The late Neogene planktonic foraminiferal zonation that we have established for the California margin based on sites drilled during ODP Leg 167 consists of eight zones representing the last ~3.5 m.y. from the late early Pliocene to the present day. We have found that the zonal scheme can be consistently applied over much of the region (Fig. 1, Fig. 8), although Zones CM5 to CM3 were not individually recognized in the northernmost site; nor were Zones CM3 and CM4 individually recognized in the southernmost sites (Holes 1011B and 1012A) of the transect (Fig. 8). The interval from the late early Pliocene to the present day has been biostratigraphically subdivided at ~0.5-m.y. intervals, employing a succession of planktonic foraminiferal changes. The planktonic foraminiferal assemblages of the California Current system continued to change during the late Neogene as a result of evolutionary appearances of new taxa within California margin waters, appearances resulting from migration of taxa into the region following evolution elsewhere, and extinction. The general similarity in the succession of planktonic foraminiferal changes in the north-south transect suggests relative uniformity of response of faunas on evolutionary time scales throughout this large region. These changes were irreversible and represent evolutionary developments of the planktonic foraminiferal assemblages through time in California margin waters. The evolutionary development of the faunal sequence was superimposed on reversible changes in planktonic foraminiferal assemblages due to climatically related paleoceanographic changes in the California Current system (Ingle, 1973b). At this stage in our investigations, it is unclear if the first and last appearances of species used to define the new zones were synchronous throughout the region. This needs to be evaluated once a detailed chronological framework is established for each hole and biostratigraphic changes are integrated with paleomagnetic stratigraphy. It is clear that a few species that were not employed for the definition of the zones exhibit meridionally diachronous first or last appearances along the margin. These include the first and last appearances of N. pachyderma C, the last appearance of G. puncticulata, the last appearance of N. humerosa praehumerosa, the first appearance of N. dutertrei, and the first appearance of G. truncatulinoides. This diachronism almost certainly was in response to the well-known meridional paleoenvironmental gradients in surface waters in the California Current system.
Changes in the planktonic foraminiferal assemblages used to define the zones reflect important biotic events within the California Current system. Our observations suggest that the appearance of most new taxa during the late Neogene were abrupt and lack any apparent evolutionary gradation from earlier, possibly ancestral forms. A clear exception is the appearance of N. asanoi, which did not appear abruptly but shows evidence of gradual evolution from its ancestor N. kagaensis during the late Pliocene in this region. Evolutionary gradation is also apparent in the G. inflata lineage in the Leg 167 sites, supporting earlier interpretations of Keller (1978a, 1979a, 1979b, 1980). It remains unclear if the abrupt first appearances of other forms used to establish the zonation, especially forms of N. pachyderma and N. kagaensis, evolved within or close to the California margin through punctuated equilibrium or if the evolution occurred elsewhere with later rapid migration into these waters. A thorough study is required of the phylogeny of late Neogene neogloboquadrinids of the northeast Pacific to help resolve these issues and to assist in the resolution of paleoceanographic history of water masses along the coast of western North America.
The sequence of late Neogene (3.5 Ma to present day) planktonic foraminiferal assemblages in Leg 167 sites indicates a continued, long dominance of G. bulloides and neogloboquadrinids within the California Current system. Furthermore, taxonomic diversity of these assemblages remained relatively low throughout the late Neogene despite known global climate oscillations that affected the relative strength of the California Current (Ingle, 1973b; Kennett, 1982). Low diversity and high dominance of the assemblages favored these and other taxa well adapted to upwelling systems exhibiting high seasonal surface-ocean variability. Neogloboquadrina is a highly plastic taxon (a generalist species) well adapted to cool-water environments exhibiting strong seasonal changes in near-surface waters (Kennett, 1976). Apparently these oceanographic conditions, as they affect planktonic foraminiferal assemblages, have persisted in the California Current system during at least the late Neogene (3.5 Ma to present day).