A programme of Venice uplift because of seawater injection into a 600–800 m deep brackish aquifer underlying the lagoon has recently been advanced. As the actual spatial variability of the hydro‐geological parameters is to a large extent unknown, a controversial issue concerns the 25–30 cm heave prediction with the possible generation of differential displacements that might jeopardize the integrity of the architectural patrimony. It is shown that significant differential displacements have occurred in Venice in the past and are still presently occurring as evidenced by SAR interferometry. The results of a stochastic analysis addressing the variability of the hydraulic conductivity of the injected formation indicate that even a highly uneven aquifer expansion does not migrate to ground surface because of the smoothing effect exerted by the overburden. The predicted differential displacements are well below the values Venice is experiencing. Any a priori alarmism appears to be unfounded and unjustified. 相似文献
A pyroxene with composition LiNiSi2O6 was synthesized at T = 1,473 K and P = 2.0 GPa; the cell parameters at T = 298 K are a = 9.4169(6) Å, b = 8.4465(7) Å, c = 5.2464(3) Å, β = 110.534(6)°, V = 390.78(3) Å3. TEM examination of the LiNiSi2O6 pyroxene showed the presence of h + k odd reflections indicative of a primitive lattice, and of antiphase domains obtained by dark field imaging of the h + k odd reflections. A HT in situ investigation was performed by examining TEM selected area diffraction patterns collected at high temperature and synchrotron radiation powder diffraction. In HTTEM the LiNiSi2O6 was examined together with LiCrSi2O6 pyroxene. In LiCrSi2O6 the h + k odd critical reflections disappear at about 340 K; they are sharp up to the transition temperature and do not change their shape until they disappear. In LiNiSi2O6 the h + k odd reflections are present up to sample deterioration at 650 K. A high temperature synchrotron radiation powder diffraction investigation was performed on LiNiSi2O6 between 298 and 773 K. The analysis of critical reflections and of changes in cell parameters shows that the space group is P-centred up to the highest temperature. The comparative analysis of the thermal and spontaneous strain contributions in P21/c and C2/c pyroxenes indicates that the high temperature strain in P-LiNiSi2O6 is very similar to that due to thermal strain only in C2/c spodumene and that a spontaneous strain contribution related to pre-transition features is not apparent in LiNiSi2O6. A different high-temperature behaviour in LiNiSi2O6 with respect to other pyroxenes is suggested, possibly in relation with the presence of Jahn–Teller distortion of the M1 polyhedron centred by low-spin Ni3+. 相似文献
The issue of determining the driving factors in gullying, apart from land use, is somewhat lagging in comparison with the study of their physical modelling and control technology. Available information focuses on the basic ideas of climatic control, anthropic determinism and internal “authigenic” dynamics. High resolution chronology of cyclic systems, common in extensively gullied areas, can provide a clue to the weight of each factor. This paper reports a study of this kind, focusing on two gully catchments in the Main Ethiopian Rift (MER), but backed by an extensive regional survey. By integrating tracing and correlation of unconformity-bounded stratigraphic units and soils with radiocarbon dating, a detailed chronology was obtained for the last 5000 years. This could be compared with proxy climate reconstructions of similar detail. Clear evidence of climatic control emerged; gully filling is triggered by decreased stream transport capacity and increased sediment supply during transitions towards drier climate phases, while gully entrenchment appears to take place at the start of moist intervals, for the reverse reasons. A broader consideration of geological setting, however, puts forward a more general interpretation. These gullies actually formed, in the beginning, as part of the land surface response to sudden, very recent tectonic events, which created accommodation space for temporary sediment stores. They should then be seen in the frame of the Discontinuous Ephemeral Stream (DES) concept; as such, they are intrinsically non-linear and complex phenomena, whose response is linearized by a strong climatic–vegetational forcing, acting on both channel flow and sediment supply. 相似文献
The interannual variations of CO2 sources and sinks in the surface waters of the Antarctic Ocean (south of 50°S) were studied between 1986 and 1994. An existing, slightly modified one-dimensional model describing the mixed-layer carbon cycle was used for this study and forced by available satellite-derived and climatological data. Between 1986 and 1994, the mean Antarctic Ocean CO2 uptake was 0.53 Pg C year−1 with an interannual variability of 0.15 Pg C year−1.Interannual variation of the Antarctic Ocean CO2 uptake is related to the Antarctic Circumpolar Wave (ACW), which affects sea surface temperature (SST), wind-speed and sea-ice extent. The CO2 uptake in the Antarctic Ocean has increased from 1986 to 1994 by 0.32 Pg C. It was found that over the 9 years, the surface ocean carbon dioxide fugacity (fCO2) increase was half that of the atmospheric CO2 increase inducing an increase of the air–sea fCO2 gradient. This effect is responsible for 60% of the Antarctic Ocean CO2 uptake increase between 1986 and 1994, as the ACW effect cancels out over the 9 years investigated. 相似文献
Orogens oblique to the direction of plate convergence are currently attributed to obliquity between the margins of one or both of the sutured continents to their direction convergence. We use a single analogue experiment and natural examples to illustrate a potential additional factor: variations in strength of the indented continent at a high angle to the convergence direction. The wavelengths of structures in laterally shortened lithosphere depend on the strength of the most competent layers. Lateral variations in crustal thickness must therefore lead to structures oblique to any applied lateral compression.
