Radiolarian assemblages in Holocene sediments from the Gulf of Mexico and Caribbean, and the Southern California borderland reflect overlying oceanographic conditions, such as general productivity, upwelling, and preservational parameters. A reconnaissance investigation of the lateral variations in the radiolarian assemblage with increasing water depth indicates that the changes are not consistent enough in either region for the establishment of a definite depth zonation; however, many radiolarian species and higher taxa show some general trends with depth.

Symbiotic taxa, and spumellaria in general, are more abundant in the thanatocoenosis of the Gulf of Mexico region than off California and are indicative of oligotrophic conditions. Low oxygen content and high dissolved silica concentration in the bottom water of the Orca basin in the Gulf of Mexico contributed to good specimen preservation in this locality. Diversity, to the family level, is greater in the California assemblages owing to a mixing of radiolarian faunas and better preservation. Deep-water radiolarian taxa are more abundant in the California assemblages than in the Gulf populations, owing to more dynamic upwelling conditions which enhance the species both in the biocoenosis and thanatocoenosis.

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