Late Quaternary Climatically Related Planktonic Foraminiferal Assemblage Changes: Hole 893A, Santa Barbara Basin, California

Abstract

Planktonic foraminiferal assemblages are described quantitatively from Hole 893A, a continuous 196.5-m uppermost Quaternary sequence from Santa Barbara Basin, Southern California. A record of glacial/interglacial cycles is clearly exhibited by changes in planktonic foraminiferal assemblages during the last -160 k.y. Faunal changes in the sequence record the northsouth movement of surface-water masses related to the changing relative strengths of the cool California Current and the warm Inshore Countercurrent (Davidson Countercurrent). Glacial maxima (Stages 2, 4, and 6) and late Stage 5 are associated with a Subarctic Assemblage when Subarctic waters had migrated to Southern California, south of Santa Barbara Basin. Average seasurface temperatures in the basin were then less than 10°C. The warmest climatic episodes (Stage 1, or Holocene, and Substage 5e, or Eemian) are both associated with the Transitional Assemblage, which occurs in present-day basinal surface-waters (average sea-surface temperature of 15°C). The only prolonged climatic interval represented by assemblages intermediate between Subarctic and Transitional assemblages is Stage 3 (60 to 25 k.y.). This interval is also marked by the highest temporal variability exhibited by the faunas during the last 160 k.y., in turn reflecting strong climatic oscillations that have also been described in the Greenland Ice Sheet. It is inferred that during Stage 3, the southern boundary of Subarctic waters and the 10°C sea-surface isotherm lay close to the Santa Barbara region. Oscillations in the position of this faunal boundary thus produced the observed high variability in planktonic foraminiferal assemblages during this interval. Comparisons with modern planktonic foraminiferal distributions suggest that average sea-surface temperatures in Santa Barbara Basin during glacial maxima were ~7°-8°C compared with ~15°C during the warmest interglacial episodes, including the present time. The range in average sea-surface temperature between glacial to interglacial extremes in this region is thus ~7°-8°C. Changes in planktonic foraminiferal assemblages during the last deglaciation (17 to 9 ka) assist in detailing aspects of climate change, including the Younger Dryas cool episode (-13 to 11.1 ka). Cooling during the Younger Dryas was not of full glacial scale; sea-surface temperatures remained higher than 10°C, but Subarctic waters remained close to the Southern California Borderland. Relatively high abundances throughout the sequence of Globigerina bulloides, a form often considered to represent a proxy for oceanic upwelling, suggest that significant coastal upwelling continued to influence the Santa Barbara Basin throughout the latest Quaternary. This includes glacial times when lower sea levels reduced connections between the basin and the open ocean.