SEDIMENTATION PATTERNS OF DIATOMS, RADIOLARIANS, AND SILICOFLAGELLATES IN SANTA-BARBARA BASIN, CALIFORNIA

We report on fluxes of siliceous microorganisms (diatoms, radiolarians , and silicoflagellates), organic carbon, calcium carbonate, biogenic silica, and lithogenic particles in the Santa Barbara Basin (34-degree s-14'N, 120-degrees-02'W), offshore of California, in a sediment trap set 540 m deep, from August 1993 to November 1994. Although total mass flux was dominated by lithogenic components throughout the sampling p eriod, we believe that over-all flux variations at 540 m were closely coupled with oceanographic conditions at the surface. Organic carbon a nd biogenic silica fluxes show distinct variations, with maxima during the upwelling period, from May to July 1994, and low fluxes from Sept ember to March. Diatoms were the main contributor to the biogenic opal fraction (mean daily flux = 3.98 x 10(5) valves m(-2) d(-1)), followe d by radiolarians (mean = 7.05 x 10(3) tests m(-2) d(-1)) and silicofl agellates (mean = 1.48 x 10(3) skeletons m(-2) d(-1)). Each group show ed a distinct pattern, with marked production maxima at different time s of the year: radiolarians in late summer and fall, silicoflagellates in winter, and diatoms in spring, In total, 150 diatoms and 165 radio larians taxa were identified. Species composition associated with flux peaks differed, reflecting seasonal changes in circulation patterns a nd the source of water masses in the basins. Spring upwelling is refle cted by the co-occurrence of deep-living intermediate radiolarian faun a (mainly Lithomelissa setosa) and diatom resting spores (mainly Chaet oceros radicans). Non-upwelling conditions, summer through winter, wit h water entering the basin from the west or the east, are represented by warm-water, surface-dwelling radiolarian fauna and diatoms that are typically found in temperate and temperate-warm offshore waters. The intra-annual sequence of events, each characterized by a distinct diat om assemblage, can be identified in the laminated sediments of the San ta Barbara Basin. Chaetoceros resting spores dominate fluxes in spring , the most productive season, in both the trap and sedimentary records . Thus the effect of preservation in the sedimentary record does not s eem to remove information from the most productive season, as is the c ase in most other areas of the world. Since species in the plankton an d in the trap occur simultaneously and are observed within the surface sediment layer in pristine conditions, we assume that dissolution is minimized by rapid descent through the water column. Dissolution seems to take place immediately below the sediment/water interface, and wea kly silicified species are removed from the sedimentary record.