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1.
Tuncer Demir Ali Seyrek Herv Guillou Stphane Scaillet Rob Westaway David Bridgland 《Global and Planetary Change》2009,68(4):254
Unspiked K–Ar dating makes the age of the Çakmaközü basalt in eastern Turkey 1818 ± 39 ka (± 2σ). This basalt overlies a staircase of four terraces of the River Murat, a Euphrates tributary, each separated vertically by 20 m. We deduce from the relationship with the basalt that these fluvial deposits aggraded during successive 40 ka climate cycles around the Pliocene–Pleistocene boundary (probably MIS 72-66). The incision and rock uplift at 0.5 mm a− 1, thus indicated, are roughly consistent with the 500 m of entrenchment of this 1.8 Ma Murat palaeo-valley into a former lake basin since the Mid-Pliocene climatic optimum. We infer that the 130 m of incision in this locality since 1.8 Ma dramatically underestimates the associated rock uplift, estimated as 600 m. The 1100 m of rock uplift and 800 m of surface uplift thus estimated since the Mid-Pliocene indicate (assuming Airy isostatic equilibrium) 5 km of thickening of the continental crust, from 37 km to the present 42 km. Eastern Anatolia was thus at a much lower altitude in the Mid-Pliocene than at present, consistent with the low-relief lacustrine palaeo-environment. We infer that the subsequent development of topography and excess crustal thickness are being caused by coupling between surface processes and induced flow in the lower crust: climate change following the Mid-Pliocene climatic optimum resulted in faster erosion that has drawn mobile lower crust beneath the study region. 相似文献
2.
Rob Westaway Herv Guillou Sema Yurtmen Anthony Beck David Bridgland Tuncer Demir Stphane Scaillet George Rowbotham 《Global and Planetary Change》2006,51(3-4):131-171
A set of 13 new unspiked K–Ar dates has been obtained for the Quaternary basaltic volcanism in the Kula area of western Turkey, providing improved age control for the fluvial deposits of the Gediz River that underlie these basalt flows. This dating is able, for the first time, to resolve different ages for the oldest basalts, assigned to category β2, that cap the earliest Gediz deposits recognised in this area, at altitudes of 140 to 210 m above present river level. In particular, the β2 basalt capping the Sarnıç Plateau is dated to 1215 ± 16 ka (± 2σ), suggesting that the youngest underlying fluvial deposits, 185 m above present river level, are no younger than marine oxygen isotope stage (MIS) 38. In contrast, the β2 basalt capping the adjacent Burgaz Plateau is dated to 1014 ± 23 ka, suggesting that the youngest underlying fluvial deposits, 140 m above present river level, date from MIS 28. The staircase of 11 high Gediz terraces capping the latter plateau is thus dated to MIS 48-28, assuming they represent consecutive 40 ka Milankovitch cycles, although it is possible that as many as two cycles are missing from this sequence such that the highest terrace is correspondingly older. Basalt flows assigned to the β3 category, capping Gediz terraces 35 and 25 m above the present river level, have been dated to 236 ± 6 ka and 180 ± 5 ka, indicating incision rates of 0.15 mm a− 1, similar to the time-averaged rates since the eruptions of the β2 basalts. The youngest basalts, assigned to category β4, are Late Holocene; our K–Ar results for them range from zero age to a maximum of 7 ± 2 ka.This fluvial incision is interpreted using numerical modelling as a consequence of uplift caused by a regional-scale increase in spatial average erosion rates to 0.1 mm a− 1, starting at 3100 ka, caused by climate deterioration, since when a total of 410 m of uplift has occurred. Parameters deduced on this basis from the observed disposition of the Early Pleistocene Gediz terraces include the local effective viscosity of the lower crust, which is 2 × 1018 Pa s, the Moho temperature of 660 °C, and the depth of the base of the brittle upper crust, which is 13 km. The thin lithosphere in this area results in high heat flow, causing this relatively shallow base of the brittle upper crust and the associated relatively thick lower-crustal layer, situated between depths of 13 and 30 km. It estimated that around 900 ka, at the start of the 100 ka Milankovitch forcing, the spatial average erosion rate increased slightly, to 0.12 mm a− 1; the associated relatively sluggish variations in uplift rates are as expected given the relatively thick lower-crustal layer.This modelling indicates that the growth of topography since the Pliocene in this study region has not involved a steady state. The landscape was significantly perturbed by the Middle Pliocene increase in erosion rates, and has subsequently adjusted towards—but not reached—a new steady state consistent with these increased erosion rates. It would not be possible to constrain what has been occurring from the Middle to Late Pleistocene or even the Early Pleistocene uplift response alone; information regarding the starting conditions is also essential, this being available in this region from the older geological record of stacked fluvial and lacustrine deposition. This result has major implications for the rigorous modelling of uplift histories in regions of rapid erosion, where preservation of information to constrain the starting conditions is unlikely. 相似文献
3.
