or less undercooling of the meteorite. The latter is assumed in most previous cooling rate determinations (e.g., Moren and Goldstein, 1978). In this paper the measured local bulk Ni concentrations are used in the computer simulation of the evolution of the Widmannstaetten pattern in order to calculate the cooling rate of the meteorite. The cooling rate obtained for Cape York is 1.3°K/my. In most previous work, a correlation is seen between the resulting taenite width and the cooling rate in one and the same meteorite. No such correlation is seen using the present method.     

Alternative PWM-estimators of the Gumbel distribution

Probability weighted moments (PWM) are widely used in hydrology for estimating parameters of statistical distributions, including the Gumbel distribution. The classical PWM-approach considers the moments β_i=E[XFⁱ] with i=0,1 for estimation of the Gumbel scale and location parameters. However, there is no reason why these probability weights (F⁰ and F¹) should provide the most efficient PWM-estimators of Gumbel parameters and quantiles. We explore an extended class of PWMs that does not impose arbitrary restrictions on the values of i. Estimation based on the extended class of PWMs is called the generalized method of probability weighted moments (GPWM) to distinguish it from the classical procedure. In fact, our investigation demonstrates that it may be advantage to use weight functions that are not of the form Fⁱ. We propose an alternative PWM-estimator of the Gumbel distribution that maintains the computational simplicity of the classical PWM method, but provides slightly more accurate quantile estimates in terms of mean square error of estimation. A simple empirical formula for the standard error of the proposed quantile estimator is presented.     

Textural and geochemical discrimination between xenotime of different origin in the Archaean Witwatersrand Basin, South Africa

Xenotime (YPO₄) of detrital, diagenetic, and hydrothermal origin within siliciclastic rocks of the Archaean Witwatersrand Basin, South Africa, has been identified on the basis of petrography and in situ ion microprobe (SHRIMP) age data. The chemical composition of xenotime, determined by in situ electron microprobe analysis, can be correlated with its origin. This allows the origin of any xenotime grain to be assessed by a non-destructive microanalytical method prior to ion microprobe geochronology. The main chemical discriminators are MREE-HREE abundance, normalised HREE slope and Eu anomaly, and, in some cases, U and Th contents. Igneous-detrital xenotime (> 2800 Ma) is distinguished from diagenetic (∼2780 Ma) and hydrothermal (< 2780 Ma) xenotime in having lower Eu, Dy, and Gd concentrations and a distinctively lower Gd/Yb ratio. Hydrothermal xenotime has distinctively lower U and Th concentrations when compared to igneous-detrital and diagenetic xenotime. Three separate hydrothermal fluid events and episodes of post-diagenetic xenotime growth are recognised in the geochemical and geochronological data, which correspond in time to the extrusion of the Ventersdorp lavas at ∼2720 Ma, the emplacement of the Bushveld Igneous Complex at ∼2061 Ma, and an event of unknown affinity at ∼2210 Ma. Although geochemical discrimination of the xenotime types from the Witwatersrand Basin, in combination with careful petrography, appears achievable, universal application of these discriminators to xenotime in other sedimentary basins remains untested.     

Effects of an environmental estrogen, 17alpha-ethinyl-estradiol,on the maternal-fetal trophic relationship in the eelpout Zoarces viviparus (L)

In the present experiment pregnant eelpout Zoarces viviparus females were exposed to 0, 5, 10, 50, 500 ng/l of the synthetic estrogen ethinylestradiol (EE2) and to 500 ng/l estradiol-17beta (E2) under flow-through conditions during 3 weeks and the maternal-fetal trophic response was investigated. The circulating yolk-precursor protein vitellogenin, measured by ELISA, increased from a mean control value of 0.017-36 mg/ml in the plasma of the motherfish exposed to the highest concentration of EE2 This increase in vitellogenin was also depicted by a 288% increase in circulating calcium levels. During pregnancy the ovary represents a new route of calcium loss from the maternal blood for the growth of the embryos. However, a significant decrease (120% in the group exposed to the highest concentration of EE2) in the calcium level in the ambient medium of the embryos, the ovarian fluid, was observed concomitant with the increase in maternal plasma calcium in the EE2-exposed females. In contrast, the level of circulating amino acids decreased in the maternal blood, with a slight concomitant increase in the ovarian fluid of the exposed fish. These and other metabolic observations indicate that exposure to different doses of EE2 and to estradiol-17beta affects the maternal-fetal trophic relationship.