Abstract The Tamboril and Olho d’Água granitic stocks are part of the abundant calc-alkalic magmatic epidote-bearing granitic rocks in the Cachoeirinha–Salgueiro Terrane (CST) in the Transversal Zone Domain, northeastern Brazil. The equigranular Olho d’Água stock is composed of medium-grained clinopyroxene-amphibole–biotite tonalite; the porphyritic Tamboril stock is medium- to coarse-grained amphibole biotite ± clinopyroxene granodiorite. Abundances of clinopyroxene and epidote vary inversely in both stocks. Amphibole–rich clots are regarded as fragments from the source region captured by granodioritic/tonalitic magma during its ascent. Epidote composition in the Olho d’Água stock (Ps18–26) and in Tamboril stock (Ps17-20) is consistent with crystallization under oxygen fugacity between QFM and HM buffers. In the Olho d’Água stock, calculated values of pressure range from 5.1 to 6.6 kbar and in the Tamboril stock from 6.2 to 7.0 kbar. Solidification temperatures estimated from plagioclase-hornblende pairs in the Olho D´Água stock range from 637 to 679 °C and for Tamboril from 587 to 641 °C. Zr-saturation temperature estimates are 788 to 819 °C (Olho d’Água) and 807 to 829 °C (Tamboril). Altogether our data suggest that the studied stocks crystallized from two distinct magmatic pulses formed from fractional melting of a single amphibolitic source. These two magma pulses underwent subsequent crystallization, in a convective magmatic chamber, at rather high pressure.

Abstract Fish otoliths store geochemical and isotopic signatures that can be used as proxies for environmental conditions in ecological investigations. We investigated the δ13C and δ18O of otoliths of four freshwater fish species (Astronotus ocellatus, Serrasalmus brandtii, Plagioscion squamosissimus, and Cichla ocellaris) from the Moxotó Reservoir in the São Francisco River, Brazil. The enriched δ13C signatures that distinguish A. ocellatus from other species could be caused by dissolved inorganic carbon (δ13CDIC) in reservoir waters. Macrophytes growing along the reservoir margins would favor photosynthetic uptake of 12C, leading to enriched δ13CDIC incorporated into the otoliths of those fish. Otolith δ18O signatures appear to reflect water column preferences, in which species preferentially inhabiting surface waters (such as S. Brandtii) show more enriched values (due to high surface evaporation ratios), while bottom water species show depleted δ18O values. Our results represent the first investigation of the isotopic compositions of fish otoliths in freshwater environments in the São Francisco River, and shed light on the interpretation of isotopic information stored in otoliths and how they can be used to infer the ecological strategies of freshwater fish.

The facies analysis and the study of the way of life of the organisms have been historically considered as the conventional methods for the study of the sedimentary environment and therefore for paleodepth estimations. The aim of this work is, first to rule out possible diagenetic alterations of the fossil material and to confirm the ability of the organisms to precipitate their shells in equilibrium with the environment. Secondly, to show how geochemical composition of water varies with depth considering previous paleoenvironmental models obtained from the facies analysis and the way of life of organisms, using innovative geochemical methods such as the study of carbon stable isotopes and rare earth elements plus yttrium in fossils. Finally, it aims to establish the reasons that produce variations in the geochemical composition of waters related to depth. In the present work, we present two case studies from contemporary marine successions deposited in different paleodepths: Arroyo Loncoche (Neuquén Basin; Fig. 1a) and Río Guanaco (Austral Basin; Fig. 1b). In both cases, paleodepths were defined based on the facies analysis and the way of life of organisms. The Arroyo Loncoche sedimentary section (Neuquén Basin) includes the complete succession of the Vaca Muerta and Chachao Formations (Fig. 2) and has been defined as ramp deposits that include different environments from basin to proximal middle ramp. The Chachao Formation is composed mainly of oyster biostromes of the genus Aetostreon sp. (Kietzmann et al., 2008, 2014; Kietzmann and Palma, 2009; Gómez Dacal et al., 2018). From the study of the way of life of oysters, Hernández-Ocaña et al. (2015) showed that this genus lived under shallow-water conditions, from the intertidal zone to 30 m depth maximum. The Río Guanaco sedimentary section of the Austral Basin is found in the southeastern area of the Santa Cruz Province, and includes the complete Springhill Formation and the lower member of the Río Mayer Formation (Fig. 2; Richiano et al., 2012). From the fossils and facies analyses, this sequence has been characterized as an external platform composed mainly of black shales, rich in belemnites of the Belemnopsis sp. fauna (Richiano et al., 2015). Jarvis (1980) postulates, that belemnites lived in depths between 120 and 330 m. A representative sedimentary succession of each study area was carried out, selecting the best and most complete outcrops of the formations (Fig. 3). In addition, a systematic analysis of the fossil material was made in order to stablish their petrographic (cathodoluminscence and scanning electron microscopy) and geochemical characteristics (rare earth elements and yttrium and stable isotopes of C and O). From cathodoluminescence and scanning electron microscopy studies it was possible to demonstrate that selected samples of Aetostreon sp. and Belemnopsis sp. were well preserved (Fig. 4). The REY results, obtained from belemnites, show that the Río Guanaco section contains higher HREY, Y/Ho ratios and the La and Eu anomalies than Arroyo Loncoche section (Fig. 5; Table 1). Oysters and belemnites precipitate their shells in equilibrium with the sea-water composition, where the difference in REY values recorded is attributed to the greater influence of continental sediment supply at the time of the formation of the oyster shells. Chemostratigraphic curves were obtained from the isotopic results (Fig. 6; Table 2). The curve of δ13C from the Río Guanaco section shows more negative values and less variations than the curve of the Arroyo Loncoche section (Fig. 6). This difference Is considered as response of the belemnites may have precipitated their shells under deeper environmental conditions, where the export of carbon from the surface to the deep ocean is continuous and generates an enrichment of 12C. From the results of the present study the following conclusions are obtained: • According to the way of life of the organisms and the interpreted environments in the sedimentary successions, we suggest that belemnites lived in a deeper marine environment, while oysters lived from the intertidal to the 30 m depth. • Both cathodoluminescence and scanning electron microscopy studies indicate a good preservation for selected samples of Aetostreon sp. and Belemnopsis sp. • The Río Guanaco section (Austral Basin) shows higher HREY values, Y/Ho rations and Eu and La anomalies than those recorded in Arroyo Loncoche (Neuquén Basin). These differences agree with the environmental conditions in relation to the continental sedimentary supply during the precipitation of the shells. • The chemostratigraphic curve from Río Guanaco section shows more negative and more homogeneous values than those recorded in the curve of Arroyo Loncoche section. This difference is attributed to belemnites having precipitated their shells in deeper environments, where the export of carbon from the surface to the deep ocean is continuous and generates an enrichment of 12C. • Isotopic and REY trends corroborate that belemnites from Austral Basin lived in a deeper marine environment than oysters from Neuquén Basin.

