Orinoco Basin Research Articles

Lithium isotopes as a probe of weathering processes: Orinoco River

Lithium isotopes have the potential to be effective tracers of weathering processes due to their large relative mass difference and therefore fractionation. In this study an attempt is made to fill a major gap in the knowledge of Li isotope fractionation during continental weathering and of the mechanisms involved. Finally the relationship between the suspended and dissolved material is made on a basin-wide scale. The Orinoco basin provides a clear contrast in reaction-limited and transport-limited weathering regimes that has already been documented by a comprehensive study on its fluvial geochemistry (Edmond et al., Geochim. Cosmochim. Acta 60 (1996) 2949–2976; Edmond et al., Geochim. Cosmochim. Acta 59 (1995) 3301–3325). Conspicuous in our new results is the difference in δ6Li of the dissolved load between the Andean (−30 to −22‰) and Shield (−22 to −7‰) tributaries, while the δ6Li of the suspended load is similar between the two. To a first approximation, during superficial weathering in high-relief, tectonically active terrains the dissolved load is high in Li and isotopically heavy (more negative δ6Li), whereas in stable Shield regions the concentrations are low and isotopically light in proportion to the increasing degree of weathering.

A pesca do Tambaqui, Colossoma macropomum, COM enfoque na área do médio Solimões, Amazonas, Brasil

Este trabalho avalia a importância da pesca do tambaqui (Colossoma macropomum) nas bacias dos rios Amazonas e Orinoco e, mais especificamente, na região do médio Solimões. As análises foram baseadas em dados de literatura e da pesca praticada pela frota de Tefé no médio Solimões, coletados no mercado de Tefé, entre 1991 e 1995, e a bordo de embarcações da frota pesqueira comercial, entre 1994 e 1995. A importância relativa da espécie pode alcançar 21% da produção de pescado em alguns centros urbanos da bacia do Orinoco, na Venezuela; 8% da Amazônia peruana; e 35% da Amazônia boliviana. No Brasil, na região do rio Madeira, a importância relativa do tambaqui subiu de 10% em 1977 para 32% entre 1984 e 1989. Em Manaus, a espécie já respondeu por 44% do pescado desembarcado. Porém, a proporção de tambaqui desembarcado nesta cidade sofreu uma grande diminuição nos anos 80 e 90, chegando a representar apenas 2,5% do total desembarcado. No médio Solimões, região apontada como a principal área de pesca para esta espécie, são utilizados diversos tipos de aparelhos e explorados diferentes habitats, de acordo com a flutuação do nível da água dos rios. A pesca do tambaqui nesta região é realizada principalmente em lagos, através de malhadeira e rede de cerco, incidindo sobre peixes jovens (<55 cm). O ambiente formado pela caída de terras nos meandros do rio ("enseada-pausada") é importante para a captura de indivíduos de maior porte.

Maternal Effects on Life-History Traits in the Amazonian Giant River Turtle Podocnemis expansa

Maternal Effects on Life-History Traits in the Amazonian Giant River Turtle Podocnemis expansa

Status and distribution of the West Indian manatee, Trichechus manatus manatus, in Colombia

Historical and recent information on the status and distribution of West Indian manatee, Trichechus manatus manatus, in Colombia was reviewed. Opportunistic and systematic interviews were also conducted. Historical information suggested that the distribution of manatees had been reduced in the Caribbean basin. Manatees can be found in the Atrato, Sinú, San Jorge, Cauca, Cesar and Magdalena rivers and the Ciénaga Grande de Santa Marta marsh in the Caribbean basin, and in the Meta River in the Orinoco basin. The Magdalena riparian system provides the largest area of suitable habitat, which also has the highest frequency of captures. Most animals (81.20%) were killed for sale or to share meat in a subsistence base. Hunting is apparently increasing but capture with nets still represents the species' major direct threat. Habitat destruction occurs in all areas. International and national laws protect the species, however, funding is inadequate for effective enforcement of present laws.

Distribution of phlebotomine sand fly genotypes (Lutzomyia shannoni, Diptera: Psychodidae) across a highly heterogeneous landscape.

