Sediment oxygen profiles in a super-oxygenated antarctic lake

Perennially ice-covered lakes are found in the McMurdo Dry Valleys of southern Victoria Land, Antarctica. In contrast to temperate lakes that have diurnal photic periods, antarctic (and arctic) lakes have a yearly photic period. An unusual feature of the antarctic lakes is the occurrence of O2 at supersaturated levels in certain portions of the water column. Here we report the first sediment O2 profiles obtained using a microelectrode from a perennially ice-covered antarctic lake. Sediment cores collected in January and October 1987 from Lake Hoare in Taylor Valley show oxygenation down to 15, and in some cases, 25 cm. The oxygenation of sediments several centimeters below the sediment-water interface is atypical for lake sediments and may be characteristic of perennially ice-covered lakes. There is a significant difference between the observed January and October sediment O2 profiles. Several explanations may account for the difference, including seasonality. A time-dependent model is presented which tests the feasibility of a seasonal cycle resulting from the long photoperiod and benthic primary production in sediments overlain by a highly oxygenated water column.     

Hostrocks to the stratabound iron-formation-hosted Starra gold-copper deposit,Australia

This paper documents the timing, geochemistry and possible origin of sodium enrichment associated with Proterozoic iron formation-hosted copper-gold ore lenses at the Starra deposit, Australia. The ore lenses are immediately underlain by variably sheared albitemagnetite-hematite-pyrite-bearing rocks, and overlain by mainly unaltered, less-sheared, meta-sediments. Evidence indicating that albite was abundant prior to deformation, and also prior to the main hydrothermal addition of Fe, S, Cu and Au, includes (1) regional albite development at this stratigraphic level; (2) alteration of albite by ore-related sericite; and (3) inclusion of unfoliated inclusions of albite and hematite in ore-related pyrite. Nevertheless, albite also developed around dolerites and within hangingwall shears during metamorphism and deformation. Least altered albite footwall rocks have SiO₂ (67.31–73.13 wt.%), Al₂O₃(9.89–12.51 wt.%), total alkali elements (6.40–9.43 wt.%) and HFS elements (e.g., Zr = 179–275 ppm; Ti/Zr = 7.6–14.1) comparable with felsic volcanics, but have high relative Na₂O contents (5.08–5.81 wt. %), and variable to high Na₂O/K₂O ratios (0.05–68.37). An episode of alkali alteration by high-Na fluids prior to the ore-related alteration is postulated to account for sodic compositions, after chemical consideration of the alternatives (1) arkosic sediments; (2) trondhjemitic volcaniclastics; and (3) Na-metasomatism of Staveley Formation sediments. During the most intense ore-alteration, isocon analysis indicates a density increase of 23% compared to the least altered albitised hostrocks. Cu, Fe, Au, Sn, W, Zn, K, Ba and Rb were enriched during the mineralising process, whereas Mn, Ca, Si, Nb and Zr were lost, and Al, Ti and Y were not changed (and were used to define the isocon). The loss of some high field strength elements during this alteration indicates that igneous rock classification diagrams based on immobile element ratios should be used with caution for albitemuscovite magnetite-pyrite assemblages associated with iron formation-hosted copper-gold deposits. Zircon was partially soluble, whereas titanium oxides were probably stable in the saline, high temperature (260°–380 °C), acid (pH = 3.9–6.0) fluids envisaged for ore-related alteration.     

