共查询到20条相似文献，搜索用时 46 毫秒
1.
Ye. I. Polyakova T. S. Klyuvitkina E. A. Novichkova H. A. Bauch H. Kassens 《Water Resources》2009,36(3):273-283
The spatial and temporal changes in the Lena River runoff over the last 9 thousand years are reconstructed through studying the freshwater microfossils in sediment cores obtained from the Laptev Sea inner shelf immediately adjacent to the Lena delta and subject to the freshening effect of river water inflowing the sea through the main arms of the delta (the Trofimovskaya, Bykovskaya, and Tumatskaya arms), the sediments having been thoroughly AMS 14C dated. The freshwater species of diatoms (predominantly the river ones) and green algae that enter the shelf with river water served as indicators of river runoff. The reconstruction of paleosalinity of the sea surface water in the regions under study is based on the relationships (established earlier) between the distribution of freshwater diatoms in the surface layers of sediments in the Arctic seas and the gradients of water salinity in summer. Data on variations in the composition of aquatic microfossil associations in sediments and the reconstructed paleosalinity in the regions of the eastern and western paleovalleys of the Lena River are used to determine the main paleohydrologic events that controlled the variations in the Lena runoff into the shelf zone of the Laptev Sea during the Holocene. 相似文献
2.
The results of studying the ionospheric response to solar flares, obtained from the data of the GPS signal observations and incoherent scatter radars and as a result of the model calculations, are presented. It is shown that, according to the GPS data, a flare can cause a decrease in the electron content at altitudes of the topside ionosphere (h > 300 km). Similar effects of formation of a negative disturbance in the ionospheric F region were also observed during the solar flares of May 21 and 23, 1967, with the Arecibo incoherent scatter radar. The mechanism by which negative disturbances appear in the topside ionosphere during solar flares has been studied in this work based on the theoretical model of the ionosphere-plasmasphere coupling. It has been indicated that the formation of the electron density negative disturbance in the topside ionosphere is caused by an intense removal of O+ ions into the overlying plasmasphere under the action of an abrupt increase in the ion production rate and thermal expansion of the ionospheric plasma. 相似文献
3.
S. I. Musatenko Yu. S. Musatenko E. V. Kurochka A. V. Lastochkin V. Ya. Cholij O. I. Maksimenko A. S. Slipchenko 《Geomagnetism and Aeronomy》2006,46(2):173-182
The emission (or modulation) line, which manifests itself during high-speed (the speed of entry into the atmosphere is about 70 km/s) meteor showers (Perseids, Orionids, Leonids), has been detected in the fluctuation spectra of ionospheric plasma radio noise at λ = 2 m. The line frequency varies from 12 to 60 Hz depending on activity of ionospheric ionization sources and local characteristics of a meteor shower, time of day, etc. The line has a width of 2–3 Hz and can have satellites. The dusty plasma characteristics have been estimated on the assumption that ionospheric noise is modulated or emitted due to dusty plasma oscillations. 相似文献
4.
Ellen M. Rathje Ismail Karatas Stephen G. Wright Jeff Bachhuber 《Soil Dynamics and Earthquake Engineering》2004,24(9-10):699
During the 1999 Kocaeli earthquake (Mw=7.4) in Turkey, coastal failures and sea inundation were observed and were particularly concentrated along the margins of Izmit Bay and Lake Sapanca, in pull-apart basins created by stepovers in the fault rupture. Geotechnical site characterization, geologic mapping, liquefaction evaluation, and slope stability analysis were carried out to identify the principal contributing factors of the coastal failures. Results from this study indicate that both liquefaction and tectonic subsidence contributed to the failures and sea inundation within the pull-apart basins. Most of the liquefaction sites were situated at the prograding nose of active delta fans, where the presence of steep slopes coupled with the loose sediments found within young active delta fan deposits resulted in liquefaction-induced slope failures and sea inundation. Liquefaction in other coastal deposits outside the actively prograding delta fans caused limited lateral spreading and only minor sea inundation. Outside the delta fans, where soils were not liquefiable, tectonic subsidence associated with normal faulting was the cause of the observed sea inundation. Generally, tectonic subsidence caused the most severe sea inundation. Based on these observations, the identification of regions susceptible to both tectonic subsidence and liquefaction are important when evaluating seismic hazards. 相似文献
5.
Ocean Dynamics - Using the Estimating Circulation and Climate of the Ocean (ECCO) Phase II product, this study investigates the energetic characteristics during eddy shedding in the Gulf of Mexico.... 相似文献
6.
