The California Current system is one of the most biologically productive modern water masses in the world's ocean. The geologic record indicates that high biogenic productivity along the California margin was established during the early Neogene and has since persisted (Ingle, 1973b; Barron, 1975). An understanding of the geological history of the California Current system has required progressive development—at sufficiently high resolution—of a stratigraphic and chronologic framework. Part of this framework has continued to be based on the biostratigraphy of planktonic foraminifers. During the Neogene, planktonic foraminifers have been an important biotic element of the plankton of the California Current system. Planktonic foraminifers are sensitive to changes in the near-surface marine environment, are important contributors to the hemipelagic sediments deposited on the California margin, and represent the primary group used in stable isotopic investigations. Nevertheless, the history of development of Neogene planktonic foraminifer faunas of the California margin is known only in broad outline.
The late Neogene planktonic foraminiferal sequences of the California margin have been the subject of investigation since the pioneering investigations of Ingle (1967, 1973a, 1973b). His studies initially focused on marine sequences exposed on land and later were extended into off-shore drilled (Deep Sea Drilling Project [DSDP]) sequences. These investigations were fundamental in establishing a basis for the Neogene biochronology of the California margin and represented a major advance toward understanding of the Neogene paleoclimatic and paleoceanographic evolution of the California Current. Because paleoceanographic changes in the California Current system have been highly sensitive to global climate change, the investigations of Ingle (1967, 1973a) represent major contributions toward understanding the history of Neogene global climate change. Changes in planktonic foraminiferal assemblages throughout the Neogene have reflected the changing relative strengths of the cool California Current from the north and warm, subtropical waters from the south.
Later investigators extended and modified the biochronology of Ingle (1973b). In particular, Keller (1978a, 1978b, 1978c, 1979a, 1980) and Keller and Ingle (1981) documented the late Neogene planktonic foraminiferal biostratigraphy for a number of deep-sea sites drilled in the North Pacific, which has helped to correlate the California sequences with those elsewhere in the North Pacific, including Japan. During these investigations, Keller (1978c) observed a greater range of taxonomic diversity in the Neogloboquadrina plexus than discovered in the previous work and began to exploit changes in the ranges of these forms for biostratigraphic subdivision of the late Neogene of the North Pacific. This followed pioneering investigations of Maiya et al. (1976), who had identified and named a suite of Neogloboquadrina species in late Neogene marine sequences exposed on land in Japan. These taxa have since been found in relatively cool waters of the entire North Pacific. Further contributions toward understanding the late Neogene planktonic foraminiferal sequence of western North America were made by other investigators studying sequences drilled by the DSDP and the Ocean Drilling Program (ODP). These have included the work of Poore (1981), who examined sequences along the western margin (including California) and Zellers (1995), who investigated sequences in the far northwest Pacific, north of California.
A further major advance was made by Lagoe and Thompson (1988) who compiled a late Neogene chronostratigraphic framework based on planktonic foraminiferal datums and coiling shifts in Neogloboquadrina pachyderma observed in marine sequences exposed on land that were calibrated with magnetostratigraphy. Several of the late Neogene datums recognized by Lagoe and Thompson (1988) have also been observed in offshore sequences drilled on the California margin.
Until now the late Neogene planktonic foraminiferal sequences of the California margin have been largely correlated with the standard Neogene zonation (N zones) of Banner and Blow (1965). This has never been satisfactory because the N zonation is based on a change in stratigraphic range of subtropical to tropical planktonic foraminifers that are absent to rare along the California margin. Thus, this zonal scheme is not directly applicable to sequences drilled off California, and correlations with these zones have been indirectly determined using other stratigraphic criteria. The only previous late Neogene foraminiferal zonation established for the California margin is that of Keller (1979a), who established this on a single drilled site (DSDP Leg 18, Site 173) off Southern California. Development of a zonal sequence that might be broadly applicable to the region has been difficult because of a combination of poor preservation and relatively low abundances of planktonic foraminifers in the margin sequences and, until now, inadequate material to develop such a zonation. Earlier drilled sites on the margin provided insufficiently continuous records and were often cored at locations that were not optimal for foraminiferal preservation.
Drilling during ODP Leg 167 provided, for the first time, a high-quality suite of late Neogene sections along the entire length of the California margin (Fig. 1; Table 1). These sequences contain a relatively continuous record of abundant planktonic foraminifers suitable for determining, at relatively high stratigraphic resolution, a biostratigraphic zonation for the California Current system. Our contribution documents the stratigraphic ranges of key planktonic foraminiferal taxa in six of these sites and establishes a new planktonic foraminiferal zonation from the late early Pliocene through the Quaternary (~3.5 Ma to present day) that is applicable to a broad sector of the California margin. A general outline of the biostratigraphy presented here began to be established at sea during Leg 167 and was based on core-catcher samples (Lyle, Koizumi, Richter, et al., 1997). Following Leg 167, we selected six of the sites from locations throughout the length of California to conduct a higher resolution biostratigraphic analysis (Fig. 1; Table 1). Biostratigraphic data on the remaining sites (Lyle, Koizumi, Richter, et al., 1997) support those presented here, although these earlier investigations were made at lower resolution; also, several of the taxa that have since been used to construct the new zonation were not employed during Leg 167.