Upper Quaternary Biostratigraphy and Formation Environmentsof the Southwestern Atlantic Core ACB-17-1447 Inferredfrom Micropaleontological Data

摘要

The foraminiferal (planktonic and benthic) and nannofosssil assemblages have been analyzed in the sediments of Core ACB-17-1447 taken from the South America continental slope north of the Rio Grande Rise piedmont during Cruise 17 of the R/V Akademik Sergey Vavilov. The core section is largely composed of carbonate and marly hemipelagic mud. The Quaternary age of the host sediments is evident from the occurrence of the planktonic foraminiferal index species Globorotalia truncatulinoides. Based on the nanno-fossilassemblages, the core sediments are attributed to the upper Pleistocene-Holocene. They contain abundant reworked Pliocene, Miocene, and Paleogene taxa transported from the slopes of the underwater Rio Grande Rise. The paleotemperature analysis of the planktonic foraminifers provided data for constructing the temperature curve that demonstrates two warm peaks. During the first warm period (Interval of 7-9 cm), the surface water temperature was as high as 26°C (Holocene optimum), which exceeds by 3-4°C its present-day values and implies the more intense warm Brazil Current. The earlier warm peak with temperatures up to 24°C recorded in the upper Pleistocene sediments (Interval of 69-71 cm) most likely reflects the 3rd oxygen-isotope stage (MIS 3), which corresponds to the interstadial phase of the last glaciation (30-40 ka ago). Based on the abundances, taxonomic diversity, and proportions of the characteristic species of benthic foraminifers, the Core ASV-17-1447 section is divided into six intervals correlated with the marine isotopic stages defined by both the planktonic foraminifers and climatic changes evident from the variations in the bottom water circulation along the southwestern slope of the Brazilian Basin during the Late Quaternary.