New insights into the early opening of the Equatorial Atlantic Gateway revealed through a magneto-cyclostratigraphy framework from the Brazilian Equatorial Margin

The opening of the Equatorial Atlantic Gateway reshaped Earth’s ocean–atmosphere system. Rifting between South America and Africa drove a shift from continental–lacustrine to evaporitic and eventually marine environments, though the timing and duration of these transitions remain debated. Here we present a high-resolution magneto–cyclostratigraphic age model from the São Luís–Grajaú Basin (Brazilian Equatorial Margin), based on two geomagnetic reversals and Milankovitch-band cycles, yielding a 405-kyr astronomically tuned timescale. This framework enables: (i) correlation of Tethyan events with the first marine incursion in the Brazilian margin at 117.2 Ma; (ii) precise dating of the M-1r subchron at 116.93 ± 0.14 Ma and subchron “2” at 116.28 ± 0.14 Ma; and (iii) constraint of Equatorial Belt humidification onset to ~116.4 Ma, ~800 kyr after the marine incursion. These results provide a robust chronostratigraphic reference for the early equatorial Atlantic opening and inform Aptian paleoclimate responses during Gondwana breakup.