Relationship between isotopic composition (Δ18O and Δ13C) and plaktonic foraminifera test size in core tops from the Brazilian Continental Margin

O tamanho de testa dos foraminíferos é uma importante fonte de variabilidade isotópica (δ18O e δ13C) em amostras de sedimento marinho comprometendo as interpretações paloeceanograficas. No presente estudo, avaliou-se a relação entre o sinal isotópico medido em diferentes frações de tamanho de testa das espécies planctônicas, Globigerinoides ruber (branca) e Globorotalia truncatulinoides (dextral) em amostras de topo de dois testemunhos localizados na Margem Continental Brasileira. Os valores de δ18O foram utilizados para estimar a profundidade de calcificação de cada fração de tamanho. Os desequilíbrios nos valores de δ13C para cada fração de tamanho foram estimados. Os valores de δ18O em G. ruber (branca) não apresentaram tendência com o tamanho sugerindo que calcifica dentro de um mesmo intervalo de profundidade (c.a. 100 m) durante a ontogenia. Os valores de δ18O em G. truncatulinoides (dextral) apresentaram aumento com o tamanho refletindo a migração ontogênica em águas da termoclina (250-400 m). Os valores e desequilíbrios de δ13C aumentaram com o tamanho nas duas espécies indicando o efeito da variação nas taxas fisiológicas durante a ontogenia. Em G. ruber (white) os valores de δ13C dos maiores tamanhos (300µm e >;355µm) refletem melhor os valores de δ13C DIC indicando que são mais apropriados para utilizar nas reconstruções paleoceanograficas.Stable oxygen (δ18O) and carbon (δ13C) isotopic signature registered in fossil planktonic foraminifera tests are widely used to reconstruct ancient oceanographic conditions. Test size is a major source of stable isotope variability in planktonic foraminifera found in sediment samples and thus can compromise paleoceanographic interpretations. Test size/stable isotope (δ18O and δ13C) relationships were evaluated in two planktonic foraminifer species (Globigerinoides ruber (white) and Globorotalia truncatulinoides (right)) in two core tops from the Brazilian Continental Margin. δ18 Omeasurements were used to predict the depth of calcification of each test size fraction. δ13C offsets for each test size fraction were then estimated. No systematic δ18O changes with size were observed in G. ruber (white) suggesting a similar calcification depth range (c.a. 100 m) during ontogeny. For G. truncatulinoides (right) δ18O values increased with size indicating ontogenetic migration along thermocline waters (250-400 m). δ13C measurements and δ13C offsets increased with size for both species reflecting well known physiological induced ontogenetic-related variability. In G. ruber (white) the largest test size fractions (300µm and >;355µm) more closely reflect δ13C DIC indicating they are best suited for paleoceanographic studies

Sedimentary processes in the head of the Cabo Polonio mega slide canyon (southwestern Atlantic margin off Uruguay)

The Southwestern Atlantic margin is characterized by several canyon systems incised on a huge contourite depositional system associated with the interaction of strong Antarctic water masses with the seafloor. So far, however, only one mega slide canyon (Cabo Polonio) has been described in the Uruguayan continental slope. In this work, sedimentary processes dominating the head of this mega slide canyon are described and interpreted based on the analysis of the composition (texture and fossils) and distribution of widespread gravel along its head and thalweg. This information is integrated with acoustic (multibeam and seismic) and hydrological data. Results suggest that the evolution of the head of the canyon presents retrogressive erosion related to debris and turbidity flows. This erosion is ongoing and/or has been active during the recent past and contourite deposits are involved in headwall erosion. The pathway of gravel along the canyon and the thalweg was reconstructed. The strong flow of the South Atlantic Central Water, and its interaction with the Antarctic Intermediate Water, dominates the modern hydrology, promoting highly energetic conditions enhancing headwall erosion. This work contributes to a better understanding of the sedimentary processes connected to an mega slide canyon in the upper slope off Uruguay and located in the northernmost distribution of the South Atlantic Contourite Depositional System

The imprint of the geological inheritage and present dynamics on uruguayan inner shelf sediments (south-western Atlantic)