Field reconnaissance of the Anti-Atlas coastline, Morocco: Fluvial and marine evidence for Late Cenozoic uplift 总被引:2,自引:2,他引:0
The available evidence regarding the disposition and chronology of Pliocene–Pleistocene fluvial terraces, coastal rock flats, raised beaches and lacustrine sediments adjoining the Anti-Atlas coastline of Morocco has been reviewed and supplemented by additional information from our own field reconnaissance. It is thus suggested that the study region has experienced uplift by 130 m since the Mid-Pliocene climatic optimum ( 3.1 Ma), by 90 m since the latest Pliocene ( 2 Ma), and by 45 m since the Mid-Pleistocene Revolution ( 0.9 Ma). Each of these phases of uplift correlates with a phase of global climate change known independently, and it is thus inferred that the observed uplift is being driven by climate through mechanisms such as erosional isostasy and the associated induced lower-crustal flow. Numerical modelling of the observed uplift history indicates that the mobile lower-crustal layer in the study region is 9 km thick, with a temperature at its base of 500 °C. The base of this mobile layer is inferred to be at 24 km depth, the deepest crust consisting of a layer of mafic underplating that does not flow under ambient conditions. The principal landform in the study region, the coastal rock platform at 60 m a.s.l., thus formed during a succession of interglacial marine highstands in the late Early Pleistocene when uplift rates were low. Although control on the ages of young sediments and landforms is currently extremely limited, being dependent on regional correlation schemes rather than on absolute dating, the study region fits the pattern, emerging worldwide, that climate change is driving the systematic growth of topographic relief evident during the Late Cenozoic. 相似文献
4.
Spectral Energy Distribution (SED) fitting is a well-developed astrophysical tool that has recently been applied to high-redshift Lyα-emitting galaxies. If rest-frame ultraviolet through near-infrared photometry is available, it allows the simultaneous determination of the star formation history and dust extinction of a galaxy. Lyα-emitter SED fitting results from the literature find star formation rates 3 M yr−1, stellar masses 109 M for the general population but 1010 M for the subset detected by IRAC, and very low dust extinction, AV 0.3, although a couple of outlying analyses prefer significantly more dust and higher intrinsic star formation rates. A checklist of 14 critical choices that must be made when performing SED fitting is discussed. 相似文献
5.
The excitation mechanism of solar five-minute oscillations is studied in the present paper. We calculated the non-adiabatic oscillations of low- and intermediate-degree (l = 1 − 25) g4-p39 modes for the Sun. Both the thermodynamic and dynamic couplings are taken into account by using our non-local and time-dependent theory of convection. The results show that all the lowfrequencyf- and p-modes with periods P > 5.4 min are pulsationally unstable, while the coupling between convection and oscillations is neglected. However, when the convection coupling is taken into account, all the g- and low-frequency f- and p-modes with periods longer than 16 minutes (except the low-degree p1-modes) and the high frequency p-modes with periods shorter than 3 minutes become stable, and the intermediate-frequency p-modes with period from 3 to 16 minutes are pulsationally unstable. The pulsation amplitude growth rates depend only on the frequency and almost do not depend on l. They achieve the maximum at ν 3700 μHz (or P 270 sec). The coupling between convection and oscillations plays a key role for stabilization of low-frequency f- and p-modes and excitation of intermediate-frequency p-modes. We propose that the solar 5-minute oscillations are not caused by any single excitation mechanism, but they are resulted from the combined effect of “regular” coupling between convection and oscillations and turbulent stochastic excitation. For low- and intermediatefrequency p-modes, the coupling between convection and oscillations dominates; while for high-frequency modes, stochastic excitation dominates. 相似文献
6.
