ODP Leg 198 Sites 1207, 1208, 1212, 1213, and 1214 on Shatsky Rise yielded the most continuous Early Cretaceous nannoplankton record yet retrieved from the Pacific Ocean and provide valuable data concerning the evolutionary succession and paleobiogeography of the largest Cretaceous marine habitat (Table T9).
Mid-Cretaceous sections (Aptian–Cenomanian) were recovered at all sites, and biostratigraphy is relatively straightforward for Zones NC6–NC11. Site 1213 included an apparently complete Berriasian–Hauterivian section, but biostratigraphic dating is problematic due to the absence or rarity of zonal fossils of both Boreal and Tethyan affinity (e.g., nannoconids, C. oblongata, T. verenae, Eiffellithus spp., etc.); Zone NK1–Subzone NC4b were identified. Short Hauterivian (Subzone NC4b) and Barremian (Subzones NC5d–NC5e) sections were also identified at Sites 1214 and 1207, respectively.
Organic-rich claystones and porcellanites dated as Zone NC6 (Aptian) were recovered at Sites 1207, 1213, and 1214 and represent Pacific expressions of OAE1a (Selli event). Older organic-rich claystones at Site 1213, in Sections 198-1213B-15R-1 and 19R-1, are assigned to Zones NK3a (lower Valanginian) and NK2a (Berriasian), respectively, and may be equivalent to organic-rich intervals from the Tethyan area (Mattioli et al., 2000; Bersezio et al., 2002; Brassell et al., 2004).
L. carniolensis is common to abundant throughout and most likely represents an oceanic-adapted taxon. Cold- to temperate-water species such as C. salebrosum, R. parvidentatum, and S. primitivum are entirely absent, indicating the persistence of tropical, warm surface water temperatures. The clearest indication of warm water is provided by the common occurrence of H. irregularis in the mid-Cretaceous, although the putative warm-water taxon, Rhagodiscus asper, is consistently present and often frequent to common. Other Tethyan taxa are absent or rare and sporadically distributed (e.g., C. oblongata, Conusphaera spp., T. verenae, and L. bollii), and other factors, probably related to neritic environments, presumably controlled their distribution. Watznaueria is almost always the most abundant coccolith in these assemblages, as it was globally at this time (Street and Bown, 2000).
Site 1213 records extended Early Cretaceous ranges for species previously thought to have become extinct during the Late Jurassic (e.g., A. cylindratus, P. grassei, and B. dorsetensis), suggesting these species became Pacific restricted prior to their extinction, significantly later than their disappearance elsewhere. W. britannica may also have been a species with Pacific affinities before reexpansion of its biogeography in the early Aptian.
The most striking feature of the Shatsky Rise Cretaceous assemblages, however, is the virtual absence of Nannoconus and Micrantholithus, both taxa that are conspicuous and often common components of many Tethyan and Atlantic nannofloras. These forms were almost certainly neritic adapted and usually absent in open-ocean settings. Their dispersal and habitation was prevented by oceanic environments, and the distribution of nannoconids in particular may have been directly controlled by water depth itself, perhaps by the presence of a benthic life cycle stage. Discussion of the global biogeochemical significance of declines in this group should be tempered by the fact that they were absent from much of the Cretaceous marine ecosystem. Micrantholithus and other typically Tethyan taxa (C. oblongata, Conusphaera spp., T. verenae, and L. bollii) may also have been neritic adapted, excluded from most truly oceanic environments away from guyots and atolls.
One new genus (Mattiolia) and thirteen new species (Zeugrhabdotus clarus, Zeugrhabdotus petrizzoae, Helicolithus leckiei, Rhagodiscus amplus, Rhagodiscus robustus, Rhagodiscus sageri, Rhagodiscus adinfinitus, Tubodiscus bellii, Tubodiscus frankiae, Gartnerago ponticula, Haqius peltatus, Mattiolia furva, and Kokia stellata) are described from the Shatsky Rise Lower Cretaceous.