EVALUATING THE EVIDENCE ON THE OPENING OF THE EQUATORIAL ATLANTIC GATEWAY AND ITS GLOBAL IMPACT

Since the 1980's there have been numerous studies attempting to evaluate the wealth of biostratigraphic and biogeographic data to confirm or contradict the dating and origin of the first open marine episode in the northern South Atlantic. Though still deemed as controversial, the opening of the Equatorial Atlantic Gateway in the late Early Cretaceous was a major event in the evolution of the Atlantic Ocean which also had a profound global impact on the ecosystems promoting evolutionary diversification and biogeographic differentiation. Macro- and microfossils preserved in the oldest (lowermost) marine beds along the South Atlantic continental margin of Brazil are of undoubted Aptian age. The oldest ammonite assemblages of the Sergipe Basin in northeastern Brazil are mainly composed of representatives of the early douvilleiceratid lineage Epicheloniceras–Eodouvilleiceras, with the typically Tethyan species Epicheloniceras cf. martinioides, Vectisites simplex and V. cf. cruzi characterizing the middle Aptian. The presence of Dufrenoyia justinae at some localities may even suggest a late early Aptian age. The diverse planktonic foraminiferal assemblages from the middle–upper Aptian of Sergipe, such as Globigerinelloides aptiensis, G. barri, G. blowi, G. ferreolensis, G. maridalensis, Hedbergella labocaensis, H. maslakovae, H. semielongata and H. similis, also show characteristic Tethyan affinities. In the Almada Basin, south of Sergipe, the co-occurrence of Globigerinelloides aptiensis, G. ferreolensis, Gubkinella graysonensis, Hedbergella infracretacea and H. sigali indicates the lower part of the G. ferreolensis Zone (mid-Aptian), The combined macro- and microfossil evidence suggests that this area of the northern South Atlantic had at least surface-water exchange with the low-latitude central Atlantic–western Tethyan region, primarily through the incipient Equatorial Atlantic Gateway, thus pushing back the first central–South Atlantic connection at least to mid-Aptian times (~115–118 Ma). A remarkable correlation is the widespread development of oceanic "dysoxic-anoxic" conditions (OAE 1b, the "Paquier" event) across the Aptian–Albian transition, and the onset of carbonate plataform growth in the entire Tethyan region, the Arabian Gulf and the western Pacific. The evolutionary implications of planktonic foraminifera will be discussed.