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131.
Presented here are stable nitrogen isotope data from a rock hyrax (Procavia capensis) middens from northwestern Namibia that record a series of rapid aridification events beginning at ca. 3800 cal yr BP, and which mark a progressive decrease in regional humidity across the Holocene. Strong correlations exist between this record and other terrestrial and marine archives from southern Africa, indicating that the observed pattern of climate change is regionally coherent. Combined, these data indicate hemispheric synchrony in tropical African climate change during the Holocene, with similar trends characterising the termination of the ‘African Humid Period’ (AHP) in both the northern and southern tropics. These findings run counter to the widely accepted model of direct low-latitude insolation forcing, which requires an anti-phase relationship to exist between the hemispheres. The combined dataset highlights: 1) the importance of forcing mechanisms influencing the high northern latitudes in effecting low-latitude climate change in Africa, and 2) the potential importance of solar forcing and variations in the Earth's geomagnetic shield in determining both long-term and rapid centennial-scale climate changes, identifying a possible mechanism for the variations marking the AHP termination in both the southern and northern tropics. 相似文献
132.
John PARNELL Stephen BOWDEN Paula LINDGREN Mark BURCHELL Daniel MILNER Mark PRICE Emily C. BALDWIN Ian A. CRAWFORD 《Meteoritics & planetary science》2010,45(8):1340-1358
Abstract– A Devonian siltstone from Orkney, Scotland, shows survival of biomarkers in high‐velocity impact experiments. The biomarkers were detected in ejecta fragments from experiments involving normal incidence of steel projectiles at 5–6 km s?1, and in projectile fragments from impact experiments into sand and water at 2–5 km s?1. The associated peak shock pressures were calculated to be in the range of 110–147 GPa for impacts of the steel projectiles into the siltstone target, and hydrocode simulations are used to show the variation of peak pressure with depth in the target and throughout the finite volume projectiles. Thermally sensitive biomarker ratios, including ratios of hopanoids and steranes, and the methylphenanthrene ratio, showed an increase in thermal maturity in the ejecta, and especially the projectile, fragments. Measurement of absolute concentrations of selected biomarkers indicates that changes in biomarker ratios reflect synthesis of new material rather than selective destruction. Their presence in ejecta and projectile fragments suggests that fossil biomarkers may survive hypervelocity impacts, and that experiments using biomarker‐rich rock have high potential for testing survival of organic matter in a range of impact scenarios. 相似文献
133.
Efficient solution of the heat equation is one of the recursive topics in computational physics. Over the years, different
software solutions have been proposed, taking advantage of today’s impressive computing power of parallel machines. In this
work, we consider a hybrid software–hardware approach making use of a field-programmable gate array platform as a heat equation
solver that can be easily attached to a PC using a PCI bus with the goal of obtaining a portable system to be used during
field experiments. The system has been successfully used for the non-destructive inspection of soils in mine detection applications
based on infrared thermography techniques. 相似文献
134.
Paula Chainho Gilda Silva Michael F. Lane José L. Costa Tadeu Pereira Carla Azeda Pedro R. Almeida Inês Metelo Maria J. Costa 《Estuaries and Coasts》2010,33(6):1314-1326
Long-term monitoring studies are needed to understand changes in ecosystem status when restoration measures are implemented.
A long-term data series (1996–2007) of the Tagus estuary (Portugal) intertidal and subtidal benthic communities was collected
in a degraded area where mitigation measures were implemented. Multivariate analysis was used to analyze spatial and temporal
patterns in benthic community composition and trends in five benthic community metrics (i.e., taxonomic richness, density,
biomass, Shannon–Wiener diversity and the AMBI index) were also examined. The results revealed a clear separation between
intertidal and subtidal assemblages, although they had 50% of taxa in common, including the most abundant. Significant positive
trends were found for all metrics showing that both intertidal and subtidal communities responded to the restoration measures
implemented. Nevertheless, biotic indices need some adaptation before being universally applied to intertidal and subtidal
habitats. 相似文献
135.
