Timing and mechanisms for the generation and modification of the anomalous topography of the Borborema Province,northeastern Brazil     

J.M. Morais Neto  K.A. Hegarty  G.D. Karner  F.F. Alkmim 《Marine and Petroleum Geology》2009

The results of apatite fission-track analysis in 14 granitic-gneissic samples from two regional transects across the Borborema Plateau, northeastern Brazil, show evidence for two dominant paleothermal events: a Late Cretaceous cooling event beginning sometime between 100 and 90 Ma, and a second cooling event in the Neogene. The distribution of the fission-track results suggests that the cooling events have a broad regional expression and are consistent with the geologic record in the Araripe Basin, western Borborema Province, which attests to a post-Albian uplift of the whole region. We hypothesize that the first event is due to the uplift and denudation of regional, permanent topography generated after the breakup of Brazil and Africa. Such topography is predicted by models of continental margin extension in which continental lithosphere thinning is followed by thickening of the adjacent hinterland lithosphere and crust (Kusznir, N.J., Karner, G.D., 2007. Continental lithospheric thinning and breakup in response to upwelling divergent mantle flow: application to the Woodlark, Newfoundland and Iberia margins. In: Karner, G.D., Manatschal, G., Pinheiro, L. (Eds), Imaging, mapping and modeling continental lithosphere extension and breakup. Special Publication 282, Geological Society, London, pp. 389–419.). In northeastern Brazil, this extension-engendered topography may have been amplified by magmatic underplating related to the Saint Helena and Ascension plumes. The Miocene cooling event (20–0 Ma) occurred at a time characterized by the transition from carbonate ramp to progradational clastic systems on the Pernambuco–Paraíba margin and the offshore Potiguar Basin. This same stratigraphic response characterizes the Neogene stratigraphy of many passive margins and attests to a global increase in the delivery of clastics to margins, the simplest explanation of which is a climate change that accentuated erosion of pre-existing topography. Thus, the present rugged landscape of northeastern Brazil is interpreted to be a product of this younger denudation event. A corollary of this study is that the history, distribution and delivery of clastics to the northern and northeastern margins of Brazil are a function of the regional development of the continental landscape during the Late Cenozoic.  相似文献   

The autonomous <Emphasis Type="Italic">Simpatico</Emphasis> system for real-time continuous water-quality and current velocity monitoring: examples of application in three Portuguese estuaries     

Erwan Garel  Susana Nunes  João Magalhães Neto  Rodrigo Fernandes  Ramiro Neves  João Carlos Marques  Óscar Ferreira 《Geo-Marine Letters》2009,29(5):331-341

This technical paper describes an autonomous recording system with remote data access for the continuous, long-term in situ monitoring of water quality and discharge, as well as current velocity. The Simpatico system has been recently deployed in three estuaries in Portugal (Mondego and Tagus in July 2007; Guadiana in March 2008), highly suitable to illustrate its multifaceted potential applications: surveillance of the effects of mitigation measures to combat eutrophication (lower Mondego Estuary); real-time implementation of boundary conditions for a water modelling system (upper Tagus Estuary); study of dam-induced impacts in terms of water quality, water discharge and sediment budgets (mouth of the Guadiana Estuary). An extract of recorded data serves as an example to discuss data reliability, as well as field maintenance of the system, with implications for labour and operational costs. The use of a new generation of copper-based anti-biofouling kits significantly reduced the need for field maintenance operations. Other examples of possible application of the system include the assessment of gas accumulation in coastal waters.  相似文献   

Response of intertidal macrobenthic communities to long term human induced changes in the Eo estuary (Asturias, Spain): implications for environmental management     

de Paz L  Neto JM  Marques JC  Laborda AJ 《Marine environmental research》2008,66(2):288-299

Long term macrobenthos data together with physical habitat parameters were analysed to investigate spatial and temporal changes at an estuary under different anthropogenic pressures, mainly increasing shellfish farming. The aim was to assess the possible impacts of these pressures on the macrobenthic communities by comparing a period before and after changes in these pressures. Benthic samples were seasonally collected in 1990 (before major anthropogenic changes), 2000 and 2005 at the same sampling stations located on three different habitats in the Eo estuary (Northern Spanish coast). Multivariate and univariate methods were used to assess spatial variability of benthic assemblages and to compare community changes over time. Data from 1990 was assumed as the reference situation to appraise the subsequent impacts. We observed a significant spatial variability of the benthic assemblages in the system as a function of habitat heterogeneity in relation to sediment composition, presence/absence and density of seagrasses, and hydrodynamic regime. Changes were detected in the community composition at all sites during this 15 year period. The extent of changes was related to initial community conditions, rather than the intensity of the pressure. The results suggest that the responses of the benthic communities to human induced perturbations occurring in the system are largely dependent on its intrinsic buffer capacity, and that these communities have been able to cope with an increasing environmental stress (organic enrichment). In conclusion, to keep shellfish farming at a sustainable level without undesirable impacts, the disturbance intensity must be kept below the system carrying capacity. This will allow natural communities to cope with pressures and thus avoid further deterioration in ecological quality.  相似文献   

Estimating periphyton standing crop in streams: a comparison of chlorophyll a sampling and visual assessments     

