Biostratigraphy and Comparison of Paleocene to Lower Eocene Calcareous Nannofossils from Broken Ridge and Ninety-East Ridge

Ocean Drilling Program Site 752 on Broken Ridge in the Indian Ocean recovered an expanded section containing Paleocene and lower Eocene calcareous nannofossils. Qualitative counts show a diverse, high-latitude assemblage of primarily moderately preserved and abundant nannofossils. The assemblage was deposited in the high southern latitudes before Broken Ridge rifted off of the Kerguelen Plateau in the middle Eocene. Minor modifications allowed Okada and Bukry's 1980 low-latitude zonation to be utilized at this site for Zones CP1a through CP10. Various markers such as the first occurrence of Discoaster diastypus and Tribrachiatus bramlettei, or the last occurrence of Fasciculithus sp., were used in past studies to determine the Paleocene/Eocene boundary. At this high-latitude site, both D. diastypus and T. bramlettei are rare or absent, so the last occurrence of Fasciculithus sp. was used to approximate the boundary. Ocean Drilling Program Site 758 cored the northern Ninety-East Ridge in the Indian Ocean. Ninety-East Ridge is a hotspot trace formed from the northward movement of the Indo-Australian Plate. This site contains a more condensed section in which much of the lower Paleocene and all of the lower Eocene are absent. A more generically diverse, but still high-latitude assemblage of nannofossils is examined in this study. First seen in Zone CP6, a large variety of Discoaster okadai is present at Site 758. Figures are presented to show the variation in morphology of this species in the Indian Ocean. Much research has been done to associate Paleogene nannofossil genera with different paleoceonographic conditions. Both Sites 752 and 758 show a shift from primarily small, cool-water, eutrophic species near the K/T boundary (such as Prinsius martinii, Prinsius dimorphosus, or Cruciplacolithus primus) to warm-water, oligotrophic species (such as Sphenolithus, Discoaster, or Zygrhablithus bijugatus). This trend is seen in Principal Component Analysis and is more obvious at Site 752 due to the presence of a complete lower Paleocene section and lower Eocene nannofossils.