Equatorial Atlantic Gateway: origin, evolution, and paleoenvironment of the Equatorial Atlantic Gateway

International Ocean Discovery Program (IODP) Expedition 388 seeks to answer first-order questions about the tectonic, climatic, and biotic evolution of the Equatorial Atlantic Gateway (EAG). The scheduled drilling operations will target sequences of Late Cretaceous and Cenozoic sediments offshore northeast Brazil, just south of the theorized final opening point of the EAG. These sequences are accessible to conventional riserless drilling in the vicinity of the Pernambuco Plateau, part of the northeastern Brazilian continental shelf. This region was chosen to satisfy two key constraints: first, that some of the oldest oceanic crust of the equatorial Atlantic and overlying early postrift sediments are present at depths shallow enough to be recovered by riserless drilling, and second, Late Cretaceous and Paleogene sediments preserved on the Pernambuco Plateau are close enough to the continental margin and at shallow enough paleowater depths (<2000 m) to provide well-preserved organic biomarkers and calcareous microfossils for multiproxy studies of greenhouse climate states. New records in this region will allow us to address major questions in four key objectives: the early rift history of the equatorial Atlantic, the biogeochemistry of the restricted equatorial Atlantic, the long-term paleoceanography of the EAG, and the limits of tropical climates and ecosystems under conditions of extreme warmth. Tackling these major questions with new drilling in the EAG region will advance our understanding of the long-term interactions between tectonics, oceanography, ocean biogeochemistry, and climate and the functioning of tropical ecosystems and climate during intervals of extreme warmth. Schedule for Expedition 388 International Ocean Discovery Program (IODP) Expedition 388 is based on IODP drilling Proposal 864-Full2 and 864-Add (available at http://iodp.tamu.edu/scienceops/expeditions/equatorial_atlantic_gateway.html). Following evaluation by the IODP Scientific Advisory Structure, the expedition was scheduled for the research vessel (R/V) JOIDES Resolution, operating under contract with the JOIDES Resolution Science Operator (JRSO). At the time of publication of this Scientific Prospectus, the expedition is scheduled to start in Recife, Brazil, on 26 June 2020 and to end in Recife, Brazil, on 26 August. A total of 61 days will be available for the transit, drilling, coring, and downhole measurements described in this report (for the current detailed schedule, see http://iodp.tamu.edu/scienceops). Further details about the facilities aboard JOIDES Resolution can be found at http://iodp.tamu. edu/publicinfo/drillship.html. Introduction Background Recovery of extended sequences of Cretaceous and Paleogene sediments from the vicinity of the Pernambuco Plateau, northeast Brazil (Figures F1, F2), will provide a unique record of equatorial Atlantic tectonic, climatic, and biotic evolution stretching back an estimated 100 My or more. Extending back to the final phases of the breakup of Gondwana, recovery of new sedimentary sequences would provide unique records from the early rifting of a supercontinent through the final opening of the Equatorial Atlantic Gateway (EAG) and the transition to a fully modern system of ocean circulation. The new records would also allow us to constrain the relationships between tectonics and rifting processes and the associated sedimentary, biogeochemical, and paleoclimate phenomena in adjacent ocean basins. Sediments from the Pernambuco Plateau would also provide a very long time series of Late Cretaceous to modern tropical paleoclimates because of the relatively small latitudinal movement of this location since the Early Cretaceous (Müller et al., 2016). The Pernambuco Plateau is the only location yet identified along the tropical Brazilian continental margin where sediments from the Late Cretaceous supergreenhouse epochs are accessible within reach of riserless drilling. Constraining the tectonic and early rift evolution of the EAG is critical for testing geodynamic models of South Atlantic rifting and passive margin development and hypotheses relating the evolution of the EAG to interhemispheric oceanic exchange and global climate via oceanic heat redistribution (MacLeod et al., 2011; Voigt et al., 2013). The proposed drilling locations are situated at the northern end of an evolving rift system in which massive evaporitic sequences were deposited during the Aptian (Tedeschi et al., 2017) and whose termination is likely controlled by the evolution of the Atlantic rift system and widening of critical oceanic gateways. The final restriction to oceanic exchange through the EAG “transform pinch point” is thought to be the Ghanaian Ridge at the Romanche fracture zone (FZ) (Granot and Dyment, 2015; Heine et al., 2013) (Figure F3) north of the Pernambuco Plateau. During the Early Cretaceous, this topographic high isolated the large deep basins of the South Atlantic from Central/equatorial Atlantic waters. Here, this pinch point will be regarded as the central point of the EAG, with the basins north and west of this referred to as the “equatorial Atlantic” and south of this— including the Pernambuco Plateau—as the “equatorial South Atlantic. ” Past Ocean Drilling Program (ODP) legs recovered organic-rich Late Cretaceous successions from both the eastern (Cote d'Ivoire-Ghana transform continental margin; ODP Leg 159) and western (Demerara Rise; ODP Leg 207) margins of the equatorial Atlantic but only north of the EAG (Figure F4). As a result, little is known about the Late Cretaceous–Paleogene environments in the equatorial South Atlantic or about the history of water mass exchange between the South and equatorial Atlantic. Expedition 388 aims to address this data gap with new ocean drilling directly in the equatorial South Atlantic. Hydrocarbon exploration in the South Atlantic has discovered major Early Cretaceous depocenters of organic carbon and evaporite sequences, typically capped by thick postrift successions (Davison, 2007). However, apart from Deep Sea Drilling Project (DSDP) Sites 356 (São Paulo Plateau; The Shipboard Scientific Party, 1977) and 530 (Angola Basin; Shipboard Scientific Party, 1984), there has been no direct recovery of organic-rich Cretaceous sediments during DSDP, ODP, or IODP operations in the South Atlantic. Key questions thus remain about the nature of organic carbon burial, biogeochemistry, and oceanography of the Cretaceous South Atlantic and its similarity or differences from the equatorial Atlantic system. There is great potential for records of these warm and lowoxygen oceanic systems to provide a natural laboratory to constrain the impact of extreme conditions on marine ecosystems, nutrient cycling, and climate feedbacks (Jenkyns, 2010; Tedeschi et al., 2017). Expedition 388 also seeks to recover long successions of clayrich hemipelagic sediments from the northeast Brazil continental margin to extract high-quality climate proxy data to determine tropical climate evolution over the past 100 My. New scientific ocean drilling in the vicinity of the Pernambuco Plateau will recover key new records of South Atlantic basin evolution and the tropical climate system to address these questions.