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301.
In order to improve the reliability of climate reconstruction, especially the climatologies outside the modern observed climate
space, an improved inverse vegetation model using a recent version of BIOME4 has been designed to quantitatively reconstruct
past climates, based on pollen biome scores from the BIOME6000 project. The method has been validated with surface pollen
spectra from Eurasia and Africa, and applied to palaeoclimate reconstruction. At 6 cal ka BP (calendar years), the climate
was generally wetter than today in southern Europe and northern Africa, especially in the summer. Winter temperatures were
higher (1–5°C) than present in southern Scandinavia, northeastern Europe, and southern Africa, but cooler in southern Eurasia
and in tropical Africa, especially in Mediterranean regions. Summer temperatures were generally higher than today in most
of Eurasia and Africa, with a significant warming from ∼3 to 5°C over northwestern and southern Europe, southern Africa, and
eastern Africa. In contrast, summers were 1–3°C cooler than present in the Mediterranean lowlands and in a band from the eastern
Black Sea to Siberia. At 21 cal ka BP, a marked hydrological change can be seen in the tropical zone, where annual precipitation
was ∼200–1,000 mm/year lower than today in equatorial East Africa compared to the present. A robust inverse relationship is
shown between precipitation change and elevation in Africa. This relationship indicates that precipitation likely had an important
role in controlling equilibrium-line altitudes (ELA) changes in the tropics during the LGM period. In Eurasia, hydrological
decreases follow a longitudinal gradient from Europe to Siberia. Winter temperatures were ∼10–17°C lower than today in Eurasia
with a more significant decrease in northern regions. In Africa, winter temperature was ∼10–15°C lower than present in the
south, while it was only reduced by ∼0–3°C in the tropical zone. Comparison of palaeoclimate reconstructions using LGM and
modern CO2 concentrations reveals that the effect of CO2 on pollen-based LGM reconstructions differs by vegetation type. Reconstructions for pollen sites in steppic vegetation in
Europe show warmer winter temperatures under LGM CO2 concentrations than under modern concentrations, and reconstructions for sites in xerophytic woods/scrub in tropical high
altitude regions of Africa are wetter for LGM CO2 concentrations than for modern concentrations, because our reconstructions account for decreased plant water use efficiency. 相似文献
302.
Chronology and depositional processes of the laminated sediment record from Lac d'Annecy, French Alps 总被引:1,自引:0,他引:1
A high resolution sediment record spanning the entire time since the ice retreat after the Last Glacial Maximum has been recovered from Lac d'Annecy. The main focus of this study is to develop a reliable chronology of the record and to evaluate the environmental variability during the period of Late Würmian ice retreat. Most of the record is laminated. These laminations are of different structure, composition, and thickness. On the basis of varve stratigraphy five sedimentation units were identified which correspond to particular stages in the deglaciation of the region. Except for one each facies type has been related to an annual cycle of deposition. Varve counting in combination with radiocarbon dating provides the time control of the record and dates the base of lacustrine deposits to 16,600 varve yrs BP. The beginning of the Late Glacial is marked by a shift from clastic to endogenic carbonate varves caused by the climatic warming. Clastic varves have been further subdivided into a succession of complex and standard varve types. These variations of clastic varve formation are triggered by the ice retreat and related hydrological variations in the watershed of the lake. Sedimentological, mineralogical and isotopic data help identify different sediment sources of the sub-layers. Proximal sediments originate from local carbonaceous bedrock whereas distal sediments have characteristics of the molassic complex of the outer Alps. The alternation of proximal and distal sediments in the varve sequence reflects the deglaciation of the Annecy area with a changing influence of local and regional glaciers. The melting of the Alpine ice sheet is the driving force for regional environmental changes which in turn control the sediment transport and deposition processes in Lac d'Annecy. 相似文献
303.
de Quadros Telmo F. P. Koppe Jair C. Strieder Adelir J. Costa João Felipe C. L. 《Natural Resources Research》2003,12(1):57-65
A large number of mineral deposits are associated with hydrothermal processes, especially auriferous deposits. In such processes, studies on percolating fluids may indicate the presence of potash (K), among other elements. In this study, aerogammaspectrometric data-processing methodologies are evaluated, especially those methods based on the suppression of the primary contribution of potassium, the result of lithological and soil variations, and to environmental conditions. Resulting maps point out the contribution of hydrothermal K. This processing procedure was used because of the association of hydrothermal K and auriferous mineralizations according to the deposit model defined for the studied region. Intensity maps locate the areas with great influence of hydrothermal K. Data integration required to improve a change in the gammaspectrometric data processing in order to positively correlate hydrothermalised areas. Data integration could distinguish high and medium favorable targets for mineral exploration of lode-Au deposits in the studied region. 相似文献
304.
