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Alcides N . Sial

Federal University of Pernambuco, Department of Geology, NEG-LABISE, Faculty Member

Federal University of Pernambuco, Department of Geology, Faculty Member

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Graduated in Geology from Federal University of Pernambuco (1966) and Ph.D. in Geology from the University of California at Davis, USA (1974). Full professor, Federal Univ. Pernambuco. Has experience on Igneous Petrology (mainly basalts and granites) and Stable Isotope Geochemistry (isotope chemostratigraphy). Developed post-doctoral work at the Universities of Texas (Austin, 1 year), Georgia at Athens (2 years) and visiting prof. Univ. Wisconsin, Madison (3 months), USA. Traineeship in Newfoundland, Canada (2 months), Tsukuba, Japan (1 month), East Kilbride, Scotland (2 weeks), Lisbon (6 months). Has published 243 articles in international/national journals (most of them in English), 4 books, 29 book chapters, organized 12 specials issues (Lithos, Chemical Geology, Earth Sciences Reviews, Gondwana Research, Precambrian Research, Journal of South America Earth Sciences among others). Editorial board of Geology (3 years), Gondwana Research (2 years), Annals of the Brazilian Academy of Science (Earth Science Section). Member Brazilian Academy of Sci., Lisbon Academy of Sci., Fellow World Academy of Science (TWAS), National Order Scientific Merit (Commendator, 1996; Great Cross, 2006), Silver Hammer Medalist (Braz. Geol. Soc.), Centennial Medal School of Engineering, Federal University of Pernambuco, Fellow of the Geol. Society of America (USA), homages from the 8th South Amer. Symp. of Isotope Geology (2011), and from the International Hutton Symposium on Granites and Related rocks (2015), José Bonifácio Golden Medal Medalist (Braz. Geol. Soc.), Professor Emeritus (UFPE, 2016), Marechal Trompowsky Medalist (IDMM-PE, 2016).
Supervisors: Supervisor
Phone: 55-81-21268243
Address: Av. Acadêmico Helio Ramos s/n

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Papers by Alcides N . Sial

Alcides N. Sial: 52 years of Scientific Activities

The Neoproterozoic and Cambrian: A Time of Upheavals, Extremes and Innovations

by Claudio Gaucher and Alcides N . Sial

Chemostratigraphy. Neoproterozoic-Cambrian evolution of the Río de la Plata Palaeocontinent.

by Claudio Gaucher, Gomez Peral Lucia Elena, and Alcides N . Sial

Chromium isotopes in carbonates — A tracer for climate change and for reconstructing the redox state of ancient seawater

by Claudio Gaucher and Alcides N . Sial

Earth and Planetary Science Letters, 2011

Strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic c... more Strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic cross correlations of time-equivalent sedimentary sequences and for calibration of the compositional evolution of seawater throughout Earth's history, mainly for the purpose of reconstructing ancient climatic changes. We here present results of a new isotopic tracer system – stable chromium isotopes – applied to a late Ediacaran marine carbonate sequence exposed in the Calera de Recalde syncline, Arroyo del Soldado Group, Uruguay. The aim was to compare Cr isotope signatures directly to δ 13 C, 87 Sr/ 86 Sr and 143 Nd/ 144 Nd fluctuations in a well defined stratigraphic profile comprising sediments that were deposited during cold–warm periods accompanied by sea-level changes in response to glaciation–deglaciation at higher latitudes. The studied section is characterized by a pronounced negative (down to − 3.3‰) δ 13 C excursion in carbonates paralleled by a decrease of 87 Sr/ 86 Sr values. Chromium isotope signatures over this section also show a correlated decrease in δ 53 Cr (δ 53 Cr = [((53 Cr/ 52 Cr) sample / (53 Cr/ 52 Cr) SRM979) − 1] × 1000) values from ~+0.29 to − 0.17‰ which mirrors a decrease in positively fractioned seawater signatures to slightly negative values characteristic of high-temperature magmatic sources. Linear correlations between δ 53 Cr and εNd(570 Ma), 87 Sr/ 86 Sr and Cr concentrations can be explained by mixing between two major input sources of Cr, Nd and Sr into the shallow seawater: 1) a source characterized by negative δ 53 Cr values of ~−0.2‰ , low 87 Sr/ 86 Sr values of ~ 0.707, and elevated 147 Sm/ 144 Nd values of ~ 0.13, recognized as a subaqueous hydrothermal dominated input source, and 2) a source characterized by positively fractionated δ 53 Cr values of ~+0.2‰, higher 87 Sr/ 86 Sr values of ~ 0.708, and lower 147 Sm/ 144 Nd values of ~ 0.11, a source which is strongly affected by continentally derived input. Chromium isotopes provide a powerful tool for reconstructing the redox state of ancient seawater since positive values indicate that, at least locally, Neoproterozoic shallow ocean waters were sufficiently oxidized to fractionate chromium and/or that oxygen levels of the atmosphere were sufficient to transform Cr(III) into the more mobile hexavalent Cr(VI) formed during weathering processes on land. The fact that 87 Sr/ 86 Sr values, despite δ 13 C fluctuations, remain low (indicative of a strong hydrothermal input into the basin at his time) implies that CO 2 limitation was the cause of negative δ 13 C and δ 53 Cr excursions in otherwise nutrient rich late Neoproterozoic basins, and that glaciation is only one more consequence of a tectonically driven, biologically mediated system. In such a scenario, glaciation acts as an amplifier of δ 53 Cr signals. These signals in marine carbonates are a sensitive tracer for redox processes in the ocean and/or on land and have the potential to contribute significantly, in combination with the other commonly used isotopic tracers, to the reconstruction of climatic changes, particularly those that are associated with major glaciation periods in Earth's history.

