Lateral heterogeneities in the upper and mid-mantle conductivity in Argentina     

Ana Osella  Alicia Favetto  Patricia Martinelli 《Pure and Applied Geophysics》1993,141(1):139-156

Regional studies of Sq variations measured at five permanent observatories in Argentina were performed to estimate the distribution of the electrical conductivity of the upper and mid-mantle, reaching a depth of approximately 800 km. After separating the total field into its parts of external and internal origin, the in duced field was modeled, applying two-dimensional techniques. Additionally, induction arrows and an equivalent apparent resistivity curve were calculated A north-south heterogeneity was detected, showing an increase of the conductivity of the shallower layers north of the profile, which may be related to the presence of high volcanic activity in the area.Also at CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas).  相似文献   

A paleoenvironmental reconstruction of Laguna Babícora, Chihuahua, Mexico based on ostracode paleoecology and trace element shell chemistry     

Manuel R. Palacios-Fest  Ana Luisa Carreño  José R. Ortega-Ramírez  Guillermo Alvarado-Valdéz 《Journal of Paleolimnology》2002,27(2):185-206

Paleoecology of Laguna Babícora, Chihuahua, Mexico was reconstructed using ostracode faunal assemblages and shell chemistry. The paleolimnological record is used to show the magnitude of paleoclimatic changes in the area from 25,000 years to the present.Faunal assemblages consist of four species of the genus Limnocythere: L. sappaensis, L. ceriotuberosa, L. bradburyi and L. platyforma, all associated with Candona caudata, Candona patzcuaro and Cypridopsis vidua. A paleosalinity index developed from these assemblages indicates that the lake's salinity fluctuated frequently from oligo- to meso-haline conditions during the last 25,000 years. This pattern and low salinity range are in good agreement with modern TDS (here used as an indicator of salinity) values recorded from 26 wells and one spring from the area (258–975 mg l^–1). To estimate paleotemperature we examined the trace element content (Mg/Ca ratios) from individual valves of L. ceriotuberosa and L. platyforma, the two species most commonly recorded in Laguna Babícora.Shell Mg/Ca ratios of 204 specimens of these two species were used to estimate water temperature (Mg/Ca) by means of experimental standard coefficients. Our data show that paleowater temperature ranged from 5.6–21.3 °C (with 2 values ranging from 0.2–4.8 °C), which suggest a close correlation with atmospheric temperatures around the lake. These results are in good agreement with a modern mean winter temperature (3.5 °C) and mean summer temperature (20 °C) recorded in the area between 1970 and 1980.  相似文献   

Paleohydrology of Andean saline lakes from sedimentological and isotopic records, Northwestern Argentina     

Blas Valero-Garcés  Antonio Delgado-Huertas  Norma Ratto  Ana Navas  Larry Edwards 《Journal of Paleolimnology》2000,24(3):343-359

