Seasonal production and morphologic variation of Neogloboquadrina pachyderma (Ehrenberg) in the Northeast Pacific

A high resolution study of seasonal variation in the flux of the planktonic foraminifer Neogloboquadrina pachyderma to the sea floor has been carried out in the northeast Pacific (50°N, 145°W). Using an automated time series sediment trap moored at 3858 m, a total of 24 consecutive two-week samples were taken from September 1982 to September 1983. Two distinct morphologic groups of N. pachyderma were recognized, based on textural and coiling differences. Variations in the shell flux of each N. pachyderma group are directly related to seasonal changes in water temperature and structure and depth of the thermocline. Group A, predominantly sinistral forms with a crystalline, heavily calcified test, is most abundant when surface water temperatures are less than 8°C and the upper 100 m of the water column are well mixed. In contrast, the less calcified, reticulate forms of Group B are predominantly dextrally coiled and are most abundant when surface water temperatures are greater than 8°C and the thermocline is well established at depths between 5 and 20 m. The association of each N. pachyderma group with different near-surface water conditions suggests they are strongly dependent on specific thermal environments. By examining the variation in the shell flux of N. pachyderma as it relates to seasonal changes in local hydrography, it is possible to better understand the ecology of this important paleoclimatic indicator.