ENSO FREQUENCY AND PERIODICITY AS INTERPRETED FROM THE 6200-YEAR-LONG RECORD FROM BAINBRIDGE CRATER LAKE, GALAPAGOS

The Galápagos Islands, located within the core of the El Niño/Southern Oscillation region, contain numerous closed-based saline lakes that are ideally situated to provide a potentially continuous, long-term record of paleo-ENSO events. The 6200-year-long stratigraphic record recovered from Bainbridge Crater Lake comprises an uninterrupted sequence of finely laminated, organic-rich, calcareous salts and pyritic muds. Superposed on the century and millennial-scale Paleolimnological, hydrological, and brine evolutionary changes that are evident from the endogenic and allogenic mineralogical fluctuations in this record, the finely laminated nature of the sequence provides an opportunity to examine fluctuations in frequency of occurrence and intensity of ENSO activity. The gray-scale intensity data of xradiographs of the core from Bainbridge were analysed using a variety of time series techniques. Fourier power spectrum wavelet analyses of the entire 6200 year record indicate a relatively complex overall periodicity both in terms of laminae frequency and thickness. However, examination of the Hurst exponents of the frequency spectra suggest persistent behaviour with a periodicity of 52 years in the younger part of the sequence up to about 3000 years ago, followed by a cyclicity of about 11 years in ENSO after 3000 years BP. Highest El Niño frequency occurred at ~1100-1400 yr BP.