Electrical resistivity model of the crust and upper mantle from a magnetotelluric survey through the central Pyrenees   总被引：2，自引：0，他引：2  

Jaume Pous  Juanjo Ledo  Alex Marcuello  Marc Daignières 《Geophysical Journal International》1995,121(3):750-762

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Joint Audio-Magnetotelluric and Passive Seismic Imaging of the Cerdanya Basin     

A. Gabàs  A. Macau  B. Benjumea  P. Queralt  J. Ledo  S. Figueras  A. Marcuello 《Surveys in Geophysics》2016,37(5):897-921

The structure of Cerdanya Basin (north-east of Iberian Peninsula) is partly known from geological cross sections, geological maps and vintage geophysical data. However, these data do not have the necessary resolution to characterize some parts of Cerdanya Basin such as the thickness of soft soil, geometry of bedrock or geometry of geological units and associated faults. For all these reasons, the main objective of this work is to improve this deficiency carrying out a detailed study in this Neogene basin applying jointly the combination of passive seismic methods (H/V spectral ratio and seismic array) and electromagnetic methods (audio-magnetotelluric and magnetotelluric method). The passive seismic techniques provide valuable information of geometry of basement along the profile. The maximum depth is located near Alp village with a bedrock depth of 500 m. The bedrock is located in surface at both sites of profile. The Neogene sediments present a shear-wave velocity between 400 and 1000 m/s, and the bedrock basement presents a shear-wave velocity values between 1700 and 2200 m/s. These results are used as a priori information to create a 2D resistivity initial model which constraints the inversion process of electromagnetic data. We have obtained a 2D resistivity model which is characterized by (1) a heterogeneous conductivity zone (<40 Ohm m) that corresponds to shallow part of the model up to 500 m depth in the centre of the profile. These values have been associated with Quaternary and Neogene sediments formed by silts, clays, conglomerates, sandstones and gravels, and (2) a deeper resistive zone (1000–3000 Ohm m) interpreted as Palaeozoic basement (sandstones, limestones and slates at NW and conglomerates and microconglomerates at SE). The resistive zone is truncated by a discontinuity at the south-east of the profile which is interpreted as the Alp-La Tet Fault. This discontinuity is represented by a more conductive zone (600 Ohm m approx.) and is explained as a combination of fractured rock and a fluid network. The result highlights that the support between different geophysical methods is essential in producing geophysical meaningful models.  相似文献   

Quantification of the 1998–1999 explosion sequence at Popocatépetl volcano,Mexico     

Vyacheslav M. Zobin  Alicia Martínez 《Journal of Volcanology and Geothermal Research》2010

The sequence of large Vulcanian explosions occurring at the andesitic Popocatépetl volcano, Mexico during November 1998 to April 1999 was studied. The size of 26 largest explosions was estimated from broadband seismic records at the distance of 4 km from the crater. The sequence began with the largest explosion (E = 2.6 × 10¹² J) occurring on 25 November at 08:05, and following largest daily explosions were characterized by gradual decrease in the energy. The energy of 20 large (E ≥ 10¹¹ J) explosions was distributed as Student's t-distribution with a geometrical mean Log E = 11.81 (J).  相似文献   

Picophytoplankton: A major contributor to planktonic biomass and primary production in a eutrophic,river-dominated estuary     

Alicia K. Gaulke  Michael S. Wetz  Hans W. Paerl 《Estuarine, Coastal and Shelf Science》2010

Biomass and primary productivity of picophytoplankton (PP; phytoplankton <3 μm) and larger phytoplankton (>3 μm) were determined during an annual cycle along the salinity gradient in North Carolina’s Neuse River Estuary (NRE), a eutrophic, microtidal estuary. The PP were a major component of total phytoplankton biomass and productivity, contributing ∼35–44% of the total chlorophyll a (Chl a) and 42–55% of the total primary productivity. Chl a and productivity of PP decreased from the upper to lower estuary, although the PP contribution relative to larger phytoplankton remained nearly constant. Significant PP growth occurred in the spring, but PP productivity and biomass were maximal in summer. PP productivity and biomass were positively correlated with temperature and dissolved inorganic phosphorus concentrations, which were maximal in summer due to release from sediments. Biomass and productivity of PP and >3 μm phytoplankton were also positively correlated, suggesting that growth conditions favoring the onset of blooms of larger phytoplankton species will similarly affect PP. High PP productivity and biomass in the NRE support the notion that PP play an important role in the production and eutrophication potentials of this estuary. High PP productivity and biomass have been noted in several other temperate estuaries, all sharing a common feature with the NRE—long residence time. These findings challenge the assumption that PP relative importance should be minimal in eutrophic systems.  相似文献   