Genetic variability of eight Colombian field populations and two laboratory colonies of a tropical forest sand fly, Lutzomyia shannoni Dyar, was assessed by comparing allozyme frequencies at 20 enzyme loci. Substantial genetic variability was noted in all strains, with mean heterozygosities of 13-21% and alleles per locus of 2.0-2.8. Four loci were monomorphic. Six populations in north and central Colombia showed close genetic similarity (Nei's distances, 0.01-0.09), despite mountainous environment, discontinuous forest habitat, and elevation differences from 125 to 1,220 m. Two samples representing the Orinoco (near Villavicencio) and Amazon (near Leticia) river basins were similar (Nei's distance, 0.08) but diverged substantially from the central six samples (Nei's distances, 0.26-0.40). Although the range of L. shannoni extends from the southeastern United States to northern Argentina, three genetically distinct, geographically discrete, groups were discerned by the current analysis: Orinoco-Amazon river basins, north-central Colombia, and eastern United States.

Spur cell anaemia and acute haemolysis in patients with hyperreactive malarious splenomegaly. Experience in an isolated Yanomamo population of Venezuela

A prospective study, aimed to investigate the aetiology of an unusual clustering of cases of severe acute haemolytic anaemia affecting a high percentage of the adult population, was carried out in two isolated Yanomamo communities of the Upper Orinoco basin in Venezuela. Twenty-six patients with active or recent episodes of severe haemolysis were evaluated. All of them exhibited massive liver and spleen enlargement and fulfilled the diagnostic criteria of the hyperreactive malarious splenomegaly (HMS) syndrome. In four cases with advanced non-alcohol-related chronic liver disease, hypersplenism, severe haemolytic anaemia and acanthocytosis, the characteristic clinical and laboratory findings of spur cell anaemia were documented. Chronic infection by the HBV and HCV was present in three of them. However, in most of the 22 additional HMS cases, the acute haemolytic condition appeared associated with the occurrence of a cold agglutinin-mediated autoimmune response. The clustering of a significant number of cases of severe acute haemolysis in HMS patients from this small isolated aboriginal community is most unusual, and represents a serious complicating factor for a population already beleaguered by a high prevalence of malaria due to multiresistant strains of Plasmodium falciparum. Moreover, the coexistence of HMS and severe chronic HBV or HCV infection may further aggravate the course of the haemolytic disorder, because of the occurrence of spur cell anaemia.

Quirípas and Mostacillas: The Evolution of Shell Beads as a Medium of Exchange in Northern South America

This article examines the evolution of quirípa, a primitive valuable from the Orinoco Basin, from a world-systems perspective. This process must be understood as a result of the economic and symbolic uses given to beads by different social actors based on their ethnic and sociopolitical backgrounds through time. The history of this primitive valuable illustrates how some aboriginal economic institutions evolved as a result of exchange among Amerindian and European economies and societies, not merely as the product of the imposition of Western economic rationality.

The Gangbanger Autobiography of Monster Kody (aka Sanyika Shakur) and Warrior Literature

As an undergraduate I took a course devoted to Beowulf. The poem—that tale of honor, courage, and slaughter—made a deep impression on me. But some of my most vivid memories of the course are of the professor. He was a small, tweed-clad man of about 60. His salt-and-pepper hair was close-cropped. He sat at a table alternately reading and interpreting. He was a captivating teacher. The poem was very real to him. I remember that he would sometimes grind his teeth as he read the bloodier passages. His interpretations were vivid. And it seemed clear to me that this small man’s heart beat in sympathy with the poem, that his vitals yearned for a more muscular age, that in his eyes Portland was but a poor, soft place in 1965. I wondered at the time if he was aware how casually Beowulf or Achilles would have burned his house, taken his Chevrolet and his daughter, and eaten him for breakfast. It seems to me that some of the recently published gang autobiographies might have served as a tonic for my professor, because these autobiographies take us into the mind and the territory of tribal warriors who live just down the block. Beowulf and Achilles live at a safe distance in time. Piegan, Crow, Sarsi, and Sioux warriors are closer—but they are bathed in the light of a glorious sunset. Yanomamo poisoned arrows are confined to the jungles of the Orinoco basin. But just take the wrong bus, and you end up in Monster Kody’s ’hood—every suburbanite’s worst nightmare: alone, after dark, in “the wrong part of town,” warrior territory. I am not writing metaphorically. I am convinced that young people in some of our urban subcultures are so alienated from the larger society that they have reinvented tribalism—and the . . . young people in some of our urban subcultures are so alienated from the larger society that they have reinvented tribalism—and the warrior culture that is often associated with tribalism.

