Natan Silva Pereira

Chromium-isotope signatures in scleractinian corals from the Rocas Atoll, Tropical South Atlantic.

Chromium-isotope compositions (expressed as δ(53) Cr) of recent and ancient skeletal and non-skeletal carbonates are currently explored as a (paleo-) redox-proxy for shallow seawater. The idea behind this approach is that biogenic and non-biogenic carbonates could potentially be used as archives recording the Cr-isotope composition of seawater in which they formed, and with this contribute to the reconstruction of past paleo-environmental changes in the marine realm, and potentially to climate changes on land. However, investigations addressing the behavior and uptake mechanism of Cr, and the potential isotope fractionations between seawater and biogenic carbonates are scarce. Here, we present a study of Cr-isotope variations in three species of corals and contemporary seawater from the Rocas Atoll, NE, Brazil. Cr-isotope values of the studied coral species (Siderastrea stellata, Porites sp., and Montastrea cavernosa) vary from -0.5 to +0.33‰ and point to significant isotopic disequilibrium with coexisting seawater characterized by a Cr-isotope value of +0.92 ± 0.2‰. This isotopic offset requires reduction of hexavalent Cr(VI) in the sequestration process of all the studied coral species. Cr-isotope values in a profile across an S. stellata colony returned homogeneous, slightly positively fractioned δ(53) Cr values of +0.07 ± 0.08‰ (n = 8, 2σ), which we interpret to reflect a constant reductive uptake during the 20-year growth period recorded in this coral. In contrast, samples across a 12-year growth profile from Porites sp. display rather heterogeneous Cr-isotope values with δ(53) Cr varying from -0.50 to +0.10‰, indicating Cr incorporation under changing redox processes during its growth intervals. We propose a mechanism whereby initial photoreduction of isotopically heavy Cr(VI) to isotopically lighter Cr(III) in the endodermal layer of corals must be followed by efficient and effective re-oxidation of reduced Cr species to favor subsequent chromate (CrO42-) substitution during the calcifying processes ultimately leading to the formation of the coral skeleton.

Detrital carbonate sedimentation of the Rocas Atoll, South Atlantic

Located 266 km offshore Brazilian coast, Rocas Reef complex is the only atoll in the South Atlantic. Two scientific expeditions carried out detailed sedimentological studies of the complex. Sand texture was dominant in the sedimentary environment, although other textures such as gravel, sandy gravel and gravelly sand, were also observed. Mean size ranged from fine sand to coarse sand (-1.23 to 2.34o), with an average value of 0.69o (sand fraction), which from a geological perspective represents a high-energy environment. The values of mean size varied in the different geomorphologic compartments, a response to variable hydrodynamics on the reef. Sorting ranged from very well sorted to poorly sorted, with a mean value of 0.97o (moderately sorted). The values of mean size and sorting are directly proportional (i.e. the larger the grain size, the better the sorting), a fact that might be controlled by the decreasing of organism diversity that contributes to the sedimentary environment as the mean size of the particles increase. Skewness parameter was used to correlate positive values to depositional environments. The Kurtosis parameter showed little relevance in this study. The sedimentary particles analyzed are exclusively composed of biogenic grains. In total eleven major groups were described, coralline algae being the most unusual.

Isotope and Elemental Chemostratigraphy

Abstract We report results of some of the latest cutting-edge research on traditional (C, O, S, N, Sr) isotope stratigraphy and some newer isotope systems (e.g., B, Ca, Mo, Fe, Cr, N), and elemental or elemental ratio (e.g., V, Mo, Hg, Rb/K, V/Cr, Zr/Ti, Mg/Ca, Mn/Sr, I/Ca) stratigraphy. The latter, in parallel with isotope stratigraphy, could be used as potential tracer for glacial events, buildup of volcanic gases (e.g., CO2) during glaciations, role of volcanism in mass extinction, salinity variation, redox state of the ocean and atmosphere, and provenance, among other applications. This paper also presents review on studies that are broad in both space and time, highlighting chemical events from the Archean to the Cenozoic. Special attention is given to Paleoproterozoic and Neoproterozoic isotope chemostratigraphy and problems including the reliability of the use of carbon isotopes for Neoproterozoic stratigraphic correlation. Some attention is also devoted to the early Paleozoic, K–T boundary, and Paleogene climatic and biogeochemical changes in the light of isotope chemostratigraphic studies.