Paleo-climatic and paleo-environmental evolution of the Neoproterozoic basal sedimentary cover on the Río de La Plata Craton, Argentina

by Gomez Peral Lucia Elena, Alcides N . Sial, Maria Julia Arrouy, Sebastian Richiano, and Daniel G. Poiré

The Sierras Bayas Group of the Tandilia System constitutes the Neoproterozoic sedimentary cover o... more The Sierras Bayas Group of the Tandilia System constitutes the Neoproterozoic sedimentary cover of the Río de La Plata Craton in Argentina that accumulated amid the breakup of the Rodinia supercontinent and subsequent assembly of Gondwanaland. Evidence for glaciation at the base of the succession in the Villa Mónica Formation (VMF) comes in the form of iron-rich laminated sediments
containing dropstones composed predominantly of basement crystalline rocks and quartzites, which are sequentially overlain by a phosphatic mudstone and a ~40 m thick stromatolitic dolomite. Subtidal facies preserve columnar forms similar to the tubestone stromatolites seen in the Neoproterozoic records. The VMF dolomites in four of our
studied sections near Olavarría-Sierras Bayas area reveal a pronounced negative-topositive δ13C up section trend suggestive of post-glacial seawater variations as recorded in Neoproterozoic marine proxies worldwide. Our sedimentological and geochemical study of the VMF sections reveal consistent carbon and oxygen isotope trends that may
be useful for detailed intra-basinal correlations....

Rare-earth element geochemistry of the Meruoca and Mucambo plutons, Ceará, northeast Brazil

Chemical Geology, 1980

Sial, A.N., Figueiredo, M.C.H. and Long, L.E., 1981. Rare-earth element geochemistry of the Meruo... more

Stable Isotope Geochemistry of Carbonate rocks from Ophiolite Melange Zone , Manipur, Norteast India

by Vinod Chandra Tewari and Alcides N . Sial

The Himalayan speleothems provide reliable record of Holocene palaeociimate and palaeomonsoon sin... more The Himalayan speleothems provide reliable record of Holocene palaeociimate and palaeomonsoon since they are not subjected to diagenesis, erosion and terrestrial deposits. Miocene to Holocene palaeociimatic, paleoceanographic, palaeomonsoonal studies from the tropical and monsoonal regions of Indian ocean and subcontinent and Southeast Asia have been attempted in recent years. The Indian monsoon intensified during Late Miocene and the resulting upwelling increased the surface productivity in the northern Indian ocean. Carbon and oxygen isotopic variations in speleothems, especially of stalagmite growth laminae, are used for interpreting the amount of rainfall. In the present paper, speleothems have been studied from the Kumaon and the Garhwal Lesser Himalaya in the NW and Shillong Plateau in the NE region. Samples were collected from caves for the sedimentological microfacies of the speleothems, carbon and oxygen isotopic ratios of the stalactites and stalagmites for palaeoenvironmental and palaeociimatic interpretations. All these caves lie in the high monsoonal region. It is, therefore, essential to study the strength of Indian Summer Monsoon (ISM) and decadal scale seasonal variations. High resolution 13tYfh dating of stalagmites from these unexplored caves has helped in calculating the amount of rainfall due to south west ISM and their exact timing of formation, and decadal scale monsoonal variation. Results of this study are presented in the paper.

