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191.
Archean greenstone-tonalite duality: Thermochemical mantle convection models or plate tectonics in the early Earth global dynamics? 总被引:2,自引:0,他引:2
Mantle convection and plate tectonics are one system, because oceanic plates are cold upper thermal boundary layers of the convection cells. As a corollary, Phanerozoic-style of plate tectonics or more likely a different version of it (i.e. a larger number of slowly moving plates, or similar number of faster plates) is expected to have operated in the hotter, vigorously convecting early Earth. Despite the recent advances in understanding the origin of Archean greenstone–granitoid terranes, the question regarding the operation of plate tectonics in the early Earth remains still controversial. Numerical model outputs for the Archean Earth range from predominantly shallow to flat subduction between 4.0 and 2.5 Ga and well-established steep subduction since 2.5 Ga [Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937–940], to no plate tectonics but rather foundering of 1000 km sectors of basaltic crust, then “resurfaced” by upper asthenospheric mantle basaltic melts that generate the observed duality of basalts and tonalities [van Thienen, P., van den Berg, A.P., Vlaar, N.J., 2004a. Production and recycling of oceanic crust in the early earth. Tectonophysics 386, 41–65; van Thienen, P., Van den Berg, A.P., Vlaar, N.J., 2004b. On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth. Tectonophysics 394, 111–124]. These model outputs can be tested against the geological record. Greenstone belt volcanics are composites of komatiite–basalt plateau sequences erupted from deep mantle plumes and bimodal basalt–dacite sequences having the geochemical signatures of convergent margins; i.e. horizontally imbricated plateau and island arc crust. Greenstone belts from 3.8 to 2.5 Ga include volcanic types reported from Cenozoic convergent margins including: boninites; arc picrites; and the association of adakites–Mg andesites- and Nb-enriched basalts.Archean cratons were intruded by voluminous norites from the Neoarchean through Proterozoic; norites are accounted for by melting of subduction metasomatized Archean continental lithospheric mantle (CLM). Deep CLM defines Archean cratons; it extends to 350 km, includes the diamond facies, and xenoliths signify a composition of the buoyant, refractory, residue of plume melting, a natural consequence of imbricated plateau-arc crust. Voluminous tonalites of Archean greenstone–granitoid terranes show a secular trend of increasing Mg#, Cr, Ni consistent with slab melts hybridizing with thicker mantle wedge as subduction angle steepens. Strike-slip faults of 1000 km scale; diachronous accretion of distinct tectonostratigraphic terranes; and broad Cordilleran-type orogens featuring multiple sutures, and oceanward migration of arcs, in the Archean Superior and Yilgarn cratons, are in common with the Altaid and Phanerozoic Cordilleran orogens. There is increasing geological evidence of the supercontinent cycle operating back to 2.7 Ga: Kenorland or Ur 2.7–2.4 Ga; Columbia 1.6–1.4 Ga; Rodinia 1100–750 Ma; and Pangea 230 Ma. High-resolution seismic reflection profiling of Archean terranes reveals a prevalence of low angle structures, and evidence for paleo-subduction zones. Collectively, the geological–geochemical–seismic records endorse the operation of plate tectonics since the early Archean. 相似文献
192.
Controlled laboratory studies of the formation of oil-mineral aggregates (OMA) in seawater demonstrate that sediment concentration and sediment size are key variables for determining the quantity of oil droplets stabilised by OMA formation. Experiments with a single sediment size and a range of sediment concentrations show that as sediment concentration increases, the quantity of oil trapped in OMA increases abruptly. In experiments with a single sediment concentration and a range of sediment sizes, the quantity of oil trapped in OMA decreases as sediment size increases. These results provide direct support to the hypothesis that there is a critical sediment concentration for OMA formation. Below this concentration, stabilisation of oil droplets by OMA decreases rapidly, while above this concentration, stabilisation is extensive. The results also support simple geometric models of OMA formation that predict that the critical sediment mass concentration increases linearly with sediment particle diameter. These results will help to place quantitative constraint on predictions of where and when OMA formation will be a factor in the natural dispersal of oil accidentally spilled into the ocean. 相似文献
193.
There is an emerging requirement for processing global navigation satellite system (GNSS) signals indoor where the signal
is very weak and subjected to spatial fading. Typically, longer coherent integration intervals provide the additional processing
gain required for the detection and processing of such weak signals. However, the arbitrary physical motion of the handset
imputed by the user limits the effectiveness of longer coherent integration intervals due to the spatial decorrelation of
the multipath-faded GNSS signal. In this paper, limits of coherent integration due to spatial decorrelation are derived and
corroborated with experimental verification. A general result is that the processing gain resulting from direct coherent integration
saturates after the antenna has moved through a certain distance, which for typical indoor propagation, is about half a carrier
wavelength. However, a refined Doppler search coupled with a prolonged coherent integration interval extends this limit, which
is effectively a manifestation of selective diversity. 相似文献
194.
