HIGH-RESOLUTION OBSERVATIONS OF THE EXTREME ULTRAVIOLET SUN

This paper presents first results of the Coronal Diagnostic Spectrometer (CDS) recently launched aboard the Solar and Heliospheric Observatory (SOHO). CDS is a twin spectrometer, operating in the extreme ultraviolet range 151–785 Å. Thus, it can detect emission lines from trace elements in the corona and transition region which will be used to provide diagnostic information on the solar atmosphere. In this paper, we present early spectra and images, to illustrate the performance of the instrument and to pave the way for future studies.     

Depositional processes and basin analysis of Messinian evaporites in Cyprus

Messinian evaporites in Cyprus resulted from the interplay of Mediterranean-wide and eustatic sea-level changes and local tectonics, in an inferred above-subduction zone setting. Distinctive Tortonian-early Messinian pre-evaporitic facies include diatomaceous marls and microbial carbonates, overlain by a variety of gypsum facies and then by lagoonal-lacustrine deposits and local palaeosols. Facies analysis and comparisons allow construction of a simple model, in which evaporites formed in semi-isolated small basins not far below global eustatic sea-level. Coarsely crystalline gypsum formedin situ along the margins of small basins and within shallow-water lagoons (< 10 m deep); this comprised common banded-stacked (i.e. layered) selenite, swallowtail selenite, botryoidal selenite and sugary-bedded selenite. Fine-grained gypsum precipitated widely and was reworked into basinal areas (< 70 m deep) by weak traction currents and low-density turbidity currents. Shallow-water derived selenite was also reworked basinwards by high-density turbidity currents and debris flows. Slumps indicate tectonic instability. More detailed basin analysis can be achieved by study of individual sub-basins. In the Polemi sub-basin in the west, a Lower Unit (up to 60 m thick) comprises basinal gypsum, interbedded with gypsum turbidites and mass flow deposits, with slumps. This is overlain by an extensive mega-rudite (up to 20 m thick) including up to metre-sized clasts of marginal gypsum facies. Above, the Upper Unit (up to 70 m thick) includes shallow-water gypsum (e.g. swallowtails), marl and minor microbial carbonates. The Pissouri sub-basin in the south-west exposes marginal facies of the Upper Unit, including deltaic elastics and palaeosols. The Maroni sub-basin in the south exhibits a basinal lower gypsum unit, with laterally equivalent marginal facies (up to 50 m thick), overlain by an extensive mega-rudite (up to 20 m thick). Finally, the Mesaoria subbasin in the north exposes relatively marginal gypsum facies in an unstable tectonic setting. Formation of the Polemi, Pissouri and Mesaoria gypsum sub-basins relates to coeval extensional faulting and graben development. Evaporites in south Cyprus (Maroni sub-basin) formed in elongate basins between former compressional lineaments created by localized Early Miocene thrusting. In the sub-basins of west, south-west and south Cyprus, large-scale slumping of marginal gypsum facies took place towards depocentres (to form megarudite debris flows), triggered by one or several phases of extensional faulting.     

U–Pb zircon,zircon Hf and whole-rock Sm–Nd isotopic constraints on the evolution of Paleoproterozoic rocks in the northern Gawler Craton

U–Pb zircon analyses from a series of orthogneisses sampled in drill core in the northern Gawler Craton provide crystallisation ages at ca 1775–1750 Ma, which is an uncommon age in the Gawler Craton. Metamorphic zircon and monazite give ages of ca 1730–1710 Ma indicating that the igneous protoliths underwent metamorphism during the craton-wide Kimban Orogeny. Isotopic Hf zircon data show that 1780–1750 Ma zircons are somewhat evolved with initial ε_Hf values –4 to +0.9, and model ages of ca 2.3 to 2.2 Ga. Isotopic whole rock Sm–Nd values from most samples have relatively evolved initial ε_Nd values of –3.7 to –1.4. In contrast, a mafic unit from drill hole Middle Bore 1 has a juvenile isotopic signature with initial ε_Hf zircon values of ca +5.2 to +8.2, and initial ε_Nd values of ～+3.5 to +3.8. The presence of 1775–1750 Ma zircon forming magmatic rocks in the northern Gawler Craton provides a possible source for similarly aged detrital zircons in Paleoproterozoic basin systems of the Gawler Craton and adjacent Curnamona Province. Previous provenance studies on these Paleoproterozoic basins have appealed to the Arunta Region of the North Australian Craton to provide 1780–1750 Ma detrital zircons, and isotopically and geochemically similar basin fill. The orthogneisses in the northern Gawler Craton also match the source criteria and display geochemical similarities between coeval magmatism in the Arunta Region of the North Australian Craton, providing further support for paleogeographic reconstructions that link the Gawler Craton and North Australian Craton during the Paleoproterozoic.     