Jos Luis Cervantes-Rodríguez Alejandro Vazdekis Jordi Cepa 《New Astronomy Reviews》2006,49(10-12):670
We present the modelling of new indicators for old and intermediate age stellar populations based on the absorption features of Mg λ 5175 Å, Fe λ 4383 Å, and Hβ λ 5270, 4860 Å. Spectral models of stellar populations are convolved with the responses of the tunable filters of OSIRIS-GTC to provide photometric indexes with great abilities to separate age and metallicity effects. The new indexes allow us to obtain 2D maps of these features on the basis of a photometric approach and are built up to unreveal most relevant stellar population parameters. 相似文献
7.
W.C. Jones P.A.R. Ade J.J. Bock J.R. Bond J. Borrill A. Boscaleri P. Cabella C.R. Contaldi B.P. Crill P. de Bernardis G. De Gasperis A. de Oliveira-Costa G. De Troia G. Di Stefano E. Hivon A.H. Jaffe T.S. Kisner A.E. Lange C.J. MacTavish S. Masi P.D. Mauskopf A. Melchiorri T.E. Montroy P. Natoli C.B. Netterfield E. Pascale F. Piacentini D. Pogosyan G. Polenta S. Prunet S. Ricciardi G. Romeo J.E. Ruhl P. Santini M. Tegmark M. Veneziani N. Vittorio 《New Astronomy Reviews》2006,50(11-12):945
The Boomerang experiment completed its final long duration balloon (LDB) flight over Antarctica in January 2003. The focal plane was upgraded to accommodate four sets of 145 GHz polarization sensitive bolometers (PSBs), identical to those to be flown on the Planck HFI instrument. Approximately, 195 hours of science observations were obtained during this flight, including 75 hours distributed over 1.84% of the sky and an additional 120 hours concentrated on a region covering 0.22% of the sky. We derive the angular power spectra of the cosmic microwave background (cmb) temperature and polarization anisotropies from these data. The temperature anisotropies are detected with high signal to noise on angular scales ranging from several degrees to 10 arcminutes. The curl-free (EE) component is detected at 4.8σ, and a two-sigma upper limit on the curl (BB) component of 8.6 μK2 is obtained on scales corresponding to 0.5°. Both the temperature and polarization anisotropies are found to be consistent with a concordance ΛCDM cosmology that is seeded by adiabatic density perturbations. In addition to the cmb observations, Boomerang03 surveyed a 300 square degree region centered on the Galactic plane. These observations represent the first light for polarization sensitive bolometers, which are currently operational in two South-Pole based polarimeters, as well as Planck HFI, at frequencies ranging from 100 to 350 GHz (3 mm to 850 μm). 相似文献
8.