The paleohydrological evolution of several high altitude, saline lakes located in the southernmost Altiplano (El Peinado and San Francisco basins, Catamarca province, NW Argentina) was reconstructed applying sedimentological, geochemical and isotopic techniques. Several playa lakes from the San Francisco basin (26° 56 S; 68° 08 W, 3800-3900 m a.s.l.) show evidence of a recent raise in the watertable that led to modern deposition of carbonate and diatomaceous muds. A 2 m - long core from El Peinado Lake (26° 29 59 S, 68°05 32 W, 3820 m a.s.l.) consists of calcitic crusts (unit 3), overlaid by an alternation of macrophyte-rich and travertine clast- rich, laminated muds (unit 2), and topped by travertine facies (unit 1). This sedimentary sequence illustrates a paleohydrological evolution from a subaerial exposure (unit 3) to a high lake stand (unit 2), and a subsequent smaller decrease in lake level (unit 1). The ¹³C_{organic matter}record also reflects the lake transgression between units 3 and 2. Although there is a general positive correlation between ¹⁸O_carbonate and salinity proxies (Na, Li and B content), the large data dispersion indicates that other factors besides evaporation effects control chemical and isotopic composition of lakewater. Consequently, the oxygen isotopic composition cannot be interpreted exclusively as an indicator of salinity or evaporation ratio. The degassing of CO₂ during groundwater discharge can explain the enriched ¹³C values for primary carbonates precipitated. The carbon budget in these high altitude, saline lakes seems to be controlled by physical rather than biological processes.The Altiplano saline lakes contain records of environmental and climatic change, although accurate ¹⁴C dating of these lacustrine sediments is hindered by the scarcity of terrestrial organic material, and the large reservoir effects. Sedimentologic evidence, a ²¹⁰Pb-based chronology, and a preliminary U/Th chronology indicate a very large reservoir effect in El Peinado, likely as a result of old groundwaters and large contributions of volcanic and geothermal ¹⁴C-free CO₂ to the lake system. Alternative chronologies are needed to place these paleorecords in a reliable chronological framework. A period of increased water balance in the San Francisco basin ended at about 1660 ± 82 yr B.P. (calendar yr U/Th age), and would correlates with the humid phase between 3000 and 1800 yr B.P detected in other sites of the southern Altiplano. Both, ²¹⁰Pb and preliminary U/Th dating favor a younger age for the paleohydrological changes in El Peinado. The arid period reflected by subaerial exposure and low lake levels in unit 3 would have ended with a large increase in effective moisture during the late 17th century. The increased lake level during deposition of unit 2 would represent the period between AD1650 - 1900, synchronous to the Little Ice Age. This chronological framework is coherent with other regional records that show an abrupt transition from more arid to more humid conditions in the early 17th century, and a change to modern conditions in the late 19th century. Although there are local differences, the Little Ice Age stands as a significant climatic event in the Andean Altiplano.  相似文献   

Considerations about antenna pattern measurements of 2-D aperture synthesis radiometers     

Camps  A. Skou  N. Torres  F. Corbella  I. Duffo  N. Vall-llossera  M. 《Geoscience and Remote Sensing Letters, IEEE》2006,3(2):259-261

Accurate measurement of the antenna voltage patterns of large-aperture synthesis radiometers is critical in order to achieve good radiometric accuracy, and a very time consuming and expensive task. Measurement requirements and a tradeoff study relating radiometric accuracy degradation and number of elements to be measured are presented.  相似文献   

Nutrient dynamics in Mediterranean temporary streams: A case study in Pardiela catchment (Degebe River, Portugal)   总被引：1，自引：0，他引：1  

Ana Isabel Lilleb  Manuela Morais  Pedro Guilherme  Rita Fonseca  Antnio Serafim  Ramiro Neves 《Limnologica》2007,37(4):337-348

Most of the streams in the Mediterranean region are temporary, following predictable seasonal of flooding and drying, with a transition from lotic conditions to shallow lentic conditions. The goal of our study was to assess the nitrogen and phosphorus dynamics in channel-bed processes of temporary streams between floods. Results show that, during winter, temperatures ranged between 9.5 and 11.2 °C and oxygen concentration ranged from 8.0 to 9.5 mg L⁻¹, whereas, during summer, temperatures varied between 21.2 and 26.8 °C and oxygen between 1.2 and 5.3 mg L⁻¹, with oxygen depletion in the pools during the night. The nitrate concentrations were far more abundant during winter (February), while ammonium concentration increased after stream fragmentation into pools (especially in July when oxygen depletion conditions favoured ammonification). Results on sediment profiles showed that the most active sediment layers for NH₄-N are the top 2–3 cm, corresponding to the sediment depositional sites of the stream. Phosphate concentrations had larger variability, yet concentrations decreased from winter to spring and increased again in summer, when the shallow water pools were formed. Sediment profiles at the sediment depositional sites showed that PO₄-P was more dynamic in the first 6 cm.

