Archaeological sites composed only of surficial lithics are widespread in arid environments. Numerical dating of such sites is challenging, however, and even establishing a relative chronology can be daunting. One potentially helpful method for assigning relative chronologies is to use lithic weathering, on the assumption that the most weathered artifacts are also the oldest. Yet, few studies have systematically assessed how local environmental processes affect weathering of surficial lithics. Using macroscopic analyses, we compared the weathering of surficial lithic assemblages from seven mid‐to‐late Holocene archaeological sites sampled from four different microenvironments in the Atacama Desert of northern Chile. Changes in polish, texture, shine, and color were used to establish significant differences in weathering between two kinds of locations: interfluves and canyon sites. Lithics from interfluve sites were moderately to highly weathered by wind and possessed a dark coating, whereas canyon lithics were mildly weathered despite greater exposure to moisture, often lacked indications of eolian abrasion, and lacked dark coatings. Our results show that lithic weathering can be used as a proxy for relative age, but only after considering local environmental factors. The power of such chronologies can be improved by combining archaeological, paleoenvironmental, geomorphological, and taphonomic data. 相似文献
During the last 2 decades, Global Positioning System (GPS) measurements have become a very important data-source for ionospheric
studies. However, it is not a direct and easy task to obtain accurate ionospheric information from these measurements because
it is necessary to perform a careful estimation of the calibration constants affecting the GPS observations, the so-called
differential code biases (DCBs). In this paper, the most common approximations used in several GPS calibration methods, e.g.
the La Plata Ionospheric Model (LPIM), are applied to a set of specially computed synthetic slant Total Electron Content datasets
to assess the accuracy of the DCB estimation in a global scale scenario. These synthetic datasets were generated using a modified
version of the NeQuick model, and have two important features: they show a realistic temporal and spatial behavior and all
a-priori DCBs are set to zero by construction. Then, after the application of the calibration method the deviations from zero
of the estimated DCBs are direct indicators of the accuracy of the method. To evaluate the effect of the solar activity radiation
level the analysis was performed for years 2001 (high solar activity) and 2006 (low solar activity). To take into account
seasonal changes of the ionosphere behavior, the analysis was repeated for three consecutive days close to each equinox and
solstice of every year. Then, a data package comprising 24 days from approximately 200 IGS permanent stations was processed.
In order to avoid unwanted geomagnetic storms effects, the selected days correspond to periods of quiet geomagnetic conditions.
The most important results of this work are: i) the estimated DCBs can be affected by errors around ±8 TECu for high solar
activity and ±3 TECu for low solar activity; and ii) DCB errors present a systematic behavior depending on the modip coordinate,
that is more evident for the positive modip region. 相似文献
For the degassing of coal seams, either prior to mining or in un-minable seams to obtain coalbed methane, it is the combination of cleat frequency, aperture, connectivity, stress, and mineral occlusions that control permeability. Unfortunately, many potential coalbeds have limited permeability and are thus marginal for economic methane extraction. Enhanced coalbed methane production, with concurrent CO2 sequestration is also challenging due to limited CO2 injectivity. Microwave energy can, in the absence of confining stress, induce fractures in coal. Here, creation of new fractures and increasing existing cleat apertures via short burst, high-energy microwave energy was evaluated for an isotropically stressed and an unstressed bituminous coal core. A microwave-transparent argon gas pressurized (1000 psi) polycarbonate vessel was constructed to apply isotropic stress simulating ~ 1800 foot depth. Cleat frequency and distribution was determined for the two cores via micro-focused X-ray computed tomography. Evaluation occurred before and after microwave exposure with and without the application of isotropic stress during exposure. Optical microscopy was performed for tomography cleat aperture calibration and also to examine lithotypes influences on fracture: initiation, propagation, frequency, and orientation. It was confirmed that new fractures are induced via high-energy microwave exposure in an unconfined bituminous core and that the aperture increased in existing cleats. Cleat/fracture volume, following microwave exposure increased from 1.8% to 16.1% of the unconfined core volume. For the first time, similar observations of fracture generation and aperture enhancement in coal were also determined for microwave exposure under isotropic stress conditions. An existing cleat aperture, determined from calibrated X-ray computed tomography increased from 0.17 mm to 0.32 mm. The cleat/fracture volume increased from 0.5% to 5.5%. Optical microscopy indicated that fracture initiated likely occurred in at least some cases at fusain microlithotypes. Presumably this was due to the open pore volumes and potential for bulk water presence or steam pressure buildup in these locations. For the major induced fractures, they were mostly horizontal (parallel to the bedding plane) and often contained within lithotype bands. Thus it appears likely that microwaves have the potential to enhance the communication between horizontal wellbore and existing cleat network, in coal seams at depth, for improved gas recovery or CO2 injection. 相似文献