Radio-continuum study of the nearby Sculptor group Galaxies. Part 2: NGC 55 at λ=20, 13, 6 and 3 cm

A series of new radio-continuum (λ=20, 13, 6 and 3 cm) mosaic images focused on the NGC 55 galactic system were produced using archived observational data from the Australia Telescope Compact Array. These new images are both very sensitive (down to rms=33 μJy) and feature high angular resolution (down to <4″). Using these newly created images, 66 previously unidentified discrete sources are identified. Of these sources, 46 were classified as background sources, 11 as H?ii regions and 6 as supernova remnant candidates. This relatively low number of SNR candidates detected coupled with the low number of large H?ii regions is consistent with the estimated low star formation rate of the galaxy at 0.06 M _⊙?year^?1. Our spectral index map shows that the core of galaxy appears to have a shallow spectral index between α=?0.2 and ?0.4. This indicates that the core of the galaxy is a region of high thermal radiation output.     

Ecology of testate amoebae in peatlands of central China and development of a transfer function for paleohydrological reconstruction

Testate amoebae are a diverse and abundant group of protozoa that constitute a large proportion of biomass in many ecosystems and probably fill important roles in ecosystem function. These microorganisms have attracted the interest of paleoecologists because the preserved shells of testate amoebae and their known hydrological preferences enable reconstruction of past hydrological change. In ombrotrophic peatlands, surface wetness reflects hydroclimate, so testate amoebae can play an important role in reconstruction of Holocene climate change. Previous studies, however, have been geographically restricted, mostly to North America and Europe. We studied the ecology of testate amoebae in peatlands from central China in relation to hydrology, pH and metal concentrations. We found that testate amoeba community structure was correlated with depth to water table (DWT) and that the hydrological preferences of species generally matched those of previous studies. We developed a weighted average DWT transfer function that enables prediction of water table depth with a cross-validated mean error of <5 cm. Our results demonstrate the potential for using testate amoebae to reconstruct paleohydrology in China. Such studies could contribute to our understanding of Holocene climate changes in China, particularly regarding past Asian monsoon activity.     

Ecophenotypic responses of benthic foraminifera to oxygen availability along an oxygen gradient in the California Borderland

Spatial variation in environmental conditions can elicit predictable size and morphological responses in marine organisms through influences on physiology. Thus, spatial and temporal variation in marine organism size and shape are often used to infer paleoenvironmental conditions, such as dissolved oxygen concentrations. Benthic foraminifera commonly serve as a tool for reconstructing past ocean oxygen levels. For example, benthic foraminiferal species assemblages, within‐ and among‐species patterns of test morphology, and geochemical analyses of carbonate tests are often employed to reconstruct past marine conditions. In this study, we measured the sizes and shapes of modern foraminifera representing four species that inhabit a steep oxygen gradient in the Santa Monica Basin on the Southern California Borderland with the aim of quantifying the influence of oxygen availability on foraminiferal morphology, both within and among species. Most foraminifera rely on aerobic respiration, but the four benthic foraminifera from the Santa Monica Basin do not show the predicted size and morphological responses to variations in dissolved oxygen concentrations based on first principles of cell physiology: Bolivina spissa shows no volume or volume‐to‐surface area response, Uvigerina peregrina increases in both test volume and volume‐to‐surface area ratio with decreasing dissolved oxygen concentrations, and both Bolivina argentea and Loxostomum pseudobeyrichi decrease in test volume, but only L. pseudobeyrichi shows a decrease in test volume‐to‐surface area ratio with decreasing oxygen concentrations as expected from physiological predictions. These findings imply that the morphological responses of individual foraminiferal species are not necessarily representative of the responses of other foraminiferal species within the community. Our findings further suggest that these species use physiological strategies such as depressed metabolic rates and alternative energy metabolisms to persist in low oxygen environments and, therefore, cannot be used in any simple way as paleo‐oxygen indicators. Should Proterozoic (1,000–542 million years ago) protists have possessed metabolic strategies similar to foraminifera, the sizes and shapes of protists in the fossil record may not usefully constrain ambient oxygen conditions during the appearance and initial taxonomic radiation of heterotrophic eukaryotes.     

