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1.
Nicolas Pouvreau Belén Martin Miguez Bernard Simon Guy Wöppelmann 《Comptes Rendus Geoscience》2006,338(11):802-808
The work of searching, recovering and quality control of ancient sea-level measurements at Brest is presented. This work enables us to complete a study carried out by Cartwright in 1972, which showed a decrease in the tidal M2 semi-diurnal amplitude of 1% per century. After including these ancient data, as well as the last four decades of observations in the analysis, our results show an increase of the amplitude of M2 after 1960 and a decrease before 1880, suggesting a long-period oscillation rather than a steady secular trend. To cite this article: N. Pouvreau et al., C. R. Geoscience 338 (2006). 相似文献
2.
Arantxa Luzón Angel González Arsenio Muñoz Belén Sánchez-Valverde 《Journal of Paleolimnology》2002,28(4):441-456
The lacustrine facies from two sections (Candasnos and Fraga) ofthe Oligocene-Miocene Torrente de Cinca lithostratigraphic Unit in thecentral part of the Ebro Basin (Spain) have been analysed to determine theinfluence of orbital parameters in lacustrine sedimentation. The unit ispredominantly composed of limestones and marls, and represents a shallowlacustrine freshwater system. The sedimentological features of the faciesstudied demonstrate that the lower part of the Candasnos section representsoffshore lacustrine subenvironments whereas the upper part, and the whole ofthe Fraga section, characterise marginal lacustrine areas. Series of stratalthickness variations of limestone, marl, and limestone/marl couplets fromboth sections have been analysed using spectral analysis. This shows thatinformation corresponding to periodic cycles only appears in the offshorefacies, that is to say, in the lower part of Candasnos section, and disappearsin the marginal facies where non-periodic cycles exist. Furthermore, thespectral analysis of the offshore facies highlights the existence of a peak inthe power spectrum with a period of around 7 (6.8 to 7.8) that can berecognised in the field as shallowing-upward lacustrine sequences.Magnetostratigraphic data from the Candasnos section allow us to establish atime span of 2,808 years for the limestone/marl couplet from the lower partof this section, and between 19,000 and 22,000 years for the periodic cycleidentified, thus representing the climatic precession cycle. Shallowingsequences from marginal areas do not correspond with any periodiccycle. 相似文献
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V. Yu. Kuznetsov E. V. Tabuns G. A. Cherkashev V. E. Bel’tenev F. E. Maksimov K. A. Kuksa L. I. Lazareva S. B. Levchenkoa I. E. Zherebtsov 《Doklady Earth Sciences》2018,478(1):26-30
The results of 230Th/U dating and mineralogical–geochemical studies of sulfide ores from the Irinovskoe hydrothermal field and the Severo-Zapadnoe ore occurrence (Mid-Atlantic Ridge) are presented. Sulfides are represented primarily by copper–sulfide ores with 12–30% Cu content; sulfur- and zinc-sulfide ores are distributed less frequently. The analysis of a change in the composition of sulfides over time has made it possible to identify three stages of formation. Each stage assumes that mineral associations are changed from high-temperature (sulfur and copper sulfide) to medium temperature (Zn–Cu and Zn-sulfide) sulfide ores. The whole age range of formation of the hydrothermal deposits falls within the time interval of about 58000–8000 for the Irinovskoe field and 69000–11000 years ago for the Severo-Zapadnoe ore occurrence. 相似文献
6.
This article points out some particular features conditioning seismic hazard assessments (SHA) in Spain, a region with low–moderate
seismicity. Although sized earthquakes occurred in the past, as evidenced by historical documents and neotectonic studies,
no large events occurred during the last decades. The absence of strong motion records corresponding to earthquakes with magnitude
larger than 5.5 is an important obstacle for the development of ground motion models constrained by local data, with the consequent
difficulty in SHA studies. In this paper, some recent developments aiming at providing solutions to these difficulties are
presented. Specifically, a strong motion databank containing a massive collection of accelerograms and response spectra from
different configurations source-path-site corresponding to earthquakes all over the world is introduced, together with software
utilities for its management. A first application of this databank is the development of specific ground motion models for
Spain and for the Mediterranean region that predict peak ground accelerations as a function of several definitions of magnitude,
distance and soil class. The predictive power of these ground motion models is tested by contrasting their estimates with
recently recorded ground motions. The comparison between our ground-motion models with others proposed in the literature for
other areas reveals a regular overestimation of the expected ground motions at Spanish sites by the non-local models. Consequently,
SHA studies based in external models may overestimate the predicted hazard at the Iberian sites. In the last part of the paper
a method for checking whether the response spectra proposed in the Spanish Building Code (NCSE-02) are consistent with actual
accelerometric data from recent low magnitude earthquakes is applied. The spectral shapes of the Spanish Building Code NCSE-02
are compared with the response spectral shapes deduced from the available accelerograms by normalising the response spectra
with the recorded PGA. It is appreciated that the NCSE-02 spectral shapes are exceeded by a large number of actual spectral
shapes for short periods (around 0.2 s), a result to be taken into account in further revisions of the NCSE-02 code. The issues
tackled in this work constitute not only an improvement for ground-motion characterisation in Spain, but also provide guidelines
of general interest for potential applications in other regions with similar seismicity. 相似文献
7.