Con el objetivo de conocer como varía la composición geoquímica de las aguas con la profundidad y que factores influyen en estos cambios, se realizaron estudios de tierras raras e isótopos estables sobre sucesiones sedimentarias definidas ambientalmente por los métodos tradicionales. A tal fin, se tomaron como casos de estudio secciones sedimentarias de las Formaciones Vaca Muerta, Chachao (Cuenca Neuquina) y Río Mayer (Cuenca Austral), depositadas en forma contemporánea pero a profundidades distintas. Los depósitos de la Cuenca Austral, corresponden a ambientes profundos (plataforma externa) y los indicadores geoquímicos fueron obtenidos a partir de ejemplares de belemnites del género Belemnopsis sp. En la Cuenca Neuquina, los estudios se realizaron en ostras del género Aetostreon sp. acumuladas en ambientes marinos relativamente más someros (rampa externa proximal a rampa media proximal). Estudios petrográficos han permitido confirmar el carácter primario de los valores isotópicos. En la Cuenca Neuquina, las ostras de ambiente marino relativamente más somero muestran un amplio rango de valores de δ13C desde negativos hasta positivos (-4,93 a 2,66‰ VPDB), en tanto que los belemnites de la Cuenca Austral, de ambiente más profundo, poseen menor variabilidad y tenores negativos de δ13C (-1,77 a -0,33‰ VPDB). Asimismo, estos últimos organismos se caracterizan por un enriquecimiento en tierras raras pesadas (valores más elevados de Y/Ho) y de las anomalías de Y, La y Eu, en comparación con los encontrados en las ostras de la Cuenca Neuquina. Se concluye que el contenido de tierras raras e itrio (REY) e isótopos estables en el agua de mar en las cuencas Neuquina y Austral durante el lapso Berriasiano- Valanginiano varió sistemáticamente con la profundidad. En el caso de las REY, este cambio estaría relacionado con la cercanía a las fuentes de aporte de material sedimentario, mientras que la variación en las curvas quimioestratigráficas de carbono se vincularía con la productividad primaria.

ABSTRACT The Tithonian-Valanginian time interval in the Mendoza Shelf (Neuquén Basin, Argentina) is well exposed in the Río Salado, Puesto Loncoche and Cuesta del Chihuido sedimentary sections. From those localities, more than fifty preserved oyster shells of the genus Aetostreon sp. were selected and sampled in order to perform the first δ13C curves for this particular time interval. Mineralogical and cathodoluminiscence properties, inner micromorphology of the valves, added to major and trace element geochemistry were analyzed in order to highlight the best C-O isotopic preservation. The δ13C isotope curves show values varying between 0 and -3‰ VPDB for the Tithonian-Berriasian basal section, and a positive excursion of ~2.4-2.7‰ VPDB in the Valanginian upper section. This δ13C up section trend is here considered in order to reveal eminent correlations with other sections from the Neuquén Basin, as well as the Weissert Event from the Tethys area, also on the basis of their ammonite faunal zones. The palaeotemperatures obtained from δ18O preserved values, added to a detailed sedimentological study suggest that observed δ13C anomaly may responds to a global climatic change from warm and dry to warm and humid conditions.

