Time dependence of negative buoyancy and the subduction of continental lithosphere   总被引：1，自引：0，他引：1  

Giorgio Ranalli  Rita Pellegrini  Sergio D&#x;Offizi 《Journal of Geodynamics》2000,30(5)

The time evolution of negative buoyancy of a subducting slab is modelled from the beginning of subduction under various kinematic conditions (dip angle and subduction velocity). The calculations take into account the thermal and density effects of the variations of the thermophysical parameters with temperature and pressure, and of phase transitions. The magnitude of the negative buoyancy increases during subduction of oceanic lithosphere, up to values in the (2–4) × 10¹³ N m⁻¹ range when the tip of the slab reaches a depth of 600–700 km. If continental material arrives at the trench and is subducted, the downward buoyancy decreases by an amount proportional to the volume of the subducted continental crust. Assuming that subduction stops when the buoyancy becomes zero, and that delamination of the continental crust or slab breakoff do not occur, the maximum downdip length of the subductable continental crust is estimated as a function of the dip angle, subduction velocity and geometry of the margin. In most cases, subduction of continental material down to depths of 100–250 km is possible, and continental subduction can continue for times up to 10–15 Ma if the velocity is low. These estimates are not significantly affected by the hypothetical occurrence of a metastable olivine wedge within the slab, and could be lower bounds if the lower continental crust is mafic and transforms to eclogite.  相似文献   

The role of constraints on joint hypocentre determination in seismotectonic studies     

Rodolfo Console  Rita Di Giovambattista  Aldo Di Sanza  Paolo Favali   Giuseppe Smriglio 《Geophysical Journal International》2000,143(1):129-141

In this paper we show how the performance of the joint hypocentre determination (JHD) method can be improved, leading to reduced instability in cases close to singularity. The method has been extended by imposing a number of constraints introduced by other authors, and adding a new one. We tested the stability of the method and the relative advantages of the various constraints by simulating a geometrical space distribution of hypocentres recorded by a regional seismic network. We then applied this method to deep earthquakes that occurred in the Southern Tyrrhenian Sea subduction zone and to the seismicity of the Northern Apennines, which is moderately deeper than the typical shallow seismicity of the Apennines. The results obtained from the analysis of synthetic data and actual earthquakes confirm that the JHD method produces less scatter in the hypocentral determinations with respect to the standard locations. The main conclusion of our study is that we can significantly reduce the systematic mislocations that result from applying JHD to very clustered seismicity if we introduce the appropriate set of constraints.  相似文献   

Effects of forestry on summertime low flows and physical fish habitat in snowmelt‐dominant headwater catchments of the Pacific Northwest     

Stefan Gronsdahl  R. Dan Moore  Jordan Rosenfeld  Rich McCleary  Rita Winkler 《水文研究》2019,33(25):3152-3168

Periods of summertime low flows are often critical for fish. This study quantified the impacts of forest clear‐cutting on summertime low flows and fish habitat and how they evolved through time in two snowmelt‐dominant headwater catchments in the southern interior of British Columbia, Canada. A paired‐catchment analysis was applied to July–September water yield, the number of days each year with flow less than 10% of mean annual discharge, and daily streamflow for each calendar day. The postharvest time series were divided into treatment periods of approximately 6–10 years, which were analysed independently to evaluate how the effects of forestry changed through time. An instream flow assessment using a physical habitat simulation‐style approach was used to relate streamflow to the availability of physical habitat for resident rainbow trout. About two decades after the onset of logging and as the extent of logging increased to approximately 50% of the catchments, reductions in daily summertime low flows became more significant for the July–September yield (43%) and for the analysis by calendar day (11–68%). Reductions in summertime low flows were most pronounced in the catchment with the longest postharvest time series. On the basis of the temporal patterns of response, we hypothesize that the delayed reductions in late‐summer flow represent the combined effects of a persistent advance in snowmelt timing in combination with at least a partial recovery of transpiration and interception loss from the regenerating forests. These results indicate that asymptotic hydrological recovery as time progresses following logging is not suitable for understanding the impacts of forest harvesting on summertime low flows. Additionally, these reductions in streamflow corresponded to persistent decreases in modelled fish habitat availability that typically ranged from 20% to 50% during the summer low‐flow period in one of the catchments, suggesting that forest harvest may have substantial delayed effects on rearing salmonids in headwater streams.  相似文献   

The effect of an arctic polynya on the Northern Hemisphere mean circulation and eddy regime: a numerical experiment     

Rita Glowienka-Hense  Andreas Hense 《Climate Dynamics》1992,7(3):155-163

The feedback of an arctic polynya, which is a large ice-free zone within the sea ice, on the hemispheric climate is studied with the ECMWF T21 GCM. For this purpose a control and an anomaly integration, in which a polynya was introduced in the Kara Sea, are compared. As the GCM, like the real atmosphere, shows a high level of low frequency variability, the mean response to the changed boundary conditions is obscured by internal noise. The necessary significance analyses are thus performed to enhance the signal-tonoise ratio within the framework of an a priori chosen guess pattern and a multivariate test statistic. The sensible and latent heat fluxes increased above the polynya, which resulted in a warming of the lower troposphere above and near the polynya. No statistically significant local or global sea-level pressure changes are associated with this heating. However we find a significant change of hemispheric extent of the geopotential fields at 300 hPa, if we use as guess patterns the eigenmodes of the barotropic vorticity equation. The different mean flow field is accompanied by significant changes of the synoptic transient eddy field. We find a significant variation in the barotropic and baroclinic forcing of the mean flow by the eddies, a change in the location and intensity of the storm tracks and in the conversion between eddy available and eddy kinetic energy. The additional heat flux from the polynya results in a reduction of the meridional heat flux by the synoptic eddies on the western Atlantic.  相似文献   