European Geostationary Navigation Overlay Service (EGNOS) is a satellite-based system developed to improve the performance
of GPS in the European region. It not only enhances the estimated positioning accuracy but also provides an integrity solution.
We assess the EGNOS performance through analysis of real measurements, performed in different road user environments. We investigate
the use of the integrity concept in future Global Navigation Satellite System (GNSS)-based applications in the road environment,
considering both suburban and dense urban areas. The analysis is motivated by the fact that there is an increasing number
of road applications, such as billing system, transportation of dangerous goods, remote tracking of trucks and snow plows,
fleet management and “pay as you go” car insurances, where the system liability has to be guaranteed. The scope of the analysis
is to investigate to which extent the use of the integrity concept can be used for such purpose; the assessment is based on
real measurement campaigns in representative environments, providing statistical results in terms of accuracy and integrity,
and in a wider sense of the feasibility of the use of EGNOS for augmenting the reliability of GNSS positioning for road applications. 相似文献
195.
During the course of a major sandstorm from April 17 to April 23, 2008 in the Taklimakan Desert, data pertaining to the mass concentrations of different-sized atmospheric particulate matter were observed continuously with Grimm 1.108, Thermo RP 1400a, TSP, and CAWS-600 instruments. The results showed that: (1) during the entire sandstorm process there were some differences between the daily mean particle concentration peaks and the hourly mean particle concentration peaks because the actual sandstorm lasted for only about 4 hr, whereas more particles were accumulated in the floating dust days before and after the actual sandstorm; (2) the intensity of the sandstorm was enhanced with the increase of wind speed, and this was related to the peak mass concentrations of atmospheric particulate matter; the wind speed directly affected the concentration of atmospheric particulate matter: the higher the wind speed, the higher the mass concentration (>0.23 μm was 39,496.5 μg/m3, and >20.0 μm was 5,390.7 μg/m3); (3) the concentration changes of PM10 and TSP were also related to the course and intensity of the sandstorm; and (4) the mass concentration of atmospheric particulate matter had the following sequence during the dust weather: clear day < floating dust < floating and blowing dust < sandstorm. Temperature, relative humidity, and barometric pressure are important factors affecting the strength of storms, which could also indirectly affect the concentration change of atmospheric particulate matter. 相似文献
196.
Cesar Villanoy Laura David Olivia Cabrera Michael Atrigenio Fernando Siringan Porfirio Aliño Maya Villaluz 《Climatic change》2012,112(2):493-505
Coral reefs and other coastal ecosystems such as seagrasses and mangroves are widely recognized to provide protection against
the devastating effects of strong waves associated with tsunamis and storms. The predicted warming climate brings to fore
the role of these ecosystems in providing protection against stronger typhoons that can result in more devastating waves of
greater amplitude. We performed a model simulation of storm generated waves on a Philippine reef, which is located along the
path of tropical storms, i.e., at least 10 typhoons on the average pass through the study site yearly. A model to simulate
wave propagation was developed using Simulating Waves Nearshore (SWAN) and DELFT3D-WAVE computer simulation software. Scenarios
involving local monsoonal wind forcing and storm conditions were simulated. In addition, as climate change may also result
to increased relative sea level, a 0.3 m and 1 m rise in sea level scenarios were also used in the wave model simulations.
Results showed that the extensive reef system in the site helped dissipate wave energy that in turn reduced wave run-up on
land. A significant reduction in wave energy was observed in both climate change, i.e., stronger wind and higher sea level,
and non-climate change scenarios. This present study was conducted in a reef whose coral cover is in excellent condition (i.e.,
50 to 80% coral cover). Estimates of coral reef growth are in the same order of magnitude as estimates of relative sea level
rise based on tide gauge and satellite altimeter data, thus it is possible that the role of reefs in attenuating wave energy
may be maintained if coral reef growth can keep up with the change in sea level. Nonetheless, to maintain reef growth, it
is imperative to manage coral reef ecosystems sustainably and to eliminate the stressors that are within human control. Minimizing
activities such as illegal and destructive blast and poison fishing methods, pollution and siltation, is crucial to minimize
the impacts of high-energy waves that may increase with climate change. 相似文献
197.
P. V. Nagamani Prakash Chauhan Nivedita Sanwlani M. M. Ali 《Journal of the Indian Society of Remote Sensing》2012,40(1):137-143
Ocean-colour remote sensing in optically shallow waters is influenced by contribution from the water column depth as well
as by the substrate type. Therefore, it is required to include the contribution from the water column and substrate bottom
type for bathymetry estimation. In this report we demonstrate the use of Artificial Neural Network (ANN) based approach to
spectrally distinguish various benthic bottom types and estimate depth of substrate bottom simultaneously in optically shallow
waters. We have used in-water radiative transfer simulation modeling to generate simulated top-of-the-water column reflectance
the four major benthic bottom types viz. sea grass, coral sand, green algae and red algae using Hydrolight simulation model.