RESUMEN El intervalo Titoniano-Valanginiano en la Plataforma Mendocina (Cuenca Neuquina, Argentina) se encuentra muy bien representado en las secciones sedimentarias estudiadas de Río Salado, Puesto Loncoche y Cuesta del Chihuido. En estas localidades más de cincuenta valvas de ostras del género Aetostreon sp. fueron seleccionadas y muestreadas con el objeto de obtener las primeras curvas de δ13C de este particular intervalo temporal. Las propiedades mineralógicas, la respuesta a la catodoluminiscencia, así como la micromorfología interna de las valvas, sumado al análisis geoquímico de los elementos mayoritarios y trazas fueron utilizados para indicar las zonas con el más alto grado de preservación isotópica del C y O. La curva de δ13C muestra valores que varían entre 0 y -3‰ VPDB para la sección basal de edad titoniana-berriasiana y una excursión positiva de ~2.4-2.7‰ VPDB para la sección cuspidal de edad valanginiana. Esta tendencia del δ13C hacia el tope de la sección revela una eminente correlación con otras secciones de la Cuenca Neuquina como así también con el Evento Weissert para el área del Tetis, también respaldada por la fauna de amonites presente. Las paleotemperaturas obtenidas aquí a partir de los datos de δ18O, sumado al estudio sedimentológico de detalle sugieren que la anomalía de δ13C podría ser la respuesta a un cambio climático desde condiciones cálidas y áridas a cálidas y húmedas registrado a nivel global.

Abstract: Petrography, stable carbon and oxygen isotopes, and major, trace, and rare earth elements geochemistry of limestones of the Alisitos Formation in the Los Torotes, Baja California were studied in order to document the elemental variations among Member C (MC) and Member E (ME) of the formation and to understand their diagenetic and depositional conditions. The major petrographic types identified are mudstone, wackestone and packstone. The limestones of the Alisitos Formation show significant variations in δ13C and δ18O values (+4.13 to +5.26‰; -14.17 to -6.84‰; respectively). The limestones from Member C (MC) show higher ΣREE content (23 ± 17, n=10) than Member E (ME) (11 ± 5.5, n=9) of the Alisitos Formation. The observed variations in ΣREE content in these limestones are due to the amount of detrital material present in them. The limestones from ME record seawater-like REE+Y patterns, whereas the MC records non-seawater-like signatures (i.e. significant enrichment of middle rare earth elements: MREE). Most of the analyzed samples from the Alisitos Formation show negative Ce anomalies (Ce/Ce*: 0.67 - 0.99, n=17). The limestones show both negative and positive Eu anomalies (Eu/Eu*: 0.58 ‒ 2.91, n=19). The positive Eu anomalies identified in many samples may be due to the influence of hydrothermal fluids or co-precipitation of hydrothermal Fe-sulfide. The limestones of ME exhibit seawater-like-REE patterns suggesting that most of the REEs present in them were derived from seawater. However, the limestones of MC were contaminated by detrital materials which effectively mask the seawater-like signatures. The detrital materials present in the limestones of MC were probably derived from the mafic to felsic source rocks.

Resumen: La petrografía, composiciones isotópicas (carbono y oxígeno) y geoquímicas (elementos mayores, traza y de tierras raras en calizas de la Formación Alisitos en el área Los Torotes, Baja California fueron estudiadas con el fin de documentar las variaciones elementales entre el Miembro C (MC) y el Miembro E (ME) con la finalidad de entender las condiciones diagenéticas y depositacionales. Los principales tipos petrográficos identificados fueron mudstone, wackestone y packstone. Las calizas de la Formación Alisitos muestran notables variaciones en δ13C y δ18O (+4.13 a +5.26‰; -14.17 a -6.84‰; respectivamente). Las calizas del Miembro C (MC) muestran contenidos más altos de elementos tierra raras totales (ΣREE por su siglas en inglés) (23 ± 17, n=10) que las del Miembro E (ME) (11 ± 5.5, n=9) en la Formación Alisitos. Las variaciones observadas en los contenidos de tierras raras totales en esas calizas son debido a la cantidad de material detrítico presente en ellas. Las calizas de ME registran patrones REE+Y similares a las aguas marinas, mientras MC registra firmas distintas a las aguas marinas, es decir, tienen un significante enriquecimiento de tierra raras medias. La mayoría de las muestras analizadas de la Formación Alisitos señalan anomalías negativas de Ce (Ce/Ce*: 0.67 ‒ 0.99, n=17). Las calizas estudiadas muestran tanto anomalía positiva como negativa de Eu (Eu/Eu*: 0.58 ‒ 2.91, n=19). Las anomalías positivas de Eu identificadas en muchas muestras podrían deberse a la influencia de fluidos hidrotermales o a la co-precipitación de sulfuro de Fe hidrotermal. Las calizas de ME exhiben patrones de REE similares a las aguas marinas lo que sugiere que las mayoría de REE presentes en estas calizas son derivadas de aguas marinas. Sin embargo, las calizas de MC fueron contaminadas por materiales detríticos los cuales ocultan efectivamente las firmas similares a las aguas marinas. El material detrítico presente en las calizas de MC se derivó probablemente de rocas fuentes máficas a félsicas.

