Preservation of Plankton Shells in an Anaerobic Basin off California

The sill depth (480 m) in the Santa Barbara Basin, California, separates two strikingly different planktonic sediment assemblages. The anaerobic part of the basin below the sill depth contains abundant aragomtic shells (pteropods and pelecy-pods) and relatively high concentrations of calcitic shells (planktonic foraminifera). In the oxygenated region above sill depth there are no aragonitic shells and concentrations of planktonic foraminifera are much reduced. Percentages of species, size classes, and morphological variants of planktonic foraminifera differ between samples from above and below the sill depth. In contrast, concentrations and faunal composition of siliceous shells (radiolarians and diatoms) apparently are essentially the same above and below the sill. The most important mechanism responsible for the differences between oxygenated and anaerobic calcareous assemblages probably is selective dissolution. Bottom transport may also be important, especially for slowly settling shells that may frequently be returned to suspension by benthomc activity in the aerobic region and consequently settle preferentially in the undisturbed anaerobic region. The accumulation rate of large planktonic fora-minifera in the anaerobic sediment provides an excellent measure of shell output. Comparison with the standing stock of planktonic foraminifera in the productive surface waters yields minimum turnover times of 2 to 3 weeks for the living planktonic foraminifera in the Santa Barbara Basin.