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41.
Francesca Sangiorgi Lucilla Capotondi Nathalie Combourieu Nebout Luigi Vigliotti Henk Brinkhuis Simona Giunta Andr F. Lotter Caterina Morigi Alessandra Negri Gert‐Jan Reichart 《第四纪科学杂志》2003,18(8):723-732
Holocene cooling events have been reconstructed for the southern Adriatic Sea (central Mediterranean) by means of analyses of organic walled dinoflagellate cysts, planktonic foraminifera, oxygen isotopes, calcareous nanoplankton, alkenones and pollen from a sediment core. Two cooling events have been detected, during which sea‐surface temperatures (SSTs) were ca. 2°C lower. Unravelling the SST signal into dominant seasonal components suggests maximum winter cooling of 2°C at around 6.0 ka, whereas the cooling at ca. 3.0 ka might be the result of a spring temperature cooling of 2–3°C. The events, lasting several hundred years, are apparently synchronous with those in the Aegean Sea, where they have been related to known cooling events from the Greenland ice‐core record. A distinct interruption in Adriatic Sea sapropel S1 is not clearly accompanied by a local drop in winter temperatures, but seems to be forced by ventilation, which probably occurred earlier in the Aegean Sea and was subsequently transmitted to the Adriatic Sea. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
42.
We characterized the abundance and species composition of sessile and mobile epifaunal assemblages in the York River, a tributary
of the Chesapeake Bay, U.S., during the summer hypoxia seasons in 1996 and 1997. We collected communities on artificial substrates
in two areas of the river that have historically experienced different exposure to hypoxia. Despite frequent hypoxic stress,
epifauna formed dense communities in both areas. Dominant species comprised a range of phyla and included the polychaetesPolydora cornuta andSabellaria vulgaris, the bryozoansMembranipora tenuis andConopeum tenuissimum, the tunicateMolgula manhattensis, the barnacleBalanus improvisus, the anemoneDiadumene leucolena, and the hydroidsEctopleura dumortieri andObelia bicuspidata. Common mobile species included the nudibranchsCratena kaoruae andDoridella obscura, the amphipodsMelita nitida andParacaprella tenuis, the polychaeteNereis succinea, and the flatwormStylochus ellipticus. We found few differences in species composition between the two areas, even though one area usually experienced lower oxygen
concentrations during hypoxic events, suggesting that hypoxia does not exclude any epifaunal species, in the York River. We
did find differences between the two study areas in percent cover and abundance of some species. While tunicates, hydroids,
and anemones were equally abundant in both areas during both study years, bryozoans and the polychaeteS. vulgaris were more abundant in the area with gereally higher oxygen, suggesting that they may be less tolerant of hypoxic stress.
The polychaeteP. cornuta was more abundant in the area that usually had lower oxygen. These results suggest that many epifaunal species have high
hypoxia tolerance, and most epifaunal species found in the lowr York River are able to survive in hypoxic areas. Epifaunal
species are not necessarily more susceptible to hypoxia than infaunal species in the York River. Epifaunal communities in
areas with brief hypoxic episodes and moderate hypoxia (0.5–2 mg O2 I−1) can persist with little change in species composition, and with few changes in abundance, as oxygen concentrations fall. 相似文献
43.
Anji Seth Sara A. Rauscher Maisa Rojas Alessandra Giannini Suzana J. Camargo 《Climatic change》2011,104(2):403-414
Twenty-first century climate model projections show an amplification of the annual cycle in tropical precipitation with increased
strength in both wet and dry seasons, but uncertainty is large and few studies have examined transition seasons. Here we analyze
coupled climate model projections of global land monsoons and show a redistribution of precipitation from spring to summer
in northern (North America, West Africa and Southeast Asia) and southern (South America, Southern Africa) regions. The annual
cycle changes are global in scale. Two mechanisms, remote (based on tropospheric stability) and local (based on low level
and surface moisture), are evaluated through the annual cycle. Increases in tropospheric stability persist from winter into
spring and are reinforced by a reduction in surface moisture conditions, suggesting that in spring both remote and local mechanisms
act to inhibit convection. This enhanced spring convective barrier leads to reduced early season rainfall; however, once sufficient
increases in moisture (by transport) are achieved, decreases in tropospheric stability result in increased precipitation during
the late rainy season. Further examination of this mechanism is needed in observations and models, as the projected changes
would have substantial implications for agriculture, water management, and disaster preparedness. 相似文献
44.
We calculate charge state distributions of Kr and Xe in a model for two different types of solar wind using the effective ionization and recombination rates provided from the OPEN_ADAS data base. The charge states of heavy elements in the solar wind are essential for estimating the efficiency of Coulomb drag in the inner corona. We find that xenon ions experience particularly low Coulomb drag from protons in the inner corona, comparable to the notoriously weak drag of protons on helium ions. It has been found long ago that helium in the solar wind can be strongly depleted near interplanetary current sheets, whereas coronal mass ejecta are sometimes strongly enriched in helium. We argue that if the extraordinary variability of the helium abundance in the solar wind is due to inefficient Coulomb drag, the xenon abundance must vary strongly. In fact, a secular decrease of the solar wind xenon abundance relative to the other heavier noble gases (Ne, Ar, Kr) has been postulated based on a comparison of noble gases in recently irradiated and ancient samples of ilmenite in the lunar regolith. We conclude that decreasing solar activity and decreasing frequency of coronal mass ejections over the solar lifetime might be responsible for a secularly decreasing abundance of xenon in the solar wind. 相似文献
45.
