SUBTROPICAL PACIFIC HOLOCENE CLIMATE VARIABILITY AND EL NIÑO: STABLE ISOTOPE AND BIOGENIC SILICA EVIDENCE FROM SANTA BARBARA BASIN

Decadal- to millennial-scale climate variability in the northeast Pacific is apparent in planktonic foraminiferal stable isotope and biogenic silica records of early to middle Holocene age from the Santa Barbara Basin, offshore southern California. Dramatic changes in climate have taken place in as little as 20-30 years during the Holocene, most notably near 7.6 ka when the Santa Barbara Basin experienced rapid cooling then warming to maximum Holocene temperatures. Since Santa Barbara Basin is affected by the Pacific Decadal Oscillation (PDO), periodicities of 20 to 60 years observed in our records are attributed to this phenomenon. Lower frequency variability (~1,500 year periodicity) appears to be related to changes in global ocean circulation or more likely to associated atmospheric climate effects.

Based on six years of sediment trap and concurrent CTD data, we have constructed a two end-member hydrographic model for the Santa Barbara Basin. The "non-El Niño" end-member is equivalent to modern spring upwelling conditions when equatorward California Current flow is strong and productivity is high. This is represented in the sediment trap and in our down-core record by high concentrations of biogenic silica (a proxy for productivity) as well as a small difference between the oxygen isotope values of Globigerina bulloides (a surface dwelling foraminifer) and Neogloboquadrina pachyderma (a thermocline dwelling foraminifer). The "El Niño" end member is equivalent to late summer and fall thermal stratification of the upper water column when the poleward California Undercurrent rises to the surface of the basin and productivity is suppressed. This is represented in the data as low silica concentrations and a large foraminiferal d¹⁸O difference. Our records suggest that upwelling and productivity are lowest in the Santa Barbara Basin between 5.2 and 3.8 ka, thus we conclude that El Niño was most intense and/or frequent during this mid-Holocene interval. This lies within the period of peak Holocene warmth and indicates that El Niño is strongest when the climate is warmest.