Palaeoceanographic changes during the late Albian-early Turonian in the Sergipe-Alagoas Basin (Northeast Brazil) and their link with global events

Abstract

The mid-Cretaceous was characterized by extreme greenhouse conditions, long-term sea level rise, and tectonic processes that influenced the dynamics of oceanic systems. The global carbon cycle was affected by major perturbations during Oceanic Anoxic Events (OAEs). We present new integrated geochemical, magnetic, and mineralogical data from a core drilled through late Albian to early Turonian sediments in the Sergipe-Alagoas Basin, off northeastern Brazil, which includes one of the few records of the OAE2 from the South Atlantic Ocean. The obtained results show a major change in sedimentation in the middle Cenomanian when mixed carbonate and siliciclastic deposits were replaced by chalk. This transition reflects a major change in palaeoceanographic conditions toward a more stable circulation mode, with better connection between the different oceanic basins, persistent currents, and bottom ventilation. Cyclicity in the ln(K/Al) record indicates insolation-driven fluctuations in aridity and humidity. A new age model, based on the stable 405 kyr-long eccentricity cycles, places the onset of the OAE2 carbon isotope excursion at ∼94.74 Ma. Sedimentation during the OAE2 seemed to be mainly controlled by long-term processes and local effects rather than global perturbations. Variations in ln(Sr/Ca) indicate increased primary productivity persisting for two long eccentricity cycles, while transient reducing conditions, inferred from loss in magnetic susceptibility compared to Fe, persisted across the earliest long eccentricity maximum phase, at least.