Spatially resolved cloud structure on Uranus: Implications of near-IR adaptive optics imaging 总被引:1,自引:1,他引:0
Seven-band near-IR adaptive optics imaging of Uranus by the Keck II telescope during 2004, with the assistance of selected Hubble Space Telescope images, provides new constraints on the uranian vertical cloud structure and CH4 mixing ratio, after tuned deconvolutions are applied to remove significant limb darkening distortions. The most strongly absorbing bands approximately agree with the stratospheric haze model of Rages et al. [Rages, K., Pollack, J.B., Tomasko, M.G., Doose, L.R., 1991. Icarus 89, 359–376]. The next most absorbing bands suggest a CH4 relative humidity of 50–60% above the 1.2-bar condensation level. Window channels imply effective cloud pressures at 12° S that vary from 9 to 3.5 bars, and reflectivity values that vary from 7 to 4%, as the assumed CH4 mixing ratio varies from 0.75 to 4%. The shape of the center-to-limb radiance profile is in best agreement with the deep cloud being translucent, with relatively low optical depth, and is most consistent with low methane mixing ratios (0.75–1%) if the cloud particles are conservative. Non-conservative particles provide good fits over a wide range of mixing ratios. If C and S are enhanced by the same factor over solar mixing ratios, then the cloud pressures inferred from near-IR observations would be less than H2S condensation pressures for methane mixing ratios of 1.3% or greater. The bright band at 45° S must be partly produced by increased particulate scattering at pressures 2 bars to be consistent with its absence in 1.9-μm images and its presence in 0.619-μm images. The reflectivity of the lower clouds declines to nearly negligible values in the northern hemisphere, where I/F observations beyond 50° N are nearly those of a clear atmosphere. The most surprising result is the general lack of scattering originating from the 1.2-bar region where methane is expected to condense. Exceptions occur for discrete features. A large and long-lived discrete feature at 34° S is associated with particulates near 700 mb and 4.5 bars. The highest discrete feature, near 26° N, reached pressures 200 mb and was eleven times brighter than the background atmosphere in K′ images. 相似文献
9.
The Transkei Basin deposits document the sediment transport around South Africa and, hence, reveal details of palaeocurrent activity of this region for the past 36 my. Thermohaline driven water masses like the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW), which are part of the global conveyor belt and the main motor for the heat transfer worldwide, have to pass the southern tip of Africa. A large amount of their sedimentary freight is deposited in the submarine Transkei Basin. By the investigation of high resolution seismic reflection data from central Transkei Basin sediments, we reconstructed depocentres and interface outlines for five different time slices since Cretaceous times. Since at least Late Eocene times, we observe an increasing activity of proto-AABW and later proto-NADW in the Transkei Basin. The current's settings were recurrently modified by various large scale global events, like the opening of the Drake Passage Gateway and the Tasman Gateway ( 34 Ma), respectively, or the closure of the Panama Isthmus ( 3 Ma). The investigations reveal a strong influence of global tectonic and climatic events on NADW and (proto-) AABW and their influence on Transkei Basin deposition. 相似文献
10.
Land clearance and hydrological change in the Sahel: SW Niger 总被引:1,自引:0,他引:1
Marc J. Leblanc Guillaume Favreau Sylvain Massuel Sarah O. Tweed Maud Loireau Bernard Cappelaere 《Global and Planetary Change》2008,61(3-4):135-150
In the West African semiarid belt of the Sahel, for the second half of the XXth century, lasting droughts (1970s–1980s) and one of the World's highest population growths have resulted in major land cover and hydrological changes that can be quantified using aerial photographs. This paper aims to provide one of the longest combined observations of land cover and hydrological changes for semiarid areas using a time series of normalised mosaics of aerial photographs dating back from 1950, field inquiries, and updated groundwater data. The 500 km2 study area in southwest Niger was chosen (i) for its rural environment representative of the rain-fed agriculture belt of the Sahel and (ii) to encompass the main hydrological study sites investigated in this region over the past two decades (Hapex-Sahel and AMMA experiments, 1990–2000s). Results have significant implications for future freshwater availability and food security in the Sahel.Between 1950 and 1992, 80% of the study area has been cleared, firstly to open new areas for agriculture and secondly for firewood supply (59% of the plateaux, 42% of the valley bottoms, and 87% of the hillslopes). Intermediate aerial photograph surveys (1960, 1975) attest an accelerated loss in the woody savannah that could not be recovered on the short term. A strong, indirect impact of land clearance is observed on the water resources. Land clearance has resulted in a modification of the soil properties and infiltration capacity and has led to an increase in Hortonian runoff collected in numerous gullies and ponds. Between 1950 and 1992, aerial photographs show a 2.5 fold increase of the drainage density with the development of large drainage systems and new ponds. Groundwater data also indicate a continuous rise in the water table, mostly noticeable since the 1980s with a mean groundwater level rise of 4 m for the 1963–2005 period (+ 15% in aquifer reserves). The relatively short 30 year time-lag between the onset of land clearance and the beginning of the water table rise is linked to the process of indirect groundwater recharge and is timed with the connectivity of the drainage network and the formation of new ponds. Finally, the sustained increase in surface runoff and groundwater recharge during the past four decades indicates that the indirect impact of land clearance on the terrestrial water balance has been stronger than that of the long-lasting Sahelian drought. As the rate of land clearance increased for the past century in semiarid Africa, its main hydrological effects may not yet be fully perceptible. 相似文献
11.
