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.

在海洋钻探计划（Ocean Drilling Program）第122航次期间，澳大利亚西北边缘袋熊高原（Wombat Plateau）的761站位获取了生物地层完整的白垩纪/第三纪边界（Cretaceous/Tertiary boundary）——所有纳米化石生物带均有出露。对该边界处密集采样的钙质纳米化石开展定量研究后发现，其组合变化迅速，模式与其他白垩纪/第三纪边界站位相似。纳米化石物种被划分为三类：第三纪种、白垩纪种及“幸存者种”。这些组合的快速连续更替过程如下：边界处的白垩纪物种被机会性幸存者种突然取代，后者随即又被新演化的第三纪类群迅速替代。最顶部的马斯特里赫特期（Maestrichtian）组合具有明显的中纬度特征，其中Micula murus数量稀少，Nephrolithus frequens则罕见至极少。边界正上方的纳米化石组合以Cyclagelosphaera spp.的大量繁盛为主导。与其他众多地点不同，此处未出现Thoracosphaera或Braarudosphaera属的繁盛现象。