The simulated remote sensing reflectance, for the four benthic bottom types having benthic bottom depth up to 30 m were generated
for moderately clear waters. A multi-layer perceptron (MLP) type neural network was trained using the simulated data. ANN
based approach was used for classification of the benthic bottom type and simultaneous inversion of bathymetry. Simulated
data was inverted to yield benthic bottom type classification with an accuracy of ~98% for the four benthic substrate types
and the substrate depth were estimated with an error of 0% for sea grass, 1% for coral sand and 1–3% for green and red algae
up to 25 m, whereas for substrate bottom deeper than 25 m depth the classification errors increased by 2–5% for three substrate
bottom types except sea grass bottom type. The initial results are promising which needs validation using the in-situ measured
remote sensing reflectance spectra for implementing further on satellite data. 相似文献
198.
P. D. Safai S. Kewat G. Pandithurai P. S. Praveen K. Ali S. Tiwari P. S. P. Rao K. B. Budhawant S. K. Saha P. C. S. Devara 《Journal of Atmospheric Chemistry》2008,61(2):101-118
Simultaneous measurements on physical, chemical and optical properties of aerosols over a tropical semi-arid location, Agra
in north India, were undertaken during December 2004. The average concentration of total suspended particulates (TSP) increased
by about 1.4 times during intense foggy/hazy days. Concentrations of SO4
2−, NO3
−, NH4
+ and Black Carbon (BC) aerosols increased by 4, 2, 3.5 and 1.7 times, respectively during that period. Aerosols were acidic
during intense foggy/hazy days but the fog water showed alkaline nature, mainly due to the neutralizing capacity of NH4 aerosols. Trajectory analyses showed that air masses were predominantly from NW direction, which might be responsible for
transport of BC from distant and surrounding local sources. Diurnal variation of BC on all days showed a morning and an evening
peak that were related to domestic cooking and vehicular emissions, apart from boundary layer changes. OPAC (Optical properties
of aerosols and clouds) model was used to compute the optical properties of aerosols. Both OPAC-derived and observed aerosol
optical depth (AOD) values showed spectral variation with high loadings in the short wavelengths (<1 μm). AOD value at 0.5 μm
wavelength was significantly high during intense foggy/hazy days (1.22) than during clear sky or less foggy/hazy days (0.63).
OPAC-derived Single scattering albedo (SSA) was 0.84 during the observational period, indicating significant contribution
of absorbing aerosols. However, the BC mass fraction to TSP increased by only 1% during intense foggy/hazy days and thereby
did not show any impact on SSA during that period. A large increase was observed in the shortwave (SW) atmospheric (ATM) forcing
during intense foggy/hazy days (+75.8 W/m2) than that during clear sky or less foggy/hazy days (+38 W/m2), mainly due to increase in absorbing aerosols. Whereas SW forcing at surface (SUF) increased from −40 W/m2 during clear sky or less foggy/hazy days to −76 W/m2 during intense foggy/hazy days, mainly due to the scattering aerosols like SO4
2-. 相似文献
199.
Previous experimental studies have found that people generally misperceive the basic dynamics of renewable resources, and in particular the accumulation of greenhouse gases (GHGs) in the atmosphere. The purpose of the present laboratory experiment is to find out why people misperceive the dynamics of CO2 accumulation and how misperceptions could be avoided. Using a simulator, 242 subjects were each asked to control total global emissions of CO2 to reach a given target for the stock of CO2 in the atmosphere. Consistent with previous investigations we find a strong tendency for people to overshoot the stated goal. Furthermore, our results point out that people need help to develop proper mental models of CO2 accumulation and they need motivation to reconsider inappropriate decision heuristics. Based on these results and the literature on conceptual change a new information strategy is designed. To motivate, it imposes cognitive conflict; and to facilitate new understanding, it provides simple analogies. A new test shows promising learning effects. The results have important implications for the Intergovernmental Panel on Climate Change (IPCC), governments, and media covering the climatic change issue as well as for general education. 相似文献
200.
The 3+1 spacetime split is used in a preliminary investigation of waves propagating in a medium of isothermal plasma in the vicinity of event horizon of Reissner-Nordström planar analogue. The general relativistic magnetohydrodynamic equations for isothermal have been investigated there. The dispersion relations are obtained by using linear perturbation and Fourier analyze techniques for non-rotating and rotating, non-magnetized and magnetized environment. The wave number, phase velocity, group velocity etc. are shown to discuss the nature of the waves. 相似文献