Mouth Of Amazon Research Articles

Lowermost Amazon River: evidence of late Quaternary sea-level fluctuations in a complex hydrodynamic system

Abstract Sedimentological, geochemical, and high-resolution seismic studies on the lowermost Amazon River (Xingu Mouth–Macapa–Estreitos de Breves) allow reconstruction of the sediment dynamics of this large river system in response to late Quaternary sea-level changes. Areas of sediment transport and deposition on an older river bottom are delineated by seismic reflectors and are related to inundation and incision caused by late Quaternary rising and falling of sea level. Older substrates have been identified as the Alter do Chao Formation (Cretaceous) and the Barreiras Formation (Tertiary). The initial AMS-radiocarbon dates yielded late Pleistocene to Holocene ages, and indicate a high sedimentation rate (0.6 cm/yr) for the last 800 yr. Three stages in the late Quaternary evolution of the Amazon mouth system can be distinguished: (i) relative sea-level fall, probably related to the Last Glacial Maximum with channel incision; (ii) deposition and consolidation of mud found near Afua during relative sea-level rise between late Pleistocene and middle Holocene, corresponding to the “freshwater lake” or the evolution of large floodplain areas in Amazonia; and (iii) stable sea-level conditions since 6000 yr BP resulting in the modern pattern of deposition and erosion, controlled by complex interactions of fluvial and oceanic processes.

Major and trace elements of stream sediments from the lowermost Amazon River

Geochemical results from river-bottom sediments taken from the lowermost Amazon River are presented. Bedload sediments were analyzed for 10 major and 38 minor elements. Mud bulk sediments exhibit a tendency to a slight enrichment in Cr, Mn, Rb, Sr, Zr, Cs, Ba and Hf, pointing to the importance of these elements in the silt fraction. Generally, Cu, Zn, Rb, Cs, Ba and Sn are clearly associated with clay minerals, while Y, Zr, Hf, Th and U are associated with heavy minerals. The location of this area in an intensive chemical weathering environment results in the strong modification of sediment composition. Chemical weathering processes strongly modify original sediment composition and can be recognized apart from provenance processes on the basis of their geochemical signature. Discriminant analysis makes the detection of less marked differences in sediment characteristics possible, thus, improving the accuracy of distinctions between groups. The Amazon Mouth and Xingu Mouth (XM) are discriminated principally by the textural maturity of their sediments. The Jari Mouth (JM) shows highest Co and Fe 2O 3 contents. Despite similarities between the Amazon North Branch (ANB) and Amazon South Branch (ASB), the ANB can be differentiated on the basis of its higher Sr, P 2O 5, and Na 2O contents, the ASB on the basis of its higher Zn, MnO, and CaO contents, and the Estreitos de Breves (EST) based on its higher abundance of Ni and Cu. Higher contents in P 2O 5, Fe, and Mn are related to laterite covers.

Sources and fate of n-alkanols and sterols in sediments of the Amazon shelf

Surficial sediments collected along the Amazon shelf during the November 1991 cruise of the AmasSeds program were analyzed for n-alkanols and 4-desmethyl sterols. Despite the high water column productivity and Amazon River inputs, total concentrations are low (usually <1 μg/g). Principal component analysis (PCA) indicates that the n-C 16 alkanol and cholesterol/cholestanol are the best biomarkers for autochthonous inputs while the n-C 28 alkanol and most C 29 sterols are the best for trace river-borne material. 24-Ethylcholesterol is the major sterol throughout the shelf, but PCA modeling shows that this C 29 sterol has mixed marine and terrigenous sources. The concomitant evaluation of alkanol and sterol biomarkers allows the identification of areas where river-borne organic matter (OM) accumulates, mainly in the outer topset northwest of the river mouth and to a smaller degree in the near shore region along the Amapá shelf. The contribution of compounds produced locally is less and associated with low polar lipid levels in the mid-topset northwest of the Amazon mouth and in the deepest water column in the foreset/bottomset area.

Cenozoic tectonics of Amazon Mouth Basin

The Cenozoic shelf margin of the Amazon Mouth Basin is characterized by a thick prograding prism of siliciclastic sediments. This prism, composed mainly of Upper Miocene and younger sediments, overlies a Lower Tertiary carbonate shelf. Two tectonic–sedimentary models for the area were developed with the aid of new deep-reflection seismic data. Gravitational tectonics dominate the regional geological framework. Tensional stresses are created near the shelf margin, and compressional features dominate at the base of the slope. The morphology of this compressional zone is closely associated with the St. Paul Fracture Zone and the boundary between continental and oceanic crusts.

