Roberto Ventura Santos

Carbon and oxygen isotope profiles across Meso-Neoproterozoic limestones from central Brazil: Bambuı́ and Paranoá groups

We present carbon and oxygen isotopic data on carbonates along three profiles of the Neoproterozoic Bambui group in central Brazil. This unit covers an area of more than 300 000 km2 and comprises carbonate–silicliclastic sediments at the base that grade into siliciclastic sediments towards the top. The Bambui group overlies by unconformity the Paranoa group, which consists mostly of siltstone, quartzite and minor limestone. The data presented here improve the stratigraphic correlation within the Bambui basin and show that it evolved in an environment significantly different from that of the Paranoa basin. Our data show large fluctuations of δ13CPDB in limestones from the Bambui Group (from +0.8 to +13.5‰) in all the three studied areas. Some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale. This observation is relevant considering the lack of fossil record and other stratigraphic markers in Neoproterozoic sequences. We also present the first isotopic profiles along the Paranoa–Bambui transition, which shows that the δ13CPDB values grade from +1.0‰ in the Paranoa group, to +2.6‰ in the lower portion of the Bambui group, increasing up to +12‰ in the upper part of this unit. Based on our carbon isotope data, as well as other geological, mineralogical and Nd isotope studies, we argue that the sediments of the Paranoa group were deposited on an open platform that was fully connected to the ocean. On the other hand, the sediments of the Bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790–600 Ma. This unit displays an increased amount of clastic sediments upwards. We argue that the high carbon isotope values observed in limestones and marlstones from the Bambui group are correlated to worldwide high carbon isotope values reported for the Neoproterozoic. However, we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the Bambui carbonates.

Monitoring present day climatic conditions in tropical caves using an Environmental Data Acquisition System (EDAS)

This paper presents data from automatic stations which have been installed for monitoring climatic parameters in caves in two areas of Brazil. These devices, initially developed at the Royal Observatory of Belgium to monitor environmental parameters in geophysical observatories, were adapted in our study to operate under tropical cave conditions and to measure temperature, atmospheric pressure and drip rate of stalactites. Similar devices were installed at the surface near to the caves to measure air temperature, atmospheric pressure and rainfall. The results reveal that the drip rate at the tip of stalactites is related to the effective rainfall (water excess). The stable drip regime observed during the dry season seems to be reproducible from one year to the other and could be related to the infiltration of water which has a long residence time in the aquifer. Regular pressure oscillations, with amplitude ranging between 1 and 2 mb, are observed in both of the monitored caves. Spectral analysis of the data suggests that these oscillations are linked to the diurnal and semi-diurnal solar tides (S1 and S2). In one cave, very small temperature variations (0.02– 0.05 8C) are also observed with a similar diurnal and semi-diurnal pattern, and we argue that the generating process of the thermal components of the S1 and S2 frequencies is a mixture of thermal convection produced by the surface meteorological variations and of an adiabatic induction of the S2 atmospheric pressure modulation. A very large annual thermal amplitude (13 8C) is observed in the other cave; this is a great motivation to study the stable isotope geochemistry of its speleothems as they probably have recorded past temperature fluctuations linked to paleoclimate variations in this area of south-western Brazil. q 2003 Elsevier Science B.V. All rights reserved.

Geographical Origin of Amazonian Freshwater Fishes Fingerprinted by 87Sr/86Sr Ratios on Fish Otoliths and Scales

Calcified structures such as otoliths and scales grow continuously throughout the lifetime of fishes. The geochemical variations present in these biogenic structures are particularly relevant for studying fish migration and origin. In order to investigate the potential of the (87)Sr/(86)Sr ratio as a precise biogeochemical tag in Amazonian fishes, we compared this ratio between the water and fish otoliths and scales of two commercial fish species, Hoplias malabaricus and Schizodon fasciatus, from three major drainage basins of the Amazon: the Madeira, Solimões, and Tapajós rivers, displaying contrasted (87)Sr/(86)Sr ratios. A comparison of the (87)Sr/(86)Sr ratios between the otoliths and scales of the same individuals revealed similar values and were very close to the Sr isotopic composition of the local river where they were captured. This indicates, first, the absence of Sr isotopic fractionation during biological uptake and incorporation into calcified structures and, second, that scales may represent an interesting nonlethal alternative for (87)Sr/(86)Sr ratio measurements in comparison to otoliths. Considering the wide range of (87)Sr/(86)Sr variations that exist across Amazonian rivers, we used variations of (87)Sr/(86)Sr to discriminate fish origin at the basin level, as well as at the sub-basin level between the river and savannah lakes of the Beni River (Madeira basin).

