Millennial-scale shifts in microtidal ecosystems during the Holocene: dynamics and drivers of change from the Po Plain coastal record (NE Italy)     

Veronica Rossi  Giulia Barbieri  Stefano Claudio Vaiani  Marco Cacciari  Luigi Bruno  Bruno Campo  Marco Marchesini  Silvia Marvelli  Alessandro Amorosi 《第四纪科学杂志》2021,36(6):961-979

Framed into a robust stratigraphic context, multivariate analyses on the Holocene palaeobiological record (pollen, benthic foraminifers, ostracods) of the Po coastal plain (NE Italy) allowed the investigation of microtidal ecosystems variability and driving parameters along a 35-km-long land–sea transect. Millennial-scale ecosystem shifts are documented by coeval changes in the meiofauna, reflecting variations in organic matter–water depth (shallow-marine environments) and degree of confinement-salinity (back-barrier settings). In-phase shifts of vegetation communities track unsteady water-table levels and river dynamics in freshwater palustrine areas. Five environmental–ecological stages followed one another crossing four tipping points that mark changes in relative sea level (RSL), climate and/or fluvial regime. At the culmination of Mediterranean RSL rise, after the 8200 event, remarkable growth of peatlands took place in the Po estuary, while low accumulation rates typified the shelf. At the transgressive–regressive turnaround (~7000 cal a bp ), the estuary turned into a delta plain with tidally influenced interdistributary embayments. River flow regime oscillations after the Climate Optimum (post-5000 cal a bp ) favoured isolation of the bays and the development of brackish wetlands surrounded by wooded peatlands. The youngest threshold (~800 cal a bp ), which led to the establishment of the modern delta, reflects a major avulsion of the Po River.  相似文献   

Wavelet‐based regularization of the extracted topographic index from high‐resolution topography for hydro‐geomorphic applications          下载免费PDF全文

Vahid Nourani  Stefano Zanardo 《水文研究》2014,28(3):1345-1357

High‐resolution topography, e.g. 1‐m digital elevation model (DEM) from light detection and ranging (LiDAR), offers opportunity for accurate identification of topographic features of relevance for hydrologic and geomorphologic modelling. Yet, the computation of some derived topographic properties, such as the topographic index (TI), is characterized by daunting challenges that hamper the full exploration of topography‐based models. Particular problems, for example, arise when a distributed (or semi‐distributed) rainfall–runoff model is applied to high‐resolution DEMs. Indeed, the characteristic dependency between landscape resolution and the computed TI distribution results in the formation of un‐physical, unconnected saturated zones, which in turn cause unrealistic representations of rainfall–runoff dynamics. In this study, we present a methodology based on a multi‐resolution wavelet transformation that, by means of a soft‐thresholding scheme on the wavelet coefficients, filters the noise of high‐resolution topography to construct regularized sets of locally smoother topography on which the TI is computed. While the methodology needs a somewhat arbitrary definition of the wavelet coefficients threshold, our study shows that when the information content (entropy) of the TI distribution is used as a filtering efficiency metric, a critical threshold automatically emerges in the landscape reconstruction. The methodology is demonstrated using 1‐m LiDAR data for the Elder Creek River basin in California. While the proposed case study uses a TOPMODEL approach, the methodology can be extended to different topography‐based models and is not limited to hydrological applications. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

A bond contact model for methane hydrate‐bearing sediments with interparticle cementation     

Mingjing Jiang  Fangyuan Zhu  Fang Liu  Stefano Utili 《国际地质力学数值与分析法杂志》2014,38(17):1823-1854

While methane hydrates (MHs) can be present in various forms in deep seabeds or permafrost regions, this paper deals with MH‐bearing sediments (MHBS) where the MH has formed bonds between sand grains. A bond model based on experimentally validated contact laws for cemented granules is introduced to describe the mechanical behavior of the MH bonds. The model parameters were derived from measured values of temperature, water pressure and MH density. Bond width and thickness adopted for each bond of the MHBS were selected based on the degree of MH saturation. The model was implemented into a 2D distinct element method code. A series of numerical biaxial standard compression tests were carried out for various degrees of MH saturation. A comparison with available experimental data shows that the model can effectively capture the essential features of the mechanical behavior of MHBS for a wide range of levels of hydrate saturation under drained and undrained conditions. In addition, the analyses presented here shed light on the following: (1) the relationship between level of cementation and debonding mechanisms taking place at the microscopic level and the observed macro‐mechanical behavior of MHBS and (2) the relationship between spatial distribution of bond breakages and contact force chains with the observed strength, dilatancy and deformability of the samples. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Measurements of rill and gully erosion in Sicily     

Costanza Di Stefano  Vito Ferro 《水文研究》2011,25(14):2221-2227

This article reports the results of a field investigation aimed to characterize the morphology of both rills monitored at Sparacia experimental area and two ephemeral gullies (EGs) located in the Tremamargi basin, Sicily, Italy. At first, the available literature data together with the measurements carried out in this investigation were used to show that the EG length is a key parameter for the estimation of the eroded volume. Then, the comparison among the pairs length and volume corresponding to measured rills, EGs and gullies showed that the exponent of the power relationship is independent of the channelized erosion type (rill, EG and gully), while a different scale factor has to be used for each erosion process. Finally, a single relationship applicable to all channelized erosion processes was deduced applying the dimensional analysis and the self‐similarity theory. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Geophysical investigation of the Sandalp rock avalanche deposits     

Laura Valentina Socco  Denis Jongmans  Daniele Boiero  Stefano Stocco  Margherita Maraschini  Ken Tokeshi  Didier Hantz 《Journal of Applied Geophysics》2010,70(4):277-291