C-, Sr-isotope stratigraphy of carbonate rocks from the Southern Espinhaço Ridge, Minas Gerais, southeastern Brazil

by Alcides N . Sial and Adolf Horn

Anais da Academia Brasileira de Ciências, 2014

Neoproterozoic carbonate rocks comprise different stratigraphic units in the southern part of the... more 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 δ 13 C values as high as 10.9‰ decreasing up section to -1.1‰ and 87 Sr/ 86 Sr values vary from 0.7072 to 0.7076, a range commonly observed in Cryogenian rocks. In section 2, dolomitic samples exhibit 87 Sr/ 86 Sr from 0.7076 to 0.7077 while in section 3, 87 Sr/ 86 Sr from 0.7074 to 0.7079. In section 4, 87 Sr/ 86 Sr values are around 0.7080. The values of 87 Sr/ 86 Sr 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.

Isotope geochemistry of Paleoproterozoic metacarbonates from Itatuba, Borborema Province, Northeastern Brazil: Evidence of marble melting within a collisional suture

by Luis H E N R I Q U E Mancini and Alcides N . Sial

Gondwana Research, 2013

Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions... more Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions. Both rocks may present similar texture and foliation to the host rock, or even cross cutting field relationships, which could be interpreted either as igneous or high-grade metamorphosed marble. Diagnostic criteria are even more complex when there is evidence of melting of the metasedimentary carbonate rock, such as has been described in the Himalayas and in the Eastern Ghats, India. In the Alto Moxotó Terrane, a high-grade gneissic domain of the Borborema Province, Northeastern Brazil, there are metacarbonates associated with banded gneisses and different metaplutonic rocks. Field evidence indicates the absence of other metasedimentary rocks associated with these marbles, thus suggesting that these carbonates were separated from other siliciclastic metasedimentary rocks. The presence of marble also suggests that it may represent the initial stage of a crustal carbon recycling into the mantle. These marbles present many field similarities to carbonatites (e.g., fluid-flow structure) and, together with metagranites and metamafic intrusions, may represent a major collisional tectonic suture. A detailed study of the carbon, oxygen and strontium isotopic composition of these marbles is presented. This study aims to identify the origin of the different isotopic components. It is argued that these rocks were subjected to temperature and pressure conditions that were sufficiently high to have melted them. The isotopic data presented here support this interpretation and indicate the mixing of two components: (i) one characterized by radiogenic Sr isotopes and mantle-like carbon isotopes, which is associated with the gneissic and mafic rocks, and (ii) another characterized by low 87 Sr/ 86 Sr ratios and highly positive δ 13 C values. Available geochemical data for the upper Paleoproterozoic indicate that the 87 Sr/ 86 Sr ratio of ocean water, varying between 0.7050 (2.25 ± 0.25 Ga) and 0.7047 (1.91 Ga), falls within the lower range of the samples from Itatuba and thus reinforces the interpretation that these marbles are sedimentary-derived and were partially contaminated by interaction with the host gneissic and mafic rocks.

Carbonate cements in contemporaneous beachrocks, Jaguaribe beach, Itamaracá island, northeastern Brazil: petrographic, geochemical and isotopic aspects

Anais Da Academia Brasileira De Ciencias, 2005

Holocene beachrocks of the Jaguaribe beach, State of Pernambuco, northeastern Brazil, consist of ... more 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

GEOQUÍMICA DE ISÓTOPOS ESTÁVEIS (C, S e O) DAS ROCHAS ENCAIXANTES E DO MINÉRIO DE Cu(Au) DO DEPÓSITO CRISTALINO, PROVÍNCIA MINERAL DE CARAJÁS, PARÁ

