139736eo.pdf (20MB) - Japan Oceanographic Data Center

scouring of the sediments by cold bottom waters may have caused these hiatuses. This bottom wateractivity can be ascribed to the partial thermal isolation of Antarctica to higher latitudes and thebeginning of large scale freezing at sea level that spurred the initiation of cold Antarctic Bottom Water(AABW). Thus, a drop of about 5°C in the bottom water temperature at the Eocene/Oligoceneboundary reflects a fundamental change in the nature of the bottom waters and signals the developmentof the psychrosphere (BENSON, 1975; SHACKLETON and KENNETT, 1975).By the middle to late Oligocene time (30-24 Ma) the global surface circulation had begun toevolve the essential present-day patterns (Fig. 10). An event of major significance at this time, thatchanged the global oceanographic patterns forever, was opening of the Drake Passage in the midOligocene, between anomalies 10 and 8 (ca 30 Ma or somewhat earlier). A recent tectonic map of theScotia Arc region (BRITISH ANTARCTIC SURVEY, 1985) suggests that the deep connection atDrake Passage began at the time of initiation of the spreading in the region, around anomaly 10 timeand was well established by anomaly 8 time. The spreading in the area continued until anomaly 5time. This event led to the enhanced thermal isolation of Antarctica and the initiation of thecircurn-Antarctic circulation.The Oligocene was probably the most influential epoch in the history of the world ocean whenthe oceanographic-cli.matic features were dramatically changed. The most important change was theshift of circum-global circulation from the tropical-temperate latitudes to the southern high latitudesaround Antarctica. This is the time of the fundamental change in the bottom water regime and the entryof the earth from a non-glacial to a predominantly glacial mode.The Neogene Indian OceanThe Neogene period (25-1.7 Ma) was characterized by the further accentuation ofoceanographic-climatic conditions that had been initiated in the Oligocene. During the early Miocenethe convergence of Eurasia and Africa finally interrupted much of the periodic influence of the IndianOcean on the Mediterranean that had continued intermittently through much of the Oligocene. By thistime the major geographic features of the Indian Ocean were also in place and the sedimentary patternsbegin to resemble the present-day patterns (Figs. 11 and 12).During the Neogene, terrigenous sedimentation in the Indian Ocean increased in importance.In the east the Bengal Fan continued to accumulate. The Miocene subduction at the Sunda-Java Trenchwith its island-arc volcanic activity supplied important volumes of volcanogenic sediments in thatregion (KIDD and DAVIES, 1978). In the west, the Indus Fan continued to grow. The sediments onthis fan seem to have been transported both by the action of turbidity currents as well as the slowerspeedsuspension currents (WHITMARSH, 1974). In the south, the Zambesi Fan began to developsometime in the middle Miocene (KIDD and DAVIES, 1978).A major middle Miocene phase of Himalayan uplift may have initiated upwelling off theArabian Peninsula by establishment of the cyclic monsoonal system (WHITMARSH, 1974). In otherareas, such as the Somali Basin, there was a general increase in the biogenic siliceous input andorganic matter in the sediments throughout the Miocene, the result of increased surface productivity ofthe water masses in the area. Intensification of the western boundary undercurrents supplied withcorrosive AABW in the late Oligocene-early Miocene have eroded large surfaces of the Somali,Mascarene, Madagascar and Mozambique Basins (LECLAIRE, 1974).A drop of the CCD in the late Miocene is signaled by the presence of carbonates in the deeperparts of the basins. The event may have been caused by an accelerated seafloor spreading along theridges and increased volcanic supply of calcium from the fresh basalt (LECLAIRE, op. cit.). Sincelate Miocene siliceous sediments have become increasing more common up to the Holocene time due tothe development of modern high productivity belts and the subsequent rise of CCD (KIDD andDAVIES, 1978).By the later part of middle Miocene and late Miocene, both the Red Sea and the Gulf of Adenhad begun to open. Earliest synrift evaporite sequences in the Red Sea seem to have had a sabkhatypeorigin. The overlying evaporitic facies indicates a restricted environment, whereas youngersediments are of more normal marine origin, reflecting the opening of the Red Sea to Indian Oceaninfluence in late early Pliocene (STOFFERS and ROSS, 1974) (Fig. 12).The Neogene oxygen-isotopic record of the Indian Ocean is consistent with the global patterns(Fig 9). A period of climatic amelioration in the early middle Miocene was followed by a ratherdramatic fall in both the surface and bottom water temperatures around 15 m.y. ago. This was63