Variation in groundwater salinity in a tidal salt marsh basin,North Inlet Estuary,South Carolina     

Erin S. Carter  Scott M. White  Alicia M. Wilson 《Estuarine, Coastal and Shelf Science》2008

Groundwater flow of freshwater from upland forests into salt marshes is influenced by hydrologic forces that operate over a wide range of temporal scales, including storm events, tidal fluctuations, seasonal variations in rainfall and evapotranspiration (ET). Groundwater salinity can be a useful first order indicator of the balance between these flow processes. A dipole-dipole electrical resistivity survey was conducted approximately monthly during 2005 to measure groundwater salinity across a portion of Crabhaul Creek, a tidal salt marsh basin at the boundary of the upland forest and the North Inlet marsh in South Carolina. The monthly electrical resistivity surveys were designed to provide a detailed, spatially continuous measurement of subsurface conductivity to a depth of 4 m in order to further investigate the seasonal variation in groundwater salinity. Resistivity models were corroborated by simultaneous measurements of salinity in nearby piezometers. The freshwater-brackish water interface was clearly imaged by the resistivity. Movement of this interface occurs on a timescale of months rather than a regular seasonal variation. The average salinity in the marsh basin is highest in late Summer (July–August) when ET is highest, and lowest during the Winter (November–December). The position of the brackish-freshwater interface changes, but is not well correlated to local rainfall or tidal cycles except under specific circumstances. A steady-state hydrological model correctly predicts the average position of the freshwater-brackish interface and suggests a linear relationship between the height of the water table and the location of the interface. These results suggest a complex relationship between precipitation events and groundwater flow from the forest into the marsh.  相似文献   

Grain size and depositional environment as predictors of permeability in coastal marine sands     

Alicia M. Wilson  Markus Huettel  Stephen Klein 《Estuarine, Coastal and Shelf Science》2008

More than half of the surface sediments covering the continental shelves are sandy, which may permit substantial sub-seafloor pore water advection. Knowledge of sediment permeability is required for quantifying advection and associated solute transport, but studies of marine sediments typically report grain size analyses rather than permeability. Here data from 23 studies were examined to determine the range in permeabilities reported for sublittoral marine sands and to assess the utility of permeability–grain size relationships in this setting. In the resulting database, the permeability of small (∼30 cm) undisturbed cores collected from the sea floor all fell between 2 × 10⁻¹² and 4 × 10⁻¹⁰ m², a range where advective transport induced by wave and current action should be pervasive. The range in grain size was very similar for near-shore (<10 m water depth) and continental shelf samples (>10 m water depth), but the permeability of the continental shelf samples was consistently lower for the same median grain size. Empirical permeability–grain size relationships generated a poor fit (r² = 0.35) for the aggregate data, but separate relationships for near-shore and continental shelf samples were significantly better, r² = 0.66 and 0.77, respectively. Permeability–grain size relationships thus may be useful for sublittoral sands, but a larger database needs to be accumulated before reliable fit parameters and variability can be predicted. Thus it is recommended that permeability be routinely determined when characterizing sedimentological properties of marine sand deposits. Concurrent determinations of sediment bulk density and porosity may further improve estimates of permeability.  相似文献   

The mixed-bed glacial landform imprint of the North Sea Lobe in the western North Sea     

David H. Roberts  Elena Grimoldi  Louise Callard  David J.A. Evans  Chris D. Clark  Heather A. Stewart  Dayton Dove  Margot Saher  Colm Ó Cofaigh  Richard C. Chiverrell  Mark D. Bateman  Steven G. Moreton  Tom Bradwell  Derek Fabel  Alicia Medialdea 《地球表面变化过程与地形》2019,44(6):1233-1258

