Planktonic foraminifera: factors controlling sinking speeds

Abstract

Sinking speeds of 330 specimens belonging to 10 extant species of planktonic foraminifera were determined in a sinking column device filled with 3°C seawater. The sinking speed is governed primarily by shell weight and presence/absence of spines. For example, preserved planktonic specimens of Orbulina universa, whose shell weight ranged from 2 to 21 μg, sank 122 to 583 m day −1, with a correlation coefficient of 0.92 on log-log scale. Progressive shell thickening during foraminiferal growth accounts for some of the higher sinking speeds. In addition, shells from sediment on the average sink about three times faster than shells (of equivalent size and species) of planktonic foraminifera collected in near-surface waters. These high values are in part due to the shells often being encrusted with clay and nannoplankton remains. In contrast, the sinking speeds of the spinose species are approximately 3-fold slower than those of the non-spinose species. Based on data from plankton tows, most planktonic foraminifera > 150 μm reach the mean ocean depth of 3800 m in 3 to 12 days depending upon shell weight and presence or absence of spines. Estimated Reynolds numbers range from 0.05 to 24.85 and most exceed a value of 0.5 which is an upper for limit Stokes' Law range, suggesting that foraminifera are out of Stokes' sinking range. The Reynolds number and drag coefficients are negatively well correlated, indicating that drag is one of the important controlling factors in the sinking regime. The presence of spines is significant in increasing drag, decreasing the Reynolds number, and hence reducing the sinking speed.