LITTORAL PALEOGEOGRAPHY AND ISOTOPIC RECO RD C-13,O-18 OF EL-NINO EVENTS IN MODERN AND HOLOCENE MOLLUSK SHELLS FROM NW PERU

Mollusks from modern and Holocene littoral deposits and from shell mid dens were collected in the coastal areas of Chao and Santa (NW Peru). The assemblages were analysed f or their heavy isotope (C-13, O-18) co ntent in order to document paleoceanographical changes. Seriated isoto pic measurements were also performed in shells of Trachycardium procer um in order to identify the isotopic signature of El Nino events in mo llusk shells. The C-14 ages obtained on fossil assemblages indicate th at the maximum Holocene sea level was reached as soon as ca. 6500 BP. Between ca. 6500 BP and 4000 BP, a lagoon developed behind a major bea ch ridge at Santa. The presence of a warm water ''panamean'' fauna eas t of the beach ridge complex and its isotopic composition (high O-18 c ontents), contrasting with the cold peruvian fauna from open littoral environments, support this interpretation. During the last 4000 years, a series of beach ridges were built west of the Santa paleolagoon, wh ile at Chao the bay was progressively infilled until 2850 BP. The isot opic signature of the 1982-83 El Nino event in two recent mollusk shel ls of Trachycardium procerum showing growth ring disturbances, is foun d in a approximately 1,7 parts per thousand depletion in O-18 as a con sequence of warm water invading the Peruvian coast. When corrected for the temperature rise, the C-13 contents indicate much lower organic c arbon fluxes during such episodes, i.e. a reduced primary productivity . Isotopic measurements in fossil shells indicate that such thermal ev ents also occurred during middle Holocene time.