ABSTRACT
Deep-sea drilling in the Antarctic region (DSDP Legs 28, 29, 35 and 36) has provided much new data about the development of circum-Antarctic circulation through the Cainozoic. The development of this circulation has had profound effects on the total global oceanic circulation and climatic change.
During the Palaeocene (t = ∼ 65 to 55 MY ago), Australia and Antarctica were joined. In the Early Eocene (t = ∼ 55 MY ago), Australia began to drift northwards from Antarctica forming an ocean, although circum-Antarctic flow was blocked by the continental South Tasman Rise and Tasmania. During the Eocene (t = 55 to 38 MY ago) the Southern Ocean was relatively warm and the continent largely non-glaciated. Cool temperate vegetation existed in some regions. By the Late Eocene (t = ∼ 39 MY ago) a shallow water connection had developed between the southern Indian and Pacific Oceans over the South Tasman Rise.
The first major climatic threshold was crossed 38 MY ago near the Eocene–Oligocene boundary, when substantial Antarctic sea-ice began to form. This resulted in a rapid temperature drop in bottom-waters of about 5°C and a major crisis in deep-sea faunas. Thermohaline oceanic circulation was initiated at this time much like that of the present day. The resulting change in climatic regime increased bottom-water activity over wide areas of the deep ocean basins creating much sediment erosion especially in western parts of oceans. A major (∼ 2 000 m) and apparently rapid deepening also occurred in the calcium carbonate compensation depth (CCD). This climatic threshold was crossed as a result of the gradual isolation of Antarctica from Australia and perhaps the opening of the Drake Passage.
By the Middle to Late Oligocene (t = ∼ 30 to 25 MY ago), deep-seated circum-Antarctic flow had developed south of the South Tasman Rise as this separated sufficiently from Victoria Land, Antarctica. Major reorganization resulted in Southern Hemisphere deep-sea sediment distribution patterns. During the Early Miocene calcareous biogenic sediments began to be displaced northwards by siliceous biogenic sediments with higher rates of sedimentation reflecting the beginning of circulation related to the development of the Antarctic Convergence. In the Middle Miocene (t = 14 to 11 MY ago) at about the time of closure of the Australian–Indonesian deep-sea passage, the Antarctic ice-cap formed and has remained a semi-permanent feature exhibiting some changes in volume. The most important of these occurred during the latest Miocene (t = 5 MY ago) when ice volumes increased beyond those of the present day. This event was related to global climatic cooling; a rapid northward movement of about 300 km of the Antarctic Convergence and a eustatic sea-level drop that may have been partly responsible for the isolation of the Mediterranean basin.
The Quaternary in the Southern Ocean marks a 42peak in activity of oceanic circulation as reflected by widespread deep-sea erosion, very high biogenic productivity at the Antarctic Convergence and resulting high rates of biogenic sedimentation, and maximum northward distribution of ice-rafted debris.