Here we present a crustal folding or buckling mechanism to explain the rootless 3–5 km high Alborz Mountains in northern Iran as well as 10 km of Late Miocene to recent subsidence in the south Caspian basin and 3–6 km of subsidence in the central Iranian basin in the context of the middle Miocene to recent Arabia–Eurasia collision. A key element of the mechanism is the presence of lateral and vertical lithospheric strength contrasts between the north Iranian continental and south Caspian oceanic crusts: when compression from the collision is applied across the region, the strong south Caspian oceanic crust, buried under > 10 km of premiddle Miocene sediment, interacts with the bottom of the mechanically strong continental upper crust of northern Iran, resulting in upward buckling of the continental crust and downward buckling of the oceanic crust. We test this mechanism using a finite-element numerical model with a Maxwell rheology and obtain results that are consistent with the geological and geophysical observations. The observations compiled here and the model results demonstrate the potential for using this region as a natural laboratory for studying the early stages of continent–oceanic collision, including processes like basin inversion, fault localization and, potentially, subduction initiation. 相似文献
12.
Paleogeographic and paleotopographic evolution of the Swiss and Eastern Alps since the Oligocene 总被引:3,自引:1,他引:2
The Alps are representing the most prominent topographic feature of central Europe in the last 30 Million years. Integration of sediment budget and thermochronological data show that crustal blocks within the Swiss and Eastern Alps experienced differential erosion and uplift, with eastward decreasing rates. Since 30 Ma, in the course of collision and slab breakoff, the Swiss and western Eastern Alps were uplifted and formed a mountain range. Moderate erosion rates stabilized 2 Myr later after adaption to the new base level. Between 28 Ma and 5 Ma, erosion rates shifted above and below different regional mean levels. These shifts of erosion rates were governed by changes of the tectonic setting. Moderate temporary drops of erosion rates and lowering of topography occurred at 21 Ma and 15 Ma in the course of lateral extension. Temporary rises occurred between 24 and 22 Ma during thrust loading and between 18 and 16 Ma during axial updoming of core complexes. By 5 Ma, a sustainable strong increase of erosion rates occurred in the Swiss and Western Alps. A much weaker increase is detected in the Eastern Alps somewhat later, which makes a tectonic trigger, probably a 2nd slab breakoff, more reasonable than a climatic trigger. However, this controversial 5-Ma event is superimposed by accelerated erosion after 2.7 Ma, governed by the onset of cyclic glaciations. Effective valley incision and sediment evacuation by glacial erosion caused isostatic compensation and pronounced young uplift of the Alps.The post-Eocene paleotopographic evolution of key areas is indirectly assessed on the base of erosion rates. Spatial change of paleotopography of the Alps and central Europe is estimated and illustrated in 3 paleogeographic maps. 相似文献
13.
The Hard X-ray Modulation Telescope (HXMT) is a broadband X-ray (1250 keV) astronomical satellite. Its core payload, the High Energy X-ray Telescope (hereafter HE), is operated in the hard X-ray energy range (20250 keV) and dedicated to the hard X-ray high-sensitivity survey observation, hard X-ray sky mapping and high-sensitivity focused observations towards particular celestial bodies. In order to achieve a high sensitivity, it is important to reduce effectively the background that is caused by the interactions between the detector and space particles (γ-ray, protons, electrons, neutrons). Combining a series of references about the near-earth space background with the up-to-date observational data, this paper presents a set of self-consistent data and energy spectrum formulae of near-earth space particles for the convenience of applications. In addition, by the simulative calculations with the software Geant 4, the background of HXMT and its variations with the time and orbit are also given. 相似文献
14.