We report for the first time the occurrence of organic-walled microfossils in Ediacaran limestones and marls of the Frecheirinha Formation, Ubajara Group, and the first Precambrian acritarchs so far reported for northeastern Brazil. The assemblage of the Frecheirinha Formation represents a low-diversity microflora comprising Leiosphaeridia, Lophosphaeridium and subordinated Bavlinella (=Sphaerocongregus). Their thermal alteration index (TAI) between 4+ and 5, suggests metamorphic temperatures exceeding 200-250°C. Higher temperatures are probably related to intruding granitic plutons (Meruoca, Mucambo). Reported δ13C values of carbonates of -3.5 ‰ VPDB (Vienna-Peedee Belemnite) at the base, passing up section into a positive plateau of up to +3.7 ‰, and corresponding 87Sr/86Sr values between 0.7075 and 0.7080 suggest an Ediacaran age. The acritarch assemblage is comparable to the Late Ediacaran Leiosphere Palynoflora (LELP) or Kotlin-Rovno assemblage, in broad agreement with chemostratigraphic data. Macrofossils belonging to the Ediacara fauna were reported from the overlying Jaibaras Group, which would constrain even further the depositional age of the Frecheirinha Formation to within ca. 575-555 Ma. A more comprehensive palynological study of the Frecheirinha Formation is necessary to confirm this age assignment.

Rochas carbonáticas do Neoproterozóico ocorrem em diferentes unidades estratigráficas na parte meridional da Serra do Espinhaço, Minas Gerais, Brasil. Análises isotópicas de C, O e Sr foram realizadas ao longo de quatro seções estratigráficas selecionadas. São elas: (i) Formação Rio Pardo Grande, na porção superior do Supergrupo Espinhaço, amostrada na seção 3, (ii) calcários laminados (Fazenda Tijucuçu) e camadas de dolomitos estromatolíticos (Formação Domingas) do Grupo Macaúbas, amostrados respectivamente ao longo das seções 1 e 2, e (iii) calcários laminados das unidades estratigráficas inferiores do Grupo Bambuí, amostrados na seção 4. Amostras de calcário laminado do Grupo Macaúbas têm valores δ13C tão elevados quanto 10,9‰, diminuindo no final da seção para -1,1‰ e os valores de 87Sr/86Sr variam entre 0,7072-0,7076, faixa comumente observada em rochas do Criogeniano. Na seção 2, as amostras de dolomito apresentam 87Sr/86Sr entre 0,7076-0,7077 enquanto na seção 3 87Sr/86Sr ocorrem valores entre 0,7074-0,7079. Na seção 4, os valores de 87Sr/86Sr estão em torno de 0,7080. Os valores de 87Sr/86Sr observados em amostras de carbonato do Grupo Macaúbas são similares aos observados na curva secular dos isótopos de Sr para o Neoproterozoico. As amostras de carbonato da base do Grupo Bambuí correlacionam-se com a assinatura isotópica do Ediacarano, após a glaciação Marinoana (ca. 635 Ma).

Neoproterozoic carbonate rocks comprise different stratigraphic units in the southern part of the Espinhaço Ridge, Minas Gerais, Brazil. C, O- and Sr-isotope analyses were carried out along four selected stratigraphic sections across these formations. These are: (i) the Rio Pardo Grande Formation in the upper portion of the Espinhaço Supergroup, sampled in section 3; (ii) Macaúbas Group laminated limestones (Tijucuçu Farm) and dolostone layers (Domingas Formation) have been respectively sampled along the so-called sections 1 and 2, and (iii) the lower stratigraphic units of the Bambuí Group, sampled in section 4. Laminated limestone samples from the Macaúbas Group have δ13C values as high as 10.9‰ decreasing up section to -1.1‰ and 87Sr/86Sr values vary from 0.7072 to 0.7076, a range commonly observed in Cryogenian rocks. In section 2, dolomitic samples exhibit 87Sr/86Sr from 0.7076 to 0.7077 while in section 3, 87Sr/86Sr from 0.7074 to 0.7079. In section 4, 87Sr/86Sr values are around 0.7080. The values of 87Sr/86Sr observed in carbonate samples from the Macaúbas Group are similar to those observed in the Sr-isotope secular curve for the Neoproterozoic. Carbonate samples from the base of the Bambuí Group correlate with Ediacaran fingerprints, after the Marinoan (ca. 635 Ma) glaciation.

