Aptian carbon-isotope record from the Sergipe-Alagoas Basin: New insights into oceanic anoxic event 1a and the timing of seawater entry into the South Atlantic

Carbon-isotope ratios derived from organic matter (delta C-13(OM)) are presented from Petrobras Well D, onshore Brazil, in the Sergipe-Alagoas Basin. These new data provide a unique view of Aptian palaeoenvironmental history of this low-latitude equatorial region and record the transition from continental to shallow-water marine settings. Comparison of the delta C-13(OM) record with that from the Campos-Santos Basins and an agecalibrated Tethyan reference curve allows construction of a detailed chemostratigraphic framework for Petrobras Well D. The evidence suggests that the interval represented by OAE 1 a was dominated by deposition of coarse immature elastic sediments and shales deposited in predominantly continental environments with some local marine influence. The refined stratigraphy of Well D suggests synchroneity between a dramatic change in local water chemistry, interpreted as due to the entrance of seawater as consequence of a eustatic sea-level rise, in the Sergipe-Alagoas Basin and deposition of major South Atlantic evaporites present in the Espirito Santo-Campos-Santos Basins (Itaimas, Retiro and Ariri Formations). Furthermore, the more soluble salts in the Ibura Member (deposited elsewhere in the onshore part of the Sergipe-Alagoas Basin) likely postdate the major South Atlantic evaporites present in the Espirito Santo, Campos and Santos Basins. Evaporite deposition in these three southern Brazilian basins would have caused a significant decrease of the global sulphate inventory, which was followed by subsequent deposition of the salts of the Ibura Formation in the onshore part of the Sergipe-Alagoas Basin. Drawdown within these different basins would have led to a eustatic rise in sea level in the world ocean followed by a eustatic fall as refilling took place.