OCEANIC CONDITIONS IN THE EASTERN EQUATORIAL PACIFIC DURING THE ONSET OF ENSO IN THE HOLOCENE

Records from South America show that modern ENSO did not exist 7ckyr (calendar kiloyears) ago and has developed progressively since then. There has been little information available on oceanic conditions in the eastern equatorial Pacific (EEP) to constrain explanations for ENSO onset. We report the first quantitative observations (planktonic foraminiferal isotopes, planktonic and benthic foraminiferal assemblages, sediment 231Pa/230Th)on thermocline and mixed layer conditions in the EEP during ENSO start up. Using a suite of 4 core locations, we found that thermocline and mixed layer temperatures were higher in the mid-Holocene and cooled towards the present. Also, thermocline and mixed layer nutrients and biological productivity were lower in the past and have increased over the past 7ckyr. Our data indicate increased upwelling of cooler waters since 7ckyr in the EEP. Modeling shows that the oceanographic changes were driven by extratropical processes, giving these a role in conditioning the development of ENSO.