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One hundred and fifty-seven samples from the sea floor off San Diego, California, were studied for both living and total (living plus dead) populations of Foraminifera.

Seven benthonic Foraminifera depth assemblages are recognized on the basis of the living distributions and abundances. The boundaries are at depths of approximately 13, 45, 100, 250, 350, and 450 fathoms. Based on the Scripps Institution’s study of hydrography it appears that the 13-fathom boundary may be interpreted as approximating the base of the turbulent zone, the 45-fathom boundary the bottom of the seasonal thermocline, the 100-fathom boundary the bottom of the California Current, the 250-fathom boundary the top of the permanent thermocline, the 350-fathom boundary the oxygen minimum layer, the 450-fathom boundary the bottom of the permanent thermocline. The shallowest assemblage is divided into two facies and difference of sediment types may be one of the principle causes of such differentiation.

Comparison of depth ranges of living and empty tests of 95 species shows that some of the tests of almost all species are transported toward deeper water after death. Total population counts are valid in defining the general composition and distribution where little or no displacement of sediment is expected, but generally these counts are not indicative of distribution of living specimens. Maximum abundance of living benthonic Foraminifera occurs between 55 and 150 fathoms and approximately coincides with the greatest number of species and genera. Temperature, food, and sediment type are considered important factors for depth distribution and size of population of living benthonic Foraminifera.

Ratios of living to total populations from sediment samples appear to be indicative of the rate of sedimentation. The ratios support the suggestion of Dietz (1952) that sediments from the land are deposited either nearshore or on the lower part of the continental slope and in basins, bypassing the outer shelf and upper continental slope. Rates of sedimentation calculated from an assumed rate of reproduction of Foraminifera are 97 years per centimeter of sediment in the San Diego Trough and 0.36 years per centimeter in the nearshore area.

Three methods of calculating the amount of sea-level change by using benthonic Foraminifera are discussed. Five to ten fathoms of deepening is suggested at some time later than the Pleistocene.

The presence of shallow-water Foraminifera and Pleistocene Foraminifera in sand layers of a clayey silt core, and in sandy silt on the surface of the floor of the San Diego Trough, where clayey silts usually are found, proves displacement of sediments from shallow to deep water.

Siltstones were cored at three stations in and near Loma Sea Valley and Coronado Canyon, and are Miocene in age based on assemblages of Foraminifera, diatoms, and Radiolaria.

One hundred and sixty species of benthonic Foraminifera are figured, of which seventy species are discussed as to their ranges of variation of forms and/or synonymies. Two new genera are described: Paradentalina and Recurvoidella; thirty-four new species are described: Ammomarginulina sandiegoensis, Arenoparrella oceanica, Asterigerinata pacifica, Bigenerina hoeglundi, Bolivina peirsonae, B. subargentea, Buccella angulata, Cassidulina bradshawi, C. subcarinata, Cassidulinoides waltoni, Cibicides phlegeri, Cornuspira lajollaensis, Eggerella scrippsi, Epistominella sandiegoensis, Globobulimina hoeglundi, Gyroidina quinqueloba, Haplophragmoides neobradyi, H. Quadratus, Involutina hoeglundi, Karreriella parkerae, Nonion lankfordi, N. parkerae, Nonionella (?) fragilis, Recurvoidella parkerae, Spiroloculina fragilis, Spiroplectammina bathyca, Textularia sandiegoensis, Trochammina chitinosa, T. discorbinoides, T. labiata, T. rhumbleri, Virgulina apertura, V. delicatula, V. sandiegoensis.

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