The Uruguayan continental shelf is characterised by a unique hydrographic system, composed of the Río de la Plata buoyant plume (RdlP-BP), and by water masses of contrasting thermohaline characteristics. Below the RdlP-BP the southward-flowing Subtropical Shelf Water and the northward-flowing Subantarctic Shelf Water converge at the Subtropical Shelf Front, which is the shelf extension of the Brazil-Malvinas Confluence. Three main sedimentary environments can be described associated with; I) “Atlantic coastal sands” (i.e. onshore from the palaeovalley); II) the SW-NE running “RdlP palaeovalley” and; III) “Relict sands” (i.e., offshore of the RdlP palaeovalley). Three exposed sedimentary units (U1 to U3) identified from acoustic profiles and sediment cores (sedimentary characteristics of stratigraphic units) almost entirely restricted to the palaeovalley. The transect here studied intersects these three environments contributing thus with the description of the morphological setting and sedimentary coverture. Physical and geochemical data were integrated and used to characterise the sedimentary facies previously described in the inner shelf and to understand environmental control on the development of these facies. Sediments from U1 show the terrestrial imprint of the RdlP and drier regional conditions, while sediments that characterised U3 indicate a sandy facies (quartz and bioclasts: whole and fragmented shells and polychaetes tubes) corresponding to an ancient coast. This last (with approximately 11 m height), is probably related to sea-level stabilization, between 20 and 25 m occurring during the Upper Pleistocene and Holocene for the South-Western Atlantic. The sediment from U3 reflects the colder and drier conditions prevailing in the region during the formation of this facies (13.7 and 9.7 cal ka BP). Eroded sediments (e.g., from U3; ancient coast) are deposited inside the palaeovalley and on the onshore region (between the palaeovalley and coastal sands). Also, U1 extends from the palaeovalley covering the onshore region. The outcrop of warmer oceanic shelf waters was probably a consequence of the geomorphology of the palaeovalley (edge of the palaeovalley) and related to the still fall presence of waters typical of the austral warm season when higher advection of Brazil Current occurs over the shelf. In this regard, the mound-like feature should induce bottom water to rise, operating as a ramp. Productivity proxies (Si/Ti, Ba/Ti, Ca/Ti and P/Ti) present the highest values in these stations (S16-S18), reflecting the imprint of the upwelling in the sediment. The information reported in this work is particularly important to better understand sedimentological dynamics in the Uruguayan inner shelf and the Southwestern Atlantic region. It is also important for elaborating more precise paleoenvironmental and palaeoceanographic reconstructions

Physical drivers and dominant oceanographic processes on the uruguayan margin (Southwestern Atlantic): a review and a conceptual model

The Uruguayan continental margin (UCM), located in the Southwestern Atlantic margin’s subtropical region, is positioned in a critical transitional region regarding the global ocean circulation (Río de la Plata (RdlP) outflow and Brazil-Malvinas Confluence), as also reflected in seafloor features (northernmost distribution of a large depositional contourite system and RdlP paleovalley). This complex oceanographic scenario occurring in a relatively small area highlights the advantage of considering the UCM as a natural laboratory for oceanographic research. The present work provides the first conceptual “control” model of the physical drivers (i.e., climate, geomorphology) and main oceanographic processes (i.e., hydrodynamics, sediment, and carbon dynamics) occurring along the UCM, reviewing and synthesizing available relevant information based on a functional integrated approach. Despite the conspicuous knowledge gaps on critical processes, a general picture of the system’s functioning is emerging for this complex biophysical setting. This includes conceptualizations of the actual controls, main processes, feedbacks, and interactions responsible for system dynamics. The structure adopted for developing our conceptual models allows permanent improvement by empirical testing of the working hypothesis and incorporating new information as scientific knowledge advances. These models can be used as a baseline for developing quantitative models and, as representations of relatively “pristine” conditions, for stressors models by identifying sources of stress and ecological responses of key system attributes under a transboundary approach.CSIC: I + D 201

Control of oceanic circulation on sediment distribution in the southwestern Atlantic margin (23 to 55º S)

In this study, we interpret the role played by ocean circulation in sediment distribution on the southwestern Atlantic margin using radiogenic Nd and Pb isotopes. The latitudinal trends for Pb and Nd isotopes reflect the different current systems acting on the margin. The utilization of the sediment fingerprinting method allowed us to associate the isotopic signatures with the main oceanographic features in the area. We recognized differences between Nd and Pb sources to the Argentinean shelf (carried by the flow of Subantarctic Shelf Water) and slopes (transported by deeper flows). Sediments from Antarctica extend up to the Uruguayan margin, carried by the Upper and Lower Circumpolar Deep Water. Our data confirm that, for shelf and intermediate areas (the upper 1200 m), the transfer of sediments from the Argentinean margin to the north of 35∘ S is limited by the Subtropical Shelf Front and the basin-wide recirculated Antarctic Intermediate Water. On the southern Brazilian inner and middle shelf, it is possible to recognize the northward influence of the Río de la Plata sediments carried by the Plata Plume Water. Another flow responsible for sediment transport and deposition on the outer shelf and slope is the southward flow of the Brazil Current. Finally, we propose that the Brazil–Malvinas Confluence and the Santos Bifurcation act as boundaries of geochemical provinces in the area. A conceptual model of sediment sources and transport is provided for the southwestern Atlantic margin

