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排序方式: 共有103条查询结果,搜索用时 15 毫秒
61.
62.
Eutrophication and hypoxia in four streams discharging in Guanabara Bay, RJ, Brazil, a case study 总被引:1,自引:0,他引:1
Four streams in the city of São Gonçalo, were sampled to evaluate their potential as sources of nutrients to Guanabara Bay aiming to contribute with the government program to decrease the levels of pollution in this area. Imbuaçu, Guaxindiba, Marimbondo and Brandoas streams were sampled in 2007, 2008 and 2009. The streams revealed to be hipereutrophic with severe limitation of primary production by nitrogen, as shown by the N/P molar ratio. Phosphate levels were abnormally high varying between 4.35 and 130.82 μM, whereas nitrate and nitrite ranged from 0.06 to 54.05 μM and from 0.28 to 19.23 μM, respectively. The streams also presented severe hypoxia and anoxia, with oxygen values varying from non-detected to 3.72 ml l−1. Heavy loads of particulate suspended material were recorded in the studied streams, ranging between 6.00 and 400.00 mg l−1. The streams were considered inexorable sources of nutrients, enhancing the severe eutrophication process in Guanabara Bay. 相似文献
63.
A detailed rock magnetic and paleomagnetic study was performed on samples from the Neoproterozoic Itajaí Basin in the state of Santa Catarina, Brazil, in order to better constrain the paleogeographic evolution of the Rio de la Plata craton between 600 and 550 Ma. However, rock magnetic properties typical of remagnetized rocks and negative response in the fold test indicated that these rocks carried a secondary chemical remanent magnetization. After detailed AF and thermal cleaning, almost all samples showed a normal polarity characteristic remanent magnetization component close to the present geomagnetic field. The main magnetic carriers are magnetite and hematite, probably of authigenic origin. The mean paleomagnetic pole of the Itajaí Basin is located at Plat = − 84°, Plong = 97.5° (A95 = 2°) and overlaps the lower Cretaceous segment of the apparent polar wander path of South America, suggesting a cause and effect with the opening of the South Atlantic Ocean. A compilation of remagnetized paleomagnetic poles from South America is presented that highlights the superposition of several large-scale remagnetization events between the Cambrian and the Cretaceous. It is suggested that some paleomagnetic poles used to calibrate the APWP of Gondwana at Precambrian times need to be revised; the indication of remagnetized areas in southern South America may offer some help in the selection of sites for future paleomagnetic investigations in Precambrian rocks. 相似文献
64.
Consolidation and Break-up of the South American Platform in Southeastern Brazil: Tectonothermal and Denudation Histories 总被引:1,自引:0,他引:1
P.C. Hackspacher L.F.B. Ribeiro M.C.S. Ribeiro A.H. Fetter J.C. Hadler Neto C.E.S. Tello E.L. Dantas 《Gondwana Research》2004,7(1):91-101
The different tectonic stages that occurred at the end of the Proterozoic and during the Phanerozoic have an important bearing on the tectonothermal history of the South American Platform and its consolidation. Geochronological data (U/Pb monazite, 40Ar/39Ar whole rock) and apatite fission-track analysis, from Precambrian rocks of the southeastern Brazilian coastline, permit the modeling of a long-term thermal history of the crust and constrain variable denudation rates.Using these data, a temperature-time diagram reflects a period of accelerated exhumation during the end of the Brasiliano Orogeny, followed by long stability and reactivation of the platform during the Rifting Phase of the South Atlantic Ocean.U/Pb zircon and monazite (blocking temperature of ca. 650° C) data from a series of igneous bodies suggest that a tangential and transpressional tectonic regime occurred between 625 and 610 Ma. During the following escape tectonics, between 610 and 590 Ma the exhumation process indicates cooling rates of ca. 12°C/Ma. 40Ar/39Ar biotite ages between 540 and 510 Ma (ca. 300°C) and a corrected fission-track age on apatites (100°C) of 480 Ma indicate an exhumation event related to block tectonics with huge vertical displacement along shear zones.A long stabilization phase, with low exhumation, and cooling rate around 0.25°C/Ma was recorded from the Cambro/Ordovician to the Mesozoic. At 65 Ma an acceleration of the exhumation through denudation and reworking of the South American surface with cooling rate of 1.5°C/Ma is observed.The uplift of the Mantiqueira and Serra do Mar mountain ranges along the southeast Brazilian coastline works as a climatic barrier provoking lateral erosional processes causing long-term scarp retreat, combined with intense, but progressive denudation towards the continent. A denudation of 2.5 to 4 km was calculated for such processes. This lateral retreat of escarpments and flexural response can provide important insights regarding marginal isostatic uplift and the evolution of offshore sedimentary basins of southeast Brazil. 相似文献
65.
