Upper Cretaceous to Quaternary calcareous nannofossils from Broken Ridge and Ninetyeast Ridge, Indian Ocean, Ocean Drilling Program Leg 121

Study of the upper Cretaceous-Recent sedimentary sequence from seven sites drilled during Ocean Drilling Program Leg 121, show that the distribution and the variation of calcareous nannofossil assemblages from Broken Ridge and Ninetyeast Ridge are related to their tectonic histories. The upper Campanian to Maestrichtian calcareous nannofossil assemblages from Broken Ridge are austral in nature, containing many of endemic Southern Ocean forms observed from the Falkland Plateau, Weddell Sea, and Kerguelen Plateau. The absence of the Lithraphidites praequadratus-L. quadratus lineage and the significant overlap of the first appearance of Nephrolithus frequens and last appearance of Reinhardtites levis negates direct use of temperate zonation schemes for these sediments. The upper Campanian to Maestrichtian of Ninetyeast Ridge contains both austral and tropical forms in different horizons. The tropical zonation scheme of Sissingh (1977) is most effective in age-dating these sediments. The Cenozoic calcareous nannofossils from Broken Ridge and Ninetyeast Ridge are similar, but some significant differences occur. In the Paleogene, some low latitude zonal marker species (Ellipsolithus macellus, Sphenolithus ciperoensis and S. distentus) are common on Ninetyeast Ridge, but are not useful on Broken Ridge. Some high latitude zonal marker species (Isthmolithus recurvus and Chiasmolithus oamaruensis) are common on Broken Ridge and the southernmost site of Ninetyeast Ridge, but are not useful on northern Ninetyeast Ridge. Comparison of the Neogene calcareous nannofossil assemblage indicates decreased abundance of discoasters, amauroliths, ceratoliths, and scyphopheres at the Broken Ridge sites and the southernmost site of Ninetyeast Ridge. With the exception of Helicosphaera ampliaperta, all of the Neogene zonal markers used by Okada and Bukry (1980) occur on Broken Ridge and Ninetyeast Ridge. Twenty-one nannoplankton datums were recognized in the continuous, largely undisturbed upper neogene from Site 758. The calibration of the datum planes with paleomagnetic and oxygen isotope records yielded absolute ages for the datums that generally agree with previous recorded ages