Seasonal change in foraminiferal production in the western equatorial Pacific: evidence from sediment trap experiments.

Abstract

Seasonal changes in the flux of individual planktonic foraminifera in the deep ocean have been studied in the western equatorial Pacific warm pool using material collected by sediment traps over a period of 1 yr (1991-1992). Mean total foraminiferal fluxes (TFFs) were 171 and 97 shells m-2 day-1 at Sites N1 (2°59.8′N, 135°01.5′E) and N2 (4°07.5′N, 136°16.6′E), respectively. The seasonal change in TFFs was much higher than for organic matter (OM) and carbonate fluxes. The major species of planktonic foraminifera were Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, Globigerinoides sacculifer, Globigerinella aequilateralis and Globigerinoides ruber. Symbiont-bearing species and N. dutertrei were predominant in the western equatorial Pacific warm pool. Comparison between the warm pool and upwelling (Deep-Sea Research II 43 (1996) 1257, Mar. Micropaleontology 33 (1998) 157) indicates that the foraminiferal assemblage trapped at the western equatorial Pacific warm pool was comparable with the South Equatorial and subtropical assemblages during a plankton tow study conducted at 140°W in 1992. It suggests that fairly oligotrophic conditions prevailed in the western equatorial Pacific warm pool in association with terrestrial nutrient inputs or temporary mixing by wind bursts. The OM flux was more positively correlated with the <63 μm carbonate fraction, mainly composed of coccoliths (r = 0.87) and biogenic opal, which suggests that OM fluxes were more affected by phytoplankton productivity. Although food availability is generally important for foraminiferal production, TFFs were poorly correlated with OM fluxes at Sites N1 and N2, which is partly attributed to the significant influence of symbiont photosynthesis. The δ18O values of P. obliquiloculata and N. dutertrei showed maxima in association with high OM fluxes during Period 2 (from August to October) and Period 4 (from late December to April) at Site N1, reflecting thickening of the mixed layer, following wind bursts. G. sacculifer showed a lunar reproduction cycle, when, between July and October, carbonate production was more predominant than biogenic opal production. A similar profile was evident for G. ruber, Orbulina universa and G. aequilateralis. These four species are generally predominant at low latitudes. Therefore, the lunar reproduction cycle may significantly affect the fluctuation in carbonate production in the equatorial ocean.