A GIS FOR THE ANTARCTIC SPECIALLY MANAGED AREA OF ADMIRALTY BAY,KING GEORGE ISLAND,ANTARCTICA 总被引:1,自引:0,他引:1
Jefferson Cardia Sim“非汉字符号”es F.A.Ferron Matthias Braun J.Arigony Neto F.E.Aquino 《地球空间信息科学学报》2001,4(2):8-14
1 IntroductionKingGeorgeIsland’s (KGI)ecosystemshavebeensubjectedtovarioustypesofexploitationandobjectofseveralscientificexpeditions,sinceitwasdiscoveredjustafterthefirstsightingofAntarctilandbyWilliamSmithin1 81 9.KGIwasnamedaf tertheBritishkingofthattime … 相似文献
66.
67.
F.R. Aquino Neto J. Trigüis D.A. Azevedo R. Rodriques B.R.T. Simoneit 《Organic Geochemistry》1992,18(6)
The biomarker analysis of three tasmanite rock samples from Tasmania, Alaska and Brazil showed an unusually high tricyclic terpane concentration relative to hopanes and steranes. A possible product-precursor relationship is advanced, associating Tasmanaceae alage with these tricyclic terpanes. These results support the search of possible precursor molecules in present day organisms which may be taxonomically related to these algae. 相似文献
68.
RésumRé Dans le district de Santa Catarina, la teneur en terres rares des fluorines filoniennes caractérise chacune des neuf générations successives mises en évidence (concentrations en Sm, rapports La/Sm ou La/Yb, anomalie en Europium). Deux cycles hydrothermaux majeurs, rapportés au rifting Sud-Atlantique (130-115 m.a.) ont été distingués à partir de l'évolution des rapports La/Sm et La/Yb et des températures d'homogénisation des inclusions aqueuses primaires. Dans cette évolution, une coupure brutale sépare les générations I–IV et V–VI. Les fluorines plus tardives sont rapportées à un évènement néocrétacé-éotertiaire. Une signature géochimique constante est notée pour les terres lourdes, quelle que soit la génération considérée (ex: Yb/Tb = 5; Gd/Yb = 1) suggérant l'existence d'une source unique pour les terres rares et probablement le fluor. A titre d'hypothèse, on pense qu'il s'agit de fluorine disséminée dans les granitoïdes altérés du socle.
In the Santa Catarina district, REE contents of vein fluorites characterize each one of the nine successive generations (Sm concentrations, La/Sm or La/Yb ratio, Eu anomaly). Two main hydrothermal cycles, related to the South Atlantic rifting (130-115 m.y.), are distinguished through the evolutions of the La/Sm and La/Yb ratios and of the homogenization temperatures of primary aqueous inclusions. In this evolution, a sharp break separates generations I–IV and V–VI. Later fluorite deposits are related to a Neo-Cretaceous-to-Eo-Tertiary event. A constant geochemical signature for heavy REE is noticed, whatever the fluorite generation (e.g. Yb/ Tb = 5; Gd/Yb = 1) suggesting an unique source for REE and possibly fluorine. This source is tentatively determined as disseminated fluorite in altered granitoids from the basement.相似文献
69.
Luiz Henrique Ronchi Clvis Vaz Parente Kazuo Fuzikawa Artur Bastos Neto 《Journal of South American Earth Sciences》2008,25(4):492-500
Precambrian magnesite occurrences hosted by metadolomites from the Orós belt, Ceará, Brazil, are part of a greenschist–amphibolite, metavolcano-sedimentary terrain, dated at 1.8 Ga, cut by Meso- to Neoproterozoic Brasiliano granites and Neoproterozoic basic sills. These rocks were affected by a shear zone between 580 and 500 Ma. The magnesite-bearing marbles can be grouped as medium-grained (1–9 mm) at the Riacho Fundo ore deposit or sparry magnesite (1–15 cm) at the Cabeça de Negro ore deposit. The sparry magnesite shows textural characteristics related to original sedimentary structures. Both types of magnesite-bearing marbles contain aqueous and aqueous-carbonic fluid inclusions that yield homogenization temperatures between 170 and 370 °C. Applying a pressure correction, these temperatures are compatible with the evolution from greenschist to amphibolite facies metamorphic conditions, as described in previous work on the Orós region. It also agrees with data in specialized literature on the metamorphism of carbonate rocks. Fluid inclusion distribution, composition, and physical-chemical characteristics suggest temperature increase, probably related to metamorphism on these rocks. The medium-grained magnesite records partial contamination of CO2-rich inclusions by relict carbonaceous material (bitumen, hydrocarbons?) that favors, but does not confirm, a syngenetic sedimentary origin and could have caused the lowering of CO2 melting point in these inclusions. Therefore, though textural evidence points to a sedimentary-diagenetic model, fluid inclusions record conditions of a metamorphic event. 相似文献