The Alpha Crucis Carbonate Ridge (ACC R): Discovery of a giant ringshaped carbonate complex on the SW Atlantic margin

Recently acquired bathymetric and high-resolution seismic data from the upper slope of Santos Basin, southern Brazilian margin, reveal a major geomorphological feature in the SW Atlantic that is interpreted as a carbonate ridge - the Alpha Crucis Carbonate Ridge (ACCR). The ACCR is the first megastructure of this type described on the SW Atlantic margin. The ~17 × 11-km-wide ring-shaped ACC R features tens of >100-m-high steep-sided carbonate mounds protruding from the surrounding seabed and flanked by elongated depressions. Comet-like marks downstream of the mound structures indicate that the area is presently influenced by the northward flow of the Intermediate Western Boundary Current (IWBC), a branch of the Subtropical Gyre that transports Antarctic Intermediate Water. Abundant carbonate sands and gravels cover the mounds and are overlain by a biologically significant community of living and dead ramified corals and associated invertebrates. The IWBC acts as a hydrodynamic factor that is responsible for both shaping the bottom and transporting coral larvae. We contend that the ACCR was formed by upward fluid flow along active sub-surface faults and fractures that formed by lateral extension generated by the ascending movement of salt diapirs at depth. The ACC R provides an important modern and accessible analogue for a seabed carbonate build-up related to sub-surface hydrocarbon systems.Acknowledgements are also due to the São Paulo Science Foundation (FAPESP grants 2014/08266-2, 2015/17763-2, and 2016/22194-0). MMdeM acknowledges the Brazilian National Research Council (CNPq, grant 300962/2018-5). The partnership between MMdeM and the US was supported by FAPESP (grant 2017/50191-8 – SPRINT Program). The partnership between MMdeM and F.J.L. was supported by CNPq (grant 401041/2014-0). The authors gratefully acknowledge support from Shell Brasil through the BIOIL project at the Oceanographic Institute of the University of São Paulo and the strategic importance of the support given by ANP through R&D levy regulation

Evaluation of the Electrical Resistivity Method for Shallow Gas-bearing Sediments Mapping in the ria of Saco do Mamanguá and Paraty-Mirim

A constante pressão exercida sobre os ambientes costeiros por parte das diferentes atividades humanas, em conjunto com novas necessidades relacionadas a estudos ambientais particularmente no que diz respeito à geração de gases de efeito estufa em sedimentos marinhos, têm gerado uma crescente demanda por conhecimento dos processos de geração, acúmulo e liberação desses gases. Dado esse desafio, torna-se necessário o desenvolvimento de novas aplicações de ferramentas geofísicas que forneçam informações além das propriedades acústicas do meio. Dessa forma, o presente trabalho tem como objetivo a avaliação do método eletrorresistivo no mapeamento estratigráfico e na detecção de feições geoelétricas indicativas da presença de gás em sedimentos rasos. Para isso, foram analisadas seções geoelétricas adquiridas no Saco do Mamanguá e na Enseada de Paraty-Mirim por meio da comparação com perfis sísmicos de alta resolução e da medição de resistividade em amostras de sedimentos. Nessas amostras foi simulada a presença de bolhas de gás e analisada a variação da resistividade com a diminuição da salinidade da água intersticial. Observou-se que um volume de bolhas de 0,20% do volume total é suficiente para aumentar em um fator de 1,8 a resistividade de uma amostra saturada em água do mar. Também verificou-se que a resistividade medida é dependente da resistividade da água intersticial. Medidas de susceptibilidade magnética mostraram, neste local, que a variação da resistividade não é devida a variações mineralógicas. As seções geoelétricas mostraram boa correlação com os perfis sísmicos até 9 m de profundidade a partir da superfície da água, onde ocorre uma camada geoelétrica de resistividade <0,35 Ohm.m. Nessa profundidade, observa-se uma interface entre camadas de resistividade elétrica possivelmente causada por uma mudança no teor de água ou de matéria orgânica. Essa interface coincide com o topo da turbidez acústica produzida pela presença de gás.Given the constant pressure exerted over coastal environments by different human activities, in addition to the need for more knowledge concerning environmental issues, particularly with respect to greenhouse gases generation in marine sediments have generated an increasing demand for more information regarding the generation, accumulation and seepage of these gases. Faced with this challenge, there is a necessity to develop new applications to geophysical tools that provide more information than merely the acoustic properties of the medium. This work aims to the evaluation of the geoelectric method in stratigraphic mapping and the detection of geoelectric features of shallow gas. For that purpose, geoelectric sections acquired in the ria of Saco do Mamanguá and Paraty-Mirim were compared with high resolution seismic profiles and resistivity measurements in sediment samples. Presence of gas bubbles inside the sedimentary matrix was simulated and resistivity variation analyzed against the decrease of interstitial water salinity. It was observed that a bubble volume of 0.20% of the total volume is sufficient to increase the resistivity by a factor of 1.8. It was also confirmed that the sample resistivity is dependent on interstitial water resistivity. Magnetic susceptibility measurements showed that resistivity variation is not due to mineralogical variations. The geoelectric sections showed good correlation with the seismic profiles up to 9 m depth from the water surface, where occurs a geoelectric layer of resistivity <0.35 Ohm.m. At this depth, it is observed an interface between resistivity layers possibly caused by a change in water or organic matter content. This interface matches the top of the acoustic turbidity produced by shallow gas occurrences