The Neotropical Fish Subfamily Cynodontinae (Teleostei: Ostariophysi: Characiformes): A Phylogenetic Study and a Revision of Cynodon and Rhaphiodon

Abstract Osteological features of cynodontine species and other characiforms were examined in order to investigate the monophyly of the Cynodontinae and the relationships among its species. A number of derived characters corroborated the hypothesis that the Cynodontinae and its three included genera Cynodon, Rhaphiodon, and Hydrolycus are monophyletic. Hydrolycus armatus is the sister group of H. tatauaia; H. scomberoides is the sister group of the clade formed by the latter two species; and H. wallacei is the sister group of the clade formed by H. armatus, H. tatauaia, and H. scomberoides. Rhaphiodon vulpinus is the sister group to Cynodon, and together they are the sister group to the clade formed by Hydrolycus species. Three Cynodon Agassiz, 1829, species are diagnosed. Cynodon gibbus Agassiz, 1829, occurs in the Rio Amazonas and Rio Orinoco basins, and the Rupununi River, Guyana. Cynodon meionactis Gery et al., 1999, occurs in the upper Maroni River, French Guiana, and C. septenarius, new species, occ...

Influence of hydrological seasonality on bacterioplankton in two neotropical floodplain lakes

Seasonal fluctuation in river stage strongly affects the ecological functioning of tropical floodplain lakes. This study was conducted to assess the influence of hydrological seasonality on bacterial production and abundance in two floodplain lakes of the Autana River, a blackwater river in the Middle Orinoco basin, Venezuela. Water samples for nutrient chemistry, chlorophyll a, and microbiological determinations were collected in two floodplain lakes and in the mainstem of the river during 1997–98. DOC and chlorophyll a concentrations were similar between mainstem and lake sites during high water when river and lakes were well connected but became different during the period of low water when the interaction was minimal. Higher values of bacterial production were observed in the floodplain lakes (0.62–1.03 μg C l−1 h−1) compared to the mainstem sites (0.17–0.19 μg C l−1 h−1) during the period of low water, while during the period of high water river and lake sites showed similar levels (0.04 μg C l−1 h−1). Bacterial numbers followed bacterial production in the floodplain lakes, reaching higher numbers during the period of low water (1.41–2.40 × 106 cells ml−1). Availability of substrate and inorganic nutrients, pH, and inputs and losses of bacterial cells could be determining the observed seasonal patterns in bacterial production and abundance. The Autana lakes exhibited a strong seasonal pattern in the chemical and biological conditions, showing higher productivity during the lentic phase that lasted between 5 and 6 months.

The South American catfish genus Auchenipterus Valenciennes, 1840 (Ostariophysi: Siluriformes: Auchenipteridae): monophyly and relationships, with a revisionary study

Abstract The Neotropical catfishes of the genus Auchenipterus Valenciennes (1840) are reviewed. The genus is hypothesized to be a monophyletic assemblage on the basis of the shared presence of grooves in the ventral surface of the head that accommodate adducted mental barbels. A possible second synapomorphy, the presence of papillae on the dorsal and medial surface of the ossified maxillary barbel of mature males, is tentatively advanced pending discovery of adult males of three species. Contrary to previous hypotheses which considered Auchenipterus to consist of a maximum of five species, we recognize 11 species, including two previously undescribed forms, A. britskii and A. menezesiAuchenipterus is broadly distributed through the Rio Orinoco, Rio Amazonas, and Rio de La Plata basins, and the coastal drainages of the Guianas, with one species in the Rio Pindare-Mirim and Rio Parnaiba basins of northeastern Brazil. Auchenipterus nuchalis, previously thought to be broadly distributed across the range of the genus, is found to rather have a restricted distribution in the eastern portions of the Amazon basin, the lower portions of the Rio Tocantins, and lower courses of some rivers in Suriname and French Guiana. Citations of A. nuchalis from elsewhere in the range of the genus are of other species. Euanemus Muller & Troschel (1842) and Ceratocheilus Miranda Ribeiro (1918) are considered synonyms of Auchenipterus. Euanemus colymbetes Muller & Troschel (1842) is considered a synonym of Auchenipterus dentatus Valenciennes (1840), and A. paysanduanus Devincenzi (1933) is placed into the synonymy of A. nigripinnis Boulenger (1895). A neotype is designated for Hypopthalmus nuchalis Spix & Agassiz (1829). Lectotypes are designated for Euanemus colymbetes and Auchenipterus nigripinnis.

