Late Pleistocene dust deposition in the Patagonian steppe - extending and refining the paleoenvironmental and tephrochronological record from Laguna Potrok Aike back to 55 ka     

Torsten Haberzettl  Flavio S. Anselmetti  Sabine W. Bowen  Michael Fey  Christoph Mayr  Bernd Zolitschka  Daniel Ariztegui  Barbara Mauz  Christian Ohlendorf  Stephanie Kastner  Andreas Lücke  Frank Schäbitz  Michael Wille 《Quaternary Science Reviews》2009,28(25-26):2927-2939

Paleoenvironmental records extending well into the last glacial period are scarce in the steppe regions of southern South America. Here, we present a continuous record for the past 55 ka from the maar lake Laguna Potrok Aike (51°58′ S, 70°23′ W, southern Patagonia, Argentina). Previous studies on a sedimentary core from a lake level terrace near the northern margin of the lake covered parts of Oxygen Isotope Stage (OIS) 3 (59–29 ka) whereas a second core from the centre of the basin comprised the last 16 ka. Tephrostratigraphical constraints and OSL ages from a third core located below the lake level terrace provide the crucial piece to close the gap between the previous coring sites. High-resolution XRF and magnetic susceptibility as well as grain size data indicate a positive hydrological balance alongside with relatively high aeolian activity during the glacial which is contemporaneous with increased dust fluxes in Antarctica. This is therefore the first evidence for contemporaneity of aeolian deposition in both the target area (Antarctica) and in the major source area of Patagonia. During the Holocene climatic conditions driving sediment deposition seem to have been more variable and less dominated by wind compared to glacial times. The identification of a minor lake level lowering at approximately 4 cal ka BP allows to refine earlier paleoenvironmental reconstructions for the Holocene. Within error margins the OSL ages are consistent with published radiocarbon-dated records offering hence a valuable tool for further studies of the sediments from Laguna Potrok Aike. The new chronology confirms the age of three tephra layers up to now only found in Laguna Potrok Aike sediments and ascribed to OIS 3.  相似文献   

Improving SST reconstructions from coral Sr/Ca records: multiple corals from Tahiti (French Polynesia)     

Sri Yudawati Cahyarini  Miriam Pfeiffer  W.-Chr. Dullo 《International Journal of Earth Sciences》2009,98(1):31-40

We reconstruct SST from coral Sr/Ca ratios measured at three coral cores taken from the lagoon of Tahiti (French Polynesia). Two coral cores were drilled from the same coral colony (one horizontally and one vertically), and a third core was drilled vertically from another coral growing at a different site. We evaluate several Sr/Ca records as proxies for regional SST variations: (1) the three single-core records from Tahiti, (2) an average Sr/Ca record computed from the two cores drilled from the same coral colony, (3) an average Sr/Ca record computed from all three Tahiti cores, and (4) an average Sr/Ca record computed from the three Tahiti cores and a fourth core taken from a different island (Rarotonga). On a monthly scale, the average Sr/Ca record including the four coral cores from Tahiti and Rarotonga shows the best correlation with regional SST. The variance of the SST reconstruction is very realistic and the residual SST is low. This suggests that reconstructing SST from average proxy records gives a better representation of regional SST variations. Of the three Tahiti cores, the one that was drilled horizontally shows the best correlation with grid-SST on an annual mean scale. All three Tahiti corals show much larger interannual SST variations than that indicated by grid-SST.  相似文献   

Patterns of Pacific decadal variability recorded by Indian Ocean corals     

Traute Crueger  Jens Zinke  Miriam Pfeiffer 《International Journal of Earth Sciences》2009,98(1):41-52

We investigate Pacific Decadal Oscillation (PDO) signals recorded by two bimonthly resolved coral δ¹⁸O series from La Réunion and Ifaty (West Madagascar), Indian Ocean from 1882 to 1993. To isolate the main PDO frequencies, we apply a band pass filter to the time series passing only periodicities from 16 to 28 years. We investigate the covariance patterns of the coral time series with sea surface temperature (SST) and sea level pressure (SLP) of the Indian and Pacific Oceans. In addition, the empirical orthogonal functions of the filtered SST and SLP fields (single and coupled) are related to the filtered coral times series. The covariance maps show the typical PDO pattern for SST and SLP, confirming the coupling between the Indian and Pacific Oceans. Both corals show the strongest signal in boreal summer. The La Réunion (Ifaty) coral better records SST (SLP) than SLP (SST) pattern variability. We suggest that the filtered La Réunion coral δ¹⁸O represents δ¹⁸O of seawater that varies with the South Equatorial Current, which, in turn, is linked with the SST PDO. The filtered Ifaty coral δ¹⁸O represents SST and is remotely linked with the SLP PDO variability. A combined coral record of the Ifaty and La Réunion boreal summer δ¹⁸O series explains about 64% of the variance of the coupled SST/SLP PDO time series. AGU OS06 special issue “Ocean’s role in climate change—a paleo perspective”.  相似文献   