8.
V. A. Bel’chikov A. Ya. Polunin Yu. A. Simonov A. V. Khristoforov 《Russian Meteorology and Hydrology》2013,38(2):119-125
Presented is a method of studying possible climate-driven changes of river runoff characteristics, according to which the atmosphere-ocean general circulation models are used for computing the climate system impact, whereas the response of a watershed to given disturbance is estimated using the model of runoff formation in a river basin. The study is carried out for the Northern Dvina River basin. Estimates of possible changes (relative to the reference period) by the middle and late 21st century of such water regime characteristics as mean annual and maximum water discharge, as well as mean discharge values for the spring, summer-fall and winter seasons, are presented. 相似文献
9.
Assessment of erosion and settling properties of fine sediments stored in cobble bed rivers: the Arc and Isère alpine rivers before and after reservoir flushing 下载免费PDF全文
Cohesive sediment dynamics in mountainous rivers is poorly understood even though these rivers are the main providers of fine particles to the oceans. Complex interactions exist between the coarse matrix of cobble bed rivers and fine sediments. Given that fine sediment load in such environments can be very high due to intense natural rainfall or snowmelt events and to man‐induced reservoir or dam flushing, a better understanding of the deposition and sedimentation processes is needed in order to reduce ecohydrological downstream impacts. We tested a field‐based approach on the Arc and Isère alpine rivers combining measurements of erosion and settling properties of river bed deposits before and after a dam flushing, with the U‐GEMS (Gust Erosion Microcosm System) and SCAF (System Characterizing Aggregates and Flocs), respectively. These measurements highlight that critical shears, rates of erosion, settling velocities and propensity of particles to flocculate are highly variable in time and space. This is reflective of the heterogeneity of the hydrodynamic conditions during particle settling, local bed roughness, and nature and size of particles. Generally the deposits were found to be stable relative to what is measured in lowland rivers. It was, however, not possible to make a conclusive assessment of the extent to which the dynamics of deposits after reservoir flushing were different from those settled after natural events. The absence of any relationships between erosion and deposition variables, making it impossible to predict one from another, underlined the need to measure all of them to have a full assessment of the fine sediment dynamics and to obtain representative input variables for numerical models. While the SCAF was found to be effective, an alternative to the U‐GEMS device will have to be found for the erodibility assessment in cobble bed rivers, in order to make more rapid measurements at higher shears. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
10.
Amor Ben Moussa Sarra Bel Haj Salem Kamel Zouari Faycel Jelassi 《Arabian Journal of Geosciences》2017,10(20):446
The major ion hydrochemistry, sodium absorption ratio (SAR), sodium percentage, and isotopic signatures of Hammamet-Nabeul groundwaters were used to identify the processes that control the mineralization, irrigation suitability, and origin of different water bodies. This investigation highlights that groundwater mineralization is mainly influenced by water-rock interaction and pollution by the return flow of irrigation water. The comparison of groundwater quality with irrigation suitability standards proves that most parts of groundwater are unacceptable for irrigation and this long-term practice may result in a significant increase of the salinity and alkalinity in the soils. Based on isotopic signatures, the shallow aquifer groundwater samples were classified into (i) waters with depleted δ18O and δ2H contents, highlighting recharge by modern precipitation, and (ii) waters with enriched stable isotope contents, reflecting the significance of recharge by contaminated water derived from the return flow of evaporated irrigation waters. The deep-aquifer groundwater samples were also classified into (i) waters with relatively enriched isotope contents derived from modern recharge and mixed with shallow-aquifer groundwater and (ii) waters with depleted stable isotope contents reflecting a paleoclimatic origin. Tritium data permit to identify three origins of recharge, i.e., contemporaneous, post-nuclear, and pre-nuclear. Carbon-14 activities demonstrate the existence of old paleoclimatic recharge related to the Holocene and Late Pleistocene humid periods. 相似文献