Las cuencas Cinturón Plegado de San Jacinto (CPSJ) y el Valle Inferior del Magdalena (VIM) en el norte de Colombia se han convertido en los mayores objetivos de recientes campañas de exploración para gas y petróleo, a pesar de que la correlación de las unidades estratigráficas diacrónicas y la génesis de las unidades no está bien entendida en la actualidad, debido principalmente a la tectónica transtensional que afecta el área. A partir del análisis de isótopos de Sr, C, O y multi-elementos en carbonatos de tres sucesiones sedimentarias del Cinturón Plegado de San Jacinto y el Valle Inferior del Magdalena se intenta contribuir al entendimiento de la evolución genética de este registro sedimentario. El segmento estudiado de la Formación Toluviejo (Eoceno-Oligoceno) presenta una composición isotópica de estroncio entre 0.707444-0.707598, valores que son bajos con respecto a sucesiones globales para dicha temporalidad. Composiciones de 143Nd/144Nd expresados como εNd=2,67 sugieren que los valores anómalos de 87Sr/86Sr son el producto de aporte de aguas que drenaban una fuente con esta composición isotópica, probablemente la Cordillera Occidental. Sin embargo, estos valores anómalos de 87Sr/86Sr en conjunto con los valores empobrecidos de δ13C pueden ser usados como marcadores geoquímicos para diferenciar regionalmente e identificar carbonatos que pertenezcan a la Formación Toluviejo en el CPSJ. Las composiciones isotópicas de 87Sr/86Sr de las formaciones Ciénaga de Oro y Porquera, que afloran en ambas cuencas, presentan valores entre 0.708231-0.708423 y 0.708352-0.708474, respectivamente. Estos valores sugieren edades del Aquitaniano-Burdigaliano para ambas unidades, aunque difieren en el ambiente de depósito.

The San Jacinto Folded Belt and Lower Magdalena basins have become major targets on recent exploratory campaigns, for fossil fuels, in northern Colombia. Despite the potential of these sedimentary basins in terms of oil and gas production, although to date the evolution of genetically correlative successions within these basins is still not fully understood. This is mainly due to the occurrence of transtential tectonics affecting this area, which has resulted in potentially diachronic sedimentary records. In this work we use carbonate Sr, C, O isotope stratigraphy and multielemental stratigraphy from three sedimentary successions along the San Jacinto Fold Belt (SJFB) and the Lower Magdalena Valley (LMV), northern Colombia to further contribute to the understanding of the genetic evolution of the sedimentary records in such basins. 87Sr/86Sr values of 0.707444 to 0.707598 characterize the Eocene-Oligocene Toluviejo Formation (SJFB basin). Those values are lower than any value reported for Cenozoic carbonate successions worldwide and, thus could not be used to infer their depositional ages. Preliminar 143Nd/144Nd compositions expressed as εNd = 2.67 suggests that these anomalously 87Sr/86Sr values are the result of continental waters draining Caribbean related undifferentiated rocks displaying these compositions in western Colombia. These anomalous 87Sr/86Sr values along with their depleted δ13C values however can be used as geochemical fingerprints to regionally differentiate and identify carbonates belonging to the Eocene-Oligocene Toluviejo Formation along the SJFB basin. The Cienaga de Oro and Porquera formations, which outcrop along both basins, display 87Sr/86Sr values between 0.708231 to 0.708423 and 0.708352 to 0.708474, respectively. These values suggest an Aquitanian - Burdigalian age for their deposition. They also suggest a coeval depositional age for these Formations, which display important differences in their sedimentary environment.

We used petrofacies analysis and stable isotope data to interpret the isotopic variations in the marine carbonate succession of the Early Cretaceous Mural Formation of northeastern Sonora (Pitaycachi section), Mexico. The petrographic study reveals a range of lithofacies from bioclastic mudstones to boundstones. Allochems consist of corals, algae, rudists, echinoids, sponge spicules, radiolarians, foraminifera and calpionellids. Samples analyzedfor stable isotope are significantly depleted with δ18O values of -15.19‰ to -6.32‰ and exhibit positive δ13C values ranging from 2.91‰ to 4.39‰. The lack of correlation between δ13C and δ18O values also supports a primary marine origin for the δ13C values of limestones from the Pitaycachi section. In the δ13C profile, the Cánova Member shows an upward increasing trend from 3.09‰ to 4.36‰ interpreted to indicate an increase in the rate of marine organic production and/or organic burial in the basin during early Albian time. The abrupt increase in carbon isotope values in the lower part of the section correlates with OAE1b. The shape of the C-isotope curve of the present study is similar to other C-isotope curves from Mexico and other continents (e.g., Vacontian basin, France) indicating that OAE1b may have been global in extent.

