Climate, agriculture, and cycles of human occupation over the last 4000 yr in southern Zacatecas, Mexico     

Michelle Elliott  Christopher T. Fisher  Roberto S. Molina Garza  Deborah M. Pearsall 《Quaternary Research》2010,74(1):26-35

Scholars attribute the growth and decline of Classic period (AD 200-900) settlements in the semi-arid northern frontier zone of Mesoamerica to rainfall cycles that controlled the extent of arable land. However, there is little empirical evidence to support this claim. We present phytolith, organic carbon, and magnetic susceptibility analyses of a 4000-yr alluvial record of climate and human land use from the Malpaso Valley, the site of one such Classic frontier community. The earliest farming occupation is detected around 500 BC and appears related to a slight increase of aridity, similar to the level of the modern day valley. By AD 500, the valley's Classic period Mesoamerican settlements were founded under these same dry conditions, which continued into the Postclassic period. This indicates that the La Quemada occupation did not develop during a period of increased rainfall, but rather an arid phase. The most dramatic changes detected in the valley resulted from the erosion associated with Spanish Colonial grazing and deforestation that began in the 16th century. The landscape of the modern Malpaso Valley is thus primarily the product of a series of intense and rapid transformations that were concentrated within the last 400 yr.  相似文献   

Spatio-temporal variability in debris-flow activity: a tree-ring study at Geisstriftbach (Swiss Alps) extending back to AD 1736     

Markus Stoffel  Michelle Bollschweiler  Susanne Widmer  Annina Sorg 《Swiss Journal of Geoscience》2010,103(2):283-292

Past debris-flow activity on the forested cone of the Geisstriftbach torrent (St. Niklaus, Valais, Swiss Alps) was assessed from growth disturbances in old conifer trees, providing a much improved record of past events. The study of 633 tree-ring sequences sampled from 252 European larch (Larix decidua Mill.), Norway spruce (Picea abies (L.) Karst.) and Silver birch (Betula pendula Roth.) trees allowed reconstruction of 53 debris-flow events since AD 1736. The spatial analysis of trees affected during particular events on the geomorphic map allowed for a spatial representation of individual events and a reconstruction of four flow patterns. Based on our results and Siegfried maps, we believe that before the formation of a dogleg near the cone apex in the late 1890s, debris flows preferentially used the channels located in the west-southwestern part of the Geisstriftbach cone. This study contributes to our understanding of debris-flow processes on cones and provides an example of how dendrogeomorphic techniques may help in the reconstruction and understanding of debris flows in Alpine areas.  相似文献   

Modeling stresses on satellites due to nonsynchronous rotation and orbital eccentricity using gravitational potential theory     

John Wahr  Zane A. Selvans  Amy C. Barr  Michelle M. Selvans 《Icarus》2009,200(1):188-206

The tidal stress at the surface of a satellite is derived from the gravitational potential of the satellite's parent planet, assuming that the satellite is fully differentiated into a silicate core, a global subsurface ocean, and a decoupled, viscoelastic lithospheric shell. We consider two types of time variability for the tidal force acting on the shell: one caused by the satellite's eccentric orbit within the planet's gravitational field (diurnal tides), and one due to nonsynchronous rotation (NSR) of the shell relative to the satellite's core, which is presumed to be tidally locked. In calculating surface stresses, this method allows the Love numbers h and ?, describing the satellite's tidal response, to be specified independently; it allows the use of frequency-dependent viscoelastic rheologies (e.g. a Maxwell solid); and its mathematical form is amenable to the inclusion of stresses due to individual tides. The lithosphere can respond to NSR forcing either viscously or elastically depending on the value of the parameter , where μ and η are the shear modulus and viscosity of the shell respectively, and ω is the NSR forcing frequency. Δ is proportional to the ratio of the forcing period to the viscous relaxation time. When Δ?1 the response is nearly fluid; when Δ?1 it is nearly elastic. In the elastic case, tensile stresses due to NSR on Europa can be as large as ∼3.3 MPa, which dominate the ∼50 kPa stresses predicted to result from Europa's diurnal tides. The faster the viscous relaxation the smaller the NSR stresses, such that diurnal stresses dominate when Δ?100. Given the uncertainty in current estimates of the NSR period and of the viscosity of Europa's ice shell, it is unclear which tide should be dominant. For Europa, tidal stresses are relatively insensitive both to the rheological structure beneath the ice layer and to the thickness of the icy shell. The phase shift between the tidal potential and the resulting stresses increases with Δ. This shift can displace the NSR stresses longitudinally by as much as 45° in the direction opposite of the satellite's rotation.  相似文献   

A method to infer past surface mass balance and topography from internal layers in martian polar layered deposits     