We describe stable isotopic data from country rocks and sulfide ores of the Cristalino Cu(Au) dep... more We describe stable isotopic data from country rocks and sulfide ores of the Cristalino Cu(Au) deposit (CD), Carajás Province, Brazil. Calcite from whole rock and sulfides grains from mafic to felsic volcanics and "hydrothermal breccias" were analyzed. The CD, an Archean (ca. 2.7Ga) IOCG deposit, occurs in hydrothermalized (e.g., alkalinization; carbonatization; sulfidation; etc.) Itacaiúnas Supergroup supracrustals. Diorites crosscut these rocks

Chemostratigraphy of the Lower Arroyo del Soldado Group (Vendian, Uruguay) and Palaeoclimatic Implications

by Claudio Gaucher and Alcides N . Sial

Gondwana Research, 2004

New C-and 0-isotopic determinations from the Vendian lower Arroyo del Soldado Group are reported ... more New C-and 0-isotopic determinations from the Vendian lower Arroyo del Soldado Group are reported and combined with sedimentologic and biostratigraphic data. On the basis of different geochemical and petrographic criteria, the primary nature of the C-and Sr-isotopic signature is shown. Positive 613C-values characterize the mainly siliciclastic upper Yerbal Formation, which contains oxide-facies BIF and a diverse assemblage of skeletal fossils, including Cloudma riernkeae. A series of positive and negative 613C-excursions occur up-section in the overlying Polanco Formation, mainly composed of limestones and limestone-dolostone rhythmites. The transition to the overlying Barriga Negra and Cerro Espuelitas Formation, which consists of conglomerates and shales/cherts/BIF respectively, is marked by a further negative excursion. On the basis of sedimentary structures, a correlation of with palaeobathymetry is established.

Preface to the special issue of Lithos on isotopes in igneous petrogenesis

Lithos, 2005

An Isotopic Insight of the Sergipano Belt Stratigraphy: Radiogenic and Carbon Isotopes

309 CHEMOSTRATIGRAPHY OF THE MINA VERDÚN GROUP AND OTHER CEMENT-GRADE PROTEROZOIC LIMESTONE DEPOSITS IN URUGUAY

by Claudio Gaucher, Alcides N . Sial, and Daniel G. Poiré

Chapter 3 The São Francisco Palaeocontinent

Developments in Precambrian Geology, 2009

Neoproterozoic to Cambrian Palaeoclimatic Events in Southwestern Gondwana

Developments in Precambrian Geology, 2009

In this review we provide a correlation scheme for Neoproterozoic palaeoclimatic events in southw... more In this review we provide a correlation scheme for Neoproterozoic palaeoclimatic events in southwestern Gondwana based on combined carbon and strontium isotope variations informed by emerging radiometric constraints on glacial deposits. Strontium isotope ratios in particular hold promise as chronostratigraphic tools if well-preserved samples can be identified, and if long-term trends through the Neoproterozoic show a regular progression to more radiogenic values. Based on analyses of high Sr limestone samples from cap carbonate lithofacies, we recognise four discrete glacial events in southwestern Gondwana, including two Sturtian, one Marinoan and one Gaskiers ice age. Notably, the Namibian Maieberg cap carbonate – considered by many to be a Marinoan archetype – sits stratigraphically below a profound positive carbon isotope excursion (called the Hüttenberg anomaly) and is correlated using Sr isotope ratios with an identical post-glacial cap in Brazil dated at ca. 740Ma by Pb-Pb carbonate techniques. If correct, the Pb-Pb age constraint places the Ghaub ice age and its equivalents into the Sturtian epoch.

δ δ18O em sedimentos ricos em foraminíferos da margem continental do Nordeste brasileiro, entre os estados do Ceará e Pernambuco

δ13C and δ18O analyses were performed in eight marine carbonatic rich foraminifera sediment sampl... more δ13C and δ18O analyses were performed in eight marine carbonatic rich foraminifera sediment samples collected in the continental margin between Ceará and Pernambuco States, Northeastern Brazil. The results show a clear dependence of the sediments δ13C and δ18O values with depth, indicating a C- and O- isotopic ocean water stratification. The results also show a dependence of the seawater temperature,