Physical characterization of Quaternary sediments on the Southern Brazilian Continental Shelf

Os estudos oceanográficos de escala regional demandam a coleta de grandes quantidades de dados, cujo tratamento e análise resultam dispendiosos em termos de tempo, recursos humanos e recursos materiais. No caso particular dos estudos que envolvem sistemas deposicionais, as análises químicas e granulométricas e as datações radiométricas são talvez os pontos mais críticos. Uma maneira de contornar essas dificuldades práticas é aproveitar as ferramentas que realizam medições automáticas não invasivas, correlacionando essas medidas indiretas feitas na totalidade das amostras com parâmetros descritivos mais completos obtidos sobre um pequeno conjunto amostral. Por meio dessa abordagem metodológica e utilizando um Multi Sensor Core Logger, foi medida a velocidade de propagação de ondas P (vp), a densidade &#947;, a resistividade elétrica e a susceptibilidade magnética de 16 testemunhos coletados na Plataforma Continental Sul Brasileira (PSB), entre 33,8°S e 25,1°S. Desse total foram escolhidos 9 testemunhos para medidas de flourescência de raios X e, deles, selecionado um subconjunto de 4 para análises granulométricas e datações por radiocarbono. Analisando os padrões de correlação entre as propriedades físicas e a distribuição granulométrica, foi possível estabelecer uma boa correspondência entre a vp e o conteúdo de areia das amostras e, em menor medida, com o diâmetro médio dos sedimentos. As curvas de variação de vp, densidade &#947; e susceptibilidade magnética permitiram realizar correlações litoestratigráficas e assim construir modelos de idade em testemunhos não datados por radiocarbono. Essas correlações, analisadas em conjunto com a correspondência entre a vp e o tamanho dos grãos, permitiram inferir que existe um padrão de granodecrescência ascendente ao longo do Holoceno nos depocentros da PSB, sem importar a latitude ou profundidade e também que as poucas exceções são produto de particularidades hidrodinâmicas locais. As razões de elementos maiores - Ti/Ca, Ti/K e Zr/Rb - foram utilizadas como proxies de aporte de material terrígeno, de intensidade do intemperismo nas áreas fonte e do tamanho dos grãos, respectivamente. A razão Zr/Rb mostrou boa correlação com o conteúdo de areia e com a vp, demonstrando o potencial de aplicabilidade como proxy para a proporção entre areia e finos. A interpretação da variação da razão Ti/Ca juntamente com a distribuição granulométrica permitiu inferir a ocorrência de mudanças na sedimentação, induzidas pela transgressão marinha do Holoceno médio, assim como a razão Ti/K mostrou boa concordância com o estabelecimento das condições climáticas modernas a partir aproximadamente de 3.000 anos cal AP, como documentado em outros trabalhos.Oceanographic studies on a regional scale demand the collection of large amounts of data, whose processing and analysis are time-consuming and costly regarding human and material resources. In the particular case of studies involving depositional systems, chemical and granulometric analyzes, and radiometric dating are perhaps the most critical issues. One way to overcome these practical difficulties is to take advantage of tools that perform non- destructive and automated measurements, correlating these indirect measurements performed on all samples with more complete descriptive parameters obtained on a small sample set. Through this methodological approach and using a Multi Sensor Core Logger, it was measured the propagation velocity of P waves (vp), the bulk density, the electrical resistivity, and the magnetic susceptibility of 16 cores collected in the Southern Brazilian Continental Shelf (SBS) between 33.8°S and 25.1°S. Nine cores were chosen for X-ray fluorescence measurements and a subset of four cores was selected for granulometric analyzes and radiocarbon dating. Analyzing the correlation patterns between the physical properties and the granulometric distribution, it was possible to establish a good correspondence between the vp and the sand content of the samples and, to a lesser extent, with the mean diameter of the sediments. Lithostratigraphic correlations were carried out through the variation curves of vp, &#947; density, and magnetic susceptibility. These correlations, analyzed together with the correspondence between vp and grain size, allowed us to infer that there is a fining upward pattern throughout the Holocene in the SBS depocenters, regardless of latitude or depth, and also that the few exceptions are owed to local hydrodynamic features. Ratios of major elements - Ti/Ca, Ti/K, and Zr/Rb - were used as proxies for the contribution of terrigenous material, the intensity of weathering in the source areas, and the grain size, respectively. The Zr/Rb ratio showed a good correlation with sand content and vp, showing its potential applicability as a proxy for the proportion between sand and fine sediments. The interpretation of the variation of the Ti/Ca ratio together with the granulometric distribution allowed inferring the occurrence of changes in sedimentation induced by the marine transgression of the middle Holocene, as well as the Ti/K ratio showed good agreement with the establishment of modern climatic conditions from approximately 3,000 cal years BP reported in other works