Integrated chemostratigraphy and biostratigraphy of the Windermere Supergroup, northwestern Canada: implications for Neoproterozoic correlations and the early evolution of animals

The thick, richly fossiliferous succession of the upper Windermere Supergroup, Mackenzie Mountains, northwestern Canada, provides a test of integrated biostratigraphic and chemostratigraphic frameworks in terminal Proterozoic correlation. The C- and Sr-isotopic abundances of lower Keele Formation carbonates approximate those for other pre-Varanger samples, confirming that the simple disc-like fossils of the underlying Twitya Formation predate all known diverse Ediacaran faunas. "Tepee" and Sheepbed carbonates record strong post-glacial isotopic excursions; in contrast, delta13C values for Gametrail through Risky carbonates vary only within the narrow range of about +l% to +2%. A second negative excursion occurs in Ingta Formation carbonates that immediately underlie the paleontologically determined Precambrian-Cambrian boundary. The upper Windermere profile as a whole compares closely with curves determined for other terminal Proterozoic successions. The lowermost diverse Ediacaran assemblages in the Sheepbed Formation correlate chemostratigraphically with the oldest fauna in Namibia, but the two assemblages differ in taxonomic composition. Blueflower assemblages correlate both chemostratigraphically and taxonomically with faunas from Australia, China, Siberia, and elsewhere. Increasing data support the hypothesis that paleontological and geochemical data together provide a reliable means of correlating terminal Proterozoic sedimentary rocks throughout the world.     

An evaluation of residential air radon concentrations and related variables in southeast Michigan,USA

Analysis of 153 residential air radon (Rn-222) screening measurements from southeast Michigan indicates that basements host Rn levels two to three times higher than upper-level rooms. Compared to unfinished basements, finished (e.g., paneled walls, tiled floors) basements apparently reduce indoor air Rn levels while partially finished basements may not. Factor analysis of residence questionnaire data explains 59 percent of the Rn data variance. The volume of pathways (e.g., foundation cracks/holes, uncapped sumps) allowing Rn seepage into the dwelling controls the largest portion, 23 percent, of the explained data variance. The residence water source explains 11 percent of the Rn data variance. Groundwater Rn levels contribute to the air Rn data variability, but the study data cannot quantitatively assess this contribution. Seven percent of the Rn data variance is likely controlled by house depressurization facilitated by residence structural properties. Residences with foundation cracks or poorly sealed joints and low-volume indoor-outdoor air exchange are more prone to this effect. Eighteen percent of the explained Rn data variance correlates with the residence's primary heat source. Evidently, operating combustion sources also induce house depressurization and allow Rn to be drawn into the house through entry paths. Twenty-four percent of the analyses equal or exceed 4 pCi/1 Rn. In residences occupied 5 years by the same individuals, 17 percent of the Rn data are 4 pCi/l; here the arithmetic mean air Rn level is 8.3 pCi/l and the average occupancy period 17.4 years.     

Dike rock generation and magma interactions in the Bir Safsaf igneous complex,south-west Egypt: Implications for the Pan-African evolution in north-east Africa

The geological setting, ages, petrography and geochemistry of late Pan-African ( 580 Ma) calc-alkaline and tholeiitic dike rocks in the Bir Safsaf igneous complex of south-west Egypt are discussed. These basaltic to rhyolitic dikes intruded contemporaneously and shortly after the intrusion of granitoids. The major and trace element data, Sr and Nd isotope relations, in combination with textural observations, confirm complex interactions between most of the intermediate calcalkaline dike melts and plutonic melts, with different degrees of mixing, assimilation, replenishment and tapping of magma chambers. Trachytic and rhyolitic dikes are strongly differentiated melts from the granitic pluton. The tholeiitic dikes evolved dominantly by fractional crystallization processes. It is inferred that open system and closed system processes operated in calc-alkaline magma chambers, and that the calc-alkaline melts came from a garnet-and amphibole-bearing mantle, modified by a subduction component. Tholeiitic rocks were formed later by fractional crystallization and assimilation processes. Magma ascent of both dike types took place in an extensional environment and the presumed subduction zone has to be seen in connection with the Atmur-Delgo suture zone.     

