Deep pockmarks as natural sediment traps: a case study from southern Santos Basin (SW Atlantic upper slope)

This study examines the role of deep pockmarks in acting as natural sediment traps. Multibeam bathymetry, single-channel seismic and sediment samples data were used for describing the morphology of pockmarks as well as the nature of sediments inside and outside these depressed features, in an area of Santos Basin (SW Atlantic upper slope), dominated by the strong flow of Brazil Current. Results show that the grain size and chemical composition of sediments inside pockmarks are distinct from the outside. Also, radiocarbon dating shows that Holocene ages are found only in samples located inside the pockmarks. Combination of sedimentological, geochemical and geochronological data allowed to recognise that deep pockmarks might present distinct sediment deposition processes when compared with those of shallow pockmarks, in which turbulence impedes sediment deposition, as reported in the literature.The authors acknowledge the crew and researchers of the two surveys held in 2016 and 2017, onboard R.V. Alpha Crucis. The São Paulo Science Foundation (FAPESP, grants 2014/08266-2, 2016/22194-7 and 2015/17763-2) funded this work. Partnerships between MM.de M., U.S. and F.L-S., are funded by FAPESP (grant 2017/50191-8) and the Brazilian National Research Council (CNPq, grant 401041/2014-0), respectively. M.M.de M. acknowledges CNPq for the research grants 303132/2014-0 and 300962/2018-5. This study was financedin partby the Coordenação deAperfeiçoamento dePessoal de Nível Superior – Brasil (CAPES) – Finance Code 001 (R.B.R. MSc. Scholarship)

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

Macrobenthic assemblages across deep-sea pockmarks and carbonate mounds at Santos Basin, SW Atlantic

The SE Brazilian continental margin is rich in geomorphological features that create different seascapes, where diverse benthic communities thrive. The seafloor is composed of a mixture of pockmarks of different sizes and shapes and tall carbonate mounds that may form extensive chains. Mounds are colonized by deep-water corals which are the main responsible organisms promoting growth over geological time. Depressions and mounds affect the benthic ecosystem in multiple ways owing to water flow, sedimentation rates and food availability. This paper presents new data on macrofaunal composition and community structure associated with deep-sea coral habitats and pockmark areas along the upper continental slope of Santos Basin. Ten sites were sampled using a 0.25 m2 box corer on board R/V Alpha-Crucis, totaling 27 sediment samples. A total of 182 taxa were found, including new records for the Southwestern Atlantic, as well as several potential new species to science. In general, we observed an association of the macrobenthic fauna with the geomorphology of the area and the most important variables, mainly substrate composition and water flux. The abundance, taxonomic composition and also the feeding modes varied across the deep-water coral sites and pockmarks, reflecting in a mosaic of benthic habitats. As deep-sea corals and pockmarks are extremely sensitive to anthropogenic influence and natural shifts, in addition to the economic value associated to fisheries, this study provides baseline information on these special habitats in Santos Basin that can be used for future research, monitoring activities, and conservation strategies