Seasonality of biogenic particle and planktonic foraminifera fluxes: response to hydrographic variability in the Kuroshio Extension, northwestern Pacific Ocean

Abstract

In order to infer the relationship between the sinking biogenic particles as well as planktonic foraminiferal fluxes to local hydrography time-series sediment trap observations were conducted at two stations, Stations WCT-3 and -7, along the Kuroshio Extension (KE) in the northwestern Pacific. The sediment trap samples were collected from 29 August 1998 to 10 August 1999 at Station WCT-3 and from 19 August 1999 to 29 August 2000 at Station WCT-7. Large seasonal, spatial and depth variations were observed in the total mass flux and the biogenic particle fluxes at both stations. The annual mean mass flux in the deep trap is ∼3 times higher than that in the shallow trap at Station WCT-3 and around twice at Station WCT-7. No distinct seasonality of biogenic particles as well as planktonic foraminiferal fluxes was observed at WCT-3 except very high fluxes during summer both in shallow and deep traps. On the other hand, a distinct bi-modal distribution of all biogenic particle fluxes was observed over almost one year at Station WCT-7, with a comparatively low value in winter and high peak in summer in both shallow and deep traps. The concentration of calcium carbonate and the organic matter decreased with depth and those of biogenic opal increased with depth at both stations. The increased fluxes of all biogenic particles in the deep traps at both stations can be explained by lateral transport. The most abundant planktonic foraminiferal species at both stations are Neogloboquadrina pachyderma (dex. and sin.), Neogloboquadrina dutertrei, Globigerina bulloides, Globigerinoides ruber, Globigerinoides sacculifer, Globigerinoides tenellus, Globigerinella aequilateralis and Globigerinita glutinata. Despite some temporal variations in relative abundance, dominant planktonic foraminiferal species showed similar distribution pattern throughout the year at both stations. Seasonal variation in species dominance also occurs at both stations, changing from G. ruber, G. tenellus and G. sacculifer in September–November to Globorotalia truncatulinoides, Globorotalia inflata and G. glutinata in January–April, and to N. pachyderma, N. dutertrei and G. bulloides in May–July. The magnitude of test fluxes and species turnover showed strong seasonality in accordance with variations in the local hydrography, which is influenced by the shift of the meandering Kuroshio and by the interaction between the subarctic water mass (Oyashio) and KE waters. The influence of local hydrography on the foraminiferal fluxes is highlighted by the simultaneous increase in total foraminiferal flux with an increase in sea-surface temperature (SST) as well as a strengthening in the stratification of the water column.