Ultraviolet spectral reflectance properties of common planetary minerals     

Edward A. Cloutis  Kaitlyn A. McCormack  Amanda R. Hendrix  Michael A. Craig  Stanley A. Mertzman  Miriam A. Riner 《Icarus》2008,197(1):321-347

Ultraviolet spectral reflectance properties (200-400 nm) of a large number of minerals known or presumed to exist on the surfaces of Mars, the Moon, and asteroids, and in many meteorites, were investigated. Ultraviolet reflectance spectra (200-400 nm) of these minerals range from slightly blue-sloped (reflectance decreasing toward longer wavelengths) to strongly red-sloped (reflectance increasing toward longer wavelengths). Most exhibit one or two absorption features that are attributable to FeO charge transfers involving Fe³⁺ or Fe²⁺. The UV region is a very sensitive indicator of the presence of even trace amounts (<0.01 wt%) of Fe³⁺ and Fe²⁺. The major Fe³⁺O absorption band occurs at shorter wavelengths (∼210-230 nm), and is more intense than the major Fe²⁺O absorption band (∼250-270 nm). Ti-bearing minerals, such as ilmenite, rutile and anatase exhibit UV absorption bands attributable to Ti⁴⁺O charge transfers. While the positions of metal-O charge transfer bands sometimes differ for different minerals, the variation is often not diagnostic enough to permit unique mineral identification. However, iron oxides and oxyhydroxides can generally be distinguished from Fe-bearing silicates in the 200-400 nm region on the basis of absorption band positions. Within a given mineral group (e.g., low-calcium pyroxene, olivine, plagioclase feldspar), changes in Fe²⁺ or Fe³⁺ abundance do not appear to result in a measurable change in absorption band minima positions. Absorption band positions can vary as a function of grain size, however, and this variation is likely due to band saturation effects. The intensity of metal-O charge transfers means that some minerals will exhibit saturated UV absorption bands even for fine-grained (<45 μm) powders. In cases where absorption bands are not saturated (e.g., Fe²⁺O bands in some plagioclase feldspars and pyroxenes), changes in Fe²⁺ content do not appear to cause variations in band position. In other minerals (e.g., olivine), changes in band positions are correlated with compositional and/or grain size variations, but this is likely due to increasing band saturation rather than compositional variations. Overall, we find that the UV spectral region is sensitive to different mineral properties than longer wavelength regions, and thus offers the potential to provide complementary capabilities and unique opportunities for planetary remote sensing.  相似文献   

Mesospheric vertical thermal structure and winds on Venus from HHSMT CO spectral-line observations     

Miriam Rengel  Paul Hartogh  Christopher Jarchow 《Planetary and Space Science》2008,56(10):1368-1384

We report vertical thermal structure and wind velocities in the Venusian mesosphere retrieved from carbon monoxide (¹²CO J=2-1 and ¹³CO J=2-1) spectral line observations obtained with the Heinrich Hertz Submillimeter Telescope (HHSMT). We observed the mesosphere of Venus from two days after the second Messenger flyby of Venus (on 5 June 2007 at 23:10 UTC) during five days. Day-to-day and day-to-night temperature variations and short-term fluctuations of the mesospheric zonal flow were evident in our data. The extensive layer of warm air detected recently by SPICAV at 90-100 km altitude is also detected in the temperature profiles reported here.These data were part of a coordinated ground-based Venus observational campaign in support of the ESA Venus Express mission. Furthermore, this study attempts to cross-calibrate space- and ground-based observations, to constrain radiative transfer and retrieval algorithms for planetary atmospheres, and to contribute to a more thorough understanding of the global patterns of circulation of the Venusian atmosphere.  相似文献   

Towards a spectral technique for determining material geometry around evolved stars: application to HD 179821     

J. Nordhaus  I. Minchev  B. Sargent  W. Forrest  E. G. Blackman  O. De Marco  J. Kastner  B. Balick  A. Frank 《Monthly notices of the Royal Astronomical Society》2008,388(2):716-722

HD 179821 is an evolved star of unknown progenitor mass range (either post-asymptotic giant branch or post-red supergiant) exhibiting a double-peaked spectral energy distribution (SED) with a sharp rise from ∼8 to  20 μm  . Such features have been associated with ejected dust shells or inwardly truncated circumstellar discs. In order to compare SEDs from both systems, we employ a spherically symmetric radiative transfer code and compare it to a radiative, inwardly truncated disc code. As a case study, we model the broad-band SED of HD 179821 using both codes. Shortward of 40 μm, we find that both models produce equivalent fits to the data. However, longward of 40 μm, the radial density distribution and corresponding broad range of disc temperatures produce excess emission above our spherically symmetric solutions and observations. For HD 179821, our best fit consists of a   T _eff= 7000 K  central source characterized by  τ _V ∼ 1.95  and surrounded by a radiatively driven, spherically symmetric dust shell. The extinction of the central source reddens the broad-band colours so that they resemble a   T _eff= 5750 K  photosphere. We believe that HD 179821 contains a hotter central star than previously thought. Our results provide an initial step towards a technique to distinguish geometric differences from spectral modelling.  相似文献   