What could form a 35-km lineament of carbonate mounds on the ocean floor?

A multibeam survey carried out in November 2019 reveals a lineament of seafloor protrusions on the Santos Basin (SW Atlantic) continental slope. Shallow seismic data and seafloor composition suggest the lineament comprises a 35 km long arrangement of Cold-Water Coral (CWC) carbonate mounds, here named the Tupana Carbonate Ridge (TCR), defined in this contribution for the first time. Insights into genetic controlling factors of the TCR led to the analysis of single-channel seismic profiles that cross the lineament. Interpretation of deep seismic reflection data suggests a stratigraphic control provided by the lateral arrangement of coastal deposits formed during the protracted progradation that characterized a significant portion of Santos Basin's depositional history. Coastal sandstones encased in low-angle shelf-margin progradational wedges constitute a potential hydrocarbon reservoir supplied by the flow of fluids toward the surface through vertical structures. The ultimate source of fluids seems to be provided by halokinetic movements on a region depleted of salts known as the Albian Gap domain. This work questions the role of seafloor hydrodynamics in shaping the TCR morphology by eroding the carbonate mounds and forming elongated contourite systems.The authors acknowledge the crew and researchers of R/V Alpha Crucis, cruises Mudbelts II, and BIOIL I. This research was carried out in association with the ongoing R&D project registered as ANP 21012–0, “MARINE LIFE - BMC - OIL AND GAS SEEPS (BIOIL)” (Universidade de São Paulo/Shell Brasil/ ANP) - Avaliação da Biologia e Geoquímica de Exsudações de Óleo e Gás na Costa Sudeste do Brasil, sponsored by Shell Brasil under the ANP R&D levy as “Compromisso de Investimentos com Pesquisa e Desenvolvimento”. Acknowledgments are also due to the São Paulo Science Foundation (Research Grant 2010/06147–5). Deep-seismic lines were provided by the Brazilian Agency of Petroleum, Natural Gas, and Biofuels (ANP) exclusively for educational use. RBR thanks Shell Brasil for the Ph.D. scholarship under number FUSP3310. MM de M and PYGS acknowledge the Brazilian Council of Scientific Research (CNPq) for the Research Grants 300962/2018–5 and 301554/2019–6, respectively

The Alpha Crucis Carbonate Ridge (ACCR): Discovery of a giant ring-shaped carbonate complex on the SW Atlantic margin