C Kilroy  DJ Booker  L Drummond  JA Wech  TH Snelder 《新西兰海洋与淡水研究杂志》2013,47(2):208-224

Periphyton standing crop is often measured as chlorophyll a but there is increasing interest in using visual assessments of periphyton cover. Visual methods are rapid and require no sample analysis, but can be regarded as subjective with high inter-operator variability. We investigated variability in periphyton standing crop across operators, rivers and time, as determined visually and from chlorophyll a measurements. We found that visual assessments (comprising percentage cover estimates of up to eight periphyton categories) distinguished sites and occasions as effectively as chlorophyll a. Furthermore, an estimate of chlorophyll a could be derived from the visual assessments. Because our surveys were conducted in only three rivers, general applicability of the derivation of estimated chlorophyll a from the visual assessments warrants further investigation. The current recommendation of 20 views was sufficient for realistic visual assessments of average cover. Overall, our results indicated that inter-operator variability in visual assessments need not be a major concern, given adequate training.  相似文献   

The Late Neoproterozoic/Early Palaeozoic evolution of the West Congo Belt of NW Angola: geochronological (U‐Pb and Ar‐Ar) and petrostructural constraints     

Patrick Monié  Delphine Bosch  Olivier Bruguier  Alain Vauchez  Yann Rolland  Pedro Nsungani  André Buta Neto 《地学学报》2012,24(3):238-247

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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  

de Carvalho Aguiar VM  Neto JA  Rangel CM 《Marine pollution bulletin》2011,62(8):1915-1919

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   

Paleomagnetism and rock magnetism of the Neoproterozoic Itajaí Basin of the Rio de la Plata craton (Brazil): Cambrian to Cretaceous widespread remagnetizations of South America     

E. Font  C.F. Ponte Neto  M. Ernesto 《Gondwana Research》2011,20(4):782-797

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   

The ‘suitcase hypothesis’: Can entrainment of meroplankton by eddies provide a pathway for gene flow between Madagascar and KwaZulu-Natal,South Africa?     

S Ockhuis  JA Huggett  G Gouws  C Sparks 《African Journal of Marine Science》2017,39(4):435-451

Similarities in the marine fauna found off the coasts of southern Madagascar and KwaZulu-Natal Province (KZN), South Africa, led to the development of the ‘suitcase project,’ with the aim of establishing whether eddies that form off southern Madagascar may package and transport biological material across the Mozambique Channel, facilitating connectivity and gene flow. Meroplankton (larval stages of fishes and benthic invertebrates) were collected on the Madagascan shelf and along a transect through a cyclonic eddy in the Mozambique Channel. The samples were analysed using microscopy and DNA barcoding, seeking to identify species known to be common to both the southeast coast of Madagascar and the east coast of South Africa and thereby to reveal potential indicators of connectivity between these regions. The greatest zooplankton biovolume in the upper 200 m occurred on the shelf, followed by in the western part of the eddy and in the region outside the eddy to the west, and was lowest in the region outside the eddy to the east. The meroplankton were dominated by taxa of coastal origin and these were also most abundant on the shelf and in the western part of the eddy, with the lowest abundances in the region outside the eddy to the east. The findings show greater zooplankton biovolumes and larval abundances and the presence of reef-associated larval assemblages on the Madagascan shelf and along the transect through the cyclonic eddy, providing support for the suitcase hypothesis that planktonic organisms are entrained within eddies as they propagate south-westwards of the Madagascan shelf.  相似文献   

Theoretical studies on metal thioarsenites and thioantimonides: synergistic interactions between transition metals and heavy metalloids     

JA Tossell 《Geochemical transactions》2000,1(1):16-7

Recently we established that the ternary complex, CuAsS(SH)(OH) has an unusually high stability and makes a large contribution to the total concentrations of both Cu and As in sulfidic solutions equilibrated with Cu and As sulfide minerals. This ternary complex has an unusual structure, containing a bond which is formally Cu(I)–As(III), along with a broken As–S bond. We have now found that complexes with similar structures exist for Au⁺ and Tl⁺ coordinated to AsS(SH)(OH)^-. However, such a direct metal–metalloid bond is not a requirement for stability. In fact, TlAsS(SH)(OH) is unstable while AuAsS(SH)(OH) is highly stable (compared to the aquo ion). Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ also form bonds to the As of AsS(SH)(OH), but without breaking any As–S bonds, and HgAsS(SH)(OH)⁺ and PbAsS(SH)(OH)⁺ are particularly stable complexes. Calculated structures are shown for these complexes, gas-phase energies are calculated, and formation constants in aqueous solution are estimated. The SbS(SH)(OH)^- ion forms analogous complexes, with similar stabilities. However, the Au⁺ complex of SbS(SH)(OH)^- is slightly less stable than the Cu⁺ complex, opposite to the order found for the AsS(SH)(OH)^- ligand. The Au⁺ and AuSH complexes of AsSSHOH^- or AsS(SH)₂ ^- may be implicated in "invisible gold" in arsenian pyrites. Vibrational frequencies are given for the AuAsS(SH)₃ ^- complex and the XANES energies of this complex and Au(SH)₂ ^- are compared. The existence of such strong complexes may explain the many correlations observed between the concentrations of coinage metals and metalloids.  相似文献   

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