Five sites (1207, 1208, 1212, 1213, and 1214) drilled during Ocean Drilling Program (ODP) Leg 198 (September–October 2001) on Shatsky Rise (northwest Pacific Ocean) cored Lower to mid-Cretaceous successions of nannofossil chalk, porcellanite, and chert, which, although poorly recovered, yielded an outstanding record of calcareous nannoplankton. Nannoplankton appeared in the Late Triassic (~225 Ma) and diversified rapidly in the Early Jurassic (205–188 Ma), but much of the remainder of the Mesozoic was characterized by steady increases in diversity and background evolutionary rates (Bown et al., 2004). The Jurassic/Cretaceous boundary interval saw considerable taxonomic reorganization of nannoplankton, with high turnover rates at species level, the appearance of several important new nannolith groups (e.g., nannoconids), the putative first record of widespread nannofossil-generated carbonates, and the onset of significant paleobiogeographic differentiation (Bown and Cooper, 1999; Bornemann et al., 2003; Bown et al., 2004). The exact nature and timing of events through this interval, in particular, have been difficult to determine precisely due to the absence of stratigraphically complete nannofossil-bearing sections. Continental shelf sections of this age are frequently incomplete as a result of sea level lowstands; only a small number of nannofossil-bearing oceanic sections have been recovered, and these are virtually restricted to the Atlantic (e.g., Bralower et al., 1989).
Nannofossil assemblages from the Leg 198 sections provide an excellent record of the final stages of the Jurassic–Cretaceous evolutionary transition and their subsequent history through the Early to mid-Cretaceous, including oceanic anoxic events (OAEs) 1a and 1b (early Aptian and early Albian, respectively). The sites, crucially, represent a record of truly oceanic nannoplankton from by far the largest Cretaceous marine habitat, the Pacific Ocean. These findings can be compared to the better known epicontinental shelf assemblages, which form the basis for much of our understanding of paleoecology (and resulting paleoceanographic proxies) and evolution (and resulting biostratigraphic events). This paper includes a discussion of the biostratigraphic analysis, which was problematic in places due to missing marker species. We also discuss the paleobiogeographic and paleoceanographic implications of these Lower Cretaceous tropical coccolithophore assemblages and shed light on the debate concerning the paleoecologically enigmatic nannolith groups, the nannoconids and braarudosphaerids (Micrantholithus and Braarudosphaera).