Ocean Dynamics - The present work focuses on the measured wave spectrum in the South China Sea (scs) during storms. The measured spectra have been obtained from surface elevation measurements... 相似文献
7.
《Continental Shelf Research》2007,27(10-11):1379-1398
Water masses in Monterey Bay are determined from the CTD casts of the Monterey Ocean Observing System (MOOS) Upper-water-column Science Experiment (MUSE) August 2000 dataset. It is shown through cluster analysis that the MUSE 2000 CTD dataset contains 5 water masses. These five water masses are: bay surface water (BSW), bay warm water (BWW), bay intermediate water (BIW), sub arctic upper water (SUW) and North Pacific deep water (NPDW). The BWW is a new water mass that exists in one area and is attributed to the effects of solar heating. The volumes occupied by each of the water masses are obtained. The BIW water is the most dominant water mass and occupies 68.8% of the volume. The statistical means and standard deviations for each water parameter, including spiciness and oxygen concentration, are calculated during separate upwelling and relaxed periods.The water mass content and structure are analyzed and studied during upwelling and a relaxed period. During upwelling, along a CTD track off Pt. Ano Nuevo, the water mass T, S distribution tended to be organized along three branches. Off Pt. Ano Nuevo the innovative coastal observation network (ICON) model showed the formation of a cyclonic eddy during the analyzed upwelling period. In time the eddy moved southwest and became absorbed into the southerly flow during the initial phases of the following wind-relaxed period. 相似文献
8.
9.
Rapid polar shifts relative to Gondwanaland are identified in the Late Ordovician and Carboniferous. These shifts form part of the “Common polar wander path” interpretation of the palaeomagnetic poles for the Gondwana continents during the Palaeozoic. For western Europe a transition occurs between the Ordovician and Silurian poles, but is of smaller magnitude than the Late Ordovician Gondwana shift. Similarly Carboniferous shifts with respect to Europe and North America are smaller than the Gondwana shifts. A third shift in Europe is dated as mid-Devonian, but could be as old as Late Silurian, and has no counterpart in Gondwanaland. The differences in timing and magnitudes of these shifts provide evidence of the predominant role of continental drift rather than polar wandering. Attempts to explain the data exclusively in terms of polar wandering lead to geologically and geometrically untenable conclusions. Whilst there were one or perhaps two supercontinents in most of the upper Palaeozoic, it seems Laurasia was itself a set of separate fragments. 相似文献
10.
N. M. Polekh O. M. Pirog S. V. Voeikov P. V. Tatarinov A. E. Stepanov V. V. Bychkov Z. F. Dumbrava 《Geomagnetism and Aeronomy》2006,46(5):593-602
Results of the studies of ionospheric parameter variations during the intense geomagnetic storm on November 7–11, 2004, in the 20°–80° N, 60°–180° E sector are presented. The data of ionospheric stations and the results of total electron content (TEC) measurements at the network of the GPS ground-based receivers and of the GPS receiver onboard the CHAMP satellite were used. Periods of total absorption and blanketing sporadic E layers were observed at high latitudes, whereas durable negative disturbances typical of geomagnetic storms of high intensity were detected at midlatitudes. In the afternoon hours of local time on November 8, 2004, a large-scale ionospheric disturbance of a frontal type was detected on the basis of foF2 and TEC measurements. The disturbance propagated southwestward at a mean velocity of about 200 m/s. The comparison of the relative amplitude of this large-scale disturbance according to the total electron content (～70%) and foF2 (～80%) measurements made it possible to assume a large vertical scale of the disturbance. 相似文献
11.
Changes in rainfall erosivity are an expected consequence of climate change. Long‐term series of the single storm erosion index, EI, may be analysed to detect trends in rainfall erosivity. An indirect approach has to be applied for estimating EI, given that long series of rainfall intensities are seldom available. In this paper, a method for estimating EI from the corresponding rainfall amount, he, was developed for Sicily. This method was then applied at 17 Sicilian locations, representative of different climatic zones of the region, to generate a long series (i.e. from 1916 to 1999 in most cases) of EI values. Linear and step (step located at 1970) trends in annual and seasonal erosivity were detected by both classical approaches (Mann–Kendall test, Wilcoxon‐Mann‐Whitney rank‐sum test) and a new empirical approach (quantile approach, QA), based on the determination of the erosivity values corresponding to selected probability levels. A power relationship between EI and he with a space‐ and time‐variable scale factor and a time‐variable process parameter yielded the most accurate predictions of EI. However, a simpler model, using a time‐variable scale factor and a constant process parameter, yielded reasonably accurate EI estimates. Annual erosivity did not increase in Sicily during the twentieth century. At the most, it decreased at a few locations (three of the 17 considered locations). Significant trends were observed more frequently for winter erosivity (six locations) than for summer erosivity (two locations), suggesting that the erosive storms of winter determined the occasional occurrence of a negative trend in annual erosivity. In general, the QA compared reasonably well with more classical approaches. The QA appears promising since step trends for different return periods may be detected but efforts are needed to statistically formalize the proposed approach. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
12.
