Tectonic controls on nitrogen cycling and ocean ventilation dynamics in the Late Cretaceous equatorial Atlantic

The Late Cretaceous opening of the Equatorial Atlantic Gateway (EAG) fundamentally reshaped oceanic circulation and biogeochemical cycles. Extensive studies have focused on nitrogen dynamics during the Cenomanian-Turonian Oceanic Anoxic Event (OAE-2), showing globally reduced nitrogen isotope (δ¹⁵N) values (<0 ‰) under ammonium-dominated conditions. However, the post-OAE-2 evolution of nitrogen cycling during the progressive deepening and widening of the EAG remains enigmatic. Here, we investigate changes in nutrient cycling and paleoceanographic conditions in the equatorial Atlantic Ocean during the Late Cretaceous to early Danian (∼102–65 Ma) based on newly generated bulk-sediment δ¹⁵N from ODP Site 1258 (western equatorial Atlantic), integrated with existing data from other Demerara Rise sites. Our dataset reveals a two-phase nitrogen-cycle transition directly tied to EAG's tectonic development. The first phase, recorded in organic-rich sediments of Cenomanian-Santonian age (Unit IV), exhibit low δ¹⁵N values (ca. −2 ‰), reflecting an ammonium-dominated nitrogen cycle in a restricted oceanic environment. The exceptionally high total organic carbon (TOC) content (5–15 %) and C_org/N ratios (>30) indicate intense ammonium recycling within a nutrient trap, which led to high productivity and efficient organic matter preservation under euxinic conditions. The second phase is recorded by a stepwise increase of δ¹⁵N from +4 ‰ to +9 ‰ during the Campanian–Maastrichtian stages, marking the shift to a nitrate-dominated nitrogen cycle driven by enhanced deep-water ventilation and partial denitrification in expanding oxygen-minimum zones. This shift correlates with declining TOC (0.08–0.25 %) and C_org/N ratios (<5), indicating weakened nutrient retention and strengthened regional vertical mixing following opening of the EAG. The observed nitrogen-cycle reorganization correlates with a critical tectonic threshold (Δ_lat > 6.1 ± 0.1° and Δ_long > 13.9 ± 1.5°, i.e., the cumulative latitudinal and longitudinal rift extension in the equatorial Atlantic) derived from plate reconstructions, beyond which sustained deep-water circulation initiated deep-ocean ventilation. Our findings establish the EAG’s tectonic pacing as the key driver of a fundamental transition to a nitrate-dominated nitrogen cycle in the nascent Atlantic Ocean, resolving the long-standing enigma of post-OAE-2 nitrogen-cycle evolution and mechanistically linking Late Cretaceous deep-ocean oxygenation to the emergence of modern OMZ-driven nitrogen cycling.