Chapter 4.4 Chemostratigraphy

by Claudio Gaucher, D. Poiré, Alcides N . Sial, and Gomez Peral Lucia Elena

Developments in Precambrian Geology, 2009

Carbon- and strontium-isotope data from the Mina Verdún, Arroyo del Soldado and Sierras Bayas gro... more Carbon- and strontium-isotope data from the Mina Verdún, Arroyo del Soldado and Sierras Bayas groups are summarised and discussed. Mn/Sr ratios and δ13C versus δ18O crossplots for the Mina Verdún Group at its stratotype show that δ13C reflects seawater composition. The δ13C curve obtained for this unit suggests a late Mesoproterozoic to Tonian age. Four positive and negative δ13C excursions

Carbon and oxygen isotope profiles across Meso-Neoproterozoic limestones from central Brazil: Bambuiand Paranoagroups

We present carbon and oxygen isotopic data on carbonates along three profiles of the Neoproterozo... more We present carbon and oxygen isotopic data on carbonates along three profiles of the Neoproterozoic Bambuigroup 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 Bambuigroup overlies by unconformity the Paranoagroup, which consists mostly of siltstone, quartzite and minor

Alcides N. Sial: 52 years of Scientific Activities

The Neoproterozoic and Cambrian: A Time of Upheavals, Extremes and Innovations

by Claudio Gaucher and Alcides N . Sial

Chemostratigraphy. Neoproterozoic-Cambrian evolution of the Río de la Plata Palaeocontinent.

by Claudio Gaucher, Gomez Peral Lucia Elena, and Alcides N . Sial

Chromium isotopes in carbonates — A tracer for climate change and for reconstructing the redox state of ancient seawater

by Claudio Gaucher and Alcides N . Sial

Earth and Planetary Science Letters, 2011

Strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic c... more Strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic cross correlations of time-equivalent sedimentary sequences and for calibration of the compositional evolution of seawater throughout Earth's history, mainly for the purpose of reconstructing ancient climatic changes. We here present results of a new isotopic tracer system – stable chromium isotopes – applied to a late Ediacaran marine carbonate sequence exposed in the Calera de Recalde syncline, Arroyo del Soldado Group, Uruguay. The aim was to compare Cr isotope signatures directly to δ 13 C, 87 Sr/ 86 Sr and 143 Nd/ 144 Nd fluctuations in a well defined stratigraphic profile comprising sediments that were deposited during cold–warm periods accompanied by sea-level changes in response to glaciation–deglaciation at higher latitudes. The studied section is characterized by a pronounced negative (down to − 3.3‰) δ 13 C excursion in carbonates paralleled by a decrease of 87 Sr/ 86 Sr values. Chromium isotope signatures over this section also show a correlated decrease in δ 53 Cr (δ 53 Cr = [((53 Cr/ 52 Cr) sample / (53 Cr/ 52 Cr) SRM979) − 1] × 1000) values from ~+0.29 to − 0.17‰ which mirrors a decrease in positively fractioned seawater signatures to slightly negative values characteristic of high-temperature magmatic sources. Linear correlations between δ 53 Cr and εNd(570 Ma), 87 Sr/ 86 Sr and Cr concentrations can be explained by mixing between two major input sources of Cr, Nd and Sr into the shallow seawater: 1) a source characterized by negative δ 53 Cr values of ~−0.2‰ , low 87 Sr/ 86 Sr values of ~ 0.707, and elevated 147 Sm/ 144 Nd values of ~ 0.13, recognized as a subaqueous hydrothermal dominated input source, and 2) a source characterized by positively fractionated δ 53 Cr values of ~+0.2‰, higher 87 Sr/ 86 Sr values of ~ 0.708, and lower 147 Sm/ 144 Nd values of ~ 0.11, a source which is strongly affected by continentally derived input. Chromium isotopes provide a powerful tool for reconstructing the redox state of ancient seawater since positive values indicate that, at least locally, Neoproterozoic shallow ocean waters were sufficiently oxidized to fractionate chromium and/or that oxygen levels of the atmosphere were sufficient to transform Cr(III) into the more mobile hexavalent Cr(VI) formed during weathering processes on land. The fact that 87 Sr/ 86 Sr values, despite δ 13 C fluctuations, remain low (indicative of a strong hydrothermal input into the basin at his time) implies that CO 2 limitation was the cause of negative δ 13 C and δ 53 Cr excursions in otherwise nutrient rich late Neoproterozoic basins, and that glaciation is only one more consequence of a tectonically driven, biologically mediated system. In such a scenario, glaciation acts as an amplifier of δ 53 Cr signals. These signals in marine carbonates are a sensitive tracer for redox processes in the ocean and/or on land and have the potential to contribute significantly, in combination with the other commonly used isotopic tracers, to the reconstruction of climatic changes, particularly those that are associated with major glaciation periods in Earth's history.

