Influence of late holocene pyroclastic eruptions on the sedimentary geochemistry of Lake Rotoiti,North Island,New Zealand     

R. A. Pickrill  C. S. Nelson  P. Stoffers  G. G. P. Craig 《Journal of Paleolimnology》1991,6(3):173-192

Lake Rotoiti in Taupo Volcanic Zone was formed by damming of the drainage system through the floor of Okataina Caldera. Basin sediments are predominantly silt or sand, with mineralogy consistent with derivation from local silicic rocks and airfall tephras. Sandy lithofacies around the shoreline are wave worked deposits. Sand and gravel lithofacies in deeper water and on steep slopes are largely relict or airfall tephras, or both. Profundal sediments are diatomaceous silts. Organic-rich diatomaceous silts also accumulate in near-shore wave-damped zones under weed beds.Short cores penetrated the Tarawera (1886 AD) and Kaharoa (1180 AD) Tephras, identified by their stratigraphic position, geochemistry and mineralogy. Localised slumping is evidenced from secondary tephras interbedded and redeposited within the basin silts. Sedimentation rates in the basins, estimated from the age of bounding tephras, are 0.9 to 4.0 mm y^-1, and are highest under the influence of inflowing water from adjacent Lake Rotorua. For several hundred years prior to the Tarawera eruption sediment accumulation rates and the sediment geochemistry remained unchanged; deposition was predominantly biogenic opaline silica with a small terrestrial component. The Tarawera eruption deposited a terrestrial-silica, aluminum-rich primary tephra across the lake, which was followed by reworked tephra from within the catchment, the effects of which were progressively diluted by biogenic opaline silica as conditions stabilised. While the major effects of the eruption have been rapidly absorbed the lake has not returned to pre-eruption background conditions. A new equilibrium has been attained in which Tarawera Tephra continues to be eroded into the lake and is the principal source for a doubling of sedimentation rates following the eruption. High arsenic levels in some diatomaceous silts are attributed to episodic venting of hydrothermal fluids or gases into the water column.  相似文献   

The Ultimate CCD for Laser Guide Star Wavefront Sensing on Extremely Large Telescopes     

James W. Beletic  Sean Adkins  Barry Burke  Robert Reich  Bernie Kosicki  Vyshnavi Suntharalingham  Charlie Bleau  Ray DuVarney  Richard Stover  Jerry Nelson  Francois Rigaut 《Experimental Astronomy》2005,19(1-3):103-109

All of the extremely large telescopes (ELTs) will utilize sodium laser guide star (LGS) adaptive optics (AO) systems. Most of these telescopes plan to use the Shack-Hartmann approach for wavefront sensing. In these AO systems, the laser spots in subapertures at the edge of the pupil will suffer from spot elongation due to the 10 km extent of the sodium layer and the large separation from the projection laser. This spot elongation will severely degrade the performance of standard geometry wavefront sensing systems. In this paper, we present a CCD with custom pixel morphology that aligns the pixels of each subaperture with the radial extension of the LGS spot. This CCD design will give better performance than a standard geometry CCDs for continuous wave lasers. In addition, this CCD design is optimal for a pulsed sodium laser. The pixel geometry enables each subaperture to follow a laser pulse traversing the sodium layer, providing optimal sampling of a limited number of detected photons. In addition to novel pixel layout, this CCD will also incorporate experimental JFET sense amplifiers and use CMOS design approaches to simplify the routing of biases, clocks and video output. This CCD will attain photon-noise limited performance at high frame rates, and is being incorporated in the plans for the Thirty Meter Telescope (TMT).  相似文献   

Chemical analysis of mineral dust grains by SIMS: Application to a cometary mission     

N. Pailer  E. Grün  D. Bahr  D. Lang 《Planetary and Space Science》1984,32(2):187-192

Analysis of a dust sample (e.g. collected during a cometary rendezvous mission) by SIMS (Secondary Ion Mass Spectroscopy) can provide information on elemental abundances (? 100 amu), the molecular composition of grain surfaces, and isotopic ratios of selected elements. This can be accomplished with dust covering as little as 10^?4 of the collector surface area. In order to demonstrate these capabilities a special experimental set-up for substrate preparation, dust collection and SIMS analysis of dust under ultrahigh vacuum conditions was developed. The comparison of elemental abundance ratios for different olivines and pyroxenes measured with the special SIMS equipment with that measured by an electron microprobe indicated an accuracy for SIMS of the elemental abundance measurements of ? 30%. By varying the energy threshold of secondary ions to be mass-analysed from 0 to 50 eV it is possible to identify molecular ions in the spectra and to estimate their abundance with respect to elemental ions on the same mass line. The ratios of molecular to elemental ions vary by a factor of 1–25. The concept for a future cometary rendezvous experiment as well as first results of chemical investigation on mineral dust samples obtained are reported.  相似文献   

The characteristics and origin of the Gould's Belt     

F. Comerón  J. Torra  A. E. Gómez 《Astrophysics and Space Science》1992,187(2):187-195

Some statistical studies on the nearby early-type stars proposed for observations by HIPPARCOS are presented. Results on spatial orientation and kinematics are in agreement with those found in the literature. An age estimate based on non-kinematical hypothesis is found to be somewhat smaller than expansion ages derives by some authors.Finally, it is suggested that Gould's Belt may have formed as a result of high-velocity cloud impact on the galactic disk, and some estimations are presented.  相似文献   

The period of rotation, shape, density, and homogeneous surface color of the Centaur 5145 Pholus     

