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Oceanographic exchanges between the Southern and Northern Atlantic during the Cenozoic inferred from mixed contourite-turbidite systems in the Brazilian Equatorial Margin
No full textThe Equatorial Atlantic Ocean is a key region to study the oceanographic and climatic changes that occurred during the Cenozoic. Using of 2D and 3D seismic reflection data and boreholes from the Ceará Basin in the Brazilian Equatorial Margin, this work reconstructs how deep-water oceanographic currents in the Equatorial Atlantic Ocean changed during the Cenozoic by investigating the evolution of contourite depositional systems (CDS) and mixed contourite-turbidite system (mCTS). Seismic data interpretation indicates a southward-flowing proto-North Brazil Current (Proto-NBC) was already established in the Eocene and inverted direction in the late Miocene. During most of the Cenozoic, the Ceará Basin went through an alternation of CDS and mCTS, which evolved in response to different sea level and oceanographic regimes. We propose a six-phase evolutionary model for the Brazilian Equatorial Margin to summarize such changes: (1) Paleocene and early Eocene, formation of the CDS under the influence of a southeast-flowing, low intensity, proto-NBC; (2) early Oligocene, still dominance of southeast-flowing currents and further development of the CDS; (3) late Oligocene, initial deposition the m-CTS with prevalence of down-slope processes in proximal regions and bottom-current (still southeast-flowing) reworking in distal regions; (4) early Miocene, the previous late Oligocene conditions are sustained until the (5) middle Miocene, when the proto-NBC becomes weaker and start to invert its direction from southeast to northwest; finally, from the late Miocene (6), the onset of the northwest flow of NBC is established and the mCTS further develops. By documenting the evolution of Cenozoic contourite and mixed contourite-turbidite systems in the Brazilian Equatorial Margin, our study contributes to the understanding of the Atlantic Meridional Overturning Circulation in the Equatorial Atlantic, providing a baseline for future investigations.
Introduction
Sediment deposition and erosion along continental margins may occur under the influence of several processes acting with down-slope or along-slope directions (Faugères et al., 1999; Rebesco and Camerlenghi, 2008; Meiburg and Kneller, 2010). Gravity-driven processes, such as turbidity currents or landslides, relate with the first group, while bottom-current processes, with the latter (Rebesco et al., 2014). The relation between down- and along-slope processes is influenced by the oceanic circulation, and the resulting sedimentary deposits have been characterized using seismic reflection data, both 2D and 3D (Faugères et al., 1999; Viana, 2002; Rebesco and Stow, 2002; Hernández-Molina et al., 2009; Rebesco, 2016). Sediments accumulated under the sole action of bottom currents form contourite depositional systems (CDS; see Hernández-Molina et al., 2006), whereas when bottom currents interact, synchronously or asynchronously, with gravity-driven flows, the resulting deposits are named mixed contourite-turbidite systems (mCTS; see Faugères et al., 1999; Rebesco et al., 2014, Rodrigues et al., 2022). CDS and mCTS have been documented in many continental margin settings across the world (Llave et al., 2007; Hernández-Molina et al., 2010; Rebesco et al., 2014; Gruetzner et al., 2016; Sansom, 2018; Miramontes et al., 2016, Miramontes et al., 2019; Pandolpho et al., 2021), particularly in the Atlantic Ocean, with examples from the North Atlantic (Heezen et al., 1966; Hollister and Heezen, 1972; Rebesco et al., 2013), Nova Scotia margin (Campbell and Mosher, 2016; Rodrigues et al., 2022), Iberian margin (Alves et al., 2003), Brazil margin (Gomes and Viana, 2002; Viana, 2002; Maselli et al., 2019; Maestrelli et al., 2020), Southern São Paulo Plateau (Borisov et al., 2013), Uruguayan Basin (Hernández-Molina et al., 2016), Argentine Basin (Von Lom-Keil et al., 2002; Hernández-Molina et al., 2009, Hernández-Molina et al., 2010), Vema Channel (Faugères et al., 1998, Faugères et al., 2002) and Scotia Sea (García et al., 2016). Problematically, modern and ancient contourite and mixed contourite-turbidite systems along the Brazilian Equatorial Margin (BEM) are still poorly known (Tallobre et al., 2016; Almeida et al., 2019).
