SEDIMENT FOCUSING AND PB-210 DATING - A NEW APPROACH

In this paper we test the utility of the mud deposition boundary depth (mud DBD) theory (Rowan ct al. 1992) as a means of maximizing samplin g efficiency in paleolimnological investigations, particularly those t hat apply to Pb-210 dating. The mud DBD is defined by the relationship between near bottom wave velocity and particle threshold velocity, wi th wave and particle threshold theory simplified to terms of exposure and depth. Mud DBD theory can be used to define the depositional zone in lakes, and within the depositional zone defined by the mud DBD: 1) there is a high probability of obtaining a representative core, 2) var iation in mass sediment accumulation rate (MSAR) is not correlated wit h water depth, and 3) variation in MSAR is considerably reduced from t he whole lake average. This suggests that mud DBD theory can account f or the effects of sediment focusing, and that the mud DBD defined depo sitional zone is the zone to which fine-grained sediments are focused. Finally, we have shown that to optimize sampling effort, 5 to 10 core s within the depositional zone are necessary for a reasonably precise estimate of the mean mass sediment accumulation rate. In addition, the use of mud DBD theory prior to sampling can dramatically reduce the c ost associated with analyzing large numbers of cores for Pb-210.