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. The breakup of Gondwana saw gradual climatic shifts, including an Aptian cooling tied to the South Atlantic’s opening and later high greenhouse temperatures due to reduced carbon burial, reflecting changes in rifting and environmental transitions, as revealed through a magneto-cyclostratigraphic age model from the Aptian São Luís-Grajaú Basin.