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How did the breakup of Gondwana change the Atlantic Ocean during the Cretaceous Period?

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Cretaceous paleogeography — Cretaceous Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cretaceous Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The breakup of the supercontinent Gondwana significantly reshaped the Atlantic Ocean during the Cretaceous Period. As Africa and South America separated, the South Atlantic Ocean began to open approximately 140 million years ago.1

Initially, these developing basins were characterized by restricted circulation, leading to the formation of evaporite salt deposits and oxygen-poor, anoxic environments that produced organic-rich black shales. By the Late Cretaceous, the final land connection between South America and Africa—located between present-day Brazil and Nigeria—was severed. This event allowed the South Atlantic to join with the widening North Atlantic, establishing the first open oceanic circulation between the two regions. Consequently, open marine conditions became the prevailing state in the Atlantic basins from the mid-Cretaceous onward.23