Nanofósseis calcários do Albiano-Cenomaniano do DSDP SITE 364 (Bacia de Kwanza - Angola): bioestratigrafia e implicações paleoceanográficas para o Atlântico Sul

Abstract

The Albian–Cenomanian interval was a period of extreme climatic conditions related to Greenhouse conditions, in this time interval significant changes in paleogeography and paleoceanography of the oceans that caused evolutions in marine ecosystems on a global scale are also registered. In this study, the assemblage of calcareous nanofossils from samples recovered at Site 364 (Kwanza Basin, Angola), drilled by the Deep Sea Drilling Project, was analyzed. An assemblage with subtropical-tropical affinity was recovered, which provides an excellent biostratigraphic record of the upper Albian–Cenomanian interval, and indicates a shallow water connection between the southern and central regions of the Atlantic Ocean, at least as far as Angola. Data show that sedimentation was predominantly limestone and pelagic during the Albian–Cenomanian interval, except in the basal section of Site 364 where black shale deposits occur interspersed with dolomitic limestone and marls. The dissolution of calcium carbonate is a significant process recorded at Site 364 and was observed in most samples, its effects are selective on the assemblage and it was not possible to verify the relationship between the black shale deposition layers and the high fertility of calcareous nanofossils . Despite dissolution, the recovered assemblage is relatively well diversified and taxa susceptible to dissolution are observed but not very abundant. Paleontological and geochemical evidence suggests an increase in surface water fertility/productivity in the intervals where dissolution effects decrease.