Impact of tropical heat sources on the South American tropospheric upper circulation and subsidence

The nonlinear adjustment of the wind and mass fields to idealized tropical heat sources is studied with a simple nonlinear primitive equation model with emphasis on the upper level circulation over South America and neighboring oceanic regions during the austral summer. Numerical experiments are performed with (1) an idealized symmetrical heat source in the Amazon region, (2) the asymmetry induced in source (1) by the SACZ, (3) the effect of the Atlantic ITCZ off the Amazon mouth, (4) the African heat source, (5) the West Pacific source, and (6) the central Pacific source during the warm phase of ENSO. The linear response is obtained through the reduction of the heat source by a factor of 10 and subsequent multiplication of the results by the same factor. Two basic questions are discussed: (1) are localized heat sources important for the development of the observed cyclonic flow in the midequatorial Atlantic and (2) where is the compensating subsidence associated with the Amazon heat source located? The nonlinearity helps organizing a weak cyclonic curvature in the midequatorial Atlantic, with the inclusion of source (2). The basic state generated by the west Pacific source, and primarily by the central Pacific source, has a large impact on the cyclonic curvature on the equatorial Atlantic. The compensating subsidence associated with the Amazon source is concentrated on the southwest side of the source. The SACZ extension helps to enhance the subsidence over the northern Argentina, and the Atlantic ITCZ enhances the subsidence over northeast Brazil and central equatorial Atlantic. Nonlinearity weakens the subsidence at the 500 hPa level inducing a more barotropic structure in the dynamical response to the heating.

Late Quaternary vegetational and climatic changes in Brazil

Brazil comprises about 50% of the South American continent including prominent ecosystems like the Amazon and the Atlantic rain forests, semideciduous forests, cerrado, Araucaria forests and campos (grasslands). Detailed palynological studies on new sediment cores located on a transect from southern to northern Brazil allow a regional reconstruction of late Quaternary vegetation and climate history. This paper presents an overview and a paleoclimatic synthesis based on 10 paleoecological records from Brazil. Several changes within different vegetation types and shifts of vegetation belts during glacial and Holocene times were found. Pollen records from the south Brazilian highlands demonstrate that campos vegetation extended from glacial to early Holocene times which was replaced by Araucaria forests only during the late Holocene. Records from the southeast Brazilian lowlands and highlands show a replacement of modern tropical semideciduous forests and cerrado vegetation by subtropical grasslands and gallery forests during the last glacial period, indicating a relatively dry climate with a longer annual dry season and a cold climate with frosts. Some areas of cerrado were replaced by semideciduous forests in southeastern Brazil only in the last millennium. During the early Holocene a longer annual dry season is found in southern and in southeastern Brazil. The latest Holocene period is the wettest since the Last Glacial Maximum for which the shortest dry season has bee inferred. Pollen records near the Amazon mouth indicate palaeovegetation of non analog composition and changes in Amazon rain forest during the late-glacial and early Holocene. Evidence of Podocarpus pollen indicates cooler conditions during the late-glacial.

Abstract: Post-Rift Hydrocarbon Systems, Greater Amazon Mouth, Brazil: Transition from Shelf to Basin and Source Distribution Controls

Abstract: Post-Rift Hydrocarbon Systems, Greater Amazon Mouth, Brazil: Transition from Shelf to Basin and Source Distribution Controls

