Sandstone weathering processes damaging prehistoric rock paintings at the Albarracin Cultural Park, NE Spain   总被引：1，自引：0，他引：1  

G. Benito  M. J. Machado  C. Sancho 《Environmental Geology》1993,22(1):71-79

The rock paintings in cliff-foot caves of the Albarracin Cultural Park are known as some of the most important evidences of the Levantine prehistoric art of Spain (8000–3000 BP). The paintings are on sandstone (Buntsandstein facies) of Triasic age, which may develop intense weathering. The analysis of the variables controlling the weathering indicate that salt and wetting-drying weathering are responsible for granular disintegration and flaking, which lead to rock painting deterioration.  相似文献   

Concentrations en PCB et DDT des sédiments superficiels de la rive suisse du lac Léman     

Par G. Burgermeister  K. Aswald  L. Machado  J. Mowrer  J. Tarradellas 《Aquatic Sciences - Research Across Boundaries》1983,45(1):233-245

The PCB concentrations of sediments can be considered as high and risk to produce a long-term contamination of the lake ecosystem. The shoreline near Lausanne area is the most polluted. Those results show that pollution by DDT is decreasing.  相似文献   

Submarine pits of the azores plateau     

Frederico Machado 《Bulletin of Volcanology》1959,21(1):109-116

  相似文献   

Reactive sulfides relationship with metals in sediments from an eutrophicated estuary in Southeast Brazil   总被引：5，自引：0，他引：5  

Machado W  Carvalho MF  Santelli RE  Maddock JE 《Marine pollution bulletin》2004,49(1-2):89-92

The potential association of acid-volatile sulfides (AVS) and reactive (HCl soluble) Fe with the distribution of reactive trace metals (Cu, Cd, Ni, Pb and Zn) was investigated in sediment cores collected in the Igua?u river estuarine system (Guanabara bay, Brazil), within the river (core R) and the bay (core B) areas. Moderate to extremely high AVS concentrations (33-314 micromol g(-1)) were found in the rapidly-accumulated sediments of this eutrophicated estuary. AVS showed significant correlations with Fe, Ni and Pb in core B, whereas no correlation between AVS and metals was observed in core R. Results suggest that the AVS:Fe molar ratio may often reflect the diagenetic conditions controlling the distribution of Cd and Cu in core B better than AVS and Fe levels themselves. A shift in the biogeochemical controls of metal distribution from the river to the open bay sediments is suggested, with a greater association of most metals with AVS and Fe in bay sediments.  相似文献   

Mini monsters     

Karina Williams 《Astronomy& Geophysics》2006,47(3):3.36-3.37

Tiny black holes could be popping in and out of existence in our atmosphere all the time. It sounds a scary prospect but, as Karina Williams explains, there is (probably) no need to be alarmed.  相似文献   

Rotation of T Tauri Stars Revisited     

L.J.R. Machado  M.T.V.T. Lago  J.J.G. Lima 《Astrophysics and Space Science》1998,261(1-4):69-70

This paper reviews the rotation of T Tauri stars. It summarizes the results of a complete bibliographical survey of all photometric and spectroscopic data available in connection with rotation studies of these stars. It is shown that T Tauri stars are slow rotators. More than half of the stars in the sample have been found to have v sin i < 25 km/s. Furthermore, the histogram for the photometric periods of rotation is clearly bimodal, with a peak at approximately 3 days and another at around 8 days. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

On the origin and age of the Steep Rock buckshot, Ontario, Canada     

A.De Barros Machado 《Chemical Geology》1987,60(1-4):337-349

The Steep Rock buckshot is a brecciated pisolithic ferruginous bauxite found in the Errington iron mine, Steep Rock Lake, Ontario, Canada. It occurs within the almost vertical ore zone included in the Archean Steep Rock Group, reaching a depth in excess of 1100 ft. (335 m). Successive authors considered its age as Precambrian. The biological observations now presented deny this supposed Precambrian age. Treatment of the concretions by HCI, followed by staining, put in evidence abundant microscopic figurate animal and vegetal remains, namely shreds of insect integument and varied fragments of vascular plant material. Comparable results were obtained by casting and by scanning electron miscroscopy (SEM) of the surface of the concretions and by thin sections. All these remains are Phanerozoic and one of them is probably not anterior to the Late Cretaceous. The macro- and micromorphology of the concretions obey models of termite constructions and the spectrum of organic remains is identical to the one found in tubulo-alveolar laterites and in termite constructions. The presence in the matrix of canaliculi attributable to thin roots gives force to the idea that the buckshot was originated near the soil surface.  相似文献   