En este trabajo se analizan las variaciones isotópicas en la sucesión marina carbonatada de la Formación Mural (Cretácico Inferior) que aflora en la sección Pitaycachi, noreste de Sonora, México, a partir de datos petrográficos y de isótopos estables. Las petrofacies de esta sucesión varían de "mudstone" bioclásticas a "boundstone", y los aloquímicos son corales, algas, rudistas, equinodermos, espículas de esponjas, radiolarios, foraminíferos y calpionélidos. Las calizas de la sucesión analizada por isótopos estables presentan valores bajos de δ18O que varían entre -15.19‰ y -6.32‰ y valores positivos de δ13C entre 2.91‰ y 4.39‰. La falta de correlación entre los valores de δ13C y δ18O indica un origen marino primario para los valores de δ13C de las calizas de la sección de Pitaycachi. El perfil de δ13C del miembro Canova de la sección estudiada muestra un incremento hacia su parte superior, que va de 3.09‰ a 4.36‰, interpretándose como un aumento en la tasa de producción de materia orgánica y/o secuestro de ella en la cuenca durante el Albiano temprano. El abrupto incremento de los valores de isótopos de carbón en la parte inferior de la sección se correlaciona con OAE1b y la forma de su curva es similar a otras reportadas de México y de otros continentes (e.g., la cuenca Vacontian en Francia), sugiriendo que OAE1b pudo haber sido un evento global.

Petrography, major (including four trace elements), stable isotopes (carbon and oxygen), and 87Sr/86Sr geochemistry of limestones of the Shahabad Formation, Bhima basin, Karnataka, southern India are reported. These limestones show a narrow range of δ13C (<img border=0 src="../../../../../img/revistas/rmcg/v25n2/a3s1.jpg">1.34-1.96<img border=0 src="../../../../../img/revistas/rmcg/v25n2/a3s2.jpg">) and δ18O (<img border=0 src="../../../../../img/revistas/rmcg/v25n2/a3s1.jpg"> -6.04 to - 7.61 <img border=0 src="../../../../../img/revistas/rmcg/v25n2/a3s2.jpg">) values. The petrographic study reveals the presence of microsparite and micro- and macrostylolites. The δ13C and 87Sr/86Sr values indicate that these limestones were deposited during the late Neoproterozoic age and the δ18 O values also are very similar to the average Proterozoic carbonate values. Mn and Sr concentrations and low Mn/Sr ratio (<1) together with the stable and radiogenic isotope data suggest that the studied samples are well-preserved or scarcely altered limestones and probably have retained their primary isotopic signatures.

En este trabajo se reportan resultados de la geoquímica de isótopos estables (carbono y oxígeno) y de elementos mayores (incluyendo cuatro elementos traza) en las calizas de la Formación de Shahabad, cuenca de Bhima, Karnataka, en India meridional. Las calizas de la cuenca de Bhima muestran un estrecho intervalo de los valores de δ13C (<img border=0 src="../../../../../img/revistas/rmcg/v25n2/a3s1.jpg">1.34 a 1.96<img border=0 src="../../../../../img/revistas/rmcg/v25n2/a3s2.jpg">) y δ18O (<img border=0 src="../../../../../img/revistas/rmcg/v25n2/a3s1.jpg"> -6.04 a -7.61<img border=0 src="../../../../../img/revistas/rmcg/v25n2/a3s2.jpg">). El estudio petrográfico revela la presencia de dolomía, microesparita y calcita recristalizada con micro y macroestilolitos. Los valores de δ13C y 87Sr/86Sr revelan que estas calizas fueron depositadas durante el Neoproterozoico Tardío y los valores de δ18O son muy parecidos a los valores promedio de los carbonatos del Proterozoico. Las concentraciones de Mn y Sr y los valores bajos de Mn/Sr (< 1), junto con los datos de isótopos estables y radiogénicos, indican que estas calizas están bien preservadas o escasamente alteradas y probablemente han conservado su firma isotópica primaria.

Após a glaciação do final do Criogeniano, a região central do Brasil foi palco de extensa deposição de carbonatos plataformais do Grupo Araras. Este grupo inclui na sua base uma seqüência de capa carbonática sucedida por depósitos transgressivos de calcilutitos betuminosos e folhelhos de plataforma profunda. Dados de fácies e estratigráficos combinados com análises isotópicas de carbono e oxigênio da seção mais completa desses depósitos transgressivos, expostos no sinclinal da Guia, foram empregados para avaliar o paleambiente deposicional e para testar a correlação desses depósitos ao longo da faixa e também com outras unidades de outros continentes. A sucessão estudada consiste de 150 m de espessura de camadas tabulares de calcilutitos e folhelhos cinza a pretos com valores de delta13C PDB negativos, entre -2,5 e -1‰. O perfil de delta13C PDB do sinclinal da Guia mostra clara correlação com a porção superior da Formação Guia da região de Cáceres, 200 km a sudoeste. O perfil de delta13C PDB do Grupo Araras é comparável aos perfis de delta13C PDB de outras unidades ediacaranas da parte sul da Faixa Paraguai, oeste do Canadá, e crátons do Congo e do Kalahari. Além disso, a distribuição de fácies, estratigrafia e o perfil isotópico do Grupo Araras são equiparáveis à porção média do Subgrupo Tsumeb na Namíbia, o que reforça a atribuição de idade Ediacarana para o Grupo Araras.