In the study of rock avalanche phenomena, numerical modelling makes use of back analyses of the rock avalanche propagation for calibration of the modelling assumptions and parameters. The back analyses require knowledge of the run-out area boundaries and the thickness distribution of the deposit. Geophysical methods can be applied to retrieve the thickness distribution, but, due to strong heterogeneities and logistic problems they are seldom applied. The aim of this work is to assess the potential of integrated geophysical methods to recognise and characterise a deposit created by two rock avalanches which occurred in the Sandalp valley (Switzerland) in 1996. The topography of the site before and after the rock avalanche is known and can be used as a benchmark. Resistivity tomography, seismic P-wave tomography, and active and passive surface wave analysis have been applied on several profiles deployed both on the rock avalanche deposit and in the surrounding area. Innovative approaches for surface wave analysis based on laterally constrained inversion and multimodal inversion have been applied to the data. A comparison of the results of the geophysical investigations with the topographic benchmark has shown the capability of the geophysical methods to locate the bottom of the deposit in the areas where the contrast with the host sediments properties is significant. In these areas, the deposit has higher resistivities and lower seismic velocities than the underlying materials. In the areas where the deposit is thicker and richer in fine-grained materials the geophysical parameters are not able to discriminate between the rock avalanche deposit and the underlying sediments. As a secondary task, the geophysical methods also allowed the bedrock pattern to be outlined.  相似文献   

The Alps in the Cretaceous: a doubly vergent pre‐collisional orogen     

Stefano Zanchetta  Eduardo Garzanti  Carlo Doglioni  Andrea Zanchi 《地学学报》2012,24(5):351-356

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Early diagenesis in rapidly accumulating sediments on the Alboran slope, ODP site 976     

Stefano M. Bernasconi  Philip A. Meyers  Gerard O’Sullivan 《Geo-Marine Letters》1998,18(3):209-214

The oxidation and reduction that occur during early diagenesis of sediments has been studied in the interstitial waters of a rapidly accumulating sedimentary sequence from the Mediterranean margin of Spain. A series of reactions that are mediated by progressively lower free energy derived from oxidation of organic matter is evident in the sedimentary sequence. Iron and manganese are rapidly reduced. Phosphate and alkalinity maxima at a subbottom depth of 15?m indicate maximal organic matter degradation. Methane first appears at ～20?m subbottom after sulfate is depleted, and its concentrations quickly climb.  相似文献   

Exploring the long-term and interannual variability of biogeochemical variables in coastal areas by means of a data assimilation approach     

Stefano Ciavatta  Roberto Pastres 《Estuarine, Coastal and Shelf Science》2011,91(3):411-422

Dynamic Harmonic Regression (DHR) models are applied here to the investigation of the interannual changes in the trend and seasonality of biogeochemical variables monitored in coastal areas. A DHR model can be regarded as a time-series component model, where the phases and amplitudes of the seasonal component, as well as the trend, are parameters that vary with time, reflecting relevant changes in the evolution of the biogeochemical variables. The model parameters and their confidence bounds are estimated by data assimilation algorithms, i.e. the Kalman filter and the Fixed Interval smoother. The DHR model structure is here identified by a preliminary spectral analysis and a subsequent minimization of the Bayesian Information Criterion, thus avoiding subjective choices of the frequencies in the seasonal component. The methodology was applied to the investigation of the long-term and interannual variability of ammonia, nitrate, orthophosphate and chlorophyll-a monitored monthly in the lagoon of Venice (Italy) during the years 1986–2008. It was found that the long-term evolutions of the biogeochemical variables were characterized by non-linear patterns and by statistically significant changes in the trend. The seasonal cycles of all the variables were characterized by a marked interannual variability. In particular, the changes in the seasonality of chlorophyll and nitrate were significantly related to the changes in the seasonality of water temperature at the study site and of nutrient concentrations in river discharges, respectively. These results indicate that the methodology could be a sound alternative to more traditional approaches for investigating the impacts of changes in environmental and anthropogenic forcings on the evolution of biogeochemical variables in coastal areas.  相似文献   

Determination of the relative thermal conductivity of solid samples by X-ray diffraction device     

Stefano Battaglia  Leonardo Leoni 《Mineralogy and Petrology》2010,100(1-2):11-16

The thermal conductivity (TC) of metal and rock samples was determined, under ambient conditions, using standard X-ray diffraction equipment in which only the diffractometer sample holder had been modified from the original. As the method is comparative, standard reference materials were used: a copper standard for the analysis of metals and marble for the rock specimens. The marble was analyzed beforehand using a quick thermal conductivity meter QTM. In experimental measurements of TC the mean reproducibility value appears to be about 4% while the accuracy is about 5% for the examined rocks, and better for metals.  相似文献   

Dissociation Constants of Protonated Oxidized Glutathione in Seawater Media at Different Salinities     

Pasquale Crea  Concetta De Stefano  Frank J. Millero  Silvio Sammartano  Virender K. Sharma 《Aquatic Geochemistry》2010,16(3):447-466

Oxidized glutathione (GSSG), which has four carboxylic and two amino groups, interacts with metal ions and may affect the bioavailability and geochemistry of metals in natural waters. In the present paper, six stepwise protonation constants K^\textH_i K^{\text{H}}_{i} for GSSG were measured as a function of salinity, S = 5–35‰ at t = 25°C (and in NaCl/MgCl₂ mixtures at different ionic strengths), in order to provide thermodynamic data for their acid base properties, which are useful for studying the interaction with metals in these media. The protonation enthalpies (ΔH _i /kJ mol⁻¹) were also determined at t = 25°C. The results were interpreted using the SIT model and Pitzer equations. The seawater model with the interaction parameters accounts for the differences between the values in NaCl and seawater. The results suggest that it is important to consider all of the ionic interactions in natural waters in examining the proton dissociation of GSSG.  相似文献