The recent availability of 2D and 3D seismic reflection data acquired for hydrocarbon exploration has made possible to better understand the sedimentary processes governing the evolution of the BEM (De Almeida et al., 2015; Almeida et al., 2019; Jovane et al., 2016; Maestrelli et al., 2020). Here, the recognition and characterization of CDS and mCTS systems are of critical importance as they may help to understand the palaeoceanographic evolution of the basin during the Cenozoic, and shed light on the oceanographic connections between the Northern and Southern hemispheres. In detail, the North Brazil Current (NBC) and the Deep Western Boundary Current (DWBC) contribute to the Atlantic Meridional Overturning Circulation (AMOC) in the Equatorial Atlantic and have influenced the evolution of the BEM since its formation (da Silveira et al., 1994; Wilson et al., 2011; Zhang et al., 2011). The AMOC is essential for the Earth's climate and is related to several climatic, depositional and oceanographic changes around the world (Guihou et al., 2011; Zhang et al., 2015; Mulitza et al., 2017; Lynch-Stieglitz, 2017). Consequently, quantifying the onset and evolution of deep-water depositional systems influenced by the NBC and the DWBC may shed light in longer-term changes in the AMOC, and thus provide new insights on Earth's climate history.
In this study, we mapped and characterized the contourite deposits and associated turbidite channels in the Ceará Basin, Brazil Equatorial Margin, through multiple high-resolution seismic reflection datasets tied to boreholes. The BEM has been developing since the early stages of the Cenozoic, and could provide a better comprehension of equatorial paleoceanography, including the onset and evolution of the deep-water bottom-current circulation in the BEM. Our study documents new contourite and mixed contourite-turbidite systems in the Equatorial Atlantic and emphasizes the importance of the BEM in recording the oceanographic changes occurred during the Cenozoic whilst contributing to the comprehension of how those systems may change and evolve through time
Middle Eocene-Lower Oligocene nannofossil biostratigraphy and paleoceanography from ODP Site 711 (Equatorial Indian Ocean)
No full textEquatorial biostratigraph
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Integrated stratigraphy (magneto-, bio- and chronostratigraphy) and geochronology of the Palaeogene pelagic succession of the Umbria–Marche Basin (central Italy)
No full textExtensive outcrops in the Umbria Marche Basin of central Italy include some of the most complete successions of Palaeogene sediments known from the Tethyan Realm. Owing to the continuous deposition in a pelagic setting, a rather modest tectonic overprint, the availability of excellent age control through magneto-, bio-, chemo- and tephrostratigraphy, and direct radioisotopic dates from interbedded volcaniclastic layers, these sediments have played a prominent role in the establishment of standard Palaeogene time scales. We present here a complete and well-preserved Palaeogene pelagic composite succession of the Umbria-Marche Basin, which provides the means for an accurate and precise calibration of the Palaeogene time scale. As a necessary step towards the compilation of a more robust database on a wide scale so as to improve the magneto-, bio- and chronostratigraphic framework of the classical southern Tethyan zonations, enabling regional and supraregional correlations, we have constructed a record of reliable Palaeogene planktonic foraminifera, calcareous nannofossil and dinocyst biohorizons commonly used in tropical to subtropical Cenozoic zonations. In addition, an age model is provided for the Palaeogene pelagic composite succession based on magnetostratigraphy, planktonic foraminifera and calcareous nannofossils, which contributes to an integrated chronology for the Palaeogene Tethyan sediments from c. 65.5 to 23 Ma
Integrated calcareous nannofossil and magnetostratigraphic record of ODP Site 709: Middle Eocene to late Oligocene paleoclimate and paleoceanography of the Equatorial Indian Ocean
No full textWe investigated the calcareous nannofossil biostratigraphy and magnetostratigraphy of middle Eocene – lower Oligocene sediments from ODP Hole 709 C, equatorial Indian Ocean. The new bio-magnetostratigraphic analyses have resulted in an accurate biochronology of the interval spanning Chrons C20r (middle Eocene) to C12r, in which 29 bioevents were investigated, in a 12 myr interval. The magnetostratigraphic signal is less clear across the Eocene-Oligocene transition (EOT) but becomes more reliable at the top of Chron C13n to Chron C12r (early Oligocene). Quantitative analyses of calcareous nannofossil assemblages allowed recognition of the Middle Eocene Climatic Optimum (MECO) and the long cooling trend leading to the glacial state starting in the early Oligocene. We identify two hiatuses, in the lower middle Eocene and across the Eocene-Oligocene Transition (EOT). Across the latter unconformity, a major transition from oligotrophic to eutrophic favoring nannofossil taxa highlights the enhanced sea surface nutrient availability during the transition to the early Oligocene glacial system. Finally, a late Oligocene warming event is recorded at this site by the increase in calcareous nannofossil taxa that preferred warm water
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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