Late neogene planktonic foraminiferal biostratigraphy of the Rio Grande Rise (South Atlantic)

Abstract

Late Neogene planktonic foraminiferal biostratigraphy of eight piston cores atop the Rio Grande Rise (2100–2300 m water depth) reveals the presence of Late Pleistocene, and intra-Pliocene, erosional unconformities. These unconformities span an aggregate time interval ranging from 1 to 12 m.y. The biostratigraphic-biochronologic framework suggests Late Neogene (pre-Pleistocene) average sedimentation rates of ca. 0.3 cm/1000 years and a continuous stratigraphic record over the interval 7–3 Ma and 14–12 Ma. Phylogenetic relationships within the Globorotalia miozea-conoidea and G. puncticulata-crassaformis lineages allow a refined 3- and 5-fold zonation of the Late Miocene (11–5 Ma) and Early Pliocene (5–3 Ma), respectively, of the mid-latitude Atlantic. The virtually simultaneous evolutionary appearance of G. conomiozea and G. mediterranea (from G. conoidea and G. miozea, respectively) at ca. 6.5 Ma and 6.3 Ma, respectively, and a brief (< 0.5 m.y.) overlap in the ranges of G. margaritae and Globoquadrina dehiscens corroborate recent findings elsewhere. The former events serve to identify the approximate position of the Tortonian/Messinian boundary, the latter to define a new, short zone at the top of the Miocene. The Miocene/Pliocene boundary is recognized in three piston cores and Site 357 on the basis of several multiple reinforcing criteria of regional (i.a., LAD of Globoquadrina dehiscens and Globorotalia mediterranea) and local (i.a., FAD of Globorotalia scitula and Globigerinoides conglobatus) significance.