Recently acquired bathymetric and high-resolution seismic data from the upper slope of Santos Basin, southern Brazilian margin, reveal a major geomorphological feature in the SW Atlantic that is interpreted as a carbonate ridge - the Alpha Crucis Carbonate Ridge (ACCR). The ACCR is the first megastructure of this type described on the SW Atlantic margin. The ~17 × 11-km-wide ring-shaped ACCR features tens of >100-m-high steep-sided carbonate mounds protruding from the surrounding seabed and flanked by elongated depressions. Comet-like marks downstream of the mound structures indicate that the area is presently influenced by the northward flow of the Intermediate Western Boundary Current (IWBC), a branch of the Subtropical Gyre that transports Antarctic Intermediate Water. Abundant carbonate sands and gravels cover the mounds and are overlain by a biologically significant community of living and dead ramified corals and associated invertebrates. The IWBC acts as a hydrodynamic factor that is responsible for both shaping the bottom and transporting coral larvae. We contend that the ACCR was formed by upward fluid flow along active sub-surface faults and fractures that formed by lateral extension generated by the ascending movement of salt diapirs at depth. The ACCR provides an important modern and accessible analogue for a seabed carbonate build-up related to sub-surface hydrocarbon systems.The authors are indebted to the crew and researchers who participated in the Jan-Feb 2019 survey aboard the R.V. Alpha Crucis for their constant support. Acknowledgements are also due to the São Paulo Science Foundation (FAPESP grants 2014/08266-2, 2015/17763-2, and 2016/22194-0). MMdeM acknowledges the Brazilian National Research Council (CNPq, grant 300962/2018-5). The partnership between MMdeM and the US was supported by FAPESP (grant 2017/50191-8 – SPRINT Program). The partnership between MMdeM and F.J.L. was supported by CNPq (grant 401041/2014-0). The authors gratefully acknowledge support from Shell Brasil through the BIOIL project at the Oceanographic Institute of the University of São Paulo and the strategic importance of the support given by ANP through R&D levy regulation. We thank Petrel-Schlumberger for providing academic licenses that enabled the seismic interpretation

Sedimentation in the adjacencies of a southwestern Atlantic giant carbonate ridge

Although carbonate mounds have been investigated for 100 years, few studies focus on the giant variety. The Alpha Crucis Carbonate Ridge (ACCR), a ~17 x 12-km ring-shaped ridge formed by hundreds of mounded structures, located between the 300 and 800-m isobaths and reaching a maximum height of 340 meters above the adjacent seafloor, is the first giant carbonate mounded feature described for the SW Atlantic margin. This study provides the first multiproxy approach to investigate sediments covering the ACCR and its adjacencies. Most of the area is located under the Intermediate Western Boundary Current (IWBC) flow, which carries the nutrient-rich Antarctic Intermediate Water (AAIW). Radiocarbon aging shows pronounced differences for the shallow layers (MIS3 for the top of the mounds and late Holocene for the adjacencies). Grain size data indicate the prevalence of sandy fractions on top of the mounds and muddy sediments in the adjacent areas. Fe/Ca and Ti/Ca proxies allowed for identifying mainly biogenic sedimentation in the area. However, the input of allochthonous terrigenous sediment is necessary for mound buildup, and values of Fe and Ti collected on the top of the mounds are significant. End-Members distributions and metal concentrations also allowed for recognition of distinct sources of sediment. εNd and Ln(Fe/K) indicated two primary terrigenous sources, the Precambrian rocks of the Brazilian shield (Cabo Frio end-member) and the multiple lithologies drained by the Rio de la Plata basin. Redox condition proxies indicated that the area is submitted to oxic conditions, probably reflecting the action of the IWBC. This work provides the first insight into an integrated grain-size and geochemical characterization of the Alpha Crucis Carbonate Ridge (southwestern Atlantic margin)

Seep hunting in the Santos Basin, Southwest Atlantic: sampling strategy and employed methods of the multidisciplinary cruise BIOIL 1

The continental margin off the southeastern Brazilian coast is punctuated by a series of geological-geomorphological features, such as subsurface saline diapirs and pockmarks at the seafloor interface, which evidence the abundant presence of oil and gas in the region. In several of these sites, hydrocarbons can be naturally released into the water column, areas are cold seep areas. These are marked by the presence of oil- and gas-dependent ecosystems, where specific organisms are able to fix carbon from hydrocarbon chemosynthesis. In addition, light hydrocarbon fluid flow through the sediment may build up authigenic carbonates that can be further colonized by cold-water corals, generating large carbonate mounds over geological time, normally positioned at the border of these pockmark features. The present work reports on a multidisciplinary oceanographic cruise carried out in the Santos Basin, SW Atlantic, to seek, map, and collect geological, chemical, and biological data from different deep-sea habitats. The cruise occurred in November 2019 on the R/V Alpha Crucis of the Oceanographic Institute of the University of São Paulo (IOUSP). We intended to discover and detail different geomorphological features, characterize free-living and symbiotic microorganisms, determine the chemosynthetic rates in relation to heterotrophic microbial production, and characterize the fauna and study their ecological and evolutionary links within and across ocean basins. All discoveries made during the cruise and their respective results will be presented separately in several papers that comprise this special volume