Terezinha Cássia de Brito Galvão Aloisio Rodrigues Pereira Arnaldo Teixeira Coelho Paula Rodrigues Pereira Joaquim Fernandes Teixeira Coelho 《Geotechnical and Geological Engineering》2011,29(1):49-55
Soil erosion, transport and deposition of sediments represent invisible threats that along time can affect negatively the
existing infrastructure of roads and dams, and the quality of air and water resources. In the last decades, a great number
of researches were devoted to study erosion and sediments transportation processes. However, they addressed mostly areas of
agriculture, water impoundments for dams, and land conservation. A lack of studies for erosion processes in urban areas is
evident, in special those aiming to diminish the deposition of sediments originated from urban areas into the rivers and air,
in special as airborne particulate. Traditionally, the costs associated in controlling erosion and sediments in urban areas
are high and in some case, out of the range for most developing countries. Factors responsible for those high prices are the
use of heavy equipments for soil grading and transportation, the use of costly building materials, and the need for employing
more specialized personnel. In the case of gullies in urban areas, another concern is to ensure proper equipment access to
the area for remediation operations. Hereby, it is presented a cost-effective method to recover an extensive gully erosion
area of about 11,000 m2 located in urban area. Estimated sediments released from this area to the water resources and air were estimated on about
66,000,000 tons. In this study area, bioengineering works were performed in 60 days, with hardly any sediment generated outside
the work area resulting from bioengineering operations. A combination of bioengineering techniques were chosen for this study
area, which was a mixture of rigid and biological products including biologs, silt fence, biodegradable straw blankets sewn
with plastic threads derived from recycled PET bottles, among others. An efficient drainage system was implemented for preventing
upland flow, sub-seepage and seepage. Biologs were used aiming to fulfill a double role, the first as barrier to soil sediments
flow, and the second as filler for empty spaces, avoiding expensive grading operations. All engineering works were concluded
in about 60 days at a cost of US0.89/m < sup > 2 < /sup > . This price is cost-effective if compared with the traditional engineering cost of approximately US 0.89/m2. This price is cost-effective if compared with the traditional engineering cost of approximately US 2.92/m2 because it includes extensive use of grading and soil transportation. An environmental advantage of the bioengineering method
is the immediate reduction of airborne particulates. Furthermore, the overall visual aesthetics improved shortly after the
implementation of the works because the bare ground was substituted by native vegetation. After 6 months the vegetation in
the gully erosion area was totally established. Monitoring was performed after 12 months and no erosion and sediment transport
was visually noticed. The ease implementation of this technique without the need for specialized workers in combination with
low prices makes this method a model for developing countries. 相似文献
136.
Andreas Reisenegger Rodrigo Fernández Paula Jofré 《Astrophysics and Space Science》2007,308(1-4):413-418
The equilibrium composition of neutron star matter is achieved through weak interactions (direct and inverse beta decays),
which proceed on relatively long time scales. If the density of a matter element is perturbed, it will relax to the new chemical
equilibrium through non-equilibrium reactions, which produce entropy that is partly released through neutrino emission, while
a similar fraction heats the matter and is eventually radiated as thermal photons. We examined two possible mechanisms causing
such density perturbations: (1) the reduction in centrifugal force caused by spin-down (particularly in millisecond pulsars),
leading to rotochemical heating, and (2) a hypothetical time-variation of the gravitational constant, as predicted by some theories of gravity and current
cosmological models, leading to gravitochemical heating. If only slow weak interactions are allowed in the neutron star (modified Urca reactions, with or without Cooper pairing),
rotochemical heating can account for the observed ultraviolet emission from the closest millisecond pulsar, PSR J0437-4715,
which also provides a constraint on |dG/dt| of the same order as the best available in the literature.
This work made use of NASA’s Astrophysics Data System Service, and received financial support from FONDECYT through regular
grants 1020840 and 1060644. 相似文献
137.