Inorganic and organic geochemical fingerprinting of sediment sources and ocean circulation on a complex continental margin (São Paulo Bight, Brazil)

In this study, we use inorganic (metal) and organic (bulk and molecular) markers in sediment samples of the south-eastern Brazilian margin to investigate the response of geochemical fingerprints to the complex hydrodynamic processes present in the area. Results indicate the potential of export of terrigenous siliciclastic and organic constituents to the upper slope, even in an area with limited fluvial supply. Metal contents and especially the ln(Ti = Al) and ln(Fe = K) ratios make it possible to recognise the extension of shelf sediments toward the upper slope. Potassium, here expressedas ln(K = Sc) and ln(K = Al) ratios used as proxies of illite–kaolinite variations, proved to be an important parameter, especially because it allowed us to decipher the imprint of the northward flow of the Intermediate Western Boundary Current (IWBC) in comparison to the southward flows of the Brazil Current (BC) and Deep Western Boundary Current (DWBC). Using organic matter analyses, we were able to evaluate the extent of terrestrial contributions to the outer shelf and slope, even without the presence of significant fluvial input. In addition, molecular markers signify a slight increase in the input of C4-derived plants to the slope sediments, transported from distant areas by the main alongshore boundary currents, indicating that the terrestrial fraction of the organic matter deposited on the slope has a distinct origin when compared to shelf sediments

Relationship betweem oxygem and carbon isotopic signature and foraminiferal tests size from two brazilian continental margin core top samples

Os isótopos de oxigênio (18O) e carbono (13C), registrados nas testas dos foraminíferos são descritores mensuráveis (proxies) da paleotemperatura e paleoprodutividade, respectivamente, amplamente utilizados em estudos paleoceanográficos. Em amostras de sedimento marinho, o tamanho de testa dos foraminíferos é uma importante fonte de variabilidade isotópica que não limita o uso destes proxies, desde que a mesma seja conhecida e entendida. No presente estudo, analisou-se o sinal de 18O e de 13C em testas da espécie bentônica, Cibicidoides wuellerstorfi, e das espécies planctônicas, Globigerinoides ruber (branca) e Globorotalia truncatulinoides (dextral) retidas em quatro frações de tamanho de malha de peneiras (150-250, 250-300, 300-355 e >355µm). Foram utilizadas amostras de topo de dois testemunhos localizados na Margem Continental Brasileira (em torno de 2000 metros de lâmina d´água). Foi comparado o sinal isotópico das amostras com o sinal isotópico da água do mar atual da região de estudo. Foi observado que C. wuellerstorfi calcifica em equilíbrio de 18O e com um desequilíbrio positivo (0,2-0,3) de 13C com respeito á água do mar de fundo. Os valores isotópicos desta espécie não variaram (13C) ou variaram levemente (18O) com o aumento no tamanho. Os valores de 18O registrados em G. ruber (branca) não apresentaram tendência com o tamanho de testa e refletiram uma profundidade aparente de calcificação na superfície do oceano (entre 0-100 metros). Os valores de 18O registrados em G. truncatulinoides (dextral) apresentaram aumento com o tamanho de testa (até 1,22) e refletiram uma profundidade aparente de calcificação em águas da termoclina (entre 200-400 metros). O sinal de 13C aumentou com o tamanho de testa tanto em G. ruber (branca) (até 1,85), como em G. truncatulinoides (dextral) (até 1,26). Os valores de 13C referentes aos maiores tamanhos de testas estão mais próximos dos valores de 13C da água do mar da profundidade aparente de calcificação estimada para cada espécie. Este trabalho corrobora a importância da seleção do tamanho de testa dos foraminíferos em estudos paleoceanográficos para a região de estudo, especialmente no sinal de 13C em foraminíferos planctônicos.Stable oxygen (18O) and carbon (13C) isotope composition registered in foraminifera test are widely used in paleoceanography studies as paleotemperature and paleoprodutivity proxies. On the sea floor, samples size test is an important source of stable isotope variability related to the ecology and physiology of foraminifera. This variability does not restrict the use of these proxies when the relation between size and stable isotopes is known and understood. 18O and 13C values were analyzed in four size sieves fractions (150-250, 250-300, 300-355 e >355µm) in foraminiferal tests of Cibicidoides wuellerstorfi (benthic specie), Globigerinoides ruber (white) and Globorotalia truncatulinoides (right) (planktonic species), in two core topes located on the Brazilian continental margin (around 2,000 meters). Modern seawater stable isotope composition was used for data interpretation. In general, the relation between stable isotope and size for each species was consistent in all samples. C. wuellerstorfi calcifies in equilibrium with 18O, while it is enriched (0.2-0.3) in 13C values with respect to deep sea isotopic values. No clear isotopic size-related changes were observed for this species. 18O values do not change with test size in G. ruber (white) and reflect an apparent calcification depth in superficial waters (0-100 meters). 18O enrichment with size was observed for G. truncatulinoides (right) (up to 1.22) and reflects an apparent calcification depth in thermocline waters (between 200-400 meters). 13C values were clearly enriched with size in both G. ruber (white) (up to 1.85) and G. truncatulinoides (right) (up to 1.26). 13C values of the largest size sieve fraction reflects most accurately 13C values of the estimated calcification depth for each specie. This work highlights the importance of selecting foraminifera test size for paleoceanographic studies, especially in what concerns about carbon isotopes in planktonic foraminifera

