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

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. Introduction Sediment deposition and erosion along continental margins may occur under the influence of several processes acting with down-slope or along-slope directions (Faugères et al., 1999; Rebesco and Camerlenghi, 2008; Meiburg and Kneller, 2010). Gravity-driven processes, such as turbidity currents or landslides, relate with the first group, while bottom-current processes, with the latter (Rebesco et al., 2014). The relation between down- and along-slope processes is influenced by the oceanic circulation, and the resulting sedimentary deposits have been characterized using seismic reflection data, both 2D and 3D (Faugères et al., 1999; Viana, 2002; Rebesco and Stow, 2002; Hernández-Molina et al., 2009; Rebesco, 2016). Sediments accumulated under the sole action of bottom currents form contourite depositional systems (CDS; see Hernández-Molina et al., 2006), whereas when bottom currents interact, synchronously or asynchronously, with gravity-driven flows, the resulting deposits are named mixed contourite-turbidite systems (mCTS; see Faugères et al., 1999; Rebesco et al., 2014, Rodrigues et al., 2022). CDS and mCTS have been documented in many continental margin settings across the world (Llave et al., 2007; Hernández-Molina et al., 2010; Rebesco et al., 2014; Gruetzner et al., 2016; Sansom, 2018; Miramontes et al., 2016, Miramontes et al., 2019; Pandolpho et al., 2021), particularly in the Atlantic Ocean, with examples from the North Atlantic (Heezen et al., 1966; Hollister and Heezen, 1972; Rebesco et al., 2013), Nova Scotia margin (Campbell and Mosher, 2016; Rodrigues et al., 2022), Iberian margin (Alves et al., 2003), Brazil margin (Gomes and Viana, 2002; Viana, 2002; Maselli et al., 2019; Maestrelli et al., 2020), Southern São Paulo Plateau (Borisov et al., 2013), Uruguayan Basin (Hernández-Molina et al., 2016), Argentine Basin (Von Lom-Keil et al., 2002; Hernández-Molina et al., 2009, Hernández-Molina et al., 2010), Vema Channel (Faugères et al., 1998, Faugères et al., 2002) and Scotia Sea (García et al., 2016). Problematically, modern and ancient contourite and mixed contourite-turbidite systems along the Brazilian Equatorial Margin (BEM) are still poorly known (Tallobre et al., 2016; Almeida et al., 2019). The recent availability of 2D and 3D seismic reflection data acquired for hydrocarbon exploration has made possible to better understand the sedimentary processes governing the evolution of the BEM (De Almeida et al., 2015; Almeida et al., 2019; Jovane et al., 2016; Maestrelli et al., 2020). Here, the recognition and characterization of CDS and mCTS systems are of critical importance as they may help to understand the palaeoceanographic evolution of the basin during the Cenozoic, and shed light on the oceanographic connections between the Northern and Southern hemispheres. In detail, the North Brazil Current (NBC) and the Deep Western Boundary Current (DWBC) contribute to the Atlantic Meridional Overturning Circulation (AMOC) in the Equatorial Atlantic and have influenced the evolution of the BEM since its formation (da Silveira et al., 1994; Wilson et al., 2011; Zhang et al., 2011). The AMOC is essential for the Earth's climate and is related to several climatic, depositional and oceanographic changes around the world (Guihou et al., 2011; Zhang et al., 2015; Mulitza et al., 2017; Lynch-Stieglitz, 2017). Consequently, quantifying the onset and evolution of deep-water depositional systems influenced by the NBC and the DWBC may shed light in longer-term changes in the AMOC, and thus provide new insights on Earth's climate history. In this study, we mapped and characterized the contourite deposits and associated turbidite channels in the Ceará Basin, Brazil Equatorial Margin, through multiple high-resolution seismic reflection datasets tied to boreholes. The BEM has been developing since the early stages of the Cenozoic, and could provide a better comprehension of equatorial paleoceanography, including the onset and evolution of the deep-water bottom-current circulation in the BEM. Our study documents new contourite and mixed contourite-turbidite systems in the Equatorial Atlantic and emphasizes the importance of the BEM in recording the oceanographic changes occurred during the Cenozoic whilst contributing to the comprehension of how those systems may change and evolve through time.