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Keywords:

  • Biogenic bloom;
  • Cainozoic;
  • Circum-Antarctic Opal Belt;
  • clay minerals;
  • ocean circulation;
  • Southern Ocean

ABSTRACT During Leg 177 of the Ocean Drilling Program (ODP), well-preserved Middle Miocene to Pleistocene carbonate-rich sediment records were recovered on a north–south transect through the south-eastern Atlantic sector of the Southern Ocean at Site 1088 on the Agulhas Ridge and Site 1092 on Meteor Rise. Both sites were dominated by the deposition of calcareous nannofossil oozes through the Miocene, indicating low biological productivity in warm to temperate surface waters. A continuous increase in the proportions of foraminifera since the latest Miocene (6·5 Ma) points to enhanced nutrient supply, possibly related to the global ‘biogenic bloom’ event across the Miocene–Pliocene boundary. Since the Late Pliocene, different styles of biological productivity developed between the sites. Enhanced deposition of biosiliceous constituents at the southern Site 1092, particularly in the Early Pleistocene, is consistent with the formation of the Circum-Antarctic Opal Belt since 2·5 Ma in a setting near the Polar Front, whereas carbonate deposition still prevailed at the northern Site 1088 situated near the Subtropical Front. Clay-mineral tracers of water-mass advection together with the pattern of sedimentation rates and hiatuses reflect distinct pulses in the development of regional ocean circulation between 14 and 12 Ma, around 8 Ma and since 2·8 Ma. These pulses can be related to Antarctic ice-sheet extension that mediates the production and flow of southern source water, and stepwise increases in North Atlantic Deep Water production that drives global conveyor circulation. At Site 1088, illite chemistry and silt/clay ratios of the terrigenous sediment fraction reflect the history of terrestrial climate in southern Africa, with humid conditions prior to the Early Late Miocene (9·7 Ma), followed by a dry episode until 7·7 Ma. The latest Miocene and Early Pliocene were characterized by a humid episode until modern aridity was established in the Late Pliocene between 4·0 and 2·8 Ma. These climate changes were related to the latitudinal migration of climate belts in response to tectonically caused reorganizations in atmospheric and ocean circulation.