The evolution of a tropical rainforest/grassland mosaic in southeastern Brazil since 28,000 C yr BP based on carbon isotopes and pollen records     

Luiz Carlos Ruiz Pessenda  Paulo Eduardo De Oliveira  Milene Mofatto  Vanda Brito de Medeiros  Ricardo Jos Francischetti Garcia  Ramon Aravena  Jos Albertino Bendassoli  Accio Zuniga Leite  Antonio Roberto Saad  Mario Lincoln Etchebehere 《Quaternary Research》2009,71(3):437-452

The lack of paleoecological records from the montane Atlantic Rainforest of coastal Brazil, a hotspot of biological diversity, has been a major obstacle to our understanding of the vegetational changes since the last glacial cycle. We present carbon isotope and pollen records to assess the impact of the glaciation on the native vegetation of the Serra do Mar rainforest in São Paulo, Brazil. From ca. 28,000 to  22,000 ¹⁴C yr BP, a subtropical forest with conifer trees is indicative of cool and humid conditions. In agreement carbon isotopic data on soil organic matter suggest the presence of C₃ plants and perhaps C₄ plants from  28,000 to  19,000 ¹⁴C yr BP. The significant increase in the sedimentation rate and algal spores from  19,450 to  19,000 ¹⁴C yr BP indicates increasing humidity, associated to an erosion process between  19,000 and  15,600 ¹⁴C yr BP. From  15,600 ¹⁴C yr BP to present there is a substantial increase in arboreal elements and herbs, indicating more humid and warmer climate. From  19,000 to  1000 ¹⁴C yr BP, δ¹³C values indicated the predominance of C₃ plants. These results are in agreement with studies in speleothems of caves, which suggest humid conditions during the last glacial maximum.  相似文献   

Soil slip susceptibility assessment using mechanical–hydrological approach and GIS techniques: an application in the Apuan Alps (Italy)     

Giacomo D’Amato Avanzi  Francesco Falaschi  Roberto Giannecchini  Alberto Puccinelli 《Natural Hazards》2009,50(3):591-603

This study aims at contributing to the soil slip susceptibility assessment in a typical basin of the southern Apuan Alps, Italy. On June 1996, this basin (Cardoso Torrent, 13 km² large) was hit by an extremely heavy rainstorm (maximum intensity of about 160 mm/h), which caused many landslides (debris slide–debris flows) and valley bottom flows (hyperconcentrated flows), destruction and deaths. Detailed surveys provided the characterization of the main factors (geological, geomorphologic, hydrological, hydrogeological and geotechnical) which contributed in triggering landslides. In order to evaluate the soil slip susceptibility in this area, a physically based model was applied and a GIS analysis of digital elevation model was performed. This approach couples a mechanical model based on an infinite slope form of the Mohr–Coulomb failure criterion, and a steady-state hydrological one (a modified version of Shalstab, which considers the cohesion of the debris material potentially involved in landsliding). GIS techniques allowed evaluating the effects of topographic convergence and drainage area on slope failure. In this way, based on the infiltration rate, the triggering of the June 1996 landslides was simulated and the critical rainfall thresholds assessed at about 200–250 mm/24 h.  相似文献   

Astrobiological significance of the sabkha life and environments of southern Tunisia     

Nunzia Stivaletta  Roberto Barbieri  Christine Picard  Marco Bosco 《Planetary and Space Science》2009,57(5-6):597-605

In high-salinity and water-scarce environments, such as in hot and dry deserts, species develop adaptive strategies that are necessary for living in such harsh conditions. Continental ephemeral salt lakes (sabkhas) with periodic flooding from subsurface groundwater followed by high salt concentrations, such as the ones of the northern Africa Chotts, rank among the geological settings wherein the combined effect of salt concentration and fluctuation of water availability make the environment unstable and can thereafter lead to extreme changes. The present study investigates the continental sabkha environments of southern Tunisia, in which ecological niches (i.e. water and salt precipitates, including halite, gypsum, and dolomite) host microbial life. Halophilic microorganisms can be trapped in the extensive saline crusts of halite and gypsum, which can be regarded as the first step of their delivery to the fossil record. The study of halophiles can provide clues for the understanding of life strategies in extreme terrestrial environments, such as sabkhas, which are potential good terrestrial analogs for evaporite-bearing Martian deposits.  相似文献   

The angular-diameter distance maximum and its redshift as constraints on Λ≠ 0 Friedmann–Lemaître–Robertson–Walker models     

Marcelo E. Araújo  William R. Stoeger 《Monthly notices of the Royal Astronomical Society》2009,394(1):438-442

