An examination of the radiative and dissipative properties of deep ocean internal tides

In this study, the energy flux and energy dissipation of deep ocean internal tides are examined. Properties of the internal tide from two distinct generation regions are contrasted: the Mid-Atlantic Ridge (MAR) and the Hawaiian Ridge. Considerable differences are noted for the baroclinic energy flux, u^′p^′, radiated from each site. Radiation from the MAR is relatively rich in high modes, with an energy flux spectral peak at mode 5 and modes 10 and greater accounting for 40% of the total flux. In contrast, Hawaiian Ridge radiation is dominantly composed of modes 1 and 2, with modes 10 and greater accounting for less than 5% of the total flux. Depth integrated energy flux levels are at the MAR site, and at the Hawaiian Ridge. Despite these differences, observed turbulent dissipation rates at these sites are similar in magnitude and depth dependence. Decay scales, estimated as , range from O(100)km to . The mean decay scale based on the MAR data is 230 km, a factor of 3 smaller than at the Hawaiian Ridge site. We demonstrate that the dissipation level scales with the energy flux available in the high modes, which is comparable at both sites, rather than the total energy flux.     

Some problems of the tectonic framework of the Guiana Shield with special regard for the Roraima Formation

Zusammenfassung Die Arbeit diskutiert Probleme der tektonischen Entwicklung des Guyana-Schildes, eines gro\en kratonischen Gebietes archaischen Alters. Seine Konsolidation fand viel früher statt als die Konsolidation des Brasilianischen Schildes. Guyana stabilisierte sich schon wÄhrend des Älteren Proterozoikums vor 1800 M. J. Vulkanite und Molasse der Roraima-Formation füllten die Depressionen der gefalteten archaischen Gebiete auf. Der Sockel des Guyana-Schildes wird durch die Guyano Eburnéenne Geosynclinal (Choubert, 1969), die Transamazonische Geosynklinale, gebildet, welche sich von Venezuela bis Französisch-Guyana mit gleichem Streichen über eine Entfernung von mehr als 1000 km verfolgen lÄ\t. Eine archaische Geosynklinale von diesem Ausma\ ist bisher aus anderen Schildgebieten der Erde unbekannt.Die archaischen Metasedimente, Metavulkanite und kristallinen Gesteine von Guyana haben allgemein eine E—W-Streichrichtung und unterscheiden sich damit von der vorherrschenden N—S-Streichrichtung der oberprÄkambrischen Gesteine des Brasilianischen Schildes.Jung-prÄkambrische Geosynklinalen und Orogenesen scheinen im Guyana-Schild zu fehlen. Die in Brasilien aktive jung-proterozoische Assynthische (Baikalische oder Brasilianische) Orogenese hat den Guyana-Schild nicht erfa\t. Dort war die letzte Orogenèse die Post-Barama-Mazaruni-Bartica-Orogenese vor 2000 M. J. Zwischen 2000 M. J. und 1800 M. J. entwickelte sich die PrÄ-Roraima-ErosionsflÄche. Dieser Phase folgte die Ablagerung der tafeligen Roraima-Formation vor 1750 M. J. mit dem anorogenen Post-Roraima-Vulkanismus (Dolerite und Gabbro-Intrusionen). Die gro\e Verbreitung der fast nicht verformten Roraima-Sedimente beweist, da\ der grö\te Teil des Guyana-Schildes wÄhrend des Älteren Proterozoikums stabilisiert war.Das Problem der Entstehung des Amazonas-Beckens und die Rolle der Transcurrent-Verwerfungen sowie das VerhÄltnis zwischen den kontinentalen Transcurrent-Verwerfungen und den Bruchzonen des mittelozeanischen Rückens werden dargestellt.

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In this paper some problems of tectonical evolution of the Guiana Shield are discussed that represents a large cratonic area of Archaic age. Its consolidation took much earlier place than the consolidation of the Brazilian Shield. The Guianas were already settled at early Proterozoic time, 1.800 m. y., when molasse and volcanic deposits had been accomplished at depressions of areas folded during the Archaic time 2.000 m. y. The basement of the Guiana Shield is formed by rocks of the vast Guiana-Eburneana Geosyncline (Choubert, 1969), named also Trans-amazonian Geosyncline which has an extension of more than 1.000 km, with the same direction. Such a huge Archaic geosyncline is unknown at an other cratonic area of the world.The predominant trend and structural lineament of the Archaic rocks is quite different than in the Brazilian Shield. The characteristic trend of the early Precambrian rocks of the Guiana Shield is approximately E-W, contrary to the main N-S structural alignement of the Brazilian Shield.Late Precambrian geosynclines and orogeny seem to be absent in the Guiana Shield. The young Proterozoic Assynthic (Baicalian) Orogeny has not more affected the Guiana Shield, where the last orogeny was the Post-Barama-Mazaruni-Bartica Orogeny, which occurred at the Archaic, 2.000 m. y.Between 2.000 m. y. and 1.800 m. y. took place the development of Pre-Roraima erosion surface. It was followed by the deposition of the tabular Roraima Formation, manifesting the stabilisation of the central part of the Shield before 1.750 m. y., when the emplacement of the gabbroic and doleritic volcanites succeeded. The large distribution of the not metamorphosed flat lying Roraima sediments proves that the Guiana Shield was already consolidated during the old Proterozoic time and was never more affected by younger orogenic movements.It is also discussed the problem of origin of the Amazonas Basin, and the important role of the Transcurrent Faults in the tectonic framework of the Guiana Shield and the probable genetic relationship between the transcurrent faults and the fracture zones of the Mid-Atlantic Ridge.

