 2001 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; December 2001; v. 29; no. 12; p. 1075–1078; 2 figures. 1075 Planktonic foraminiferal response to the 1997–1998 El Nin ˜o: A sediment-trap record from the Santa Barbara Basin David E. Black Department of Geology, University of Akron, Akron, Ohio 44325, USA Robert C. Thunell Eric J. Tappa    Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA Figure 1. Bathymetric map (100 m contours) of Santa Barbara Basin and location of sediment-trap mooring (triangle). ABSTRACT Here we report results on a sediment-trap study from the San- ta Barbara Basin in which we examine the response of planktonic foraminifera to El Nin ˜ o conditions. Planktonic foraminifera census counts were performed on biweekly sediment-trap samples collect- ed over the interval spanning late summer 1995 through early spring 1998, a period that captured the transition from ‘‘normal’’ conditions in 1996 to full El Nin ˜ o conditions during early 1997 through early 1998. Intra-annual variations in the foraminifera as- semblage reflect changes in seasonal climate variability, significant differences occurring in both foraminiferal relative abundance and flux values between non–El Nin ˜o and El Nin ˜ o years. Warmer wa- ters in the basin during late 1997 were reflected by a significantly increased presence of warmer-water species generally absent under normal conditions. Surprisingly, despite greatly reduced upwelling conditions, foraminiferal flux values during the spring 1997 up- welling season were more than four times greater than the year before. The contrast between normal and El Nin ˜ o foraminifer pop- ulations has important implications for reconstructing the past his- tory of both El Nin ˜ o events and more general climate histories from the Santa Barbara Basin varved sediment record. Keywords: Santa Barbara Basin, El Nin ˜o–Southern Oscillation, fora- minifera, sediment traps, paleoclimate. INTRODUCTION The El Nin ˜o–Southern Oscillation (ENSO) is the largest source of interannual variability in the modern global climate system (e.g., Cane, 1986; Enfield, 1989). The connection between ENSO and cli- mate is not restricted to the region directly in contact with the El Nin ˜o current, but affects climate patterns on a global scale (Cane, 1986). An improved knowledge of the physical processes that occur during ENSO development has allowed for short-term forecasts (Zebiak and Cane, 1987; Barnett et al., 1988; Zebiak, 1989), but past variations in the ENSO system are not well understood (Cole et al., 1992). Santa Barbara Basin sediments are best known for their use in creating glacial-interglacial and millennial-scale paleoclimate recon- structions (e.g., Kennett and Ingram, 1995; Behl and Kennett, 1996). Laminae-thickness and sediment-fabric studies have been used to char- acterize forcing mechanisms and their climatic responses on subdecadal and multidecadal time scales (Biondi et al., 1997; Schaaf and Thurow, 1997; Schimmelmann et al., 1998). High-resolution sediment-based studies of microfossil assemblages, stable oxygen isotopes, biomarkers, and other proxies have also been used to generate detailed records of subtropical North Pacific climate variability (e.g., Kennedy and Bras- sell, 1992; Schimmelmann and Kastner, 1993; Weinheimer et al., 1999; Field and Baumgartner, 2000; Zhao et al., 2000). However, in order to better interpret the sediment record, we must improve our understand- ing of how discrete climate events like El Nin ˜o affect the production of material that eventually becomes the sediment record. We have been carrying out a sediment-trapping and hydrographic monitoring program in the Santa Barbara Basin with the goal of un- derstanding how seasonal and interannual climate fluctuations affect sediment production in the basin (Thunell, 1998). This study presents the results of planktonic foraminiferal analyses performed on sediment- trap samples from the Santa Barbara Basin collected between Septem- ber 1995 and April 1998, a period encompassing the 1997–1998 El Nin ˜o. STUDY SITE The Santa Barbara Basin, bounded by the California coast to the north and the northern Channel Islands to the south, is the northernmost basin of the Southern California Borderlands (Fig. 1). Seasonal changes in the positions of the North Pacific High and the adjacent continental thermal low result in changes in wind speed and direction, which in turn control the strength of the California Current (Huyer, 1983). Dur- ing spring and early summer, winds are strongest and from the north, causing Ekman-induced upwelling at this time. During fall and winter, the northerly component of the wind weakens, and upwelling within the basin is greatly reduced or largely shut off. During El Nin ˜o, the California Current weakens, and warm tropical-subtropical waters propagate northward along the western margin of North America. MATERIALS AND METHODS A times-series sediment-trapping program was started in the Santa Barbara Basin in June 1993. Since then samples taken over a two-week period have been collected using an automated sediment trap deployed in the center of the basin (34°14'N, 120°02'W; 490 m water depth; see Thunell et al. [1995] and Thunell [1998] for earlier results). Throughout the study period, conductivity-temperature-depth (CTD) measurements of the upper 100 m of the water column were made on a biweekly basis at the mooring location. Upon recovery, samples were stored in sealed containers and re- frigerated. Whole samples were split, and a one-sixteenth split was used for faunal population census counts. Counts were made of all species present in the 125 m fraction of each sample (ranging from 19 to 2810 individuals, averaging 619 individuals per sample); a stan- dard binocular microscope was used.