The Amazon shelf setting: tropical, energetic, and influenced by a large river

A Multidisciplinary Amazon Shelf SEDiment Study (AmasSeds) investigated the oceanic processes near the mouth of the Amazon River in order to understand the fate of its enormous discharge of water, solutes and particulates. In addition to receiving a large fluvial discharge, the continental shelf near the Amazon mouth is situated on the equator and experiences an extremely energetic physical regime. As such, it represents an end member in the spectrum of coastal marine settings, with regard to latitude, energy and discharge. The oceanic processes occurring on the Amazon shelf reflect these environmental characteristics. A range of interdisciplinary interactions was observed on the Amazon shelf in response to its low latitude, among them: plume dynamics have little influence from Coriolis acceleration, riverine particles experience intense weathering conditions, high primary productivity occurs during all seasons, and shoreline sedimentation involves mangrove vegetation. The high energy conditions of the Amazon shelf result in: water motions dependent on suspended-sediment distributions, tracemetal adsorption controlled by seabed dynamics, severe restriction of macrobenthos, and deep physical reworking of sedimentary strata (to 1 m or more). The great discharge of fluvial materials (water, solutes, particulates) directly or indirectly causes three-dimensional estuarine-like processes and very high rates of primary productivity, sediment accumulation and carbon burial to occur on the shelf. Although AmasSeds research can link a wide range of interdisciplinary oceanic processes to latitude, energetics or discharge, in many cases the observed process is strongly influenced by a coupling of these characteristics. For example, the dominance of Fe and Mn oxides in controlling redox reactions is a result of tropical weathering that concentrates the Fe and Mn and of intense seabed reworking that regularly reoxidizes these phases. Therefore, the importance of Fe and Mn oxides is a result of both latitude and energy considerations. A fourth characteristic of the Amazon system is its tectonic setting, which determines physiographic features such as drainage-basin size and shelf width. Low latitude and great discharge characterize all areas of the wet tropics; energy expenditure and tectonic setting vary with specific location. All four characteristics must be considered when extrapolating AmasSeds observations to other areas and attempting to predict or interpret oceanic processes.

Origin of the Rift Under the Amazon Basin as a Result of Continental Collision During Pan- African Time

A 1000-km-long positive gravity anomaly occupies the axis of the Amazon Basin and has been taken to indicate the presence of an underlying intracontinental rift. The oldest known sediments in the basin form the Trombetas formation of Late Ordovician and Silurian age (450 to 410 Ma). This unit can be interpreted as a product of thermal subsidence, suggesting that the underlying rift may have originated in Cambrian time (about 510 Ma). Overlying younger sediments and intrusives in the basin reveal a history that can be linked to the Phanerozoic evolution of the Andes and include a Devonian sequence (about 390 Ma), perhaps related to a Patagonian collision, and a thick Carbo-Permian sequence (320 to 260 Ma) deposited at the time of collision between Gondwanaland and Laurentia. Renewed rifting and a thick sedimentary sequence near the Amazon mouth records the opening of the South Atlantic at about 130 Ma and later South Atlantic continental margin history. We suggest that the Sub-Amazonal Rift originated as a result of continental collision along the margin of the West African Craton. The West African Craton was involved in arc and continental collisions along its entire periphery during latest Precambrian and Cambrian times between 700 and 500 Ma. This activity culminated latest and was most intense in an area near the site of the present mouth of the Amazon River, where the suture zone marking final continental collision is cryptic for a distance of 1500 km between Ghana and Sierra Leone. This cryptic suture zone includes the entire Sao Luis province boundary in Brazil. The coincidence in time between rift formation and continental collision, location of the rift at the site of the most intense tectonism, and its trend at a high angle to the collisional suture zone lead us to conclude that the Sub-Amazonal Rift originated as a result of the Pan-African collision. This conclusion adds one more to the growing list of intracontinental rift systems that have been attributed to local modification of within-plate stress as a result of continental collision. Rifts of this origin, which have been called impactogens, include those of the active Rhine, Baikal, and Shanxi systems, as well as those of the 1.0 Ga Mid-Continental rift system of North America.

Amazon rift and Pisco-Juruá fault: Their relation to the separation of North America from Gondwana

Research Article| May 01, 1983 Amazon rift and Pisco-Juruá fault: Their relation to the separation of North America from Gondwana Peter Szatmari Peter Szatmari 1PETROBRÁS Research Center (CENPES), Rio de Janeiro, Brazil 21910 Search for other works by this author on: GSW Google Scholar Geology (1983) 11 (5): 300–304. https://doi.org/10.1130/0091-7613(1983)11<300:ARAPFT>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter Szatmari; Amazon rift and Pisco-Juruá fault: Their relation to the separation of North America from Gondwana. Geology 1983;; 11 (5): 300–304. doi: https://doi.org/10.1130/0091-7613(1983)11<300:ARAPFT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Amazon fault passes eastward from south of Guayaquil on the Pacific to the Amazon mouth on the Atlantic coast. Along its eastern segment, a rift with pyroxenite intrusions opened up at the start of Paleozoic time; its Early Jurassic reactivation produced more than 100,000 km3 of diabase sills, intruded into the evaporitic upper Paleozoic simultaneously with North Atlantic rifting. Pliocene reactivation faulted the coastal Tertiary and sheared the subducted slab beneath the Pacific end of the fault. The northeast-trending Pisco-Juruá fault, first described here, cuts across the continent from Pisco on the Pacific to the Guyana-Surinam border on the Atlantic coast. In early Mesozoic time the fault formed the southwestern continuation of the North Atlantic Rift; its Guyanan end opened up as the North Atlantic Rift opened. The separation of North and South America caused northwestern South America to move southwest along the Pisco-Juruá fault, creating the Tacutu graben in the Guyanan Shield, the gently folded Juruá zone across the Amazon Basin, and the Pisco-Abancay deflection in the Andes. Pliocene reactivation at the Pacific end of the fault sheared the subducted slab of the Nazca plate and may have contributed to the formation of the Nazca Ridge. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Brazilian continental margin