Michelle R. Koutnik  Edwin D. Waddington 《Icarus》2009,204(2):458-470

Internal layers in ice masses can be detected with ice-penetrating radar. In a flowing ice mass, each horizon represents a past surface that has been subsequently buried by accumulation, and strained by ice flow. These layers retain information about relative spatial patterns of accumulation and ablation (mass balance). Internal layers are necessary to accurately infer mass-balance patterns because the ice-surface shape only weakly reflects spatial variations in mass balance. Additional rate-controlling information, such as the layer age, the ice temperature, or the ice-grain sizes and ice-crystal fabric, can be used to infer the absolute rate of mass balance. To infer mass balance from the shapes of internal layers, we solve an inverse problem. The solution to the inverse problem is the best set or sets of unknown boundary conditions or initial conditions that, when used in our calculation of ice-surface elevation and internal-layer shape, generate appropriate predictions of observations that are available. We also show that internal layers can be used to infer martian paleo-surface topography from a past era of ice flow, even though the topography may have been largely altered by subsequent erosion. We have successfully inferred accumulation rates and surface topography from internal layers in Antarctica. Using synthetic data, we demonstrate the ability of this method to solve the corresponding inverse problem to infer accumulation and ablation rates, as well as the surface topography, for martian ice. If past ice flow has affected the shapes of martian internal layers, this method is necessary to infer the spatial pattern and rate of mass balance.  相似文献   

Simulations of glitches in isolated pulsars     

Michelle B. Larson †  Bennett Link 《Monthly notices of the Royal Astronomical Society》2002,333(3):613-622

Many radio pulsars exhibit glitches wherein the star's spin rate increases fractionally by ∼10⁻¹⁰–10⁻⁶. Glitches are ascribed to variable coupling between the neutron star crust and its superfluid interior. With the aim of distinguishing among different theoretical explanations for the glitch phenomenon, we study the response of a neutron star to two types of perturbations to the vortex array that exists in the superfluid interior: (1) thermal motion of vortices pinned to inner crust nuclei, initiated by sudden heating of the crust, (e.g., a starquake), and (2) mechanical motion of vortices (e.g., from crust cracking by superfluid stresses). Both mechanisms produce acceptable fits to glitch observations in four pulsars, with the exception of the 1989 glitch in the Crab pulsar, which is best fitted by the thermal excitation model. The two models make different predictions for the generation of internal heat and subsequent enhancement of surface emission. The mechanical glitch model predicts a negligible temperature increase. For a pure and highly conductive crust, the thermal glitch model predicts a surface temperature increase of as much as ∼2 per cent, occurring several weeks after the glitch. If the thermal conductivity of the crust is lowered by a high concentration of impurities, however, the surface temperature increases by ∼10 per cent about a decade after a thermal glitch. A thermal glitch in an impure crust is consistent with the surface emission limits following the 2000 January glitch in the Vela pulsar. Future surface emission measurements coordinated with radio observations will constrain glitch mechanisms and the conductivity of the crust.  相似文献   

WHEN A RESIDENCE IS NOT A HOUSE: EXAMINING RESIDENCE-BASED MIGRATION DEFINITIONS     

Michelle Behr  Patricia Gober 《The Professional geographer》1982,34(2):178-184

Traditional definitions of migration contain implicit assumptions: origin and destination places are fixed residences, households inhabit one residence at any given time, and dwelling units form the center of individual activity spaces. Though these assumptions hold for certain movers, they restrict migration studies to a narrow range of movement. It is argued here that such assumptions bias present notions of migrant behavior and selectivity. A more realistic appraisal might consider integration in sets of spatial networks rather than movement between two discrete points in space.  相似文献   

Characterization and quantification of inorganic constituents of tropical peats and organic-rich deposits from Tasek Bera (Peninsular Malaysia): implications for coals     

Raphael A. J. Wüst  Colin R. Ward  R. Marc Bustin  Michelle I. Hawke 《International Journal of Coal Geology》2002,49(4)

