Neogene and Quaternary calcareous nannofossil biostratigraphy and paleoecology of Ocean Drilling Program Leg 117: Biostratigraphic and magnetostratigraphic synthesis of the northwestern Arabian Sea

The portion of the Arabian Sea studied during ODP Leg 117 experiences upwelling associated with the southwest (summer) monsoon. The upwelling associated with the summer monsoon makes this region one of the most fertile areas in the world for the production of nannoplankton. Calcareous nannofossils are abundant in most core samples recovered during Leg 117. The combined data from the investigations of calcareous nannofossils, planktonic foraminifers, radiolarians, and magnetostratigraphy provide sedimentation rates for all twelve sites visited during Leg 117. On the Oman Margin, the effect of upwelling on the microplankton was established by the middle Miocene. However, the effects of upwelling on the Owen Ridge region were not realized until later, in the early late Miocene. A transition in the upwelling regime took place between the Pliocene and Pleistocene. While the Miocene and Pliocene sediments are dominated by the siliceous component, the Pleistocene sediments appear to be dominated by the calcareous component. Some of the more typical Neogene warm-water nannoplankton are either missing or are extremely rare. Coccolithus pelagicus, a typical cold-water indicator is extremely abundant in many samples of late Pliocene to early Pleistocene age. Ages for selected nannofossil events were calculated. These are the first ages for nannofossil datums from Oman Margin sediments. $\sp{18}$Oxygen and $\sp{13}$Carbon isotopic analyses of foraminiferal carbonate provide a detailed record of glacial and interglacial history of the late Pleistocene at Site 723 which can be correlated with nannofossil assemblage changes. Small gephyrocapsids were most abundant during regressions or warm (interglacial) periods. Emiliania huxleyi, Gephyrocapsa aperta, Gephyrocapsa oceanica, Helicosphaera spp., and Neosphaera coccolithomorpha were most abundant during glacial periods. Gephyrocapsa caribbeanica appears to have been most abundant during transitional periods (transgressions and regressions). The Milankovitch-mechanism may be responsible for overprinting a 21,000 year cyclicity of more intensive monsoonal upwelling which affects the composition of the nannofossil assemblages