Seasonal succession, vertical distribution, and dissolution of planktonic foraminifera along the Subarctic Front: Implications for paleoceanographic reconstruction in the northwestern Pacific

The regional spatial and depth distribution of planktonic foraminifera were studied through sediment trap observations at two stations, Stations WCT-5 (about 41° N, 150° E) and WCT-6 (about 42° N, 155° E), along the Subarctic Front in the northwestern Pacific Ocean. The sediment traps were moored, at 1000 and 4500 m depths at Station WCT-5, and 1091, 2051 and 4871 m depths at Station WCT-6. The samples were collected from 17 August 1998 to 10 August 1999 at Station WCT-5 and from 15 August 1999 to 6 August 2000 at WCT-6. At Station WCT-5, two distinct annual periods of increased total foraminiferal and species production were evident: the period of highest production was in autumn, with a secondary period in early summer. At Station WCT-6, high total foraminiferal shell fluxes occurred in April–May and September–October. The April–May “spring bloom” of planktonic foraminifera apparently began each year as the sea surface temperature (SST) increased after winter and the decrease of thickness of the mixed water layer during June–July in every year following the cessation of bloom. The most common foraminiferal species in the study region were Globigerina quinqueloba, Neogloboquadrina pachyderma, Neogloboquadrina dutertrei, Neogloboquadrina incompta, Globigerinita glutinata, Globigerina bulloides, Globigerina falconensis, Globorotalia theyeri, Globorotalia scitula, and Globigerinella calida. Foraminiferal fluxes displayed a distinct seasonality at all trap depths at both stations. At Station WCT-5 in 1998–1999, foraminiferal species were scarce in the surface water during spring and summer, and the dominant foraminiferal species were most productive during winter and autumn. At Station WCT-6, the dominant foraminiferal species were most productive during late winter to spring, and all species showed a peak in middle spring (May). A comparison of seasonal patterns of dominant foraminiferal species between the northwestern and northeastern Pacific demonstrated that foraminiferal productivity was a good proxy for sea surface temperature and mixing of water column. Significant dissolution of foraminiferal tests was evident in the deepwater column and was used to estimate the foraminiferal lysocline depth (∼4500 m) in the northeastern Pacific basin.