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Resumo O Escudo das Guianas representa uma ampla área cratÔnica da Era Arcaica. Sua consolidaÇÃo já ocorrera muito antes de consolidaÇÃo do Escudo Brasileiro. As Guianas tornaram-se estáveis durante a Era Proterozóica inferior, 1.700 m. a. quando deposiÇÕes vulcânicas e molasse de Roraima foram concluidas em depressÕes de áreas dobradas durante a Era Arcaica, O embasamento do Escudo das Guianas é formado pelo imenso Geossinclínio Guiano-Eburneano (Choubert, 1969) ou Transamazoniano, que representa um geossinclinal de mais de 1.000 km de extensÃo, de mesma direÇÃo. Geossinclinal Arqueano de tamanha amplitude nÃo é conhecido em outra regiÃo cratÔnica de mundo.As rochas metassedimentares, metavulcânicas e cristalinas arcaicas tÊm um alinhamento aproximadamente E-W, diferindo do principal alinhamento estrutural N—S do Escudo Brasileiro.Geossinclínos e orogÊnia do Pré-Cambriana superior parecem estar ausentas no Escudo das Guianas. A orogénia do Proterozóico superior (Assíntica ou Baicaliana) nÃo mais afetou o Escudo das Guianas, onde a Última orogÊnia foi a orogÊnia Barama-Mazaruni-Bartica, que ocorreu há 2000 m. a. Entre 2000 m. a. e 1800 m. a. deu-se o desenvolvimento da superfície de erosÃo Pré-Roraima. Esta fase foi seguida pela deposiÇÃo da FormaÇÃo Roraima tabular, o que revela a estabilizaÇÃo da parte central do Escudo, há 1.750 m. a. quando ocorreu o emplacement dos vulcanitos gabróicos e doleríticos. A grande distribuiÇÃo dos sedimentos Roraima quase horizontals nÃo metamorfizados, prova que o Escudo de Guianas já estava consolidado durante o Proterozóico inferior e nÃo foi mais afetado por movimentos orogÊnicos mais jovens.Foi discutido também o papel importante das falhas transcorrentes na constituiÇÃo tectÔnica do Escudo das Guianas; o problema da origem da Bacia AmazÔnica, e a provável relaÇÃo genética entre as falhas transcorrentes as zonas de fratura da Cadeia Meio-Atlântica.

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— - . , . , . . 1800 . Roraima , . - (Choubert, 1969), , , 1000 . . , E-W N-S .- , , . - — — . Post Barama-Mazaruni-Bartica Orogenese 2000 . 2000 1800 - — . Roraima — 1700 - ( ). - , . Transcurrent, Transcurrent .

     

In situ Mixing of Organic Matter Decreases Hydraulic Conductivity of Denitrification Walls in Sand Aquifers

In a previous study, a denitrification wall was constructed in a sand aquifer using sawdust as the carbon substrate. Ground water bypassed around this sawdust wall due to reduced hydraulic conductivity. We investigated potential reasons for this by testing two new walls and conducting laboratory studies. The first wall was constructed by mixing aquifer material in situ without substrate addition to investigate the effects of the construction technique (mixed wall). A second, biochip wall, was constructed using coarse wood chips to determine the effect of size of the particles in the amendment on hydraulic conductivity. The aquifer hydraulic conductivity was 35.4 m/d, while in the mixed wall it was 2.8 m/d and in the biochip wall 3.4 m/d. This indicated that the mixing of the aquifer sands below the water table allowed the particles to re-sort themselves into a matrix with a significantly lower hydraulic conductivity than the process that originally formed the aquifer. The addition of a coarser substrate in the biochip wall significantly increased total porosity and decreased bulk density, but hydraulic conductivity remained low compared to the aquifer. Laboratory cores of aquifer sand mixed under dry and wet conditions mimicked the reduction in hydraulic conductivity observed in the field within the mixed wall. The addition of sawdust to the laboratory cores resulted in a significantly higher hydraulic conductivity when mixed dry compared to cores mixed wet. This reduction in the hydraulic conductivity of the sand/sawdust cores mixed under saturated conditions repeated what occurred in the field in the original sawdust wall. This indicated that laboratory investigations can be a useful tool to highlight potential reductions in field hydraulic conductivities that may occur when differing materials are mixed under field conditions.     

A three-dimensional numerical groundwater flow model to assess the feasibility of managed aquifer recharge in the Tamne River basin of Ghana

Hydrogeology Journal - Increasing population growth and global climatic changes threaten water security in semiarid regions such as Northern Ghana. The Tamnean Plutonic Suite aquifer is the main...