1 Journal of Paleolimnology 27: 1–7, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. A ~6100 14 C yr record of El Niño activity from the Galápagos Islands Melanie A. Riedinger 1 , Miriam Steinitz-Kannan 2 , William M. Last 3 & Mark Brenner 4 1 Department of Biology, Northeastern Illinois University, Chicago, IL 60625-4699, USA (E-mail: M-Riedinger@neiu.edu) 2 Department of Biological Sciences, Northern Kentucky University, Highland Heights, KY 41099-0400, USA (E-mail: kannan@nku.edu) 3 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada (E-mail: WM_Last@UManitoba.ca) 4 Department of Geological Sciences, University of Florida, Gainesville, FL 32611-2120, USA (E-mail: brenner@ufl.edu) Received 23 July 2001; accepted 19 September 2001 Key words: El Niño, ENSO, Galápagos Islands, hypersaline lake, laminations, mid-Holocene climate Abstract Lithostratigrahic and mineralogic analyses of sediments from hypersaline Bainbridge Crater Lake, Galápagos Is- lands, provide evidence of past El Niño frequency and intensity. Laminated sediments indicate that at least 435 moderate to very strong El Niño events have occurred since 6100 14 C yr BP (~7130 cal yr BP), and that frequency and intensity of events increased at about 3000 14 C yr BP (~3100 cal yr BP). El Niño activity was present between 6100 and 4000 14 C yr BP (~4600 cal yr BP) but infrequent. The Bainbridge record indicates that there has been considerable millennial-scale variability in El Niño since the middle Holocene. Introduction Our understanding of Holocene climate variability in the tropics is constrained by the limited number of high- resolution climate records from this region that extend back beyond the last few centuries (Thompson et al., 1992; Dunbar et al., 1994; Rodbell et al., 1999). Within the neotropics, long-term records are crucial for exam- ining historical changes in the linked oceanic-atmos- pheric processes that regulate the El Niño/Southern Oscillation (ENSO) phenomenon and impact global cli- mate. Since the 1982–1983 El Niño event, a wealth of biological, geological, and archaeological data has been used to document historical ENSO activity (Thompson et al., 1984; DeVries, 1987; Wells, 1987; Cook, 1992; Martin et al., 1993; Sandweiss et al., 1996). The vary- ing nature and resolution of these diverse records have generated much debate regarding the timing of onset and Holocene periodicity of ENSO (Ortlieb et al., 1993; Rapid communication DeVries et al., 1997; Wells & Noller, 1997; Sandweiss et al., 1998). A common shortcoming of some data sets is that the postulated ENSO signal might result from both ENSO and non-ENSO events, and a clear cause- and-effect relationship to ENSO activity is not estab- lished. This becomes more problematic for records derived from outside the core region of ENSO activ- ity in the equatorial Pacific, or from areas subject to considerable environmental or climatic fluctuations. The Galápagos Islands, located within the core ENSO region, consistently experience positive precipitation and sea surface temperature (SST) anomalies during El Niño events, and possess a number of hypersaline lakes that experience reduced salinity during these periods (Tupiza, 1985). Galápagos lake sediment deposits archive the history of salinity fluctuations, and thus provide a record of ENSO activity. We present here a ~6100 14 C yr high- resolution record of El Niño frequency and intensity based on lithostratigraphic and mineralogic analyses of