Paleoceanographic and biogeographic evolution of the Southern Ocean during the Cenozoic, and Cenozoic microfossil datums

Abstract

The paleoclimatic history of Antarctica is intimately linked with the evolution of the Circum-Antarctic paleocirculation system and hence with Cenozoic plate tectonism. The development of the Circum-Antarctic circulation system occurred as southern land masses moved away, creating unrestricted latitudinal flow. Changing boundary conditions in this region included the opening of the Australian—Antarctic seaway as Australia moved northwards, the opening of Drake Passage and the development of the Kerguelen Plateau. The formation, and later development, of the Circum-Antarctic Current thermally isolated Antarctica by decoupling the warmer subtropical gyres from the colder Subantarctic—Antarctic gyres. The climatic history of the Southern Ocean is, in general, one of decreasing temperatures, including the development of increased antarctic glaciation and later ice sheet formation, a climatic regime which itself had a profound effect on environmental evolution, and thus the biogeography at high southern latitudes. Environmental characteristics resulting from the development of this climatic regime included extensive seasonal sea-ice production, the cooling of waters surrounding the continent, and meteorologically forced upwelling of nutrient-rich intermediate waters which increase biogenic productivity in the Southern Ocean. The thermal barriers in high southern latitudes represented by the Antarctic Convergence and the Subtropical Convergence also became major biogeographic barriers strongly affecting the distribution of planktonic organisms. The development of the Circum-Antarctic Current also removed biogeographic provinciality previously occurring within different sectors of the Southern Ocean as a result of previous land barriers. As a result, cool-water planktonic assemblages of the Southern Hemisphere show little interoceanic difference. During the Cenozoic, major changes have occurred in planktonic microfossil biogeography in the Southern Ocean as reflected in the biostratigraphic sequences. Datum levels representing the first or last appearances of microfossil species (planktonic foraminifera, calcareous nannofossils, radiolaria, diatoms and silicoflagellates) have been compiled for Antarctic and Subantarctic sections. This compilation provides a series of integrated datums and hence a historical sequence of microfossil events through the Cenozoic. The datum levels of Quaternary and Pliocene age, their nature, relative position and age are now well known mainly because of integration with magnetostratigraphy because of superior sediment records. In contrast, the relative position of Miocene and older datum levels is much less firmly established. There are several reasons for this, including poorer stratigraphic control because of a lack of magnetostratigraphic framework; few section that contain siliceous and calcareous microfossil assemblages in association to establish detailed relation; and poor stratigraphic representation especially in the Early Cenozoic. In general, most datum levels do not extend over both the Antarctic and Subantarctic regions. Instead, microfossil events, if they occur in both regions, can be highly diachronous between the two water-mass regimes. This is most strongly developed in the calcareous Cenozoic microfossil groups which, as in the present-day, show major differences across the water-mass boundaries.