In Mediterranean temporary streams, nutrient dynamics vary seasonally, as the system transits from lotic conditions to shallow lentic conditions, evidencing the regeneration of nutrients from organic and inorganic matter during the flow cessation period.  相似文献   

Late Pleistocene–Holocene environments in Valle Carbajal, Tierra del Fuego, Argentina   总被引：1，自引：0，他引：1  

Ana María Borromei  Andrea Coronato  Mirta Quattrocchio  Jorge Rabassa  Silvia Grill  Claudio Roig   《Journal of South American Earth Sciences》2007,23(4):321-335

The authors discuss Late Pleistocene–Holocene depositional environments in one of the Fuegian Andes valleys on the basis of palynological, geomorphological, and sedimentological analyses from two sites located near the Beagle Channel. The results obtained at these localities reinforce and refine the Late Pleistocene–Holocene climatic pattern previously recorded there. A colder period, associated with the Younger Dryas stadial event, is suggested by low Nothofagus pollen frequency, and communities of grass, low scrub, and shrub heath expanded into the low/middle slopes (10,310 ¹⁴C yr BP). By ca. 9500 ¹⁴C yr BP, warmer and drier conditions occurred, as evidenced by the development of open-grown vegetation in the valley floors (pollen zone O-3), followed by the expansion of open Nothofagus woodland (pollen zone O-2) in the middle Holocene. The milder climate subsequently changed, as indicated by the spreading of the closed forest and mire (pollen zone O-1), to more humid and cooler conditions during the last ca. 5000 yr BP.  相似文献   

The Alleret maar (Massif Central, France): A new lacustrine sequence of the early Middle Pleistocene in western Europe     

Jean-Franois Pastre  Agns Gauthier  Sbastien Nomade  Paul Orth  Ana Andrieu  Franck Goupille  Herv Guillou  Stphane Kunesch  Stphane Scaillet  Paul Randall Renne 《Comptes Rendus Geoscience》2007,339(16):987-997

The Alleret maar (Massif Central, France) provides a long lacustrine sequence (40.6 m) attributed to the early Middle Pleistocene. Sediment, pollen and diatoms analysis of its upper part (AL2 core, 14.6 m) indicates two temperate phases marked by high lake levels, forest development and vegetation expansion. They are separated by a cold period during which lake level drops, coarse sediment input increases and steppic and xerophilous plants develop. Pollen data suggests that this sequence belongs to the upper part of the Cromerian complex. These results are in agreement with the 557 ± 3 ka (±12 ka, including all errors) ⁴⁰Ar/³⁹Ar age obtained from an interbedded tephra layer emitted by the Mont-Dore/Sancy strato-volcano and establish that this sequence probably covers the MIS 15 substages.  相似文献   

On the transmissivity of the atmosphere in the water vapour absorption (6–6.5 μ) band     

T. N. Krishnamurti  A. Papetti  M. E. S. de Torres 《Pure and Applied Geophysics》1964,59(1):225-230

Summary The atmospheric transmissivity in the water vapour absorption band (6–6.5 ) is evaluated using the recent Tiros III data and based onGoody's statistical model.Work completed while on leave from the Department of Meteorology, University of California, Los Angeles.  相似文献   

Climate‐driven cyclicity in an Early Cretaceous synrift lacustrine series (Aguilón sub‐basin,NE Spain)     

Ana R. Soria  Arsenio Muñoz  Carlos L. Liesa  Aránzazu Luzón  Alfonso Meléndez  M. Nieves Meléndez 《地学学报》2012,24(5):407-416

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Identification of high energy gamma-ray sources and source populations in the era of deep all-sky coverage     

Olaf Reimer  Diego F. Torres 《Astrophysics and Space Science》2007,309(1-4):57-62

A large fraction of the anticipated source detections by the Gamma-ray Large Area Space Telescope (GLAST-LAT) will initially be unidentified. We argue that traditional approaches to identify individuals and/or populations of gamma ray sources will encounter procedural limitations. Those limitations are discussed on the background of source identifications from EGRET observations. Generally, our ability to classify (faint) source populations in the anticipated GLAST dataset with the required degree of statistical confidence will be hampered by sheer source wealth. A new paradigm for achieving the classification of gamma ray source populations is discussed.  相似文献