Coral-based climate records from tropical South Atlantic: 2009/2010 ENSO event in C and O isotopes from Porites corals (Rocas Atoll, Brazil)

Coral skeletons contain records of past environmental conditions due to their long life span and well calibrated geochemical signatures. C and O isotope records of corals are especially interesting, because they can highlight multidecadal variability of local climate conditions beyond the instrumental record, with high fidelity and sub-annual resolution. Although, in order to get an optimal geochemical signal in coral skeleton, sampling strategies must be followed. Here we report one of the first coral-based isotopic record from the Equatorial South Atlantic from two colonies of Porites astreoides from the Rocas Atoll (offshore Brazil), a new location for climate reconstruction. We present time series of isotopic variation from profiles along the corallite valley of one colony and the apex of the corallite fan of the other colony. Significant differences in the isotopic values between the two colonies are observed, yet both record the 2009/2010 El Niño event - a period of widespread coral bleaching - as anomalously negative δ18O values (up to -1 permil). δ13C is found to be measurably affected by the El Niño event in one colony, by more positive values (+0.39 ‰), and together with a bloom of endolithic algae, may indicate physiological alteration of this colony. Our findings indicate that corals from the Rocas Atoll can be used for monitoring climate oscillations in the tropical South Atlantic Ocean.

Correlations of some Neoproterozoic carbonate-dominated successions in South America based on high-resolution chemostratigraphy

This report reviews and incorporates new elemental and isotope chemostratigraphic data for correlation of Neoproterozoic carbonate-dominated successions in South America (Argentina, Bolivia, Brazil, Paraguay and Uruguay). These thick mixed carbonate/siliciclastic successions were largely deposited in epicontinental basins or accumulated on passive margins on the edges of cratons (e.g. Sao Francisco, Amazonia, Rio Apa Block, Pampia and Rio de la Plata paleocontinents) during extensional events related to the rifting of the Rodinia Supercontinent. From the stratigraphic point of view, these successions occur as three mega-sequences: glaciogenic, marine carbonate platform (above glaciomarine diamictites or rift successions), and dominantly continental to transitional siliciclastics. In the orogenic belts surrounding cratons, carbonate-dominated successions with important volcanoclastic/siliciclastic contribution have been, in most cases, strongly deformed. The precise ages of these successions remain a matter of debate, but recently new paleontological and geochronological data have considerably constrained depositional intervals. Here, we report high-resolution C, O, Sr, and S isotope trends measured in well-preserved sample sets and mainly use Sr and C isotopes in concert with lithostratigraphic/biostratigraphic observations to provide detailed correlations of these successions. The establishing of a high-level and definite chemostratigraphic correlation between Neoproterozoic basins in South America is the main goal of this work.

Isótopos Estáveis de Carbono e Oxigênio de Foraminíferos do Complexo Recifal do Atol das Rocas, Atlântico Sul

Foraminifers have the potential to be used as natural archives, allowing the access to environmental conditions of marine systems of periods prior to the beginning of the instrumental records. In order to stablish these organisms as natural archives, it is necessary to check the fidelity of different groups of foraminifera in recording environmental conditions by geochemical information. In this study, we investigated the C and O isotopic composition of nine foraminifera genera from carbonate sediment samples collected in different parts of the Atol das Rocas, Tropical South Atlantic Ocean. The observed isotopic range for all foraminifera groups varied from -2.47 to 4.88‰ for δC and from -4.01 to -0.71‰ for δO. The δC values clearly distinguish the foraminifers from the Miliolida and Rotaliida Order. This difference might be related to the dependence of rotallids to their photosynthetic symbionts, promoting the uptake of C from photosynthetic activity for the construction of their shells, while the miliolids uptake the C directly from seawater, with less photosynthetic influence of their symbionts. Sea surface temperature (SST) calculations, based on δO values, ranged from 22.4 to 39.3 °C, in which the genera that best represent the SST range of the Rocas atoll were Sorites, Spiroculina, Triloculina, Gypsina and Amphistegina.