Experimental investigation of the effects of temperature and ionic strength on Mo isotope fractionation during adsorption to manganese oxides

The Mo stable isotope system is being applied to study changes in ocean redox. Such applications implicitly assume that Mo isotope fractionation in aqueous systems is relatively insensitive to frequently changing environmental variables such as temperature (T) and ionic strength (I). A major driver of fractionation is the adsorption of Mo to Mn oxyhydroxide surfaces [Barling J. and Anbar A. D. (2004) Molybdenum isotope fractionation during adsorption by manganese oxides. Earth Planet. Sci. Lett.217(3-4), 315-329]. Here, we report the results of experiments that determine the extent to which Mo isotope fractionation during adsorption of Mo to the Mn oxyhydroxide mineral birnessite is sensitive to T and I. The results are compared to new predictions from quantum chemical computations. We measured fractionation from 1 to 50 °C at I = 0.1 m and found that Δ^97/95Mo_{dissolved-adsorbed} varies from 1.9‰ to 1.6‰ over this temperature range. Experiments were also performed at 25 °C in synthetic seawater (I = 0.7); fractionation at this condition was the same within analytical error as in low ionic strength experiments. These findings confirm that the Mo isotope fractionation during adsorption to Mn oxyhydroxides is relatively insensitive to variations and T and I over environmentally relevant ranges. To relate these findings to potential mechanisms of Mo isotope fractionation, we also report results for density functional theory computations of the fractionation between and various possible structures of molybdic acid as a function of temperature. Because no plausible species fractionates from with a magnitude matching the experiments, we are left with three possibilities to explain the fractionation: (1) solvation effects on the vibrational frequencies of aqueous species considered thus far are significant, such that our calculations in vacuo yield inaccurate fractionations; (2) a trace aqueous species not yet considered fractionates from and then adsorbs to birnessite; or (3) a surface complex not present in solution forms on birnessite in which Mo is not tetrahedrally coordinated. Our findings help validate assumptions underlying paleoceanographic applications of the Mo isotope system and also lead us closer to understanding the mechanism of isotope fractionation during adsorption of Mo to Mn oxyhydroxides.     

Variogram Model Selection via Nonparametric Derivative Estimation

Before optimal linear prediction can be performed on spatial data sets, the variogram is usually estimated at various lags and a parametric model is fitted to those estimates. Apart from possible a priori knowledge about the process and the user's subjectivity, there is no standard methodology for choosing among valid variogram models like the spherical or the exponential ones. This paper discusses the nonparametric estimation of the variogram and its derivative, based on the spectral representation of positive definite functions. The use of the estimated derivative to help choose among valid parametric variogram models is presented. Once a model is selected, its parameters can be estimated—for example, by generalized least squares. A small simulation study is performed that demonstrates the usefulness of estimating the derivative to help model selection and illustrates the issue of aliasing. MATLAB software for nonparametric variogram derivative estimation is available at http://www-math.mit.edu/~gorsich/derivative.html. An application to the Walker Lake data set is also presented.     

Elemental Concentrations in Japanese Silicate Rock Standards JA-1, JR-1 and JB-2

Data on three Japanese geochemical reference samples (JR-1, JA-1, and JB-2) are presented. Ten major and thirty-five trace element concentrations were determined using x-ray fluorescence, instrumental thermal neutron activation, thermal neutron capture prompt gamma-ray spectrometry, delayed neutron assay, automated thermal neutron activation analysis, inductively coupled plasma emission, atomic absorption, and ion selective electrode. Good agreement between the various methods and with recent literature values were obtained using error weighted mean concentrations for the samples.     

Distinguishing extensive and intensive properties for meaningful geocomputation and mapping

A most fundamental and far-reaching trait of geographic information is the distinction between extensive and intensive properties. In common understanding, originating in Physics and Chemistry, extensive properties increase with the size of their supporting objects, while intensive properties are independent of this size. It has long been recognized that the decision whether analytical and cartographic measures can be meaningfully applied depends on whether an attribute is considered intensive or extensive. For example, the choice of a map type as well as the application of basic geocomputational operations, such as spatial intersections, aggregations or algebraic operations such as sums and weighted averages, strongly depend on this semantic distinction. So far, however, the distinction can only be drawn in the head of an analyst. We still lack practical ways of automation for composing GIS workflows and to scale up mapping and geocomputation over many data sources, e.g. in statistical portals. In this article, we test a machine-learning model that is capable of labeling extensive/intensive region attributes with high accuracy based on simple characteristics extractable from geodata files. Furthermore, we propose an ontology pattern that captures central applicability constraints for automating data conversion and mapping using Semantic Web technology.     

