Oceanographic exchanges between the Southern and Northern Atlantic during the Cenozoic inferred from mixed contourite-turbidite systems in the Brazilian Equatorial Margin

Abstract

The 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.