The plethora of recent cosmologically relevant data has indicated that our Universe is very well fitted by a standard Friedmann–Lemaître–Robertson–Walker (FLRW) model, with     and  Ω_Λ≈ 0.73  – or, more generally, by nearly flat FLRW models with parameters close to these values. Additional independent cosmological information, particularly the maximum of the angular-diameter (observer area) distance and the redshift at which it occurs, would improve and confirm these results, once sufficient precise Type Ia supernovae data in the range  1.5 < z < 1.8  become available. We obtain characteristic FLRW-closed functional forms for   C = C ( z )  and     , the angular-diameter distance and the density per source counted, respectively, when  Λ≠ 0  , analogous to those we have for  Λ= 0  . More importantly, we verify that for flat FLRW models z _max– as is already known but rarely recognized – the redshift of C _max, the maximum of the angular-diameter distance, uniquely gives  Ω_Λ  , the amount of vacuum energy in the universe, independent of H ₀, the Hubble parameter. For non-flat models, determination of both z _max and C _max gives both  Ω_Λ  and Ω_M, the amount of matter in the universe, as long as we know H ₀ independently. Finally, determination of C _max automatically gives a very simple observational criterion for whether or not the universe is flat – presuming that it is FLRW.  相似文献   

The H i gas content of galaxies around Abell 370, a galaxy cluster at z= 0.37     

Philip Lah  Michael B. Pracy  Jayaram N. Chengalur  Frank H. Briggs  Matthew Colless  Roberto De Propris  Shaun Ferris  Brian P. Schmidt  Bradley E. Tucker 《Monthly notices of the Royal Astronomical Society》2009,399(3):1447-1470

  相似文献   

Gemini/GMOS IFU gas velocity 'tomography' of the narrow line region of nearby active galaxies     

F. K. B. Barbosa  T. Storchi-Bergmann  R. Cid Fernandes  C. Winge  H. Schmitt 《Monthly notices of the Royal Astronomical Society》2009,396(1):2-18

  相似文献   

Pollution and eutrophication history AD 1800–2005 as recorded in sediments from five lakes in Central Chile     

Lucien von Gunten  Martin Grosjean  Urs Eggenberger  Philipp Grob  Roberto Urrutia  Arturo Morales 《Global and Planetary Change》2009,68(3):198-208

One of the fundamental problems to quantifying past impact of anthropogenic activities is that long series of observational data for pollutant deposition and changes in the nutrient cycling of ecosystems (eutrophication) are often not available. Lake sediments may provide suitable archives to decipher the history of local and regional pollution and eutrophication.Here we provide quantitative high-resolution data for the history of airborne pollutants and eutrophication from sediments of five lakes in Central Chile between ca. AD 1800–2005. We use spheroidal carbonaceous particles (SCPs) from fossil fuel combustion and excess atmospheric Cu deposition from mining activities as a proxy for atmospheric deposition. Organic carbon and nitrogen flux rates to the sediments and C/N ratios are used as a proxy for aquatic primary production and eutrophication.We show that the lake sediment SCP and Cu records are highly consistent and depict in great detail the local and regional history of urban, industrial and transportation history as reported in independent documentary sources and statistics. The pre-industrial and pre-1950 background concentrations (and flux rates) of the substances can be quantified. We can also show that technical measures taken in the early 1980s to trim down Cu emissions from the copper mines reduced the excess atmospheric Cu fallout to the lakes by about 50%.Eutrophication of the lakes did not start before ca. 1980. Prior to that time, warm season temperatures explain most of the variance in TOC and N flux to the sediments. The three dimictic lakes show only moderate eutrophication responses to enhanced N supply (as atmospheric fallout; enrichment factors for TOC and N 1.1–2.6), suggesting that mainly phosphorus controls aquatic primary production. The meromictic lake, where phosphorus recycling is likely, shows the largest response (enrichment factors for TOC and N between 9–20).While all five lakes show overall consistent and similar trends for the pollution history during the 19th and 20th century, there are significant differences in the details of the individual profiles. This suggests that local sources are highly important and the common regional (background) signal is relatively marginal. This is very different from Europe.  相似文献   

Reionization: characteristic scales, topology and observability     

Ilian T. Iliev  Ue-Li Pen  Patrick McDonald  Paul R. Shapiro  Garrelt Mellema  Marcelo A. Alvarez 《Astrophysics and Space Science》2009,320(1-3):39-43

Recently the numerical simulations of the process of reionization of the universe at z>6 have made a qualitative leap forward, reaching sufficient sizes and dynamic range to determine the characteristic scales of this process. This allowed making the first realistic predictions for a variety of observational signatures. We discuss recent results from large-scale radiative transfer and structure formation simulations on the observability of high-redshift Ly-α sources. We also briefly discuss the dependence of the characteristic scales and topology of the ionized and neutral patches on the reionization parameters.  相似文献   

General relativistic singularity-free cosmological model     

Marcelo Samuel Berman 《Astrophysics and Space Science》2009,321(3-4):157-160

We “explain”, using a Classical approach, how the Universe was created out of “nothing”, i.e., with no input of initial energy nor mass. The inflationary phase, with exponential expansion, is accounted for, automatically, by our equation of state for the very early Universe. This is a Universe with no-initial infinite singularity of energy density.  相似文献   

Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars   总被引：1，自引：0，他引：1  

Lynn M. Carter  Bruce A. Campbell  Roger J. Phillips  Ali Safaeinili  Chris H. Okubo  Roberto Seu  Roberto Orosei 《Icarus》2009,199(2):295-302

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155° E and east MFF between 209 and 213° E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD data.  相似文献