Energy release in solar flares     

J. C. Brown  E. Correia  F. Fárník  H. Garcia  J. -C. Hénoux  T. N. La Rosa  M. E. Machado  H. Nakajima  E. R. Priest 《Solar physics》1994,153(1-2):19-31

Team 2 of the Ottawa FLARES 22 Workshop dealt with observational and theoretical aspects of the characteristics and processes of energy release in flares. Main results summarized in this article stress the global character of the flaring phenomenon in active regions, the importance of discontinuities in magnetic connectivity, the role of field-aligned currents in free energy storage, and the fragmentation of energy release in time and space.Report of Team 2, Flares 22 Workshop, Ottawa, May 25–28, 1993.  相似文献   

The Queens' flare: Its structure and development; precursors,pre-flare brightenings,and aftermaths     

Cornelis de Jager  Marcos E. Machado  Aert Schadee  Keith T. Strong  Zdeněk Švestka  Bruce E. Woodgate  W. Van Tend 《Solar physics》1983,84(1-2):205-235

We continue previous research on the limb flare of 30 April, 1980, 20:20 UT, observed in X-rays by several instruments aboard the Solar Maximum Mission (SMM). It is shown quantitatively that the flare originated in an emerging magnetically confined kernel (diameter ～ 20″) which existed for about ten to fifteen minutes, and from which energetic electrons streamed, in at least two injections, into a previously existing complicated magnetic loop system thus forming a less bright but extended and long-lived tongue. The tongue had a length of ～ 35 000 km and lasted ～ 90 min in X-rays (～ 10 keV); at lower energies (～ 0.7 keV) it was larger (～ 80 000 km) and lasted longer. The total number of energetic electrons (≈ 10³⁷) initially present in the kernel is of the same order as the number present in the tongue after the kernel's decline. This gives evidence that the energetic electrons in the tongue originated mainly in the kernel. The electron number densities in the kernel and tongue at maximum brightness were ～ 4.5 × 10¹¹ and ～ 1 × 10¹¹ cm^#X2212;3, respectively. During the first eight minutes of its existence the tongue was hotter than the kernel, but it cooled off gradually. Its decline in intensity and temperature was exponential; energy was lost by radiation and by conduction through the footpoints of the loop system. These footpoints have a cross-section of only ～ 3 × 10⁶ km². This small value, as well as photographs in a Civ UV emission line, suggests a highly filamentary structure of the system; this is further supported by the finding that the tongue had a ‘filling factor’ of ～ 10^#X2212;2. Several faint X-ray brightenings (? 0.005 of the flare's maximum intensity) were observed at various locations along the solar limb for several hours before and after the flare. At ～ 30 min before the flare's onset a faint (? 0.02) flare precursor occurred, coinciding in place and shape with the flare. First the kernel precursor was brightest but the tongue precursor increased continuously in brightness and was the brightest part of the precursor some 10–15 min after the first visibility of the kernel precursor, until the start of the main flare. This suggests (weak) continuous electron acceleration in the tongue during a period of at least 30 min. The main flare was caused by strong emergence of magnetic field followed by two consecutive field line reconnections and accelerations in a small loop system, causing footpoint heating. Subsequently plasma streamed (convectively) into a pre-existing system of larger loops, forming the tongue.  相似文献   

Energetics of a compact flare     

Marcos E. Machado 《Solar physics》1983,89(1):133-147

We study the spatial and spectral characteristics of the 3.5 to 30.0 keV emission in a solar flare of 9 May, 1980. We find that: (a) A classical thick target interpretation of the hard X-ray burst at energies E 10 keV implies that approximately all the electrons contained within the flare loop(s) have to be accelerated per second. (b) A thermal model interpretation does not fit the data, unless its characteristics are such that it does not represent an efficient alternative to the acceleration model. We thus conclude that: (c) Acceleration does take place during the early phase of the impulsive hard X-ray event, but substantial amount of the emission at low (<20 keV) energies is of thermal origin. (d) We show the evolution of the energy content in the flare volume, and find that the energy input requirements are such that 10² erg cm^-3 s^-1 have to be released within the flare structure(s), for a period of time comparable to that of the hard X-ray burst emission. We also point out that although the main flare component ( 90% of the soft X-ray emission) was confined to a compact magnetic kernel, there are evidences of interaction of this structure with a larger field structure connecting towards the leading portion of the active region, where secondary H brightenings were observed.  相似文献