After the late Cryogenian glaciation the central region of Brazil was the site of extensive deposition of platformal carbonates of the Araras Group. This group includes a basal cap carbonate sequence succeeded by transgressive, deep platform deposits of bituminous lime mudstone and shale. Facies and stratigraphic data combined with carbon and oxygen isotopic analyses of the most complete section of the transgressive deposits, exposed in the Guia syncline, were used to evaluate the depositional paleoenvironment and to test the correlation of these deposits along the belt and with other units worldwide. The studied succession consists of 150 m thick tabular beds of black to grey lime mudstone and shale with predominantly negative delta13C PDB values around -2.5 to -1‰ . The delta13C PDB profile of Guia syncline shows a clear correlation with the upper portion of Guia Formation in the Cáceres region, about 200 km to the southwest. The delta13C PDB profile of the Araras Group is comparable with delta13C PDB profiles of Ediacaran units of the southern Paraguay Belt, western Canada, and the Congo and Kalahari cratons. Moreover, facies distribution, stratigraphy and the carbon isotopic profile of the Araras Group match the middle Tsumeb Subgroup in Namibia, which reinforces the Ediacaran age assigned to the Araras Group.

Amphistegina radiata, Peneroplis planatus e Globigerinoides ruber, presentes em cinqüenta amostras de sedimentos superficiais da margem continental do Estado do Ceará, Brasil, foram analisados quanto à composição isotópica do Oxigênio e Carbono para investigar parâmetros oceanográficos, e um possível valor de delta18O do oceano. Foi feita uma comparação entre as equações lineares de Craig e Gordon (1965) e de Wolff et al. (1998), e verificou-se que a primeira equação foi mais apropriada para o cálculo de delta18O do oceano, na área estudada, encontrando-se um valor de 0,2‰ SMOW. Os menores valores de delta18O estão associados à desembocadura dos rios, refletindo a influência continental. Valores de delta18O de -0,3‰ a -1,5‰ PDB (bentônicos) e -0,6‰ a -2,4‰ PDB (planctônicos), em toda a área, indicam variação na temperatura das massas de água entre 20 a 22ºC, nas águas de fundo, e entre 24 a 27ºC nas águas de superfície em média. Valores de delta13C de +3,2‰ a -0,2‰ PDB (bentônicos), em toda a área, indicam variação na distribuição da Utilização Aparente do Oxigênio (AOU) na margem continental, e indicam que os ambientes de decomposição bacteriológica da matéria orgânica não são contínuos ao longo da área de estudo.

Specimens of Recent foraminifera of Amphistegina radiata, Peneroplis planatus and Globigerinoides ruber, from fifty samples of surface sediments of the continental margin of the State of Ceará, Brazil, have been analyzed for carbon and oxygen isotopes to investigate oceanographic parameters and determine the values of delta18O of the oceanic water. From a comparison between values of delta18O obtained for ocean water using the linear equations by (Craig and Gordon 1965) and the one by Wolff et al. (1998), it became evident that the former yielded a more reliable value (0.2‰ SMOW) than the latter. Lower values of delta18O for the ocean water in this continental margin resulted from continental water influence. Values of 18O (-0.3‰ to -1.5‰ PDB for benthic foraminifera and -0.6‰ to -2.4‰ PDB for planktic foraminifera), attest to a variation of temperatures of oceanic water masses, in average, between 20 to 22ºC in deep water and 24 to 27ºC, in surface water. Values of delta13C from +3.2‰ to -0.2‰ PDB (benthic foraminifera) reflect a variation in the apparent oxygen utilization (AOU) in the continental margin and indicate that the environments of bacteriological decomposition of organic matter are not continuous along the investigated area.