Riparian flooded forests of the Orinoco and Amazon basins: a comparative review

This paper compares the non-deltaic, riparian-flooded forests of the Orinoco and Amazon River basins. Ecological relationships between these forests and their environments that can be useful in establishing schemes for biodiversity conservation are identified. Adaptations of species to flow seasonality, flooding intensity, sedimentation pattern and nutrient depletion are described. The variability and diversity of riparian-flooded forests is related to (i) landscape evolution (regional-scale, long-term), (ii) water quality (basin scale, long-term) and (iii) hydrology and geomorphology (sector-scale, medium-term). The floristic analysis has produced a preliminary list of 242 tree species common to the riparian-flooded forests of both basins. This relatively high number of species is related to connectivity between the riparian corridors of both basins and the effective operation of dispersal mechanisms. Highly oligotrophic environments add uniqueness at the regional scale through the evolution of endemic species presenting adaptations not only to flooding but also to nutrient depletion. The process of genetic diversification and the evolution of genotypes adapted to flooding are suggested to explain longitudinal gradients at tributary junctions and floodplain-upland ecotones where current fluvial dynamics are unpredictable over ecological time scales. The paper presents information that may be used to devise appropriate measures to evaluate sites for riparian biodiversity conservation and management.

Onchocerciasis hyperendemic in the Unturán Mountains: the value of recombinant antigens in describing a new transmission area in southern Venezuela

A recently described hyperendemic onchocerciasis area, located in the Unturán Mountains (between the Siapa and Orinoco basins) of southern Venezuela was studied using a cocktail of 3 low molecular weight onchocercal recombinant antigens (OvMBP/10, OvMBP/11, and OvMBP/29). The resulting seroepidemiological data were compared with those from a hypoendemic community (Altamira) situated in the northern coastal mountain range. Parasitological (skin biopsy) and serological (enzyme-linked immunosorbent assay, ELISA) methods for the specific diagnosis of Onchocerca volvulus in these 2 very different endemic areas were, respectively, 88% and 96% sensitive in Unturán, and 57% and 91% sensitive in Altamira. The mean microfilarial load, the mean optical density (OD), and the seropositivity rates all increased significantly with age in both communities. The serological variables (mean OD and prevalence of anti- O. volvulus antibodies) were both significantly higher in Unturán than in Altamira for children and young adults (aged < 25 years), although above this age no differences between communities were detected. Seroprevalence had already reached 50% in the under 15 year-olds examined at Unturán but was just 5% at Altamira for the same age-class. The prevalence of specific antibodies (mainly a marker of exposure to risk of infection) exceeded 85% in the remaining age-categories at the hyperendemic area. This is in agreement with the high community microfilarial load recorded in Unturán (>20 mf/mg) and the presence of sclerosing keratitis and hanging groin, suggesting that onchocerciasis is a public health problem in this community. The ELISA test used here, based on a cocktail of 3 low molecular weight onchocercal recombinant antigens, appears, therefore, to constitute a practical tool for the description of endemicity levels in remote areas, particularly given the fact that finger-prick blood samples are routinely taken from children in the Upper Orinoco region for surveys of malaria incidence. Such studies could aid in defining the true extent of the Amazon focus (still unknown) and providing priority indicators for the selection of communities where onchocerciasis control programmes should be implemented.