305.
We use hydrocode modeling to investigate dynamic models for the collapse of the Chicxulub impact crater. Our aim is to integrate the results from numerical simulations with kinematic models derived from seismic reflection and wide-angle velocity data to further our understanding of the formation of large impact craters. In our simulations, we model the collapse of a 100-km diameter, bowl-shaped cavity formed in comprehensively fractured crustal material. To facilitate wholesale collapse, we require that the strength of the target be significantly weakened. In the present model, we achieve this using acoustic fluidization, where strong vibrations produced by the expanding shock wave cause extreme pressure fluctuations in the target. At times and positions where the overburden pressure is sufficiently counteracted, the frictional resistance is reduced, enabling the rock debris to flow. Our simulations produce a collapsed crater that contains most of the features that we observe in the seismic data at Chicxulub. In particular, we observe a topographic peak ring, formed as material that is originally part of the central uplift collapses outward and is thrust over the inwardly collapsing transient crater rim. This model for peak-ring generation has not been previously demonstrated by numerical simulations and predicts that the peak ring is composed of deeply derived material and that the stratigraphy within the peak ring is overturned. 相似文献
306.
Laurence C. Breaker William W. Broenkow William E. Watson Young-Heon Jo 《Estuaries and Coasts》2008,31(2):239-257
Elkhorn Slough is a shallow, tidally forced estuary that is directly connected to Monterey Bay. It is ebb-dominated and, due
to continued erosion, the tidal prism has tripled over the past 40 years. Water level measurements at four locations are used
to examine tidal and nontidal oscillations in Elkhorn Slough. The tidal response of Elkhorn Slough differs from that of Monterey
Bay primarily due to the generation of a relatively large number of shallow-water tidal constituents that are due to tidal
distortion caused by friction along the bottom and lateral boundaries, intertidal storage, and nonlinear advection. The shallow-water
constituents range from 3 to almost 15 cycles per day (cpd) and include a rich variety of overtides and compound tides, whose
amplitudes generally increase toward the head of the slough. The tidal harmonics are seasonally dependent, with lower amplitudes
during the fall and winter and higher amplitudes in summer. The tidal constituents were examined using two types of spectral
decomposition, the conventional power spectrum and the more recent Hilbert spectrum. Unlike the power spectrum, the Hilbert
spectrum does not reveal any harmonic structure in the data. Energy associated with tidal distortion in this case appears
to be broadly distributed across the spectral continuum. At least four nontidal oscillations occur in Elkhorn Slough with
frequencies of 26.0, 39.7, 52.7, and 66.9 cpd. The Hilbert spectrum reveals maxima at 26, 39.7, and 66.9 cpd, but not at 52.7 cpd,
suggesting that it is harmonically related to the oscillation at 26.0 cpd. The nontidal oscillations fall into the range of
frequencies associated with the natural oscillations of Monterey Bay. However, evolutionary power spectra indicate that they
appear to be permanent features of the system and thus are not necessarily consistent with seiche-like oscillations that are
often transient and subject to damping. These oscillations could be caused by several factors including edge waves along the
coast of Monterey Bay, long-period surface waves of atmospheric origin that enter the bay from offshore, or breaking internal
waves in and around the Monterey Submarine Canyon. In conclusion, detailed hydrodynamic models are needed to provide a better
understanding of how tidal harmonics are generated and preserved in Elkhorn Slough, and to determine the origin of the natural
oscillations in Monterey Bay. 相似文献
307.
João A. Hackerott Mostafa Bakhoday Paskyabi Joachim Reuder Amauri P. de Oliveira Stephan T. Kral Edson P. Marques Filho Michel dos Santos Mesquita Ricardo de Camargo 《Boundary-Layer Meteorology》2017,165(2):211-231
We discuss scalar similarities and dissimilarities based on analysis of the dissipation terms in the variance budget equations, considering the turbulent kinetic energy and the variances of temperature, specific humidity and specific CO\(_2\) content. For this purpose, 124 high-frequency sampled segments are selected from the Boundary Layer Late Afternoon and Sunset Turbulence experiment. The consequences of dissipation similarity in the variance transport are also discussed and quantified. The results show that, for the convective atmospheric surface layer, the non-dimensional dissipation terms can be expressed in the framework of Monin–Obukhov similarity theory and are independent of whether the variable is temperature or moisture. The scalar similarity in the dissipation term implies that the characteristic scales of the atmospheric surface layer can be estimated from the respective rate of variance dissipation, the characteristic scale of temperature, and the dissipation rate of temperature variance. 相似文献
308.