Inner part observation of theβ pictoris disk

The inner regions of the circumstellar disk around Pictoris might be dust-free due to a possible planet which may have cleared up dust particles. We present a new observational technique based on the use of an anti-blooming CCD in order to directly image this zone. The structure of the disk is revealed down to 2 arcsec from the star (30 AU). We show that the Pic disk brightness is neutral inV, R, andI _C , but drops down when going inward inB, possibly related to a change in the dust composition. Also, a slight disk asymmetry is present but inverted from the outer to the inner zones.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Once more on quasar periodicities

In an earlier paper Arp, Bi, Chu and Zhu investigated various quasar samples and recognized a periodicity of remarkable degree in them. Their conclusion was that the existence of such a periodicity is a counterevidence against the cosmological origin of quasar redshifts. Since this question is of great importance in a cosmological context, here we reanalyze some of the samples together with a galaxy sample of the pencil-beam survey of Broadhurstet al. Our result is that on the plane of cosmological parameters (₀, ₀) there is a non-negligible region where two quasar samples and the galaxy sample are simultaneously fairly periodic. Pure periodicity is still compatible with cosmological principles, at least on length scales much longer than the period length. So the regularities can fit into a cosmological context.     

The project “spice”

Proposed to both the French and the European Space Agency as one possible small mission, SPICE is a project for a dedicated small satellite for a near-IR spectroscopic all-sky survey. The instrument would cover the spectral range 1.8 - 3.6µm, possibly extended to 1.8 - 7µm, at a resolution of 100, with pixels of 1 arc-min. The excellent sensitivity (0.02 MJy sr-1) results from: i) the quasi-zero level of background due to the efficient passive cooling of the whole experiment; ii) the use of large format arrays; iii) the non-stop observing mode (drift-scanning). The spectral domain, complementing the one of ISO, partially opaque from the ground, is specially rich in spectral features tracing stars and all components of the Interstellar Medium (molecular, atomic and ionized gas, dust). With a cooling below 80 K of the focal instrument, then it becomes possible to consider doubling the spectral domain and to cover the whole 1.8 - 7µm range.     

Geomorphic evolution of a part of Middle Ganga Plain,Bihar: A case study using remote sensing data

An insight into the geomorphic evolution of any area can be obtained by detailed landform mapping. In the present study, an area in the Middle Ganga Plain has been selected for the study using mainly remote sensing data. Various fluvial landforms have been mapped and the changes in planform of rivers over approximately 50 years have been evaluated. Both fluvial processes and tectonic activities are considered to have collectively influenced the migration of the rivers in this region. Digital enhancements of Landsat MSS and TM data are found to be quite useful in identification and mapping of subtle fluvial palaeofeatures. The present study demonstrates the utility of remote sensing in examining the geomorphic evolution of the area.     

Stellar irradiance variations due to surface temperature inhomogeneities

Observations of rotational modulation of continuum brightness and photospheric and chromospheric spectral-line profiles of late-type stars indicate the presence of very inhomogeneous surface temperature distributions. We present three stellar examples (VY Ari, HR 7275, HU Vir) where time-series photometry is used to trace the evolution of spotted regions. Simultaneous spectroscopy and Doppler imaging for one of the three stars (HU Virgo, Fig. 1) makes it possible to compute the temperature distribution of the photosphere and the relative intensity distribution of parts of the chromosphere (from CaII K and H line profiles). The combination of time-series spot modeling and Doppler imaging enabled us to determine thesign and amount of differential surface rotation on HU Vir. We found a big, cool polar spot (see figure below) and a differential (surface) rotation law where higher-latitude regions rotate faster than lower-latitude regions (opposite to what we see on the Sun). Currently, this ensemble of techniques - time-series photometry and photospheric and chromospheric Doppler imaging - is only applicable to stars overactive by approximately a factor of 100 as compared to the active Sun, e.g. the evolved components in RS CVn-type binaries and some rapidly-rotating, single, pre-main sequence stars or giant stars. Stellar rotation is a fundamental parameter for (magnetic) activity. Starspots, or any other surface inhomogeneities, allow one to derive very precise stellar rotation rates and, if coupled with seismological observations of solar-type stars, could provide information on the internal angular momentum distribution in overactive late-type stars.To be published in Astronomy & Astrophysics.