14C content in vegetation in the vicinities of Brazilian nuclear power reactors

(14)C specific activities were measured in grass samples collected around Brazilian nuclear power reactors. The specific activity values varied between 227 and 299 Bq/kg C. Except for two samples which showed (14)C specific activities 22% above background values, half of the samples showed background specific activities, and the other half had a (14)C excess of 1-18%. The highest specific activities were found close to the nuclear power plants and along the main wind directions (NE and NNE). The activity values were found to decrease with increasing distance from the reactors. The unexpectedly high (14)C excess values found in two samples were related to the local topography, which favors (14)C accumulation and limits the dispersion of the plume. The results indicate a clear (14)C anthropogenic signal within 5 km around the nuclear power plants which is most prominent along northeastwards, the prevailing wind direction.

Insights into the distribution of shallow-marine to estuarine early Miocene oysters from southwestern Patagonia: Sedimentologic and stable isotope constraints

ABSTRACT The lower Miocene Estancia 25 de Mayo Formation consists of shallow-marine to estuarine deposits that bear numerous oyster shell beds distributed throughout the succession. Facies analysis reveals that oysters grew in the nearshore paleoenvironments of both the lower Quién Sabe and the upper Bandurrias members. Two oyster species were identified: Crassostrea(?) hatcheri, distributed in the lower two thirds of the column, and Crassostrea orbignyi, distributed in the upper third of the column within the transition from marine to the fluvial deposits of the overlying Santa Cruz Formation. Petrographic, cathodoluminiscence, and carbon and oxygen stable isotope analyses of individual growth increments were performed on carbonate from all the oyster beds, together with consecutive time-series analyses for one specimen of each species. The isotopic composition of the shell seems to be associated with the microstructure of the growth increments. Selective diagenetic alteration affected chalky growth increments; whereas translucent (foliated and prismatic) growth increments are well preserved and can be used to infer paleoenvironmental conditions. Isotopic data indicate that C.(?) hatcheri lived in normal marine waters with a range of paleotemperatures from 10.6 to 20.5 °C. Monospecific beds of this oyster are related to opportunistic and quick colonization of the sea bottom. More negative &dgr;13C and &dgr;18O values in C. orbignyi shells likely reflect decreased paleosalinities, and the resultant environmental stress controlled the development of these low-diversity oyster accumulations. The separate stratigraphic distribution of each species was therefore determined by the paleoenvironmental conditions.

A high-resolution history of the South American Monsoon from Last Glacial Maximum to the Holocene

The exact extent, by which the hydrologic cycle in the Neotropics was affected by external forcing during the last deglaciation, remains poorly understood. Here we present a new paleo-rainfall reconstruction based on high-resolution speleothem δ18O records from the core region of the South American Monsoon System (SAMS), documenting the changing hydrological conditions over tropical South America (SA), in particular during abrupt millennial-scale events. This new record provides the best-resolved and most accurately constrained geochronology of any proxy from South America for this time period, spanning from the Last Glacial Maximum (LGM) to the mid-Holocene.

Human mobility on the Brazilian coast: an analysis of strontium isotopes in archaeological human remains from Forte Marechal Luz Sambaqui

This study investigated strontium isotopes in the dental enamel of 32 human skeletons from Forte Marechal Luz sambaqui (shellmound), Santa Catarina, Brazil, aiming at identifying local and non-local individuals. The archeological site presents pot sherds in the uppermost archeological layers. Dental enamel was also examined from specimens of terrestrial fauna ((87)Sr/(86)Sr = 0.71046 to 0.71273) and marine fauna ((87)Sr/(86)Sr = 0.70917). The (87)Sr/(86)Sr isotope ratio for individuals classified as locals ranged from 0. 70905 to 0. 71064 and was closer to the isotope ratio of the seawater than to the ratio of the terrestrial fauna, indicating a strong influence of marine strontium on the inhabitants of this sambaqui. The results indicate the existence of three non-local individuals ((87)Sr/(86)Sr = 0.70761 to 0.70835), buried in both the level without pottery and the layer with pottery, possibly originated from the Santa Catarina Plateau, close to the municipality of Lages, or from the Curitiba Plateau. The occurrence of a slight difference between the isotope ratios of local individuals buried in the archeological layer without pottery, when compared to those in the layer with pottery, suggests a possible change in dietary patterns between these two moments in the sites occupation.