Os recifes de arenito holocênicos da praia de Jaguaribe, Pernambuco, Nordeste do Brasil, consistem de camadas de areias de praia dispostas de forma horizontal, cimentadas por carbonatos, com aproximadamente 40 cm de espessura. O cimento ocorre sob três tipos: a) calcítico, circundando os grãos siliciclásticos, b) micrítico, com uma franja acicular; e c) preenchendo os poros na forma de calcita criptocristalina. A cimentação, precoce, aconteceu ao nível freático abaixo da crista praial, onde condições geoquímicas, hidrodinâmicas e, talvez, microbiológicas favoreceram uma rápida precipitação de aragonita e/ou calcita magnesiana. delta13C varia de -1.8 a +1.5‰ para carbonato dissolvido em água intersticiale de +0.2 a +2.1‰ para componentes bioclásticos.delta18O varia de -2.8 a +0.5‰ na água do mar, água docee intersticial. delta13C e feições diagenéticas sugerem que a cimentação ocorreu em água meteórica-vadosa e/ou marinha-freática, por perda de CO2 durante a evaporação da água intersticial. Localmente, cimento calcítico pouco magnesiano superimposto sugere influência de água doce subseqüente. A composição do cimento das camadas de recifes de arenito empilhadas verticalmente na praia de Jaguaribe mostra que a camada do topo do recife de arenito é mais velha do a situada no presente nível freático. Isto implica em uma cimentação de cima para baixo, retratando provavelmente rebaixamento do nível do mar.

Holocene beachrocks of the Jaguaribe beach, State of Pernambuco, northeastern Brazil, consist of horizontal, cemented layers approximately 40 cm thick. The cement shows three textural varieties: (a) calciferous, surrounding siliciclastic grains, (b) micritic, with an acicular fringe; and (c) cryptocrystalline calcite in pores. Early cementation took place at the water table below beach ridges, where geochemical, hydrodynamic and, perhaps, also microbiological conditions favored rapid precipitation of aragonite and/or high-Mg calcite. delta13C values range from -1.8 to +1.5‰ for dissolved carbonate in interstitial water and from +0.2 to +2.1‰ for bioclastic components. delta18O values range from -2.8 to +0.5‰ for seawater, freshwater and interstitial water. delta13C values and diagenetic features suggest that cementation occurred in meteoric-vadose and/or marine-phreatic water by loss of CO2 during evaporation of the interstitial water. Locally, superimposed low-Mg calcite cements point to subsequent freshwater influence. Total-rock cement composition of vertically stacked beachrock beds at the Jaguaribe beach shows that the highest beachrock bed is older than the one (of same petrographic composition) situated at the current groundwater level. This implies a downward progression of cementation, which probably followed the sea-level fall after a local high stand.

Valores de isótopos de C e 87Sr/86Sr de cinco seqüências de carbonatos do Complexo São Caetano, nordeste do Brasil; foram usados para estimar a sua idade de deposição e relacionar variações da composição isotópica na água do mar com eventos tectônicos globais. Três estágios de variação de isótopos de carbono foram identificados de base para o topo numa seção quimioestratigráfica composta: (1) estágio em que delta13C varia de +2 a +3.7‰PDB (media 3‰PDB), (2) estágio no qual delta13C varia consideravelmente (de -2 a +3‰PDB) e (3) estágio apresentando valores constantes de cerca de +3.7‰PDB. Valores de 87Sr/86Sr ~ 0.70600 caracterizam o estágio isotópico 1, tornando-se mais variáveis no estágio isotópico 2 (entre ~ 0.70600 e ~ 0.70700). Finalmente, valores em torno de 0.70600 caracterizam o estágio isotópico 3. A quimioestratigrafía de C e Sr permite concluir que: (a) as curvas de variação seculares dos carbonatos do Complexo São Caetano registram flutuações primárias na composição isotópica de C e Sr da água do mar durante a passagem Meso-Neoproterozóico na Província Borborema; (b) o ciclo orogênico Cariris velhos/Greenville, a ocorrência de rifteamento e formação de arcos magmáticos continentais, e a formação do supercontinente Rodinia, controlaram a composição isotópica de C e Sr da água do mar durante os estágios isotópicos 1, 2, 3.

C-isotope and 87Sr/86Sr values for five carbonate successions from the São Caetano Complex, northeastern Brazil, were used to constrain their depositional age and to determine large variations in the C- and Sr-isotopic composition of seawater under the framework of global tectonic events. Three C-isotope stages were identified from base to top in a composed chemostratigraphic section: (1) stage in which delta13C values vary from +2 to +3.7‰ PDB and average 3‰ PDB, (2) stage with delta13C values displaying stronger oscillations (from -2‰ to +‰ PDB), and (3) stage with an isotopic plateau with values around +3.7‰ PDB. Constant 87Sr/86Sr values (~ 0.70600) characterize C-isotope stage 1, whereas slightly fluctuating values (from 0.70600 to 0.70700) characterize C-isotope stage 2. Finally, 87Sr/86Sr values averaging 0.70600 characterize C-isotope stage 3. The C- and Sr- chemostratigraphic pathways permit to state: (a) the C- and Sr-isotope secular curves registered primary fluctuations of the isotope composition of seawater during late Mesoproterozoic- early Neoproterozoic transition in the Borborema Province, and (b) onset of the Cariris Velhos/Greenville cycle, widespread oceanic rifting, continental magmatic arc formation and onset of the agglutination of Rodinia supercontinent, mostly controlled the C- and Sr-isotope composition of seawater during the C-isotope stages 1, 2 and 3.