EL NIÑO OCCURRENCES DURING THE MIS 11: PALEONTOLOGICAL EVIDENCE FROM NORTHERN CHILE

The presence of warm-water mollusk species within faunal assemblage assigned to the Marine Isotope Stage 11 in several localities of northern Chile (23°-30° S) provides evidence for recurrent El Niño conditions ca. 440-400 ka. MIS 11 thermally anomalous molluscan assemblages (TAMA) are found in some coastal areas which allowed the development of larvae of extralimital species. As a few of the same warm-water species found in the MIS 11 TAMA presently appear, during El Niño years, in the most protected area of Mejillones Peninsula (23°S), it is established that enhanced poleward coastal currents drive larval transport from areas located several hundred km to the north. Under present geomorphological and climatic conditions, individuals of these warm-water species can only survive for some time (unless a La Niña event occurs immediately after a warm event, as observed in June 1998), and are not able to reproduce themselves and colonize the area. During the MIS 11, coastal lagoons and wide beach-ridge systems offered favorable environments for the development of TAMAs around Mejillones peninsula. The large and dense populations of MIS 11 fossil shells pertaining to extralimital species of mollusks suggest either that climate conditions were warmer than at present during that interglaciation, or that strong ENSO events were able to frequently bring stocks of larvae and re-furnish the area. The first interpretation is not supported by the fact that many MIS 11 coastal deposits, formerly located in exposed areas, only contain shells of cool-water species, identical to that presently living in the area. Hence, it can be assumed that during the MIS 11, like nowadays, the coastal waters in northern Chile were cooled by the Humboldt Current and upwelling processes. The surprising number of extralimital species of mollusks found in the MIS 11 sediments, and the long distance (more than 2000 km) separating the (present) southern limit (5°S) of biogeographic distribution of several of these species may indicate a greater strength of the ENSO impacts with respect to present Holocene situation. Geochemical studies on fossil shells from MIS 11 deposits, presently underway, may soon provide indications on seasonal and interannual variations of SST and on the occurrence of short-term anomalies which might be related to ENSO conditions.