Jean-Pierre Peulvast Vanda Claudino Sales Franois Btard Yanni Gunnell 《Global and Planetary Change》2008,62(1-2):39-60
The Brazilian Northeast affords good opportunities for obtaining reliable timings and rates of landscape evolution based on stratigraphic correlations across a vast region. The landscape formed in the context of an episodically fluctuating but continuously falling base level since the Cenomanian. After formation of the transform passive margin in Aptian times, landscape development was further driven by a swell-like uplift with its crest situated 300 km from the coastline. The seaward flank of this swell or broad monocline between the interior Araripe and coastal Potiguar basins was eroded, and currently forms a deeply embayed plain bordered by a semi-circular, north-facing erosional escarpment. The post-Cenomanian uplift caused an inversion of the Cretaceous basins and generated a landscape in which the most elevated landforms correspond either to resistant Mesozoic sedimentary caprock, or to eroded stumps of syn-rift Cretaceous footwall uplands. Denudation in the last 90 My never exceeded mean rates of 10 m·My− 1 and exhumed a number of Cretaceous stratigraphic unconformities. As a result, some topographic surfaces at low elevations are effectively Mesozoic land surfaces that became re-exposed in Cenozoic times. The Neogene Barreiras Formation forms a continuous and mostly clastic apron near the coast. It testifies to the last peak of erosion in the hinterland and coincided with the onset of more arid climates at 13 Ma or earlier. The semi-circular escarpment is not directly related to the initial breakup rift flanks, which had been mostly eroded before the end of the Mesozoic, but the cause and exact timing of post-Cenomanian crustal upwarping are poorly constrained. It could perhaps have been a flexural response of the low-rigidity lithosphere to sediment loads on the margin, and thus a slowly ongoing process since the late Cretaceous. Uplift could instead be the consequence of a more discrete dynamic event related either to Oligocene magmatism in the region, or to continental-scale far-field stresses determined by Andean convergence. 相似文献
15.
Ilian T. Iliev Ue-Li Pen J. Richard Bond Garrelt Mellema Paul R. Shapiro 《New Astronomy Reviews》2006,50(11-12):909
We present the first calculation of the kinetic Sunyaev–Zel’dovich (kSZ) effect due to the inhomogeneus reionization of the universe based on detailed large-scale radiative transfer simulations of reionization. The resulting sky power spectra peak at ℓ = 2000–8000 with maximum values of [ℓ(ℓ + 1)Cℓ/(2π)]max 4–7 × 10 −13. The scale roughly corresponds to the typical ionized bubble sizes observed in our simulations, of 5–20 Mpc. The kSZ anisotropy signal from reionization dominates the primary CMB signal above ℓ = 3000. At large-scales the patchy kSZ signal depends only on the source efficiencies. It is higher when sources are more efficient at producing ionizing photons, since such sources produce larger ionized regions, on average, than less efficient sources. The introduction of sub-grid gas clumping in the radiative transfer simulations produce significantly more power at small-scales, but has little effect at large-scales. The patchy reionization kSZ signal is dominated by the post-reionization signal from fully-ionized gas, but the two contributions are of similar order at scales ℓ 3000 − 104, indicating that the kSZ anisotropies from reionization are an important component of the total kSZ signal at these scales. 相似文献
16.
This is one of the first applications of geochemical proxies to define changes in vegetation, hydrology and atmospheric dust influence on a peat deposit in the southern hemisphere. A 6.6 m deep peat record from Lynch's Crater in NE-Queensland, Australia, provides a sensitive 5000 to 30,000 cal years BP archive of environmental change. The deposit consists of 1.5 m of ombrotrophic peat underlain by a minerotrophic peat. Within the minerotrophic section, sponge spicules and diatom fragments offer evidence of prolonged flooding of the peat environment resulting in several layers containing (up to 50%) high inorganic material. The Ca and Mg data display episodes of enhanced dust influx and nutrient recycling and support previous palynological studies that show a Pleistocene to Holocene switch from sclerophyll woodlands to rainforest vegetation. 相似文献
17.