Paleo-climatic and paleo-environmental evolution of the Neoproterozoic basal sedimentary cover on the Río de La Plata Craton, Argentina

by Gomez Peral Lucia Elena, Alcides N . Sial, Maria Julia Arrouy, Sebastian Richiano, and Daniel G. Poiré

The Sierras Bayas Group of the Tandilia System constitutes the Neoproterozoic sedimentary cover o... more The Sierras Bayas Group of the Tandilia System constitutes the Neoproterozoic sedimentary cover of the Río de La Plata Craton in Argentina that accumulated amid the breakup of the Rodinia supercontinent and subsequent assembly of Gondwanaland. Evidence for glaciation at the base of the succession in the Villa Mónica Formation (VMF) comes in the form of iron-rich laminated sediments
containing dropstones composed predominantly of basement crystalline rocks and quartzites, which are sequentially overlain by a phosphatic mudstone and a ~40 m thick stromatolitic dolomite. Subtidal facies preserve columnar forms similar to the tubestone stromatolites seen in the Neoproterozoic records. The VMF dolomites in four of our
studied sections near Olavarría-Sierras Bayas area reveal a pronounced negative-topositive δ13C up section trend suggestive of post-glacial seawater variations as recorded in Neoproterozoic marine proxies worldwide. Our sedimentological and geochemical study of the VMF sections reveal consistent carbon and oxygen isotope trends that may
be useful for detailed intra-basinal correlations....

Rare-earth element geochemistry of the Meruoca and Mucambo plutons, Ceará, northeast Brazil

Chemical Geology, 1980

Sial, A.N., Figueiredo, M.C.H. and Long, L.E., 1981. Rare-earth element geochemistry of the Meruo... more

Stable Isotope Geochemistry of Carbonate rocks from Ophiolite Melange Zone , Manipur, Norteast India

by Vinod Chandra Tewari and Alcides N . Sial

The Himalayan speleothems provide reliable record of Holocene palaeociimate and palaeomonsoon sin... more The Himalayan speleothems provide reliable record of Holocene palaeociimate and palaeomonsoon since they are not subjected to diagenesis, erosion and terrestrial deposits. Miocene to Holocene palaeociimatic, paleoceanographic, palaeomonsoonal studies from the tropical and monsoonal regions of Indian ocean and subcontinent and Southeast Asia have been attempted in recent years. The Indian monsoon intensified during Late Miocene and the resulting upwelling increased the surface productivity in the northern Indian ocean. Carbon and oxygen isotopic variations in speleothems, especially of stalagmite growth laminae, are used for interpreting the amount of rainfall. In the present paper, speleothems have been studied from the Kumaon and the Garhwal Lesser Himalaya in the NW and Shillong Plateau in the NE region. Samples were collected from caves for the sedimentological microfacies of the speleothems, carbon and oxygen isotopic ratios of the stalactites and stalagmites for palaeoenvironmental and palaeociimatic interpretations. All these caves lie in the high monsoonal region. It is, therefore, essential to study the strength of Indian Summer Monsoon (ISM) and decadal scale seasonal variations. High resolution 13tYfh dating of stalagmites from these unexplored caves has helped in calculating the amount of rainfall due to south west ISM and their exact timing of formation, and decadal scale monsoonal variation. Results of this study are presented in the paper.

C-, Sr-isotope stratigraphy of carbonate rocks from the Southern Espinhaço Ridge, Minas Gerais, southeastern Brazil

by Alcides N . Sial and Adolf Horn

Anais da Academia Brasileira de Ciências, 2014

Neoproterozoic carbonate rocks comprise different stratigraphic units in the southern part of the... more 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 δ 13 C values as high as 10.9‰ decreasing up section to -1.1‰ and 87 Sr/ 86 Sr values vary from 0.7072 to 0.7076, a range commonly observed in Cryogenian rocks. In section 2, dolomitic samples exhibit 87 Sr/ 86 Sr from 0.7076 to 0.7077 while in section 3, 87 Sr/ 86 Sr from 0.7074 to 0.7079. In section 4, 87 Sr/ 86 Sr values are around 0.7080. The values of 87 Sr/ 86 Sr 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.