As a first step towards accurate quantification of the polysomatic states of biopyriboles, we have studied the polysomatic transformation between amphibole and hydrous triple-chain silicate (TCS) in the synthetic system Na2O-MgO-SiO2-H2O (NMSH). The reaction is: 4Na2Mg4Si6O16(OH)2 TCS 3Na2.67Mg5.33Si8O21.33(OH)2.67. Amphibole We have characterised a polysomatic intergrowth of amphibole and TCS (synthesized at 2 kbar/(653° C) by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), infrared spectroscopy and 29Si magic-angle-spinning (MAS) NMR spectroscopy. The sample is a fine-scale lamellar intergrowth of double- and triple-chain structures; lamellae are 27 Å to hundreds of Ångströms wide. The 29Si MAS NMR spectrum of the intergrowth is explicitly a superposition of the individual amphibole and TCS spectra. By ensuring that the recycle delay time used considers the longest spin-lattice relaxation time (ca. 900 s), the relative amounts of double- and triple-chain structures can be quantified by simple deconvolution of the spectrum. The relative amounts of double- and triple-chain structures are 42 ± 5 and 58 ± 5 mol%, respectively. With regard to quantifying populations of chain multiplicities in biopyriboles, we believe that 29Si NMR is more accurate than the conventional HRTEM fringe-counting method (Maresch and Czank 1983, 1988), and is far superior to XRD and infrared spectroscopy, which suffer from high sensitivity to particle size and calibration problems. 29Si MAS NMR can provide an accurate means of monitoring the progress of polysomatic reactions in biopyriboles. It is likely to be most effective for samples containing only a few different chain multiplicities (e.g. m = 1, 2, 3 and perhaps 4), such as occur in natural pyroxenes and amphiboles. 相似文献
309.
Textural evidence for the partial breakdown of staurolite-biotite and andalusite-biotite assemblages to cordierite-orthoamphibole implies high temperature metasomatic depletion of K2O in semi-pelitic rocks from Springton, South Australia. The origin of the reaction textures is discussed with reference to K2O-T diagrams derived from the topologically equivalent K2O–(-H2O) diagram showing both discontinuous and Fe–Mg continuous reactions. The involvement of fluids in the metasomatic process is implied by the scale of K2O removal and suggests that the outcrop pattern of cordierite-gedrite rocks reflects, at least in part, a heterogeneous distribution of advecting fluids in the metamorphic pile at high temperatures.Mineral abbreviations used in text and figures ab
albite
- alm
almandine
- als
aluminosilicate
- and
andalusite
- anth
anthophyllite
- bt
biotite
- cd
cordierite
- fe-bt
Fe-rich biotite
- fe-cd
Fe-rich cordierite
- fe-oa
Fe-rich orthoamphibole
- fe-st
Fe-staurolite
- gt
garnet
- ksp
potassium feldspar
- ky
kyanite
- mg-cd
Mg-rich cordierite
- mg-oa
Mg-rich orthoamphibole
- mg-st
Mg-rich staurolite
- mu
muscovite
- oa
orthoamphibole
- phl
phlogopite
- plag
plagioclase
- py
pyrope
- sill
sillimanite
- st
staurolite
- v
vapour 相似文献
310.
Linear stacking procedures are used to retrieve the attenuation of 91 modes belonging to the 3rd, 4th and 5th Rayleigh overtones branches in the 80–160 s period range, and contributing to the so-called PhaseX wave group. Our data show in general slightly less attenuation than expected from available models. Data space inversion shows that, when combined with previously measured fundamental modeQ's, this new dataset improves resolution significantly in the 1000–2000 km depth range. Based on this remark, we carry out a number of parameter space inversions. Our results suggest a narrow (80–200 km) zone of high attenuation (Q
=75–90), low attenuation in the intermediate mantle (670–1500 km); (Q
350), and lower values in the deeper mantle (Q
200). 相似文献