The nature of savanna heterogeneity in the Orinoco Basin

Detrended Canonical Correspondence Analysis (DCCA) was used to describe the relationship between savanna environments and vegetation community composition in the Orinoco Basin. TWINSPAN derived three major types of savanna vegetation, each with its own floristic and physiognomic features. The first group reflected a plant association with species of Trachypogon occurring in moderately infertile soils with low water availability and high bulk density. This group is distributed mostly on hilltops and dissected plains of the northern Orinoco savannas. The second group has the highest species diversity in habitats occurring in the northern Orinoco with extremely infertile soils. The third group, on the other hand, has a considerable number of species belonging to the Leguminosae and Cyperaceae families and linked to the habitats of the eastern Colombian savannas with the highest soil water availability, a shorter dry season and higher precipitation. The regional analysis of the Orinoco savannas suggests edaphic controls to be important. Changes in the underlying geology have affected topography and soil formation, which results in variation in water and nutrient status. The moisture regime and hydrological features are acting as the major regional determinants, while nutrient levels and distinctive surface soil properties provide the subregional determinants. The local boundary, at any given site, is strongly determined by nutrient shortage and acidity. Also, biogeographic and floristic considerations were taken into account to explain differences in species composition.

The Nature of Savanna Heterogeneity in the Orinoco Basin

The Nature of Savanna Heterogeneity in the Orinoco Basin

Epidemiologic, genetic, and clinical associations among phenotypically distinct populations of Leishmania (Viannia) in Colombia.

Phenotypic characterization of 511 strains of Leishmania, subgenus Viannia, isolated from Colombian patients was conducted based on electrophoretic polymorphisms of 13 isoenzymes. Ninety-one Colombian strains of L. braziliensis were the most heterogeneous, constituting seven zymodemes while 397 L. panamensis and 22 L. guyanensis strains yielded five and three zymodemes, respectively. Phosphogluconate dehydrogenase, nucleoside hydrolase, and superoxide dismutase were the most polymorphic enzymes in this collection of strains, and together with glucose-6-phosphate dehydrogenase, allowed the discrimination of the three aforementioned species. Hierarchical cluster analysis of the zymodemes using Jaccard's coefficient of similarities revealed two clusters, one constituted by L. braziliensis zymodemes, and another by three subgroups consisting of zymodemes of L. panamensis closely related to the species reference strain, another consisting of L. guyanensis zymodemes, and a third group distinguished by new electromorphs of proline iminopeptidase and aspartate aminotransferase that reacted with the L. panamensis-specific monoclonal antibody B-11. Multiple zymodemes of L. panamensis and L. guyanensis were found to be sympatrically transmitted in foci along the Pacific coast. Leishmania braziliensis variants were ubiquitous throughout the territory of Colombia; L. panamensis was prevalent in the western region and L. guyanensis was prevalent in the Orinoco and Amazon river basins in the eastern half of the country. Distinct zymodemes of L. panamensis predominated in the northern and southern regions of the Pacific coast. Nine zymodemes of all three species were isolated from mucosal lesions. Zymodeme 1.1 of L. braziliensis had the highest frequency of mucosal involvement (10% of the cases), and disease caused by this zymodeme had the longest mean time of evolution (31 months; P = 0.002). In addition to being useful in describing epidemiologic relationships, the intraspecific heterogeneity of strains of the Viannia subgenus within and among foci can be used to understand such fundamental questions as the pathogenicity of different populations of parasites, and the induction of cross-protection against related parasites.