Isotope geochemistry of Paleoproterozoic metacarbonates from Itatuba, Borborema Province, Northeastern Brazil: Evidence of marble melting within a collisional suture

by Luis H E N R I Q U E Mancini and Alcides N . Sial

Gondwana Research, 2013

Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions... more Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions. Both rocks may present similar texture and foliation to the host rock, or even cross cutting field relationships, which could be interpreted either as igneous or high-grade metamorphosed marble. Diagnostic criteria are even more complex when there is evidence of melting of the metasedimentary carbonate rock, such as has been described in the Himalayas and in the Eastern Ghats, India. In the Alto Moxotó Terrane, a high-grade gneissic domain of the Borborema Province, Northeastern Brazil, there are metacarbonates associated with banded gneisses and different metaplutonic rocks. Field evidence indicates the absence of other metasedimentary rocks associated with these marbles, thus suggesting that these carbonates were separated from other siliciclastic metasedimentary rocks. The presence of marble also suggests that it may represent the initial stage of a crustal carbon recycling into the mantle. These marbles present many field similarities to carbonatites (e.g., fluid-flow structure) and, together with metagranites and metamafic intrusions, may represent a major collisional tectonic suture. A detailed study of the carbon, oxygen and strontium isotopic composition of these marbles is presented. This study aims to identify the origin of the different isotopic components. It is argued that these rocks were subjected to temperature and pressure conditions that were sufficiently high to have melted them. The isotopic data presented here support this interpretation and indicate the mixing of two components: (i) one characterized by radiogenic Sr isotopes and mantle-like carbon isotopes, which is associated with the gneissic and mafic rocks, and (ii) another characterized by low 87 Sr/ 86 Sr ratios and highly positive δ 13 C values. Available geochemical data for the upper Paleoproterozoic indicate that the 87 Sr/ 86 Sr ratio of ocean water, varying between 0.7050 (2.25 ± 0.25 Ga) and 0.7047 (1.91 Ga), falls within the lower range of the samples from Itatuba and thus reinforces the interpretation that these marbles are sedimentary-derived and were partially contaminated by interaction with the host gneissic and mafic rocks.

Carbonate cements in contemporaneous beachrocks, Jaguaribe beach, Itamaracá island, northeastern Brazil: petrographic, geochemical and isotopic aspects

Anais Da Academia Brasileira De Ciencias, 2005

Holocene beachrocks of the Jaguaribe beach, State of Pernambuco, northeastern Brazil, consist of ... more 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

GEOQUÍMICA DE ISÓTOPOS ESTÁVEIS (C, S e O) DAS ROCHAS ENCAIXANTES E DO MINÉRIO DE Cu(Au) DO DEPÓSITO CRISTALINO, PROVÍNCIA MINERAL DE CARAJÁS, PARÁ

We describe stable isotopic data from country rocks and sulfide ores of the Cristalino Cu(Au) dep... more We describe stable isotopic data from country rocks and sulfide ores of the Cristalino Cu(Au) deposit (CD), Carajás Province, Brazil. Calcite from whole rock and sulfides grains from mafic to felsic volcanics and "hydrothermal breccias" were analyzed. The CD, an Archean (ca. 2.7Ga) IOCG deposit, occurs in hydrothermalized (e.g., alkalinization; carbonatization; sulfidation; etc.) Itacaiúnas Supergroup supracrustals. Diorites crosscut these rocks