A multi-disciplinary study of deformation of the basaltic cover over fine-grained valley fills: a case study from Eastern Sardinia,Italy     

Rita Deiana  Iginio Dieni  Francesco Massari  Maria Teresa Perri  Matteo Rossi  Alessandro Brovelli 《International Journal of Earth Sciences》2016,105(4):1245-1255

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Tithonian age of dinosaur fossils in central Patagonian,Chile: U–Pb SHRIMP geochronology     

Manuel Suárez  Rita De La Cruz  Mark Fanning  Fernando Novas  Leonardo Salgado 《International Journal of Earth Sciences》2016,105(8):2273-2284

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The balance between microzooplankton grazing and phytoplankton growth in a highly productive estuarine fjord   总被引：1，自引：4，他引：1  

Andrew W. Leising  Rita Horner  James J. Pierson  James Postel  Claudia Halsband-Lenk 《Progress in Oceanography》2005,67(3-4):366

During 24, three-day cruises to Dabob Bay, Washington State, USA, from February 4 to April 26, 2002, and February 4 to May 1 2003, we examined the relative growth and grazing rates of phytoplankton and microzooplankton using dilution experiments. Experiments were conducted over two time intervals: 8–10 h during the nighttime only, or 24 h from noon to noon. We used water from two depths during each cruise: from the surface mixed layer, and from a deep layer below the seasonal thermocline. During 2002, there was one mid-sized bloom consisting mainly of Thalassiosira spp. in early February, and a larger bloom in April comprised of two Chaetoceros spp. and Phaeocystis sp. During 2003, there were also two blooms, one in early February, which was again dominated by Thalassiosira spp., and a second larger bloom in mid-April, comprised mainly of Thalassiosira spp. and Chaetoceros spp. During all four of these blooms, and for both water source depths, specific grazing rates of microzooplankton were most often as high or higher than the calculated phytoplankton specific growth rates. The major microzooplankton categories that could have accounted for this were (1) a large Gyrodinium spp., (2) a group of fusiform-shaped mid-sized Protoperidinium species, and (3) three loosely defined taxonomic groups consisting of naked ciliates, tintinnids, and unidentified heterotrophic dinoflagellates. Based on our measurements, it appears that the microzooplankton community grazing pressure can often exert significant control on phytoplankton biomass, even during the extremely productive spring bloom periods and under several different diatom-dominated bloom types. These results suggest that even in highly productive estuarine ecosystems, which are often nurseries to economically important fisheries species, microzooplankton play a critical role and may significantly alter the availability and efficiency of transfer of energy to higher trophic levels.  相似文献   

The Biancavilla-Montalto ingnimbrite (Etna,Sicily)     

D De Rita  G Frazzetta  R Romano 《Bulletin of Volcanology》1991,53(2):121-131

Stratigraphic, granulometric, and SEM grain texture studies have been carried out on pyroclastic deposits in the Biancavilla and Montalto areas on the lower SW flank of Etna. These studies support the interpretation that the deposits were emplaced as pyroclastic flows during the final stage of the Ellittico eruptive activity (14.18±26 ka). Based on the high percentage of juvenile components and granulometric characteristics, the deposits are classified as normal ignimbrite. Four separate flow units have been distinguished based on the presence of fine-grained basal layers. A multivariate G-mode statistical analysis has been used to discriminate particle morphology populations between flow units and infer temporal changes in the nature of the eruptive processes. The initial flow (I) was emplaced at a high temperature and involved a juvenile gas phase exolved from the magma. Subsequent flows (II, III, IV) exhibit evidence for the interaction of external water.  相似文献   

Validation of very high cloud droplet number concentrations in air masses transported thousands of kilometres over the ocean     

Bengt G. Martinsson  Göran Frank  Sven&#;Inge Cederfelt  Olle H. Berg  Besim Mentes  Giorgos Papaspiropoulos  Erik Swietlicki  Jingchuan Zhou  Michael Flynn  Keith N. Bower  Tom W. Choularton  Jyrki Mäkelä  Aki Virkkula  Rita Van Dingenen 《Tellus. Series B, Chemical and physical meteorology》2000,52(2):801-814

The microstructure of orographic clouds related to the aerosol present was studied during the second Aerosol Characterisation Experiment (ACE‐2). Very high cloud droplet number concentrations (almost 3000 cm⁻³) were observed. These high concentrations occurred when clouds formed on a hill slope at Tenerife in polluted air masses originating in Europe that had transported the order of 1000 km over the Atlantic Ocean. The validity of the measured droplet number concentrations was investigated by comparing with measurements of the aerosol upstream of the cloud and cloud interstitial aerosol. Guided by distributions of the ratios between the measurements, three criteria of typically 30% in maximum deviation were applied to the measurements to test their validity. Agreement was found for 88% of the cases. The validated data set spans droplet number concentrations of 150–3000 cm⁻³. The updraught velocity during the cloud formation was estimated to 2.2 m s⁻¹ by model calculations, which is typical of cumuliform clouds. The results of the present study are discussed in relation to cloud droplet number concentrations previously reported in the literature. The importance of promoting the mechanistic understanding of the aerosol/cloud interaction and the use of validation procedures of cloud microphysical parameters is stressed in relation to the assessment of the indirect climatic effect of aerosols.  相似文献   

Data sets for the Upper Penticton Creek watershed experiment: A paired-catchment study to support investigations of watershed response to forest dynamics and climatic variability in an inland snow-dominated region     

R. Dan Moore  Diana Allen  Lucy MacKenzie  David Spittlehouse  Rita Winkler 《水文研究》2021,35(10):e14391

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