John Ball Robinson Jr. 《Pure and Applied Geophysics》1970,80(1):319-334
Summary From meteorological IGY data for the calendar year 1958, the mean meridional eddy transport of enthalpy was evaluated for the Southern Hemisphere. Levels chosen for the study were 1000, 850, 700, 500, 400, 300, 200, 150 and 100 mb. Data from 84 Southern Hemisphere and 25 equatorial Northern Hemisphere stations were used. Yearly mean quantities related to meridional eddy enthalpy flux were computed and analyzed.It was found that around 40° S there is a double-maximum zone of poleward, meridional, transient eddy enthalpy flux, the stronger transport occurring at 850 mb, and the weaker near 200 mb. The countergradient transient eddy flux regions in the low latitude mid-troposphere and in the middle and upper latitude lower stratosphere, found in previous Northern Hemisphere investigations, were observed to exist in the Southern Hemisphere also. The standing eddy heat transport, as expected, was very weak except at high latitudes where Antarctic continentality effected a large double-maximum poleward flux centered near the surface and in the lower stratosphere. The total vertically integrated enthalpy transport by the eddies was found to be poleward everywhere, reaching a maximum between 35° and 40° S. 相似文献
13.
N.N. Ambraseys 《Journal of Seismology》2002,6(4):571-578
An examination of historical sources confirms thatrelatively little can be found about seismic sea-waves in the Marmara Searegion. We find that during the last 20 centuries there have been 16earthquakes which seem to be associated with seismic sea-waves, six ofwhich were damaging. Most of these events were reported from the northcentral and north-east coast of the Sea of Marmara and from the Gulf ofIzmit, from where there is some evidence that the source mechanism ofsome of them was coastal mass failures and sliding of submarine sediments.Another 16 sea-waves reported in literature are found to be spuriousevents. 相似文献
14.
V. K. Roldugin Yu. P. Maltsev A. A. Ostapenko A. V. Roldugin 《Geomagnetism and Aeronomy》2006,46(4):438-449
Pulsations in the electron fluxes and magnetic field on two geosynchronous satellites and at several ground-based observatories have been studied. Relativistic electron flux pulsations with a period of 6 min originated after the shock arrival from the solar wind at 0258 UT on February 11, 2000, had a regular character, and propagated from the dayside to the nightside at a velocity of about 60 deg min?1. Magnetic field oscillations on the satellites and on the Earth had a more complicated structure. Field oscillations morphologically similar to electron oscillations are observed only on one satellite in one component and at one-two ground-based stations, where a periodicity of 10–12 min was mainly observed. It is assumed that magnetic pulsations are Alfvén resonances generated by a fast magnetosonic wave. 相似文献
15.
Jacinto GS Sotto LP Senal MI San Diego-McGlone ML Escobar MT Amano A Miller TW 《Marine pollution bulletin》2011,63(5-12):243-248
Herein we present results from one of the first extensive bay-wide oceanographic surveys of Manila Bay, wherein 31 stations were sampled during the northeast monsoon (cold and dry season). A band of hypoxic bottom water (dissolved oxygen<2.8 mg/L) spanned the midsection of the bay from east to west. Bottom nitrate concentrations (5.7-16.8 μM; avg. 11.1 μM) and total organic carbon values in sediments (1.7-3.1%; avg. 2.4%) were high in the midsection, which coincided with the band of hypoxic bottom water. Physical processes and site-specific accumulation of organic material likely lead to hypoxic conditions in Manila Bay, even during the northeast monsoon period when the water column is relatively well mixed. The results of this study complement the previously reported widespread hypoxia that occurs during the rainy season. Thus, hypoxia may be pervasive in the bay throughout the year, although it varies in intensity and spatial extent. 相似文献
16.