Chemostratigraphy of the Lower Arroyo del Soldado Group (Vendian, Uruguay) and Palaeoclimatic Implications

by Claudio Gaucher and Alcides N . Sial

Gondwana Research, 2004

New C-and 0-isotopic determinations from the Vendian lower Arroyo del Soldado Group are reported ... more New C-and 0-isotopic determinations from the Vendian lower Arroyo del Soldado Group are reported and combined with sedimentologic and biostratigraphic data. On the basis of different geochemical and petrographic criteria, the primary nature of the C-and Sr-isotopic signature is shown. Positive 613C-values characterize the mainly siliciclastic upper Yerbal Formation, which contains oxide-facies BIF and a diverse assemblage of skeletal fossils, including Cloudma riernkeae. A series of positive and negative 613C-excursions occur up-section in the overlying Polanco Formation, mainly composed of limestones and limestone-dolostone rhythmites. The transition to the overlying Barriga Negra and Cerro Espuelitas Formation, which consists of conglomerates and shales/cherts/BIF respectively, is marked by a further negative excursion. On the basis of sedimentary structures, a correlation of with palaeobathymetry is established.

Preface to the special issue of Lithos on isotopes in igneous petrogenesis

Lithos, 2005

An Isotopic Insight of the Sergipano Belt Stratigraphy: Radiogenic and Carbon Isotopes

309 CHEMOSTRATIGRAPHY OF THE MINA VERDÚN GROUP AND OTHER CEMENT-GRADE PROTEROZOIC LIMESTONE DEPOSITS IN URUGUAY

by Claudio Gaucher, Alcides N . Sial, and Daniel G. Poiré

Chapter 3 The São Francisco Palaeocontinent

Developments in Precambrian Geology, 2009

Neoproterozoic to Cambrian Palaeoclimatic Events in Southwestern Gondwana

Developments in Precambrian Geology, 2009

In this review we provide a correlation scheme for Neoproterozoic palaeoclimatic events in southw... more In this review we provide a correlation scheme for Neoproterozoic palaeoclimatic events in southwestern Gondwana based on combined carbon and strontium isotope variations informed by emerging radiometric constraints on glacial deposits. Strontium isotope ratios in particular hold promise as chronostratigraphic tools if well-preserved samples can be identified, and if long-term trends through the Neoproterozoic show a regular progression to more radiogenic values. Based on analyses of high Sr limestone samples from cap carbonate lithofacies, we recognise four discrete glacial events in southwestern Gondwana, including two Sturtian, one Marinoan and one Gaskiers ice age. Notably, the Namibian Maieberg cap carbonate – considered by many to be a Marinoan archetype – sits stratigraphically below a profound positive carbon isotope excursion (called the Hüttenberg anomaly) and is correlated using Sr isotope ratios with an identical post-glacial cap in Brazil dated at ca. 740Ma by Pb-Pb carbonate techniques. If correct, the Pb-Pb age constraint places the Ghaub ice age and its equivalents into the Sturtian epoch.

δ δ18O em sedimentos ricos em foraminíferos da margem continental do Nordeste brasileiro, entre os estados do Ceará e Pernambuco

δ13C and δ18O analyses were performed in eight marine carbonatic rich foraminifera sediment sampl... more δ13C and δ18O analyses were performed in eight marine carbonatic rich foraminifera sediment samples collected in the continental margin between Ceará and Pernambuco States, Northeastern Brazil. The results show a clear dependence of the sediments δ13C and δ18O values with depth, indicating a C- and O- isotopic ocean water stratification. The results also show a dependence of the seawater temperature,

Chapter 4.4 Chemostratigraphy

by Claudio Gaucher, D. Poiré, Alcides N . Sial, and Gomez Peral Lucia Elena

Developments in Precambrian Geology, 2009

Carbon- and strontium-isotope data from the Mina Verdún, Arroyo del Soldado and Sierras Bayas gro... more Carbon- and strontium-isotope data from the Mina Verdún, Arroyo del Soldado and Sierras Bayas groups are summarised and discussed. Mn/Sr ratios and δ13C versus δ18O crossplots for the Mina Verdún Group at its stratotype show that δ13C reflects seawater composition. The δ13C curve obtained for this unit suggests a late Mesoproterozoic to Tonian age. Four positive and negative δ13C excursions

Carbon and oxygen isotope profiles across Meso-Neoproterozoic limestones from central Brazil: Bambuiand Paranoagroups

We present carbon and oxygen isotopic data on carbonates along three profiles of the Neoproterozo... more We present carbon and oxygen isotopic data on carbonates along three profiles of the Neoproterozoic Bambuigroup 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 Bambuigroup overlies by unconformity the Paranoagroup, which consists mostly of siltstone, quartzite and minor