Palaeoceanographical changes recorded by Cenozoic deep-sea ostracod assemblages from the South Atlantic and the Southern Ocean (ODP Sites 1087 and 1088)

Cenozoic palaeoceanography of the SE Atlantic and Southern Oceans has been investigated using Late Eocene/Early Oligocene to Quaternary ostracod assem blages from 49 samples of ODP Sites 1087 and 1088. Although the overall abu ndance of ostracods is relatively low (means of 17 and 49 specimens per sam ple at Sites 1087 and 1088, respectively) and there is an apparently high l evel of endemism (ranging from 50% to 80% at Sites 1087 and 1088), three ma jor changes in the faunal assemblages are identified at Site 1087 (denoted A, B and C) and two at Site 1088 (denoted B' and C'). The assemblage bounda ries, detected on the basis of stepwise changes in the abundance, diversity , dominance. endemism, faunal turnover and relative abundance of common tax a, coincide broadly with previously identified, ostracod-based palaeoceanog raphical 'events' discussed by Benson and co-workers over the last two deca des. The data do not extend sufficiently far back to record the initiation of Assemblage A, but the faunal change between Assemblages A and B, marked by a decline in abundance, species diversity and faunal turnover, occurs wi thin the Middle Miocene (NN5-6). It coincides with a previously documented palaeoceanographical 'event' at 16-14 Ma which, we suggest, may be related to the initiation of North Atlantic Deep Water (NADW) production and/or an expansion of the East Antarctic ice sheet. Assemblage B' is subdivided into the two Sub-assemblages B'(1) and B'(2) mainly on the basis of an increase in diversity, a peak in faunal turnover and a drop in the relative abundan ce of the genus Krithe in early tate Miocene time (NN9, c. 10.5Ma). The B'( 1)/B'(2) Sub-assemblage boundary cannot be related to any previously docume nted faunal change in deep-sea ostracods. Changes associated with the bound aries between Assemblages B and C, and B' and C', which we believe to be sy nchronous, both include a decrease in diversity and abundance. In addition, two strong turnover peaks occur near the B'/C' boundary at Site 1088. The BIC and B'/C' boundaries coincide with a previously documented mid-Pliocene 'event' (3.5 Ma) (NN15-16) which may be linked to putative closure of the Straits of Panama and increased production of NADW, the latter in turn lead ing to increased production of Antarctic Intermediate Water (AAIW) and Anta rctic Bottom Water (AABW). Alternatively, fluctuations in size of the Antar ctic ice sheet during possible Pliocene warm periods could indirectly be re sponsible for the observed mid-Pliocene faunal changes.