Redeposition of pollen grains in lake sediment

Evidence from sediment traps shows that in dimictic Frains Lake pollen grains and the bottom mud in which they are contained are resuspended during spring and autumn mixing, while in Sayles Lake, a nonstratified lake nearby, similar quantities of pollen-bearing sediment are resuspended at irregular intervals throughout the year. In the nonstratified lake resuspended material is poorly mixed in the lake water, while in the dimictic lake, during the seasons of water mixing, resuspended sediment is similar in amount, percentage a5 .h weight, and pollen composition at various levels throughout the water column. Resuspension occurs without sorting or differential movement of individual pollen grains. The pollen content of redeposited sediment serves as a tracer, showing that sediment is moved from the littoral zone to the deeper basin of the lake. In the littoral zone annual stirring may involve the uppermost 6-12 mm of sediment; even in the deeper part of the basin, the uppermost millimeter at least is stirred by this process every year.