When nonwetting fluid displaces wetting fluid in a porous rock many rapid pore-scale displacement events occur. These events are often referred to as Haines jumps and any drainage process in porous media is an ensemble of such events. However, the relevance of Haines jumps for larger scale models is often questioned. A common counter argument is that the high fluid velocities caused by a Haines jump would average-out when a bulk representative volume is considered. In this work, we examine this counter argument in detail and investigate the transient dynamics that occur during a Haines jump. In order to obtain fluid–fluid displacement data in a porous geometry, we use a micromodel system equipped with a high speed camera and couple the results to a pore-scale modeling tool called the Direct HydroDynamic (DHD) simulator. We measure the duration of a Haines jump and the distance over which fluid velocities are influenced because this sets characteristic time and length scales for fluid–fluid displacement. The simulation results are validated against experimental data and then used to explore the influence of interfacial tension and nonwetting phase viscosity on the speed of a Haines jump. We find that the speed decreases with increasing nonwetting phase viscosity or decreasing interfacial tension; however, for the same capillary number the reduction in speed can differ by an order of magnitude or more depending on whether viscosity is increased or interfacial tension is reduced. Therefore, the results suggest that capillary number alone cannot explain pore-scale displacement. One reason for this is that the interfacial and viscous forces associated with fluid–fluid displacement act over different length scales, which are not accounted for in the pore-scale definition of capillary number. We also find by analyzing different pore morphologies that the characteristic time scale of a Haines jump is dependent on the spatial configuration of fluid prior to an event. Simulation results are then used to measure the velocity field surrounding a Haines jump and thus, measure the zone of influence, which extends over a distance greater than a single pore. Overall, we find that the time and length scales of a Haines jump are inversely proportional, which is important to consider when calculating the spatial and temporal averages of pore-scale parameters during fluid–fluid displacement. 相似文献
17.
The parameters of earthquakes in northeast of Russia that occurred in 1735–1910 and were published in New Catalog of Earthquakes in USSR from Ancient Times trough 1975 have been specified based on the initial data presented (1977). Special attention has been paid to the conformity between the initial data, the available solution methods, and the resulting parameter values. The macroseismic data from the preinstrumental and instrumental observational periods have been compared. 相似文献
18.
Summary The direction of the geomagnetic field has changed substantially less in India than in Europe since the end of the 18th century.Whereas the European geomagnetic time series indicate a tendency of secular variation from which a remarkable westward drift of the non-axial field can be deduced, in India the field direction seems to have remained more or less fixed over the last two centuries. The drift of the non-axial field seems to be very small, or even eastward. 相似文献
19.
Jin Huijun Jin Xiaoying He Ruixia Luo Dongliang Chang Xiaoli Wang Shaoling Marchenko Sergey S Yang Sizhong Yi Chaolu Li Shijie Harris Stuart A 《中国科学:地球科学(英文版)》2019,62(8):1207-1223
The formation and evolution of permafrost in China during the last 20 ka were reconstructed on the basis of large amount of paleo-permafrost remains and paleo-periglacial evidence, as well as paleo-glacial landforms, paleo-flora and paleofauna records. The results indicate that, during the local Last Glacial Maximum(LLGM) or local Last Permafrost Maximum(LLPMax), the extent of permafrost of China reached 5.3×106-5.4×106 km2, or thrice that of today, but permafrost shrank to only0.80×106-0.85×106 km2, or 50% that of present, during the local Holocene Megathermal Period(LHMP), or the local Last Permafrost Minimum(LLPMin). On the basis of the dating of periglacial remains and their distributive features, the extent of permafrost in China was delineated for the two periods of LLGM(LLPMax) and LHMP(LLPMin), and the evolution of permafrost in China was divided into seven periods as follows:(1) LLGM in Late Pleistocene(ca. 20000 to 13000-10800 a BP)with extensive evidence for the presence of intensive ice-wedge expansion for outlining its LLPMax extent;(2) A period of dramatically changing climate during the early Holocene(10800 to 8500-7000 a BP) when permafrost remained relatively stable but with a general trend of shrinking areal extent;(3) The LHMP in the Mid-Holocene(8500-7000 to 4000-3000 a BP)when permafrost degraded intensively and extensively, and shrank to the LLPMin;(4) Neoglaciation during the late Holocene(4000-3000 to 1000 a BP, when permafrost again expanded;(5) Medieval Warming Period(MWP) in the late Holocene(1000-500 a BP) when permafrost was in a relative decline;(6) Little Ice Age(LIA) in the late Holocene(500-100 a BP), when permafrost relatively expanded, and;(7) Recent warming(during the 20 th century), when permafrost continuously degraded and still is degrading. The paleo-climate, geography and paleopermafrost extents and other features were reconstructed for each of these seven periods. 相似文献