During the last glacial cycle an intriguing feature of the British-Irish Ice Sheet was the North Sea Lobe (NSL); fed from the Firth of Forth and which flowed south and parallel to the English east coast. The controls on the formation and behaviour of the NSL have long been debated, but in the southern North Sea recent work suggests the NSL formed a dynamic, oscillating terrestrial margin operating over a deforming bed. Further north, however, little is known of the behaviour of the NSL or under what conditions it operated. This paper analyses new acoustic, sedimentary and geomorphic data in order to evaluate the glacial landsystem imprint and deglacial history of the NSL offshore from NE England. Subglacial tills (AF2/3) form a discontinuous mosaic interspersed with bedrock outcrops across the seafloor, with the partial excavation and advection of subglacial sediment during both advance and retreat producing mega-scale glacial lineations and grounding zone wedges. The resultant ‘mixed-bed’ glacial landsystem is the product of a dynamic switch from a terrestrial piedmont-lobe margin with a net surplus of sediment to a partially erosive, quasi-stable, marine-terminating, ice stream lobe as the NSL withdrew northwards. Glaciomarine sediments (AF4) drape the underlying subglacial mixed-bed imprint and point to a switch to tidewater conditions between 19.9 and 16.5 ka cal BP as the North Sea became inundated. The dominant controls on NSL recession during this period were changing ice flux through the Firth of Forth ice stream onset zone and water depths at the grounding line; the development of the mixed-bed landsystem being a response to grounding line instability. © 2018 John Wiley & Sons, Ltd.  相似文献   

Lithic raw material usages during the Middle Stone Age at Dakhleh Oasis,Egypt     

Alicia L. Hawkins  Maxine R. Kleindienst 《Geoarchaeology》2002,17(6):601-624

Dakhleh Oasis, the largest of the Egyptian Western Desert, presents an opportunity for diachronic study of lithic raw material preferences during the Middle Stone Age (MSA) of the Eastern Sahara. Archaeological aggregates and raw material sources are exposed and easily mapped. Diverse and abundant lithic raw materials derive from sandstone, shale, and limestone sources and are also found in secondary geological contexts. Three main raw materials were used, and there was strong preference for one of these: Tarawan chert. Easily available only in the north‐central oasis, this material was transported substantial distances even when other materials that were known and used by MSA peoples could be found closer at hand. There is little evidence for use of raw materials exogenous to Dakhleh Oasis. This pattern of usage does not appear to change from the older MSA units to the Aterian Dakhleh Unit. © 2002 Wiley Periodicals, Inc.  相似文献   

From Lightning Rod to Community Asset? The Creation of a Protected Area in Northern California and its Evolving Relationship to a Nearby Community     

Erin Clover Kelly  Alicia Christiansen 《社会与自然资源》2019,32(6):657-674

Researchers have described shifting relationships between communities and Protected Areas (PAs), from excluding communities to consultative relationships to participatory models. This research focuses on a PA created on formerly industrial timberland that was the focus of conflict between environmental activists and the timber industry. In a nearby formerly timber-dependent community, we assessed community use and support of the PA, and its perceived purposes and benefits, through a survey of residents and interviews with key stakeholders. Despite intense conflict surrounding its creation, we found support for the PA, and our case suggests a new possible relationship between PAs and communities, which is mitigating and helping to resolve natural resource conflict. We also found that community well-being benefits of the PA were unclear; we suggest that PA management should include cultural, economic, and historical context in objectives and interpretive materials; and should work to create economic opportunities connected to the PA.  相似文献   

Ichthyoplankton spatial pattern in the inner shelf off Bahía Blanca Estuary, SW Atlantic Ocean     

Mnica Susana Hoffmeyer  Menndez María Clara  Biancalana Florencia  Nizovoy Alicia Mabel  Torres Eduardo Ramn 《Estuarine, Coastal and Shelf Science》2009,84(3):181-392

This study focuses on the composition, abundance and distribution of ichthyoplankton in the inner shelf area off Bahía Blanca Estuary on the SW Atlantic Ocean during late spring. Eggs and larvae of Brevoortia aurea, Engraulis anchoita, Parona signata, Sciaenidae spp. – such as Cynoscion guatucupa and Micropogonias furnieri –, and Odontesthes argentinensis were found. Species richness was low probably as a result of season and shallow depths. Ichthyoplankton abundance reached values close to 10 000 per 10 m⁻³ (eggs) and 4000 per 10 m⁻³ (larvae) and displayed a spatial distribution pattern with maximum abundance values restricted to a band parallel to the coast. Differences between egg and larval patterns, probably derived from a different displacement and hydrodynamic behavior, were observed. Egg and larvae distribution patterns were found related with spawning areas and to directly depend on salinity and mesozooplankton. The larvae distribution pattern, in particular, was found to inversely depend on particulate organic carbon. In addition, the geographic location of egg and larvae maxima strongly coincided with a saline front reported for this area in springtime, thus suggesting a direct relationship with it.  相似文献