Eocene paleoceanographic evolution at 20–30°S in the Atlantic Ocean

Abstract

Oceanographic changes in Eocene bottom water have been traced with oxygen and carbon isotope values and gradients and benthic foraminiferal assemblages. The South Atlantic sections chosen for this study are located on an east-west traverse. Depositional depths range from middle bathyal through abyssal. The DSDP sites are located on the Walvis Ridge (Site 363), in the Angola Basin (Sites 522, 523), the Brazil Basin (Site 19, Hole 20C) and on the Rio Grande Rise (Site 357). In abyssal sites of the South Atlantic the Eocene bottom water environment can be divided into two distinct regimes — the early eocene and the late Eocene. The middle Eocene is transitional. Faunal distribution pattern indicate that source areas where bottom water formed were probably different between the two regimes. During the early Eocene warm, saline water formed possibly in the Tethyan Ocean “(east of Gibraltar)”. There is good evidence that the late Eocene bottom water of the South Atlantic, recorded at lower bathyal down to middle abyssal environments, was formed in southern high latitude regions. First invasion of cooler bottom water is recorded at upper abyssal depths around 46 Ma (earliest Zone P12; Site 19). From 40 Ma (late Zone P15) onwards this signal is recorded at all sites except middle bathyal depth Site 357. Selected faunal parameters allow reconstruction of bottom water oxygenation. Episodically increased abundance of epifaunal species indicate a “renewal” of the bottom water. Accompanying these faunal changes, δ 13C values of benthic foraminefera increase, whereas vertical Δ 13C benthic-planktonic foraminifera gradients decrease.