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.
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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.
© 2019, The Author(s).
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