Abstract

Sediment and microfossils from three subantarctic deep-sea cores recovered in the Southeast Indian Ocean have revealed a detailed climatic and glacial history for the Pliocene Epoch. Two cores contain sediment sequences that overlap and are of middle Matuyama through Gilbert a age, and one core extends into sediment that was deposited at the beginning of the Gilbert epoch (5.1 m.y. B.P.). Quartz grains that are >62 µm in size are found throughout the cores, and examination of their surfaces by scanning electron microscopy reveals that most of them are of either primary glacial or glacial marine origin. The remaining grains have features similar to grains transported mainly in a subaqueous environment. Glacially derived quartz grains become more abundant in sediment younger than the Gilbert a event; this trend continues in sediment of Gauss through middle Matuyama age. The greater abundance of ice-rafted quartz grains in sediment younger than the Gilbert a event may reflect a major late Cenozoic increase in antarctic glaciation.

Radiolarian faunas from sediment intervals of Gauss age indicate temperatures comparable to modern surface-water temperatures at the same latitudes. Warmer water faunas, however, are found in sediment of early Matuyama and late Gauss age, whereas cooler water faunas are found in middle Matuyama and upper Gilbert sediment. The warmest interval occurs directly below sediment of Gilbert c age. This interval, which is significantly warmer than the present-day surface-water temperatures at the same latitude, is followed by a marked cooling between the Gilbert b and a events and precedes the increase in glacially derived quartz grains during the Gilbert a event. There are indications that antarctic glaciation preceded major cooling of the Southern Ocean and contributed to the long-term world-wide cooling postulated for late Cenozoic time. The marked increase in ice-rafted sediment during late Gilbert time may be related to an increase in antarctic glaciation perhaps to the point where permanent ice shelves developed, the formation of which may have greatly increased antarctic bottom-water activity in the circumpolar region.

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