Along the east Pacific margin two dominant factors influence the distribution of planktonic radiolarians: east boundary ocean currents and the physiography of the southern California borderland. The east boundary current system is mainly wind driven on the surface and geostrophically controlled at depth. It is stratified into distinct water masses owing to differences in salinity, temperature, and current direction. The California borderland is a unique geomorphic province of successive basins and ridges with local circulation patterns. These factors influence present-day radiolarian assemblages but have also influenced such assemblages during the Cenozoic.

Nassellarian and spumellarian radiolarians reflect the temperature and depth of the water masses at the time of deposition. The California borderland serves as an environment similar to, yet distinct from, the boundary currents. Thus, this area seemingly has isolated species and increased their chance for allopatric speciation. These borderland species would eventually have been dispersed into the equatorial region.

Samples studies are from the DSDP Sites 33, 77, 173, 289, 468, and 469, where deposition was influenced by east boundary currents, and from the U.S. Geological Survey dart-core samples from the southern California borderland, to trace the development of the water masses through time. Once the dynamics of the water masses is determined, speciation and extinction events may be more easily postulated. Whereas previous investigations have dealt with present-day circulation, this study is the first attempt to map the influence of the dynamic current systems through time.

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