Lanthanoid behaviour in an acidic landscape     

Mats E. Åström  Miriam Nystrand  Peter Österholm  Jason K. Reynolds 《Geochimica et cosmochimica acta》2010,74(3):829-177

Lanthanoids were studied in a boreal landscape where an abundance of acid sulfate soils and Histosols provide a unique opportunity to increase the understanding of how these metals behave in acidic soils and waters and interact with soil and aqueous organic matter. In the acid sulfate soils lanthanoids are mobile as reflected in high to very high concentrations in soil water and runoff (typically a few mg l⁻¹ but up to 12 mg l⁻¹) and abundant release by several relatively weak extractants (ammonium acetate EDTA, sodium pyrophosphate, hydroxylamine hydrochloride) applied on bulk soil. Normalisation with the lanthanoid pool in the underlying parent materials (sulphide-bearing sediments deposited in brackish-water) and soil water showed that the extensive release/retention in the acidic soil was accompanied by large, and variable, fractionation trends across the lanthanoid series. In low-order streams draining these soils, the lanthanoid concentrations were high and, as indicated by frontal ultrafiltration and geochemical modelling, largely dissolved (<1 kDa) in the form of the species LnSO₄⁺ and Ln³⁺. In other moderately acidic stream waters (pH 4.3-4.6), organic complexation was predicted to be important in the <1 kDa fraction (especially for the heavy lanthanoids) and strongly dominating in the colloidal phase (1 kDa-0.45 μm). Along the main stem of a stream in focus (catchment area of 223 km²), lanthanoid concentrations increased downstream, in particular during high flows, caused by a downstream increase in the proportion of acid sulfate soils which are extensively flushed during wet periods. The geochemical models applied to the colloidal Ln-organic phase were not successful in predicting the measured fractionation patterns.  相似文献   

VLA Imaging of the Disk Surrounding the Nearby Young Star TW Hydrae     

Wilner DJ  Ho PT  Kastner JH  Rodríguez LF 《The Astrophysical journal》2000,534(1):L101-L104

The TW Hydrae system is perhaps the closest analog to the early solar nebula. We have used the Very Large Array to image TW Hya at wavelengths of 7 mm and 3.6 cm with resolutions of 0&farcs;1 ( approximately 5 AU) and 1&farcs;0 ( approximately 50 AU), respectively. The 7 mm emission is extended and appears dominated by a dusty disk of radius greater than 50 AU surrounding the star. The 3.6 cm emission is unresolved and likely arises from an ionized wind or gyrosynchrotron activity. The dust spectrum and spatially resolved 7 mm images of the TW Hya disk are fitted by a simple model with temperature and surface density described by radial power laws, T&parl0;r&parr0;~r-0.5 and Sigma&parl0;r&parr0;~r-1. These properties are consistent with an irradiated gaseous accretion disk of mass approximately 0.03 M middle dot in circle with an accretion rate approximately 10-8 M middle dot in circle yr-1 and viscosity parameter alpha=0.01. The estimates of mass and mass accretion rates are uncertain since the gas-to-dust ratio in the TW Hya disk may have evolved from the standard interstellar value.  相似文献   

Luminescence dating of canal sediments from Angkor Borei, Mekong Delta, Southern Cambodia     

David C.W. Sanderson  Paul Bishop  Miriam Stark  Sally Alexander  Dan Penny 《Quaternary Geochronology》2007,2(1-4):322-329

Work to apply luminescence dating to archaeological sites in the Lower Mekong Delta has continued with a programme aimed at dating ancient canal sediments and brick monuments in the vicinity of ancient city of Angkor Borei. Following the successful application of OSL dating to the Paris 2 canal near Angkor Borei further fieldwork and analysis has been undertaken. The infill and substrate of the larger Paris 4 canal connecting Angkor Borei to Oc Eo, some 80 km to the south in Vietnam has been sampled and subjected to luminescence analysis. Field spectroscopy and underwater bleaching experiments were also conducted in the Baray and Angkor Borei in 2004. The results show that both illumination intensities and spectral distributions are severely altered by as little as 1.5 m of turbid water, and that OSL bleaching rates for both quartz and feldspars are reduced. Since quartz resetting is heavily dependant on the UV components in daylight, which have preferentially attenuated the effects of turbid water on OSL zeroing rates are especially marked. The new data from the Paris 4 canal, which has been dated by OSL to be between the first millenium BC and the late first millenium AD are significant to understanding the archaeological development of the Fu Nan state in the Lower Mekong Delta, and the sequence of development of the canal network linking inland agrarian sites and coastal trading centres.  相似文献   

Sense of place in the neighborhood,in locations of urban revitalization   总被引：4，自引：0，他引：4  

Miriam?BilligEmail author 《GeoJournal》2005,64(2):117-130

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