Felice Arena Alfredo Ascanelli Vincenzo Nava Diego Pavone Alessandra Romolo 《Coastal Engineering》2008,55(12):1052-1061
High waves at ocean occur during a complex space–time evolution of wave groups. In this paper the nonlinear structure of three-dimensional sea wave groups at intermediate water depth is investigated. To this purpose, the Boccotti's Quasi-Determinism theory is firstly applied to describe the linear wave groups when a given exceptionally high crest occurs. Then, the second-order correction to the linear solution is derived for the general condition of three-dimensional wave groups, at a finite water depth. Several numerical applications, finally, have been carried out in order to show how both the spectral bandwidth and the directional spreading modify the nonlinear high waves at different water depth. 相似文献
46.
Raulin Jean Pierre Makhmutov Vladimir S. Kaufmann Pierre Pacini Alessandra Abe Lüthi Thomas Hudson Hugh S. Gary Dale E. 《Solar physics》2004,223(1-2):181-199
We present a report on the strong X5.3 solar flare which occurred on 25 August 2001, producing high-level γ-ray activity, nuclear lines and a dramatic long-duration white-light continuum. The bulk of millimeter radio fluxes reached a peak of ∼100 000 solar flux units at 89.4 GHz, and a few thousands of solar flux units were detected in the submillimeter range during the impulsive phase. In this paper we focus on and discuss (i) the implications inferred from high frequency radio observations during the impulsive phase; (ii) the dynamics of the low corona active region during the impulsive phase. In particular we found that 4–5 × 1036 accelerated (>20 keV) electrons s−1 radiating in a 1000–1100 G region, are needed to explain the millimeter to submillimeter-wave emissions. We present evidence that the magnetic field in the active region was very dynamic, and that strong non-thermal processes were triggered by the appearance of new, compact, low-lying (few thousand kilometers) loop systems, suggesting the acceleration site(s) were also located in the low solar atmosphere. 相似文献
47.
48.
Thomas Monecke Sven Petersen Mark D. Hannington Marco Anzidei Alessandra Esposito Guido Giordano Dieter Garbe-Sch?nberg Nico Augustin Bernd Melchert Mike Hocking 《Bulletin of Volcanology》2012,74(9):1937-1944
Explosions of hot water, steam, and gas are common periodic events of subaerial geothermal systems. These highly destructive events may cause loss of life and substantial damage to infrastructure, especially in densely populated areas and where geothermal systems are actively exploited for energy. We report on the occurrence of a large number of explosion craters associated with the offshore venting of gas and thermal waters at the volcanic island of Panarea, Italy, demonstrating that violent explosions similar to those observed on land also are common in the shallow submarine environment. With diameters ranging from 5 to over 100?m, the observed circular seafloor depressions record a history of major gas explosions caused by frequent perturbation of the submarine geothermal system over the past 10,000?years. Estimates of the total gas flux indicate that the Panarea geothermal system released over 70?Mt of CO2 over this period of time, suggesting that CO2 venting at submerged arc volcanoes contributes significantly to the global atmospheric budget of this greenhouse gas. The findings at Panarea highlight that shallow submarine gas explosions represent a previously unrecognized volcanic hazard around populated volcanic islands that needs to be taken into account in the development of risk management strategies. 相似文献
49.
According to the most modern trend, performance‐based seismic design is aimed at the evaluation of the seismic structural reliability defined as the mean annual frequency (MAF) of exceeding a threshold level of damage, i.e. a limit state. The methodology for the evaluation of the MAF of exceeding a limit state is herein applied with reference to concentrically ‘V’‐braced steel frames designed according to different criteria. In particular, two design approaches are examined. The first approach corresponds to the provisions suggested by Eurocode 8 (prEN 1998—Eurocode 8: design of structures for earthquake resistance. Part 1: general rules, seismic actions and rules for buildings), while the second approach is based on a rigorous application of capacity design criteria aiming at the control of the failure mode (J. Earthquake Eng. 2008; 12 :1246–1266; J. Earthquake Eng. 2008; 12 :728–759). The aim of the presented work is to focus on the seismic reliability obtained through these design methodologies. The probabilistic performance evaluation is based on an appropriate combination of probabilistic seismic hazard analysis, probabilistic seismic demand analysis (PSDA) and probabilistic seismic capacity analysis. Regarding PSDA, nonlinear dynamic analyses have been carried out in order to obtain the parameters describing the probability distribution laws of demand, conditioned to given values of the earthquake intensity measure. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
50.