Carbon and oxygen stable isotope composition of bulk sediment and abundance of Braarudospahaera in ODP Site 122-762 (Table 1)

Nannofossil assemblages enriched in Braarudosphaera occur in lower Oligocene to lower Miocene sediments at Ocean Drilling Program Sites 762 and 763 on the central Exmouth Plateau. Braarudosphaerids appear here rather abruptly in the lower Oligocene (in Zone NP21). They reach their greatest numbers in the lower Oligocene (in Zones NP22 and NP23), where they comprise up to 10% of some samples. Braarudosphaera bigelowii is the overwhelmingly dominant species, occurring together with rare specimens of B. discula and Micrantholithus pinguis. The holococcoliths Peritrachelina joidesa and Lantemithus minutus are also associated with the Braarudosphaera enrichment. There are two populations of B. bigelowii: one of normal size (10-14 µm) and one of large specimens (20-22 µm). The larger braarudosphaerids are more common than the smaller forms. Braarudosphaera-rich sediments are absent at Wombat Plateau sites during the same time interval. We attribute this to latitudinal control, because the Wombat sites are about 4° north of the central Exmouth Plateau sites. We believe that the occurrence of braarudosphaerids is related to an Oligocene to early Miocene oceanographic event on the Exmouth Plateau. We suspect that mid-ocean up welling of cool, low-salinity, nutrient-rich water along a divergent zone created the Braarudosphaera-nch sediments in the South Atlantic and Indian oceans.

Abundance of calcareous nannofossils in ODP Hole 122-761C (Table 1)

A biostratigraphically complete (all nannofossil biozones present) Cretaceous/Tertiary boundary was recovered at Site 761 on the Wombat Plateau, northwest Australian margin, during Ocean Drilling Program Leg 122. A quantitative study of calcareous nannofossils on closely spaced samples across the boundary reveals a rapid change in assemblages in a similar fashion to other Cretaceous/Tertiary boundary sites. Nannofossil species were placed into three categories: Tertiary, Cretaceous, and 'survivors'. The rapid sequential turnover in these assemblages is as follows: Cretaceous species are abruptly replaced at the boundary by opportunistic survivor species, which in turn are abruptly replaced by newly evolved Tertiary taxa. The uppermost Maestrichtian assemblage is distinctly mid-latitudinal with few Micula murus and rare to few Nephrolithus frequens. The nannofossil assemblage immediately above the boundary is dominated by an abundance bloom of Cyclagelosphaera spp. No Thoracosphaera or Braarudosphaera abundance blooms are present as at many other localities. The change from a survivor- to Tertiary-dominated assemblage is coincident with the CPla/CPlb nannofossil subzonal boundary, which is marked by the simultaneous first occurrence of several species including Cruciplacolithus tenuis and C. primus. The latter is found to first occur below C. tenuis in the most complete Cretaceous/Tertiary sections. A hiatus between Subzones CPla and CPlb is interpreted to explain this discrepancy.