Since the Carboniferous, tropical latitudes have been the site of formation of many economic coal deposits, most of which have a restricted range of mineralogical composition as a result of their depositional environment, climatic conditions, and diagenesis. Mineralogical and microscopic investigations of tropical peats from Tasek Bera, Peninsular Malaysia, were performed in order to better understand some of these factors controlling the nature, distribution and association of inorganic matter in peat-forming environments. Distribution and nature of the inorganic fraction of peat deposits give insight into the weathering conditions and detrital input into the mire system. Because the inorganic composition of peat deposits is determined by plant communities, height of water table, and climate, the results of the quantitative and qualitative analysis can be used to reconstruct palaeoclimatic conditions.Tasek Bera is a peat-accumulating basin in humid tropical Malaysia with organic deposits of low- to high-ash yield and thus representative of many ancient peat-forming environments. Clay minerals dominate the mineralogical composition of the peat and organic-rich sediments, while quartz and clays dominate the underlying siliciclastic deposits. Kaolinite is the most abundant clay mineral in the organic deposits with minor amounts of illite and vermiculite. Particle size analyses indicate that >50% of the inorganic detrital fraction is <2 μm. Most detrital quartz grains range in size from fine silt to fine sand. The fine sand fraction accounts for a maximum of 5 wt.% of the inorganic constituents. In addition, abundant biogenic and non-biogenic, Al- and Si-rich amorphous matter occur. In the ombrotrophic (low-nutrient) environment, biogenic inorganic material contributes up to >75% of the ash constituents. As a consequence, the vegetational communities make an important contribution to the inorganic and overall ash composition of peats and coals. The ash content of the often inundated peat consists on average of 10% opaline silica from diatoms and sponge spicules, while the ash of the top deposits may have up to 50% biogenic silica. Hence, Al- and Si-hydroxides and the opaline silica from diatoms and sponges represent a large repository of Al and Si, which may form the basis of mineral transformation, neoformation and alteration processes during coalification of the peat deposits. As a result, most coal deposits from paleotropical environments are anticipated to have little to no biogenic inorganic material but high amounts of secondary clays, such as kaolinite (detrital kaolinite, resilisified kaolinite, or desilisified gibbsite) or illite, and various amounts of detrital and authigenetic quartz.  相似文献   

The ELP solution for the main problem of the moon and some applications     

Michelle Chapront-Touze 《Celestial Mechanics and Dynamical Astronomy》1982,26(1):63-69

First derivatives of the ELP 2000 solution of the main problem have been obtained. They are used for computation of the lunar motion perturbations due to the Earth's oblateness and to secular terms in the solar eccentricity and perigee.Proceedings of the Conference on Analytical Methods and Ephemerides: Theory and Observations of the Moon and Planets. Facultés universitaires Notre Dame de la Paix, Namur, Belgium, 28–31 July, 1980.  相似文献   

The geology of the Oceanographer Transform: The ridge-transform intersection     

J. A. Karson  P. J. Fox  H. Sloan  K. T. Crane  W. S. F. Kidd  E. Bonatti  J. B. Stroup  D. J. Fornari  D. Elthon  P. Hamlyn  J. F. Casey  D. G. Gallo  D. Needham  R. Sartori  OTTER 《Marine Geophysical Researches》1984,6(2):109-141

Seven dives in the submersible ALVIN and four deep-towed (ANGUS) camera lowerings have been made at the eastern ridge-transform intersection of the Oceanographer Transform with the axis of the Mid-Atlantic Ridge. These data constrain our understanding of the processes that create and shape the distinctive morphology that is characteristic of slowly-slipping ridge-transform-ridge plate boundaries. Although the geological relationships observed in the rift valley floor in the study area are similar to those reported for the FAMOUS area, we observe a distinct change in the character of the rift valley floor with increasing proximity to the transform. Over a distance of approximately ten kilometers the volcanic constructional terrain becomes increasingly more disrupted by faulting and degraded by mass wasting. Moreover, proximal to the transform boundary, faults with orientations oblique to the trend of the rift valley are recognized. The morphology of the eastern rift valley wall is characterized by inward-facing scarps that are ridge-axis parallel, but the western rift valley wall, adjacent to the active transform zone, is characterized by a complex fault pattern defined by faults exhibiting a wide range of orientations. However, even for transform parallel faults no evidence for strike-slip displacement is observed throughout the study area and evidence for normal (dip-slip) displacement is ubiquitous. Basalts, semi-consolidated sediments (chalks, debris slide deposits) and serpentinized ultramafic rocks are recovered from localities within or proximal to the rift valley. The axis of accretion-principal transform displacement zone intersection is not clearly established, but appears to be located along the E-W trending, southern flank of the deep nodal basin that defines the intersection of the transform valley with the rift floor.  相似文献   

Buckle propagation in pipelines with non-uniform thickness   总被引：1，自引：0，他引：1  

Jianghong  Michelle S.   《Ocean Engineering》2001,28(10)

Finite element solutions for the steady-state buckle propagation pressure in a pipeline with non-uniform thickness are given. The results are useful in finding buckle propagation pressures in corroded pipelines. It has been found that when corrosion is equal to or less than 10% of the original pipeline thickness, the pipeline collapses in an overall shell buckling mode; otherwise, the pipeline experiences local buckling. The propagation pressure decreases with both the thickness and angular extent of the reduced section, but the rate of decrease with thickness reduction is almost independent of the angular extent of corrosion when it is greater than 90°.  相似文献