The relative abundances of stratigraphically important planktonic foraminiferal species are shown for each of the six sites in Table 2, Table 3, Table 4, Table 5, Table 6, Table 7. Their stratigraphic ranges are plotted in Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, and the ranges of specific taxa have been employed to construct the new zonation. A sequence of eight zones has been established for the late early Pliocene through the Quaternary (~3.5 Ma to present day; Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8). Thus the planktonic foraminiferal sequence is subdivided into a series of zones, each representing ~0.5 m.y. The average duration of the well-recognized succession of subtropical to tropical Neogene planktonic foraminiferal zones (Banner and Blow, 1965; Berggren et al., 1985) is approximately double this.
Biostratigraphic data generated during Leg 167 indicated the need and potential to construct a relatively high-resolution late Neogene planktonic foraminiferal zonation that is broadly applicable to the California margin. This zonation, constructed after the expedition, appears to be broadly applicable to the California margin despite the relatively large meridional range of the sequences (31°N to 41°N). In constructing the new zonation, we tried to use species that are easily identified, exhibit consistent ranges throughout the region, sequentially appear and disappear in the same order, and are plentiful enough to recognize zonal boundaries with confidence. The zonation is based primarily on changes in the neogloboquadrinid complex, a dominant and rapidly evolving group within the California Current system (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7). A number of other groups exhibiting evolution and biostratigraphic change within the sequences were not employed for the zonation because they occur in insufficient abundances and because of the difficulty this poses in plotting with confidence their first and last appearances. Such taxa include Globorotalia truncatulinoides, Globorotalia inflata, N. dutertrei, and N. humerosa praehumerosa. These forms are shown to have discontinuous ranges throughout (Table 2, Table 3, Table 4, Table 5, Table 6, Table 7), probably because of their migration into these waters only during relatively brief warm episodes.
The zonal sequence is made up of a series of range zones, overlap zones, and gap zones (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7). To simplify utilization, the zones have been assigned a number code with numbers increasing with age. Each numerical zone has a prefix—for example, CM = California margin. We recognize that convention in biostratigraphy employs numerical zones increasing with decreasing age, in accord with the natural evolutionary development of assemblages. This is not practicable in the present study because the late Neogene planktonic foraminiferal sequence in Leg 167 sites is not complete, extending downward only to the late early Pliocene. Older, biosiliceous-rich sediments in the sequences generally lack planktonic foraminifers. Thus, the number code, as established, can be continued back earlier in the Neogene as a result of future work. This numerical code, increasing with age, is thus similar to that employed in magnetostratigraphy and oxygen isotope stratigraphy.
Our planktonic foraminiferal zonation provides a higher resolution biostratigraphic subdivision than that of Keller (1979a) for DSDP Leg 18, Site 173, drilled off the coast of Southern California. The late Neogene zonal scheme of Keller (1979a) is made up of one zone representing the Quaternary and based on the first appearance of G. truncatulinoides. We found this species to be stratigraphically unreliable as a zonal marker in the California margin sequences because of its rarity and highly discontinuous stratigraphic range. We recognize three zones in the Quaternary. Keller (1979a) established three zones for the Pliocene based on changes in stratigraphic range of species of Globorotalia (Globoconella) (Globoconella puncticulata; Globoconella inflata, transitional variety; and G. inflata, modern variety). We did not use these forms to construct the zonation because their insufficient stratigraphic continuity did not allow us to confidently plot their ranges over the broader region of the margin. We established five zones ranging from the late-early Pliocene through the late Pliocene. These zones remain to be correlated with biostratigraphic zones based on other microfossil groups in these sequences and placed within a chronological framework by integration with paleomagnetic stratigraphy. This will enable the biostratigraphic events of the California Current system to be placed within a global framework.