S.C. Tegler  W. Romanishin  J. Rall  M. Nelson 《Icarus》2005,175(2):390-396

We present optical photometry of the Centaur 5145 Pholus during 2003 May and 2004 April using the facility CCD camera on the 1.8-m Vatican Advanced Technology Telescope on Mt. Graham, Arizona. We derive a double-peaked lightcurve and a rotation period of 9.980±0.002 h for Pholus, consistent with periods of 9.9825±0.004 and 9.9823±0.0012 h by Buie and Bus (1992, Icarus 100, 288-294) and Farnham (2001, Icarus 152, 238-245). We find a lightcurve peak-to-peak amplitude of 0.60 mag, significantly larger than peak-to-peak amplitude determinations of 0.15 and 0.39 mag by Buie and Bus and Farnham. We use the three observed amplitudes and an amplitude-aspect model to derive four possible rotational pole positions as well as axial ratios of a/b=1.9 and c/b=0.9. If we assume an albedo of 0.04, we find Pholus has dimensions of 310×160×150 km. If we assume Pholus is a strengthless rubble-pile and its non-spherical shape is due to rotational distortion, our axial ratios and period measurements indicate Pholus has a density of 0.5 g cm⁻³, suggestive of an ice-rich, porous interior. By combining B-band and R-band lightcurves, we find B−R=1.94±0.01 and any B−R color variation over the surface of Pholus must be smaller than 0.06 mag (i.e., much smaller than the 1.0<B−R<2.0 range seen among the Centaur and Kuiper belt object populations). By combining our V−R measurements with values in the literature, we find no evidence for any color variegation between the northern and southern hemispheres of Pholus. Observations of the Kuiper belt object 2004 DW (90482) over a time interval of seven hours show no color variation Our observations add to the growing body of evidence that individual Centaurs and KBOs exhibit homogeneous surface colors and hence gray impact craters on radiation reddened crusts are probably not responsible for the surprising range of colors seen among the Centaur and Kuiper belt object populations.  相似文献   

Convection theory before relaxation     

L. Herrera  N. Falcón 《Astrophysics and Space Science》1995,234(1):139-152

The well known mixing length theory of convection is revised for times shorter than the effective time of relaxation into diffusion.Using the Cattaneo equation for the heat flux, we show that the revised theory predicts, before relaxation, a convective flux which is quite different from the flux obtained with the Maxwell-Fourier law.Implications of this result in the outcome of some astrophysical processes are commented upon.  相似文献   

Propagation of muon induced Cherenkov photons through a diffusive medium     

Rodín Porrata   《Astroparticle Physics》2005,23(6):566-571

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An inflationary cosmological model in Dunn's scalar-tensor theory of gravitation     

Ø. Grøn 《Astrophysics and Space Science》1988,140(2):429-430

The gravitational field equations in Dunn's scalar-tensor theory of gravitation are generalized by including a cosmological constant. The resulting equations are solved for a Robertson-Walker line-element with flat three-space. The solution represents a cosmological model that develops into an inflationary era.  相似文献   

A review of studies of the earliest glaciation of Iceland     

Áslaug Geirsdóttir  Jón Eiriksson 《地学学报》1996,8(5):400-414

Glaciers and volcanoes have been the trademark of Iceland for centuries. The glaciers now cover ≅10% of the country and the volcanic regions are covered by lava flows every 5–10 kyr on average. Naturalists have concentrated on these two aspects, making notes on volcanic eruptions and traveling along the glacier margins ever since the first settlement of Iceland in the ninth century in some of the earliest episodes of geological research. Systematic studies of the glaciers began in the latter half of the 18th century. Even earlier, features such as striations and moraines were discovered and described in locations remote from the contemporary glacier margins. These features were interpreted as the effect of a much more extensive ice sheet on the island. At the beginning of the 20th century the discussion of the origin of sediments was unseparable from the age problem. From 1910 to 1950 very few new data were presented on the glaciation history of Iceland. A few sporadic publications either accepted or rejected previous conclusions. Systematic geological mapping of the country started in the wake of World War II encouraged by the acceptance of the plate tectonic theory in the late sixties. One of the most important results of this revived mapping effort was the identification of interbedded sediments interpreted as glacial deposits. Around 1975 Iceland was known in the geological literature for preserving more numerous glacial deposits than found elsewhere on land in the northern hemisphere. Over the last few years, systematic lithofacies analysis of sediments interbedded within lava flows has been conducted with the aim of delineating the earliest glaciation of Iceland and the periodicity of glaciation through the Tertiary and Quaternary. The results show a gradual growth of ice from south-east towards the north and west during the Pliocene-Pleistocene transition interrupted by periods of recessions and intermittent ice-free conditions.  相似文献   

Description and ESR dating of the Holsteinian interglaciation     

G. Linke  O. Katzenberger  R. Grün 《Quaternary Science Reviews》1985,4(4):319-331

The classical region of the Holsteinian interglacial is in the vicinity of Hamburg and the Lower Elbe. It is defined on the basis of pollen and is clearly distinguishable from the Eemian interglacial (Hallik, 1960; Müller, 1974). The Holsteinian interglacial is represented by a sequence of sediments up to 100 m in thickness. These consist of limnic, fluvial, and marine beds and show a transgression up to the height of present sea-level.Twenty-seven molluscs from Holsteinian deposits were used for ESR dating. The ages show that the Holsteinian may be correlated with stage 7 of the deep sea record V28-238 (Shackleton and Opdyke, 1973).  相似文献