The World Landslide Forum (WLF) is a triennial mainstream conference aimed at gathering scientists, stakeholders, policy makers
and industry members dealing with the management of landslide risk. The First World Landslide Forum, organised by the International
Consortium of Landslides, UNESCO, WMO, Food and Agriculture Organization, UNISDR, UNU, UNEP, IBRD, UNDP, ICSU, WFEO, KU and
the Japan Landslide Society, was held in 2008 at the United Nations University, Tokyo. The 1st WLF adopted the 2008 Tokyo
Declaration “Strengthening the International Programme on Landslides with UNISDR”. The Second World Landslide Forum has the
objective to further develop the outcomes of the First Forum in Tokyo 2008 by providing a global cross-cutting information
and cooperation platform for all types of organisations representing academia, United Nations organisations, governments,
private enterprises and individuals that contribute to landslide research, practice, education and decision making and are
willing to strengthen landslide and other related Earth system risk reduction strategies. The emphasis of this forum will
be “Putting Science into Practice” with special attention given to actual implementation of technology and research in everyday
applications and procedures with the direct involvement of researchers, engineers, private enterprises, stakeholders as well
as policy and decision makers. The abstract submission and pre-registration of participants was quite successful with 649
received abstracts and more than 800 participants at May 2011. The full organisation of the event is now in progress taking
into consideration the new figure and interest in scientific community and stakeholders. 相似文献
Quartz silt is widespread in terrestrial sediments [1]. Its ubiquity has led to its neglect as a geomaterial, and studies of silt as such are relatively rare, but it presents an interesting and continuing petrological problem. Is silt a specific geological material, is it defined by a formation process, and a set of size parameters? In the world of clastic quartz sedimentology there are obvious mode sizes; there is a sand mode at around 300–500 μm and a silt mode, an order of magnitude smaller, at 30–50 μm. Are these both defined by specific geological processes? 相似文献
Based on a series of experiments conducted by two regional climate models (RCA4 and LMDZ) with and without soil moisture-atmosphere coupling, we investigate the role of soil moisture on the occurrence of surface air temperature extremes and its persistence in Southeastern South America. Our analysis reveals that both factors, soil moisture-atmosphere coupling and relatively drier soil conditions, enhance the temperature extremes. In addition, the existence of soil-atmosphere coupling and the associated soil moisture variability is crucial for the development of the extremes in SESA. The key role of soil-atmosphere coupling is also reflected in the intrinsic persistence of hot days, which is greater in simulations with interactive soil moisture than in those with prescribed soil conditions. In the absence of soil-atmosphere coupling, the imprint of the anomalous dry (and also wet) soil conditions on the intensity and persistence of hot days is weaker.
Through the continuing development of improved detectors and detector arrays, far-infrared/submillimeter astronomical space
missions have had enormous successes in recent years. Despite these advances, the diffraction-limited angular resolving power
has remained virtually constant. The advent of telescopes with apertures of several meters will improve this capability, but
will still leave image resolution many orders of magnitude poorer than in most other spectral ranges. Here we point out that
the only foreseeable way to improve image quality to rival that of modern optical telescopes will be with interferometers
whose light collectors are connected by tethers. After making the scientific case for high spatial resolution far-infrared/submillimeter
imaging and the use of interferometry as the most immediate way of producing results, we discuss recent advances in dynamic
analysis and control of tethered formations, and argue that the further development and testing of tethers in space is a first
step toward providing improved far-infrared/submillimeter angular resolution and astronomical image quality. 相似文献
In the Central Mediterranean two back-arc basins, the Liguro-Provençal (LPb) and the Tyrrhenian basin (Tb), opened progressively and consecutively from the late Eocene–Oligocene to the present. Evolution in space and time of rifting and drifting processes, along three different transects across these basins, shows differences in the style of extension: LPb opened with the formation of a narrow, single rift, while in the Tb deformation and magmatism is spread over a wide area. Moreover at the Northern end of the Tb the locus of extension progressively migrated towards the east whereas in the Southern Tb the locus of extension and magmatism migrated inside the basin, inducing continental break-up and drifting of the previously formed older conjugate basins. We propose that these different styles of back-arc extension depend upon internal conditions, such as prerift rheology linked with its geological heritage, and external conditions, e.g. the style of subduction. 相似文献