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181.
182.
The Mfolozi Estuary on the KwaZulu-Natal coast of South Africa is the most turbid estuary in Natal due to poor catchment
management, leading to large quantities of suspended particulate matter (SPM) entering the estuary from the Mfolozi River.
This paper quantities some of the solute and sediment dynamics in the Mfolozi Estuary where the main documented environmental
concern is the periodic input of SPM from the Mfolozi Estuary to the St. Lucia system, causing reduction of light penetration
and endangering biological productivity in this important nature reserve. Synoptic water level results have allowed reach
mean bed shear stresses and velocities to be calculated for an observed neap tidal cycle. Results indicate that ebb velocities
dominate the sediment transport processes in the estuary when fluvial input in the Mfolozi River is of the order of 15–20
m3 s–1. Observed and predicted flood tide velocities are too low (<0.35 m s–1) to suspend and transport significant amounts of SPM. Observed results indicate that although the SPM load entering the estuary
is dominantly from the Mfolozi River, the Msunduzi River flow plays a major role in the composition of the estuary's salinity
and velocity fields. It is calculated that the Mfolozi Estuary would fill with sediment in 1.3 years if it was cut off from
the sea. The major fluvial flood events help maintain the estuary by periodically pushing sediment seawards (spit progrades
seawards 5 m yr–1) and scouring and maintaining the main flow channel in the estuary. During low fluvial flow conditions, tidal flow velocities
will become the dominant control on sediment transport in the estuary. Interchange of SPM between the St. Lucia and Mfolozi
estuaries under present conditions is complicated by the strong transverse velocity shear between the two systems at their
combined mouth. This is creating a salinity-maintained axial convergence front that suppresses mixing of solutes and SPM between
the systems for up to 10 h of the tidal cycle during observed conditions.
Received: 22 May 1995 · Accepted: 31 July 1995 相似文献
183.
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185.
Rancholabrean and/or Holocene remains record the presence of the following species of Carnivora in the Black Belt; Canis Latrans, Urocyon cinereoargentatus, Ursus amenricanus, Tremarctors floridanus. Arctodus simus, Felis amnicola, and Lynx rufus . A temporal size reduction was noted in the case of Canis latrans and Ursus americans 相似文献
186.
BARBARA WOHLFARTH SVANTE BJÖRCK GÖRAN POSSNERT BJÖRN HOLMQUIST 《Boreas: An International Journal of Quaternary Research》1998,27(4):243-257
More than 50 varve-thickness diagrams, which were established from glacial varved clays in south-eastern Sweden were correlated with each other to form an 800-year long floating varve chronology. AMS |214|0C measurements on terrestrial macrofossils from the varved clays enabled synchronization of the record with other high-resolution archives. The synchronization indicates that the chronology spans between c. 13 150 and c. 12 350 calendar years BP and covers the later part of the Allerørd and the early part of the Younger Dryas. Calibrated radiocarbon dates, which were obtained on varved clays south of the floating chronology, indicate that the ice recession in south-eastern Sweden may have started during late Bølling. Our results indicate a longer time-span in varve years for the deglaciation than has been previously estimated 相似文献
187.
Thomas R. Gregory Christopher W. Smart Malcolm B. Hart Guillaume Massé Lindsay L. Vare Simon T. Belt 《第四纪科学杂志》2010,25(7):1192-1192
188.
When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared with under wind-sea conditions. Using measurements from the Östergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A 3-year simulation and two case studies using the wavefield from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2 °C annually on average) whereas the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg−1 ). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space. 相似文献
189.
Patrycja Jernas Dorthe Klitgaard Kristensen Katrine Husum Lindsay Wilson Nalan Koç 《Boreas: An International Journal of Quaternary Research》2013,42(1):236-255
The western and northern Svalbard continental margins (European Arctic) are environmentally sensitive areas that are dependent on the northward flow of Atlantic Water, the largest heat source of the Arctic Ocean. Two marine sediment records from the Svalbard shelf: Kongsfjorden Trough and Hinlopen Trough, were analysed with regard to the benthic foraminiferal content and lithology to assess the palaeoceanographic evolution during the past two millennia with decadal to multi‐decadal temporal resolution. In both records, an overall gradual decrease of E. excavatum f. clavata during the past two millennia reflects a change towards generally warmer and less glacially influenced conditions, presumably related to enhanced inflow of Atlantic Water (AW). The influence of AW also varied on centennial time scales, as evidenced by faunal and sedimentary shifts occurring almost synchronously at both locations. The period from AD 700 to 1200 was characterized by enhanced inflow of AW, followed by the development of highly productive oceanographic fronts at both localities from AD 1200 to 1500. In contrast, the subsequent interval (AD 1500–1900) shows particularly harsh conditions in the Hinlopen Trough, with significantly reduced foraminiferal flux and sediment input related to perennial sea ice cover. In Kongsfjorden, less severe conditions were observed, indicating that the AW advection continued. The synchronicity of changes in both records demonstrates the effect of the variability in inflow of AW to the Svalbard region during the past 2000 years. Moreover, the records seem to follow climate anomalies, for example the Little Ice Age and Medieval Warm Period, found in the North Atlantic realm. 相似文献
190.
Jianhong Xu J Ramón Arrowsmith Jie Chen Lindsay M. Schoenbohm Tao Li Zhaode Yuan Lewis A. Owen 《地球表面变化过程与地形》2021,46(1):280-295
Selecting an appropriate hillslope transport model and calibrating model parameters are essential for morphological dating of fault and fluvial scarps. In this paper, we refine the method of profile-based morphologic dating by updating the representation of nonlinearity in sediment flux dependence on the hillslope gradient. We apply this revised method to fluvial scarps bounding fluvial terraces offset along the Kongur Normal Fault in the semi-arid high-altitude Pamir mountains, northwestern China. One of these terraces, the T3 surface, is dated to 7.0+1.9/−1.6 kyr using 10Be cosmogenic depth profile analysis. Well-preserved, dated terraces make this an ideal site to test the utility of morphological modelling in constraining ages of the young terrace risers. To do this, 35 topographic swath profiles across the terrace risers are extracted from a 0.2 m-resolution digital elevation model produced using structure from motion from photos collected by an unmanned aerial vehicle. The best estimates of morphological age are 13.9 ± 1.3 m2 for the riser T3/T4 and 11.9 ± 1.3 m2 for T2/T3 using a linear diffusion approach. These two morphological ages overlap within uncertainty and fail to distinguish between two young terrace risers. Alternatively, we employed a nonlinear diffusion model, calibrated with transport constant k = 1 m2 kyr−1, nonlinearity n = 2, and critical gradient Sc = tan(33°). This nonlinear model produces ages of 7.3 ± 0.5 kyr for T3/T4 and 4.0 ± 0.2 kyr for T2/T3; these ages are consistent with terrace surface ages deduced by using vertical offset divided by independently determining average throw rate. This comparison shows the advantage of a nonlinear model in defining ages of young scarps. Furthermore, we explored the minor effect of heterogeneous degradation along steep sections of the scarp profiles. The nonlinear scarp modelling scheme we develop in this paper is suitable for studying scarp degradation in other regions. © 2020 John Wiley & Sons, Ltd. 相似文献