“Lost Altogether to the Civilised World”: Race and the Cabanagem in Northern Brazil, 1750 to 1850

The colonisation of the tropical lowlands of South America was until fairly recently often portrayed by historians as a straightforward catalogue of genocide, epidemics, and forced catechisation. See, for example, the work of John Hemming: Red Gold (London: Macmillan, 1978), Amazon Frontier (London: Macmillan, 1983), “Indians and the Frontier” in L. Bethell, ed., Colonial Brazil (Cambridge: Cambridge University Press, 1987), 145–89. In its emphasis on the undeniable fact of the violent subjection of much of Amerindia, this orthodoxy obscured the vital detail of the varieties of historical experience in the Amazon and Orinoco basins and the Guianas, projecting an illusion of uniformity and direction to historical processes which were in fact much more complicated and open-ended. N. Whitehead, “Ethnic Transformation and Historical Discontinuity in Native Amazonia and Guayana, 1500–1900,” L'Homme, 126–8, 13:4 (avril–decembre 1993), 285–305. A particularly good example of the complexity of “conquest” from the Indian perspective in the period immediately prior to the Cabanagem is N. Farage, As Muralhas dos Sertoes: Os Povos Indigenas no Rio Branco e a Colonizacao (Rio: Paz e Terra, 1991). Historians, anthropologists, and archaeologists have more recently emphasised the historically contingent nature of ethnic identity and traced out its empirical consequence, the interweaving of identity construction and ethnicity with colonialism, economic expansion and state formation from the sixteenth century onwards. General volumes covering all or most of the tropical lowlands are A. Roosevelt, ed., Amazonian Indians from Prehistory to the Present (Tucson: University of Arizona Press, 1994); E.V. de Castro and M.C. da Cunha, eds., Amazonia: Etnologia e Historia Indigena (Sao Paulo: FAPESP, 1993); M.C. da Cunha, ed., Historia dos Indios no Brasil (Sao Paulo: Companhia das Letras 1992) and G. Urban and J. Sherzer, eds., Nation States and Indians in Latin America (Austin: University of Texas Press, 1991). For the Orinoco and Guianas, see chapters by Whitehead and Arvelo-Jimenez and Biord in Roosevelt, Amazonian Indians ; Dreyfus, in Castro and da Cunha, Amazonia ; and N. Whitehead, Lords of the Tiger Spirit: A History of the Caribs in Colonial Venezuela and Guayana, 1498–1820 (Dordrecht and Providence: Foris Publications, 1988). Price's trilogy on the Saramaka Maroons of Surinam, First Time (Baltimore: Johns Hopkins University Press, 1983), To Slay the Hydra (Ann Arbor: Karoma Publishers, 1983), and Alabi's World (Baltimore: Johns Hopkins University Press 1990) is an example of a similar approach to an Afro-American ethnic formation. As a result, in certain parts of the tropical lowlands—notably the Guianas—it has been possible to produce a much more sophisticated reading of regional history, a “thick description” which can relate the fluidity and specificity of local circumstances to wider historical processes and can thus serve as the basis for a properly comparative mapping of the past.

97/03948 Environmental impact of Orimulsion combustion in large utility boilers

There is considerable worldwide interest in the practical use of Orimulsion as a replacement fuel in both oil and coal fired utility boilers. Practical experience of such applications has been gained in Canada, UK, Japan, Europe and USA. Fundamental work has demonstrated the different combustion characteristics of Orimulsion which has been termed the {open_quotes}fourth{close_quotes} fossil fuel to the fossil fuels normally used for power generation and how, in certain circumstances, these can be used to advantage in the application of Orimulsion in utility boiler combustion systems. Orimulsion is an emulsify ed fuel prepared from naturally occurring bitumen deposits located in the Orinoco Basin in Venezuela and comprises approximately 70% bitumen and 30% water. Compared to the heavier fuel oils the sulphur content of Orimulsion is medium to high, the ash content is high with high levels of Vanadium and Nickel. The ash content is enhanced by the addition of Magnesium compounds, to the commercial fuel, to mitigate against the potential in boiler corrosion effects arising form the Va, Na and S content in the fuel.