An analogue experiment was performed to explore this phenomenon. A two-layer ‘indented continent’ was modelled by a brittle upper crust of sand above a lower crust of high-viscosity polymer floating on a single layer of low-viscosity syrup representing the mantle. The well-known strike-slip structures allowing lateral escape to distant weak boundaries were hindered by lateral boundaries in front of the indenter. This allowed us to focus on the effects of a thickness change built into the ‘indented continent’ along a zone parallel to the direction in which a vertical rigid wall advancing at a steady rate represented the indenter. Vertical escape led to an ‘orogenic belt’ oblique to the advancing wall; this obliquity influences subsequent lateral escape. Model scaling and interpretations are based on Extended Thin Sheet Approximation (ETSA) and standard theories of faulting.
Four sectors of the Alpine–Himalayan orogen (Iran, Tunisia, the Eastern Alps and the Himalaya) are oblique to the continental convergence direction, and we point to thickness changes at high angles to the suture that may account for this geometry. As crustal thicknesses north of oblique sectors of the Himalayas are not yet known, we speculate on them.
We infer from the main difference between our experiment and all our examples chosen from nature that vertical orogenic escape was oblique to our model suture but can be parallel to natural sutures. 相似文献
Purpose of this paper is to clarify how Planetary Nebulae (PNe) are very interesting laboratories to study cosmic gas dynamics.
I first recall the history of PNe which are generated from low and intermediate mass stars through successive mass loss processes
starting in the Reg Giant phase of evolution and continuing also after the termination of the pulsed AGB phase, where most
of the nebular mass is believed to be ejected. The correpondings tellar winds are the ingredients of the nebula. Their initial
properties and subsequent mutual interactions, under the action of the evolving stellar radiation field, are responsible for
the properties of the nebula. The observed structures of PNe are considered in detail. Larger scale macroscopic structures
(MACS) are examined separately from quite smaller scale microscopic structures (MICS). The formation of MACS, at least in
cases of round to moderately elliptical PNe, is shown to be reasonably well understood in terms of existing hydrodynamical
models. Considering the kinematical behaviour, MICS can be separated into FLIERs (Fast Low Ionization Emitting Regions) and
SLOWERs (slowly moving). Attention is focussed on FLIERs and on the proposed mechanisms to interpret them. Recent observations
with the Hubble Space Telescope have provided us with a wealth of detailed (subarcsec) information on the nebular structures.
The inner structure of FLIERs is here illustrated to consist of substructures of various shapes with an high degree of individually
from object to object, also within the same PN. These new data call for deeper thoretical efforts to solve the problems of
cosmic gas dynamics, posed by their observed properties. An ample account is given of the most relevant original works, in
an effort to allow the non specialist reader to quickly become acquainted with the status of art in the various aspects of
the subject.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
The general characterisation of the global radioxenon background is of interest for the verification of the Comprehensive Nuclear-Test-Ban Treaty. Since the major background sources are only a few isotope production facilities, their source term has an emphasized influence on the worldwide monitoring process of radioxenon. In this work, two different datasets of source terms are applied through atmospheric transport modelling, to estimate the concentration at two radioxenon detection stations in Germany and Sweden. One dataset relies on estimated average annual emissions; the other includes monthly resolved measurements from an isotope production facility in Fleurus, Belgium. The quality of the estimations is then validated by comparing them to the radioxenon concentrations that have been sampled at two monitoring stations over the course of 1 year. 相似文献