The Brazilian continental margin, with its interesting morphology, structure and sediments, has become better known only during the last two decades. Six physiographical provinces can be recognized at the continental margin and the adjacent coast: (1) Cabo Orange-Parnaiba delta; (2) Parnaiba delta-Cabo Sa˜o Roque; (3) Cabo Sa˜o Roque-Belmonte; (4) Belmonte-Cabo Frio; (5) Cabo Frio-Cabo Santa Marta; and (6) Cabo Santa Marta-Chui. The shelf is rather wide near the Amazon Mouth, becoming narrower eastwards, continuing very narrow along the northeastern and eastern coast, and becoming wider again in the south towards the Plate River. Prominent morphological features along the margin are the Amazon cone, the marginal plateaus off northeastern Brazil, the Sa˜o Francisco cone and canyon, the Abrolhos Bank, and the deep-sea plateaus of Pernambuco and Sa˜o Paulo. On the shelf proper a number of relief elements exist, such as sand waves east of the Amazon, submarine terraces at various places, and irregularities of structural origin. The shelf break is rather smooth in the far north and south, more abrupt in the remainder. Surface sediments of the Brazilian shelf show five distinct facies types: littoral quartz sands, mud, transition sand-mud, coralline algae, and biodetrital. The terrigenous elastic fractions dominate off the Amazon and in southern Brazil; between these areas they occupy a very narrow strip near the coast. The carbonate facies, predominantly composed of calcareous algae, is abundant between the Parnaiba delta and Cabo Frio; to the south this facies is more biodetrital and restricted to the outer shelf. Economically important on the Brazilian continental margin besides oil, are sands and gravels, carbonate deposits, evaporites and some subsurface coal. Other possible mineral resources could be phosphate, heavy minerals and clays for ceramics.

Tocantins and Araguaya Rivers, Brazil

T N the spring of I922, an expedition explored the Tocantins and 1 Araguaya rivers with a view to ascertaining to what extent they were naarigable and whether economic mineraIs existed in their vicinity. These rivers traverse the greater part of Brazil) flowing from about I8? S. until the Atlantic Ocean is reached at the equator. The city of Para (Belem) is at the mouth of the Tocantirls, sometimes called the river Para, which flows into the sea south of Marajo island, whilst the true mouth of the Amazon lies to its north. The Araguaya, which joins the Tocantins some 500 miles abo+re Para, has its source quite 2? south of where the Tocantins rises, is considerably wider, and has a greater arolume of water, so that it might be considered the major river, arld have its name therefore given to the mouth at Para. H. W. Bates, in his book ' The Naturalist on the River Amazons,' describes his journey in I848 up the lower part of the Tocantins from Para to Arumatheua, and his description of the inhabitants and scenery are as accurate to-day as they were seventy-five years ago. The party which left England consIsted of Captain O'Brien, Mr. A. S. Quaife, and the allthor, whilst on leaving Para the number was increased by two Brazilians (a pilot and fireman) to assist in navigating the small steam launch, 25 feet in length, brought out from England. At Para we obtained introductions to the ofiicials of the few towns on the lower Tocantins, and these were of great assistance in obtaining quarters, food, wood, and a certain amount of information. We found it almost impossible to acquire reliable knowledge of the rapids which were known to exist; indeed, there was such ignorance that many people said these were faIls which no boats could pass. A man was discovered, however? who had come over these rapids in the big canoes or batalaos which bring the Brasil nuts (castarlha) down to the port of Alcobaca, and from him we learned that these boats draw from 3 to 6 feet of water when fully loaded, and that our launch, which only required 2 feet of water, would therefore pass easily. We left Para on g May I922, and, after encountering very rough