《Astroparticle Physics》2009,32(2):89-99
Atmospheric parameters, such as pressure (P), temperature (T) and density (ρ∝P/T), affect the development of extensive air showers initiated by energetic cosmic rays. We have studied the impact of atmospheric variations on extensive air showers by means of the surface detector of the Pierre Auger Observatory. The rate of events shows a 10% seasonal modulation and 2% diurnal one. We find that the observed behaviour is explained by a model including the effects associated with the variations of P and ρ. The former affects the longitudinal development of air showers while the latter influences the Molière radius and hence the lateral distribution of the shower particles. The model is validated with full simulations of extensive air showers using atmospheric profiles measured at the site of the Pierre Auger Observatory. 相似文献
18.
Owing to the very gently sloping nature of the flood plain in the lower White Nile valley, which is underlain by a former lake-bed, the depositional record in that area is unusually well preserved. In Egypt and along the Blue Nile phases of erosion have destroyed segments of the sedimentary record, but the White Nile sequence is a good proxy for both the main Nile and the Blue Nile. During the last 15 ka, at least, times of high flow in the Blue Nile and main Nile were synchronous with those in the White Nile.Not all the White Nile flood deposits have been preserved but calibrated radiocarbon dates obtained on fossil freshwater and amphibious Pila shells and fish bones indicate that White Nile levels were high around 14.7–13.1 ka, 9.7–9.0 ka, 7.9–7.6 ka, 6.3 ka and 3.2–2.8 ka. The Blue Nile record is more fragmentary and that of the main Nile even more so except for the Holocene Nile delta. Calibrated radiocarbon ages for high Blue Nile flows indicate very high flood levels towards 13.9–13.2 ka, 8.6 ka, 7.7 ka and 6.3 ka.Incision by the Blue Nile and main Nile has caused progressive incision in the White Nile amounting to at least 4 m since the terminal Pleistocene 15 ka ago and at least 2 m over the past 9 ka. The Blue Nile seems to have cut down at least 10 m since 15 ka and at least 4 m since 9 ka. The time-transgressive and relatively late inception of plant domestication in the Nile valley may partly reflect this history of incision. Nile incision would propagate upstream into the White Nile valley, draining previously swampy areas along the valley floor, which would then become accessible to cultivation. 相似文献
19.
Lyman alpha emitters (LAEs) are high-redshift galaxies that are believed to be actively star-forming and low in mass. Although such a population of galaxies would not be expected to lie on the Hubble Sequence, there is much we can learn from their morphological properties, including the size and distribution of their star-forming regions, the spatial correlation between emission in different bandpasses, and the possible presence of a merger/interaction with one or more neighboring galaxies. Early morphological studies suggest that most LAEs are small in size (1 kpc) and concentrated, but some (20–45%) are seen to display clumpy/irregular morphologies extending over larger radii. Constructing a more detailed picture will require a standard set of depth-independent morphological diagnostics for high-redshift galaxies. These diagnostics can then be measured differentially with redshift, shedding light on the evolution of the galaxy formation process with time. 相似文献
20.
M. D. Roberts 《Astroparticle Physics》2000,12(4):471
Monte Carlo simulations show that the pulse profile of
erenkov photons measured near the core of an extensive air shower is sensitive to the secondary muon/electron ratio of the cascade.
erenkov pulses can easily be measured with a single large area mirror viewed by a photomultiplier tube subtending a small field of view (1°). Even for such a simple experiment, exposed to EAS from a range of core locations and arrival directions, strong statistical differences are shown to exist between the pulse parameter distributions of primary protons and those of heavier primary particles. A range of primary energies can be investigated by varying the zenith angle of observations. In this paper, results from simulations of primaries in the energy range 20 TeV to 400 TeV are presented, although in principle the technique could be extended to include the knee of the spectrum. At the lower end of this energy range results can be compared to direct measurements of the composition, while measurements at the upper end can augment results from existing ground based experiments. 相似文献