Eileen Hemphill-Haley and Elisabeth Fourtanier


Drilling at Site 893 provided the first opportunity to examine a pre-Holocene record of diatom biostratigraphy and paleoclimatology from Santa Barbara Basin. The diatoms studied were taken at approximately 500- to 1000-yr intervals in the core, spanning the past 114,000 yr in Hole 893A, and provide evidence for past changes in climate and diatom productivity in the Southern California Bight. Diatom preservation ranges from excellent in finely laminated sediment to poor in bioturbated deposits that were more thoroughly exposed to silica-undersaturated intermediate waters. Dominant taxa in modern Santa Barbara Basin, Chaetoceros spp. and Thalassionema nitzschioides, are likewise dominant in the Quaternary record at Site 893. The Pleistocene/Holocene transition is identified by the appearance of several subtropical species, particularly Fragilariopsis doliola, which persist throughout the Holocene. This pattern is consistent with observations off northern California and Oregon, suggesting that the introduction of F. doliola is a reliable chronostratigraphic marker along much of the length of the California Current. Diatom flux was high during part of Oxygen Isotope Stage 3 (30–46 ka), as determined from diatom mass accumulation rates, but it was greatly diminished preceding the transition to Oxygen Isotope Stage 2 (24–26 ka). During the glacial maximum at 20 ka, diatom flux remained several orders of magnitude lower than during Stage 3. High numbers of benthic diatoms do not correlate with low eustatic sea level, including the glacial maximum. Instead, gradual decreases in relative abundances of benthic diatoms from Oxygen Isotope Stage 5d to Stage 2 (~105–15 ka) could be attributed to preservation differences between younger and older deposits, or to a long-term trend in decreasing downslope transport through time. Allochthonous freshwater diatoms are rare throughout the Hole 893A record, but several small increases during Oxygen Isotope Stages 4 (~65 ka) and 5 (~85 ka) may record increased wind transport of continental diatoms during cold intervals.

A new diatom variety, Thalassiosira simonsenii var. minor. Hemphill-Haley var. nov., and a new combination, Paralia sol (Kützing) Hemphill-Haley comb. nov., are proposed.

Date of initial receipt: 6 September 1994
Date of acceptance: 3 January 1995

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