Paula Chadwick 《Astronomy& Geophysics》2005,46(6):6.12-6.15
Very-high-energy gamma-ray astronomy from the surface of the Earth is now a reality, with the HESS collaboration starting to map the sky and track down these elusive and mysterious sources. Paula Chadwick reports on the story so far. 相似文献
138.
Paula Lindgren John Parnell Craig Norman Darren F. Mark Martin Baron Jens Orm Erik Sturkell James Conliffe Wesley Fraser 《Meteoritics & planetary science》2007,42(11):1961-1969
Abstract— The Ordovician Lockne impact structure is located in central Sweden. The target lithology consisted of limestone and black unconsolidated shale overlaying a Precambrian crystalline basement. The Precambrian basement is uranium‐rich, and the black shale is both uranium‐ and organic‐rich. This circumstance makes Lockne a good candidate for testing the occurrence of U‐Th‐rich bitumen nodules in an impact structure setting. U‐Th‐rich bitumen nodules are formed through irradiation; hence the increase in the complexity of organic matter by a radioactive (uranium‐ and thorium‐rich) mineral phase. U‐Th‐rich bitumen nodules were detected in crystalline impact breccia and resurge deposits from the impact structure, but samples of non‐impact‐affected rocks from outside the impact structure do not contain any U‐Th‐rich bitumen nodules. This implies that in the Lockne impact structure, the nodules are associated with impact‐related processes. U‐Th‐rich bitumen nodules occur throughout the geological record and are not restricted to an impact structure setting, but our studies at Lockne show that this process of irradiation can readily occur in impact structures where fracturing of rocks and a post‐impact hydrothermal system enhances fluid circulation. The irradiation of organic matter by radioactive minerals has previously been proposed as a process for concentration of carbon on the early Earth. Impact structures are suggested as sites for prebiotic chemistry and primitive evolution, and irradiation by radioactive minerals could be an important mechanism for carbon concentration at impact sites. 相似文献
139.
C.M. Denardini H.C. Aveiro P.D.S.C. Almeida L.C.A. Resende L.M. Guizelli J. Moro J.H.A. Sobral M.A. Abdu 《Journal of Atmospheric and Solar》2011,73(11-12):1555-1559
We analyze the daytime efficiency of the interplanetary electric field (IEF) penetration to the equatorial ionosphere based on a correlation analysis carried out between different levels of decomposition applied to IEF intensity measured at the ACE spacecraft and ionospheric electric field intensity inferred from ground based magnetometers located in the equatorial region in Brazil. We compare the time variations of those two electric field intensities by means of a scale-by-scale decomposed time series through wavelet multi-resolution analysis. Efficiency is here defined as the fraction of the variation of the IEF intensity that has penetrated into the equatorial region, and it increases with increasing fraction. Two cases of prompt penetration electric fields (PPEF) are analyzed: one occurring on March 31, 2001, and other on April 17, 2002. Our results show that the penetration effect with time scale ranging around 1 h is maximized in relation to other scales. 相似文献
140.
Paula Lindgren Martin R. Lee Mahmood R. Sofe Michael E. Zolensky 《Meteoritics & planetary science》2013,48(6):1074-1090
Clasts in the CM2 carbonaceous chondrite Lonewolf Nunataks (LON) 94101 have been characterized using scanning and transmission electron microscopy and electron microprobe analysis to determine their degrees of aqueous alteration, and the timing of alteration relative to incorporation of clasts into the host. The provenance of the clasts, and the mechanism by which they were incorporated and mixed with their host material are also considered. Results show that at least five distinct types of clasts occur in LON 94101, of which four have been aqueously altered to various degrees and one is largely anhydrous. The fact that they have had different alteration histories implies that the main part of aqueous activity occurred prior to the mixing and assimilation of the clasts with their host. Further, the presence of such a variety of clasts suggests complex mixing in a dynamic environment involving material from various sources. Two of the clasts, one containing approximately 46 vol% carbonate and the other featuring crystals of pyrrhotite up to approximately 1 mm in size, are examples of unusual lithologies and indicate concentration of chemical elements in discrete areas of the parent body(ies), possibly by flow of aqueous solutions. 相似文献