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

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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.

Introduction

ODP Site 1090 is located in the central part of the present-day Subantarctic Zone (Fig. 1). The water depth of Site 1090 (3703 m) places it near the boundary between the North Atlantic Deep Water and the underlying Circumpolar Deep Water (see Shipboard Scientific Party, 1999). Site 1090 represents an important tie-point in correlating lower latitude sequences to Antarctic ones and in calibrating the Southern Ocean Paleogene–Neogene biostratigraphic data to the geomagnetic polarity time scale. A 330-m-thick section previously dated as Middle Eocene–Early Miocene (see Shipboard Scientific Party, 1999) was recovered in multiple cores (Holes 1090B, D, and E; see Fig. 2) encompassing a time interval crucial to the long-term evolution of the Antarctic ocean-cryosphere system during the Cenozoic. In particular, the presence of a well-preserved Eocene–Oligocene transition permits a detailed study of the time interval when the emplacement of a major Antarctic ice-sheet took place.

In this paper we describe and interpret the distribution of planktic foraminiferal assemblages from Site 1090, calibrating the identified bioevents to the available magnetostratigraphic data (see Shipboard Scientific Party, 1999), and tentatively correlating them to the Berggren et al. (1995) time scale and biochronology. A single benthic foraminiferal event has been used to corroborate the biostratigraphic interpretation based on planktic foraminifera across the Middle–Late Eocene transition.

Section snippets

Materials and methods

A total of 230 samples have been studied from an interval spanning from the bottom of Core 1090B to 71.55 mcd (metres composite depth). Overall recovery at Site 1090 was 93.4% (Shipboard Scientific Party, 1999). A complete splice covers approximately the upper 242 m of the section. Below the splice in Hole 1090B, recovery averaged 92.3%.

For this study one sample per section (ca. 1.5 m) has been studied, for most of the surveyed interval. Forty-cc samples were soaked in tap water and washed

Results and discussion

Relatively rich planktic foraminiferal assemblages occur from the bottom of Core 1090B to 360 mcd where preservation varies from moderate to good. From here upward, the record of planktic foraminifera gradually becomes less continuous. Above 330 mcd assemblages are steadily rare to scattered and moderately to poorly preserved. Several extended barren intervals occur from 330 mcd to 90 mcd. This trend clearly reflects the sharp lithological change occurring at ca. 340 mcd where mud-bearing

Summary

The analysis of planktic foraminiferal assemblages from the Middle Eocene–Early Pliocene ODP Site 1090 allowed a biostratigraphic subdivision based on the AP Zonation of Stott and Kennett (1990) for the Middle–Late Eocene. A calibration to the tropical–subtropical P zonations of Blow (1969) and Berggren et al. (1995) is tentatively made for the whole surveyed interval.

The age/depth curve based on our magnetobiostratigraphic interpretation of polarity data given by Shipboard Scientific Party

Acknowledgements

We thank Brian Huber and an anonymous referee for their critical review that greatly improved the manuscript. Thanks are due to S. Iaccarino, I. Premoli Silva, and J.P. Kennett for their opinions on the identification of Globorotalia sphericomiozea. This paper benefitted from funds COFIN ’99 to S. Monechi. SG is grateful to the Alfred Wegener Institute for financial support during his stay in Bremerhaven.

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