Time-series sediment trap experiments were carried out in the western to central equatorial Pacific (Sites MT3, MT4, MT5, and MT7) from 1999 to 2002, over the transitional time period from La Niña to El Niño conditions, in order to evaluate temporal variability in planktic foraminiferal fluxes and their assemblages. The foraminiferal test flux follows a trophic gradient from higher fluxes in the Equatorial Upwelling Region (EUR) and lower fluxes in the Western Pacific Warm Pool (WPWP) region. Globigerinoides ruber commonly dominates in all sites through the experimental period. Trilobatus sacculifer, Globigerinita glutinata, and Neogloboquadrina dutertrei occurred especially in the WPWP. In contrast, Globigerina bulloides and Pulleniatina obliquiloculata characterized the fauna in the EUR.

A change in hydrologic conditions from La Niña to El Niño was documented along the sites during the sampled time interval. Simultaneous with an eastward advancement of the WPWP, the EUR retreated to the east. Rapid decreases in the fluxes of G. ruber, G. bulloides, and P. obliquiloculata were recognized immediately after the more oligotrophic WPWP conditions prevailed at Sites MT4, MT5, and MT7. Fluxes of the total planktic foraminifer assemblage increased both in the EUR and in the western side of the WPWP under full El Niño conditions during the second half of 2002. Such increases in test fluxes in the western side of the WPWP were mainly attributed to G. ruber and N. dutertrei, suggesting inputs of eutrophic waters from the northern coastal area of Papua New Guinea. Test fluxes under El Niño conditions in the WPWP were at the same level as those in the EUR, giving rise to strongly increased carbonate fluxes in the western equatorial Pacific.

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