Relationship betweem oxygem and carbon isotopic signature and foraminiferal tests size from two brazilian continental margin core top samples

Os isótopos de oxigênio (18O) e carbono (13C), registrados nas testas dos foraminíferos são descritores mensuráveis (proxies) da paleotemperatura e paleoprodutividade, respectivamente, amplamente utilizados em estudos paleoceanográficos. Em amostras de sedimento marinho, o tamanho de testa dos foraminíferos é uma importante fonte de variabilidade isotópica que não limita o uso destes proxies, desde que a mesma seja conhecida e entendida. No presente estudo, analisou-se o sinal de 18O e de 13C em testas da espécie bentônica, Cibicidoides wuellerstorfi, e das espécies planctônicas, Globigerinoides ruber (branca) e Globorotalia truncatulinoides (dextral) retidas em quatro frações de tamanho de malha de peneiras (150-250, 250-300, 300-355 e >355µm). Foram utilizadas amostras de topo de dois testemunhos localizados na Margem Continental Brasileira (em torno de 2000 metros de lâmina d´água). Foi comparado o sinal isotópico das amostras com o sinal isotópico da água do mar atual da região de estudo. Foi observado que C. wuellerstorfi calcifica em equilíbrio de 18O e com um desequilíbrio positivo (0,2-0,3) de 13C com respeito á água do mar de fundo. Os valores isotópicos desta espécie não variaram (13C) ou variaram levemente (18O) com o aumento no tamanho. Os valores de 18O registrados em G. ruber (branca) não apresentaram tendência com o tamanho de testa e refletiram uma profundidade aparente de calcificação na superfície do oceano (entre 0-100 metros). Os valores de 18O registrados em G. truncatulinoides (dextral) apresentaram aumento com o tamanho de testa (até 1,22) e refletiram uma profundidade aparente de calcificação em águas da termoclina (entre 200-400 metros). O sinal de 13C aumentou com o tamanho de testa tanto em G. ruber (branca) (até 1,85), como em G. truncatulinoides (dextral) (até 1,26). Os valores de 13C referentes aos maiores tamanhos de testas estão mais próximos dos valores de 13C da água do mar da profundidade aparente de calcificação estimada para cada espécie. Este trabalho corrobora a importância da seleção do tamanho de testa dos foraminíferos em estudos paleoceanográficos para a região de estudo, especialmente no sinal de 13C em foraminíferos planctônicos.Stable oxygen (18O) and carbon (13C) isotope composition registered in foraminifera test are widely used in paleoceanography studies as paleotemperature and paleoprodutivity proxies. On the sea floor, samples size test is an important source of stable isotope variability related to the ecology and physiology of foraminifera. This variability does not restrict the use of these proxies when the relation between size and stable isotopes is known and understood. 18O and 13C values were analyzed in four size sieves fractions (150-250, 250-300, 300-355 e >355µm) in foraminiferal tests of Cibicidoides wuellerstorfi (benthic specie), Globigerinoides ruber (white) and Globorotalia truncatulinoides (right) (planktonic species), in two core topes located on the Brazilian continental margin (around 2,000 meters). Modern seawater stable isotope composition was used for data interpretation. In general, the relation between stable isotope and size for each species was consistent in all samples. C. wuellerstorfi calcifies in equilibrium with 18O, while it is enriched (0.2-0.3) in 13C values with respect to deep sea isotopic values. No clear isotopic size-related changes were observed for this species. 18O values do not change with test size in G. ruber (white) and reflect an apparent calcification depth in superficial waters (0-100 meters). 18O enrichment with size was observed for G. truncatulinoides (right) (up to 1.22) and reflects an apparent calcification depth in thermocline waters (between 200-400 meters). 13C values were clearly enriched with size in both G. ruber (white) (up to 1.85) and G. truncatulinoides (right) (up to 1.26). 13C values of the largest size sieve fraction reflects most accurately 13C values of the estimated calcification depth for each specie. This work highlights the importance of selecting foraminifera test size for paleoceanographic studies, especially in what concerns about carbon isotopes in planktonic foraminifera