Bolboforma as monitors of Cenozoic palaeoceanographic changes in the Southern Ocean

Bolboform distribution in space and time is analysed from a circum-Antarctic belt of lower Eocene to uppermost Miocene sediments from the Southwest Pacific, Southeast Pacific (Bellinghausen Basin), Maud Rise and South Atlantic, and Indian Ocean (Kerguelen Plateau) regions. A correlation panel using established Cenozoic planktic foraminiferal and calcareous nannofossil zones is linked to the Bolboforma zonation scheme for the Southern Ocean. Evolution of surface watermasses and their boundaries, the major oceanic fronts, are tracked using the microfossil distributions and general sediment characteristics, and are in turn linked to the Bolboforma distribution. Specifically, the migration of both the Antarctic Polar Front and the Subtropical Front northwards from continental Antarctica, starting in the earliest Oligocene and culminating in the late Miocene establishment of 'modern' conditions, links the bolboforms in the Southern Ocean with subantarctic watermass conditions between the equivalent of today's Subantarctic and Subtropical fronts. The occurrence of bolboforms in sediments including subtropical microfossil assemblages from north of the Subtropical Front, as in the Tasman Sea, is anomalous. We suggest that they were transported into this region by subsurface (intermediate depth) waters generated by subduction of subantarctic surface water at southern oceanic fronts. The distribution of bolboforms more widely outside the Southern Ocean may define the pathways of Southern Component Intermediate Water (Antarctic Intermediate Water) during the Tertiary, and may account for their bipolar distribution. Bolboforms exhibit a circum-Antarctic distribution prior to the development of a full circum-Antarctic circulation system following the opening of the Tasmanian and Drake gateways. It is inferred that a West Antarctic Seaway between the Ross Sea and Weddell Sea embayments may have afforded oceanic connection between the Southwest Pacific and the southern Atlantic during the Eocene-Oligocene, facilitating dispersal and such a distribution. (C) 2002 Elsevier Science B.V. All rights reserved.