The history of a continent from UPb ages of zircons from Orinoco River sand and SmNd isotopes in Orinoco basin river sediments

We report SHRIMP UPb ages of 49 zircons from a sand sample from the lower Orinoco River, Venezuela, and Nd model ages of the fine sediment load from the main river and tributaries. The UPb ages reflect individual magmatic or metamorphic events, the SmNd model ages reflect average crustal-residence ages of the sediment sources. Together they allow delineation of the crust-formation history of the basement precursors of the sediments. The UPb ages range from 2.83 to 0.15 Ga, and most are concordant or nearly so. Discrete age groupings occur at ∼2.8, ∼2.1, and ∼1.1 Ga. The oldest group contains only three samples but is isolated from its closest neighbors by a ∼600 Ma age gap. Larger age groupings at ∼2.1 and ∼1.1 Ga make up about a third and a quarter of the total number of analyses, respectively. The remaining analyses scatter along concordia, and most are younger than 1.6 Ga. The ∼2.8 and ∼2.1 Ga ages correspond to periods of crust formation of the Imataca and Trans-Amazonian provinces of the Guyana Shield, respectively, and record intervals of short but intensive continental growth. These ages coincide with ∼2.9 and ∼2.1 Ga Nd model ages of sediments from tributaries draining the Archean and Proterozoic provinces of the Guyana Shield, respectively, indicating that the UPb ages record the geological history of the crystalline basement of the Orinoco basin. Zircons with ages corresponding to the major orogenies of the North Atlantic continents (the Superior at ∼2.7 Ga and Hudsonian at 1.7–1.9 Ga) were not found in the Orinoco sample. The age distribution may indicate that South and North America were separated throughout their history. Nd model ages of sediments from the lower Orinoco River and Andean tributaries are ∼1.9 Ga, broadly within the range displayed by major rivers and dusts. This age does not coincide with known thermal events in the region and reflects mixing of sources with different crust-formation ages. The igneous and metamorphic history of these sources, as recorded by the detrital zircons, is that of the Orinoco basin basement. This implies that, despite evidence of fast sedimentary recycling, global similarities in Nd crustal-residence ages, and the probability of cross-continent mixing through continental drift, the sedimentary material carried by individual rivers is mainly derived from the crystalline basement in the basin. The global semblance in Nd isotope ratios in major river sediments and atmospheric dusts results from the averaging effect of large-scale sampling of the continents, which are heterogeneous in age on smaller regional scales. A large portion of the continental crust in the Orinoco basin formed during the Trans-Amazonian orogeny at 2.0–2.1 Ga, and smaller portions formed both earlier, at ∼2.8 Ga, and later, after 1.6 Ga. These observations, which are consistent with the relative sizes of crustal age provinces in the Orinoco basin, indicate that sediments from the lower Orinoco and Andean tributaries contain 25–35% of material added to the crust since Trans-Amazonian times. Nd model ages of these sediments underestimate the average crust-formation age of the basement of the Orinoco basin by only about 10%. If this relationship holds in other river basins, then Nd model ages of major rivers and wind blown particulates indicate that the mean age of the continental crust is ∼1.9–2 Ga.

Nutrient status of Amazonian caatinga forests in a seasonally dry area: nutrient fluxes in litter fall and analyses of soils

Amazonian caatinga, a low-stature forest type confined to seasonally waterlogged white-sand soils, occurs at La Esmeralda (Orinoco basin) under a lower rainfall than at San Carlos (RIo Negro basin), where many previous studies have been made. The nutrient status of forest types at La Esmeralda are compared and related to those of forests at San Carlos. At La Esmeralda, stature declines from well-drained palm-rich forest (PRF), via waterlogged tall caatinga (TC), to superficially waterlogged short caatinga (SC). Along this sequence, total fine litter fall decreased (7.6, 6.7, and 5.2 Mg·ha n1 ·year n1 ), N flux decreased (66, 59, and 40 kg·ha n1 ·year n1 ), Ca flux increased (27, 44, and 85 kg·ha n1 ·year n1 ), P flux decreased nonsignificantly, and there was no trend in Mg or K fluxes. Resin-exchangeable P concentrations of 0n10 cm soil samples increased along the sequence (20, 28, and 30 μg·g n1 ). The conclusion that N is in increasingly short supply as a result of waterlogging agrees with results previously obtained at San Carlos. The P flux in leaf litter fall of forests at La Esmeralda (range 1.0n1.3 kg·ha n1 ·year n1 ) is lower than that of tall caatinga at San Carlos (2.0 kg·ha n1 ·year n1 ) or itypicali rain forests (mean 2.6 kg·ha n1 ·year n1 ), suggesting that P is also relatively scarce.