Temporal and vertical flux changes of radiolarians in the western and central equatorial Pacific during the 1999 La Niña conditions

Abstract

Temporal flux changes of radiolarians were investigated employing time-series sediment traps deployed in the western and central equatorial Pacific during January to November 1999. On board R/V Mirai the sediment traps were deployed at four sites located between 135°E and 175°E, i.e., across the western Pacific warm pool (WPWP) region and the Equatorial Upwelling region (EUR). Total radiolarian fluxes showed higher values at Sites MT1, 2, 5 than that at Site MT3. Radiolarian production reflected levels of biological productivity, i.e., the levels of nutrient supply. The radiolarian flux species compositions at each site did not vary much seasonaly whereas levels of total radiolarian flux varied significantly. Several useful environmental indicator species are presented in this study. For instance, Lithomelissa setosa and Pseudocubus obeliscus are good eutrophic indicators. They are important tracers of the present WPWP migration in the western and central equatorial Pacific. The flux pattern of Lophophaena cylindrica was nearly the same as that of total radiolarians, and this species could represent the whole radiolarian productivity in the western and central equatorial Pacific. Furthermore, in order to understand the vertical radiolarian transport, the sediment trap samples were compared with the samples from plankton tows as well as core tops obtained using a multiple corer. The changes of Nassellaria-Spumellaria ratios between the suspended and sinking radiolarian populations were not significant, but there were significant differences between the sinking and surface sediment populations. It suggested that in the western and central equatorial Pacific the significant dissolution of radiolarians took place in surface sediments. The radiolarian assemblages were affected by selective dissolution. It appears that every species receives different levels of dissolution during their vertical transportation. The results provide important information when radiolarian microfossils are used to reconstruct paleoceanographic conditions.