Middle Eocene–Early Pliocene Subantarctic planktic foraminiferal biostratigraphy of Site 1090, Agulhas Ridge

Abstract

The analysis of planktic foraminiferal assemblages from Site 1090 (ODP Leg 177), located in the central part of the Subantarctic Zone south of South Africa, provided a geochronology of a 330-m-thick sequence spanning the Middle Eocene to Early Pliocene. A sequence of discrete bioevents enables the calibration of the Antarctic Paleogene (AP) Zonation with lower latitude biozonal schemes for the Middle–Late Eocene interval. In spite of the poor recovery of planktic foraminiferal assemblages, a correlation with the lower latitude standard planktic foraminiferal zonations has been attempted for the whole surveyed interval. Identified bioevents have been tentatively calibrated to the geomagnetic polarity time scale following the biochronology of Berggren et al. (1995). Besides planktic foraminiferal bioevents, the disappearance of the benthic foraminifera Nuttallides truempyi has been used to approximate the Middle/Late Eocene boundary. A hiatus of at least 11.7 Myr occurs between ∼78 and ∼71 m composite depth extending from the Early Miocene to the latest Miocene–Early Pliocene. Middle Eocene assemblages exhibit a temperate affinity, while the loss of several planktic foraminiferal species by late Middle to early Late Eocene time reflects cooling. During the Late Eocene–Oligocene intense dissolution caused impoverishment of planktic foraminiferal assemblages possibly following the emplacement of cold, corrosive bottom waters. Two warming peaks are, however, observed: the late Middle Eocene is marked by the invasion of the warmer water Acarinina spinuloinflata and Hantkenina alabamensis at 40.5 Ma, while the middle Late Eocene experienced the immigration of some globigerinathekids including Globigerinatheka luterbacheri and Globigerinatheka cf. semiinvoluta at 34.3 Ma. A more continuous record is observed for the Early Miocene and the Late Miocene–Early Pliocene where planktic foraminiferal assemblages show a distinct affinity with southern mid- to high-latitude faunas.