Paleomagnetic poles for the Carboniferous Brush Creek limestone and Buffalo siltstone from southwestern Pennsylvania

We report partial results of a larger project being undertaken in the Appalachian Basin to determine the character of the geomagnetic field in the Carboniferous. The Brush Creek limestone, which contains abundant terrigenous matter, yields reliable results from three different sites, while the overlying Buffalo siltstone appears at one locality. AF demagnetization curves, IRM acquisition curves, thermomagnetic analysis, and optical reflection microscopy indicate that the remanence carriers are magnetites or titanomagnetites, which are probably primary in origin. Stepwise AF demagnetization reveals that the best demagnetizing fields are 15–20 and 20–30 mT for the limestone and siltstone units, respectively. A total of 93 specimens from 20 samples (oriented hand samples and field-drilled cores) yields a paleopole at S 36.1, W 55.7, with an alpha-95 of 4.2° and a k of 13.1 for the Brush Creek limestone. The Buffalo siltstone paleopole is located at S 27.4, W 57.0, with alpha-95 = 6.1° and k = 13.0 using 45 specimens from 31 oriented hand samples. These results agree fairly well with the mean reversed Carboniferous paleopole and Noltimier's coal pole, but are sufficiently different to raise the possibility of doing magnetostratigraphic correlations.     

Topography and ice sheet growth

This paper uses a numerical ice sheet model to investigate the role of topography in influencing ice sheet growth. The model is applied to the maritime, mid-latitude uplands of Scotland and relies on a series of assumptions about mass balance, topography, and ice flow. It is driven by an imposed pattern of temperature change. The model is able to predict effectively the extent and thickness of the Loch Lomond ice sheet, using a palaeotemperature curve based on Coleoptera assemblages. A series of experiments with a stepped, constant July air temperature depression suggests that in Scotland a change in excess of ?3·0°C is necessary to initiate ice sheet growth; that steady state ice caps build up at changes of ?3 to ? 6·5°C; and that large ice sheets build up at changes of more than ? 6·625°C. The bifurcation revealed by the last two types of behaviour is the result of topography. Both the vertical amplitude and the spatial distribution of bedrock basins and ridges are important in determining the pattern, rate, and extent of ice sheet growth. The implication is that topography plays an important role in determining the dynamics of ice sheet growth.     

Estimating permeability from field measurements of seismic attenuation in fractured chalk

Broadband (100–4000 Hz) cross‐hole seismic data have been acquired at a borehole test site where extensive hydrological investigations have previously been performed, including in situ estimates of permeability. The rock type is homogeneous chalk and fractures and bedding planes have been identified from well logs. High values of seismic attenuation, Q= 22 ≤ 27 ≤ 33, were observed over a 10 m depth interval where fracture permeability values of 20–50 darcy had been recorded. An attempt has been made to separate the attenuation due to scattering and intrinsic mechanisms. The estimated values of intrinsic attenuation, Q= 31 ≤ 43 ≤ 71, have been reproduced using a number of current theories of seismic‐wave propagation and fluid‐flow‐induced seismic attenuation in cracked and fractured media. A model that considers wavelength‐scale pressure gradients is the preferred attenuation mechanism. Model parameters were obtained from the hydro‐geological and seismic data. However, we conclude that it is not possible to use seismic Q to measure rock permeability remotely, principally because of the inherent uncertainties arising from model parameterisations.