Chapter 3 The São Francisco Palaeocontinent

Abstract The Sao Francisco and the Congo cratons represent sectors of a Neoproterozoic palaeocontinent preserved from the Brasiliano-Pan African orogeny, recorded in their marginal belts. The boundaries between the Sao Francisco Craton and the surrounding belts are marked by intensive folding and overthrusting. In the interior of the craton, Neoproterozoic sedimentary covers are horizontal or gently deformed, forming isolated basins. The term Sao Francisco Basin includes Palaeo- and Mesoproterozoic successions but in this chapter it is restricted to the Neoproterozoic sedimentary successions of the Sao Francisco Supergroup, characterised by a glaciogenic unit at the base and an argillaceous–carbonatic–arkosic unit at the top. The Neoproterozoic evolution of the Sao Francisco Craton, including its marginal belts and sedimentary cover, is reviewed. The lithostratigraphy, Neoproterozoic mineralisations (lead, zinc, fluorine, barium and phosphates) and geochronological and stable isotope studies are also presented. The geological record of glacial events and stable isotope data of related cap carbonates are also discussed.

Scientific Drilling in the South Atlantic

The South Atlantic is a classic example of continental breakup in which the mechanisms that drove the initial fragmentation still need to be detailed. Mantle plumes may have played a key role in this process, as implied by the Rio Grande Rise (RGR) and Walvis Ridge (WR) hot spot tracks. Their role in the South Atlantic opening is uncertain, as questions remain about their origin and relationship with the structure and dynamics of the upper mantle. Recent findings indicate that the western margin of continental South America did not behave as a rigid block but instead stretched significantly, allowing the emplacement of igneous and exhumed mantle rocks. Continental rifting may have rafted fragments of the continents into the ocean, as implied by recently dredged continental rocks from the RGR.

Isótopos estáveis (C, O, S) e geoquímica de rocha total de carbonatitos da Província Ígnea Alto Paranaíba – SE Brasil

2Faculdade de Ciências e Tecnologia, Universidade Federal de Goiás – UFG, Goiânia (GO), Brazil. E-mail: jabrod@gmail.com, tcjbrod@gmail.com, elisa.barbosa@uol.com.br 3Centro Regional para o Desenvolvimento Tecnológico e Inovação, Universidade Federal de Goiás – UFG, Goiânia (GO), Brazil. E-mail: jabrod@gmail.com 4Instituto de Geociências, Campus Universitário Darcy Ribeiro, Universidade de Brasília – UnB, Brasília (DF), Brazil. E-mail: lucieth@gmail.com, rventura@unb.br 5CICTERRA, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Córdoba, Argentina. E-mail: ipetrinovic@yahoo.com 6Laboratório de Estudos Geodinamicos e Ambientais, Campus Universitário Darcy Ribeiro, Universidade de Brasília – UnB, Brasília (DF), Brazil. E-mail: lmancini@unb.brThe present work investigates the relationship between whole-rock geochemistry and stable isotope composition from carbonatites belonging to the Tapira, Araxa, Salitre, Serra Negra, Catalao I, and Catalao II alkaline-carbonatite complexes of the Alto Paranaiba Igneous Province (APIP), central Brazil and from the Jacupiranga Complex, of the Ponta Grossa Province, southeast Brazil. The APIP complexes are ultrapotassic, comprising bebedourites, phoscorites, nelsonites, and carbonatites, whereas Jacupiranga is a sodic complex composed of ijolite-series rocks, syenites, carbonatites, and alkaline gabbros. The geochemistry data allied to mineralogical constraints allowed us to classify the carbonatites into five groups, and to devise a chemical index (BaO/(BaO+SrO)) to gauge the magmatic evolution of the studied carbonatites.The APIP carbonatites evolve from apatite-rich calciocarbonatites toward Ba-, Sr-, and rare earth element (REE)-rich magnesiocarbonatites. This evolution is mostly driven by apatite, phlogopite, dolomite, and calcite fractionation and consequent enrichment in monazite, norsethite, and strontianite. Stable isotope data show a wide diversity of petrogenetic processes in play at the APIP, relatively to the Jacupiranga Complex, which is interpreted as a result of the shallower intrusion levels of the APIP complexes. Such shallower emplacement, at low lithostatic pressure, allowed for a complex interplay of fractional crystallization, liquid immiscibility, degassing, and interaction with hydrothermal and carbohydrothermal systems.