Paleocene and Eocene diatom biosttarigraphy of ODP Hole 121-752A, eastern Indian Ocean

The Paleocene/Eocene boundary was recovered for the first time in diatom-bearing sediments at Broken Ridge, Site 752. Diatom assemblages are documented throughout the 180-m-thick sequence of upper Paleocene to lower Eocene sediments. Age control available from magnetostratigraphy, calcareous nannofossils, and planktonic foraminifers allows calibration of diatom datum levels to absolute time. A partly new/partly revised diatom zonation is proposed for the Paleocene/early Eocene based on the results of Site 752 and consideration of other studies. The diatom zones are defined as follows (from the youngest to the oldest): Pyxilla gracilis Zone (first occurrence of Craspedodiscus undulatus to first occurrence Pyxilla gracilis); Hemiaulus incurvus Zone (first occurrence Pyxilla gracilis to first occurrence Hemiaulus incurvus); Hemiaulus peripterus Zone (first occurrence Hemiaulus incurvus to first occurrence Hemiaulus peripterus var. peripterus). Three new taxa are described: Anaulus fennerae n. sp., Stictodiscus bipolaris n. sp., and Hemiaulus peripterus var. longispinus n. var.

Chemical composition of basement rocks from ODP Leg 121, Ninetyeast Ridge

Data and observation from Drifting Program Leg 121 and plate-tectonic reconstructions indicate that the Ninetyeast Ridge (Indian Ocean) was derived from the interaction of a deep-seated Dupal hotspot and a nearby spreading-ridge axis. The 5000-km-long ridge, from lat 34°S to lat 10°N, was drilled at three sites during Leg 121. About 178 m of basalt, >38 to >80 Ma, were recovered from a total penetration of ~310 m. Shipboard petrographic and geochemical studies showed that each site has distinctive characteristics. Most of the cored lavas have a tholeiitic basalt composition. Incompatible-element abundanes and ratios show systematic trends, consistent with an origin for the Ninetyeast Ridge lavas by mixing between a depleted component-Indian Ocean mid-ocean ridge basalt-and an enriched component-oceanic-island basalt similar to that observed in the youngest alkalic basalts from the Kerguelen archipelago. Preliminary shore-based trace element abundance and isotopic data are compatible with this hypothesis, although Pb isotopes indicate the involvement of another component. The long-lasting and more or less continuous activity of the Kerguelen-Heard plume (ca. 115 Ma), now located under Heard Island, south of the Southeast Indian Ridge, provides evidence that the source of the Dupal anomaly is deep seated.