Stellar chromospheres,coronae, and winds (invited review)

Within the last few years there have been important advances in our understanding of the chromospheres, coronae, and winds of late-type stars. This progress has been the result of ultraviolet and X-ray observations obtained from space with theInternational Ultraviolet Explorer andEinstein satellites. In addition, ground-based astronomy has contributed much to synoptic studies of stellar activity. In this review I summarize some of the major recent developments in this field.Presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Marine mammals from northeast Atlantic: relationship between their trophic status as determined by delta13C and delta15N measurements and their trace metal concentrations

The relationship between trophic position through delta13C and delta15N and trace metal concentrations (Zn, Cd, Cu and Hg) was investigated in the tissues of six marine mammal species from the Northeast Atlantic: striped dolphin Stenella coeruleoalba, common dolphin, Delphinus delphis, Atlantic white-sided dolphin Lagenorhynchus acutus, harbour porpoise Phocoena phocoena, white beaked-dolphin Lagenorhynchus albirostris, grey seal Halichoerus grypus stranded on French Channel and Irish coasts. White-beaked dolphins, harbour porpoises, white-sided dolphins, common and striped dolphins display the same relative and decreasing trophic position, as measured by delta15N values, along both the Irish and French channel coasts, reflecting conservative trophic habits between these two places. Hepatic and renal Cd concentrations were significantly correlated to muscle delta13C and delta15N values while Hg, Zn and Cu did not. These results suggest that Cd accumulation is partly linked to the diet while other factors such as age or body condition might explain Hg, Zn or Cu variability in marine mammals. Combined stable isotope and trace metal analyses appear to be useful tools for the study of marine mammal ecology.     

Drying climate in Ghana over the period 1960–2005: evidence from the resampling-based Mann-Kendall test at local and regional levels

Abstract

Trends in rainfall series were investigated at 16 stations in Ghana over the period 1960–2005. Time series were first de-correlated using an effective pre-whitening methodology and then submitted to the resampling-based Mann-Kendall test. Field significances were assessed using the regional average Kendall statistic. Although no significant changes were observed in annual rainfall, the analysis reveals: (a) a reduction in the number of wet season days totalling less than 20 mm of rainfall, between latitudes 6° and 9.5°N; (b) a delay (about 0.5 d year^‐1) in the wet season onset at several locations throughout the country; and (c) a lengthening (about 0.1 d year^‐1) of rainless periods during the wet season in the south and centre of Ghana. All these changes, which remained insignificant at more than half of the individual stations, were found to be regionally significant at the 95% confidence level. The results highlight the importance of evaluating regional significance when investigating climate trends.

Editor Z.W. Kundzewicz

Citation Lacombe, G., McCartney, M., and Forkuor, G., 2012. Drying climate in Ghana over the period 1960–2005: evidence from the resampling-based Mann-Kendall test at local and regional levels. Hydrological Sciences Journal, 57 (8), 1594–1609.     

Terrestrial age measurements using natural thermoluminescence of a drained zone under the fusion crust of Antarctic ordinary chondrites

Abstract— Miono et al. (1990) and Miono and Nakanishi (1994) have proposed that the build‐up of natural thermoluminescence (TL) in a drained layer directly below the meteorite fusion crust can be used to determine terrestrial ages of meteorites in the 40 to 200 ka range. We have measured the natural TL of the drained layer of 15 meteorites. The data indicate that this technique could be used to determine terrestrial ages of meteorites with ages <200 ka, after which TL equilibrium is reached. Comparison of TL build‐up with terrestrial ages for a suite of Antarctic meteorites suggests that the meteorites have been exposed to temperatures of 0 to 5 °C. The close correspondence between natural TL levels and surface exposure TL growth curves suggest that Allan Hills meteorites with ages <200 ka have spent a significant portion of their terrestrial history exposed on the ice surface, rather than being buried in the ice sheet. The technique is, however, sensitive to thermal history; and, for Antarctic meteorites with terrestrial ages <200 ka, natural TL of the drained zone largely reflects exposure on the ice surface.