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 (δ13COM) 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 shallowwater marine settings. Comparison of the δ13COM 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 1a was dominated by deposition of coarse immature clastic 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 sealevel rise, in the Sergipe-Alagoas Basin and deposition of major South Atlantic evaporites present in the Espírito Santo–Campos–Santos Basins (Itaúnas, 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 Espírito 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