Beyond the Nadir: Early Cretaceous-Cenozoic archives of the northern Equatorial Atlantic Gateway

This proposal seeks to build a stratigraphic framework from seismic reflection data to prepare for a future ocean drilling proposal in West Africa. This drilling proposal will aim to understand (1) the timing of separation of South America and Africa, forming the 'Equatorial Atlantic Gateway' when the ocean basins of the North Atlantic and South Atlantic were connected for the first time; (2) ocean conditions (temperature, chemistry, oxygen content) during extreme warm periods in Earth history; and (3) to understand the environmental consequences of a potential meteorite impact ~65 million years ago.

The first objective, to understand the opening of the Equatorial Atlantic Gateway (EAG), is important as this event had a major influence on Earth's climate and weather patterns, because of changing ocean circulation conditions. However, there are major uncertainties in our understanding of this event, particularly when it formed, as it occurred some time during the 'Cretaceous Normal Superchron' (CNR) - a period of unusual stability in Earth's magnetic field. Magnetic field reversals are important for dating the age of ocean crust, but this is not possible during the CNR. Our seismic reflection data, coupled with biostratigraphic data from exploration boreholes, will provide new constraints on the tectonic events that led to the EAG opening. It will also highlight areas where we can drill and directly sample and date the sediments that record these events, as well as the oldest ocean crust that records the separation of the continents. It will also provide new insights into the sequence of tectonic events that occurred during the opening of the EAG and the long term deepening of the gateway through time.

The second objective is to understand the state of the Earth's oceans during the Cretaceous period and Cenozoic. During the Cretaceous period in particular, the global climate and the ocean of the Atlantic was much warmer than in the present day, and it was characterised by periods of intense anoxia (low oxygen levels) and burial of organic carbon. These events, called Oceanic Anoxic Events (OAEs), appear to have been triggered by episodes of intense greenhouse gas emissions from massive volcanic eruptions and lasted hundreds of thousands to perhaps a million years. Although they've been drilled many times, we don't have good information on how extensive they were - how much of the water column was affected for example - or on feedback processes recorded in these sediments. We plan to identify sites for drilling along a depth transect, and where seismic data indicates that they are exceptionally thick, to obtain important new information on these events.

Finally, we aim to build on an existing IODP proposal (1004-APL) to understand the consequences of a meteorite impact at the end of the Cretaceous period. The Nadir Crater formed at approximately the same time as the dinosaur-killing Chicxulub Crater in Mexico and may be related to that event. The crater has been identified on 2D seismic data but newly acquired 3D data will illuminate the crater morphology and character much more clearly, allowing us to build more robust models of the environmental consequences of this event (landslides, ejecta, tsunamis). This will also highlight alternative (for proposal 1004-APL) or additional (for the new proposal) drill sites to test the impact crater hypothesis and understand its consequences. Drilling the crater will be necessary to test whether this feature was indeed formed by an impact crater and, if so, the precise age of the crater. This will allow us to date it precisely and to test whether the Chicxulub event was actually a binary system (two asteroids) or part of a longer lived 'impact cluster' over a course of hundreds of thousands or several million years.