Determining Sediment Accumulation and Mixing Rates Using 210Pb, 137Cs, and Other Tracers: Problems Due to Postdepositional Mobility or Coring Artifacts

Abstract

Sediment profiles of 137Cs in 12 lakes and of radionuclides (60Co, 134Cs, 226Ra) added experimentally to four lakes at the Experimental Lakes Area, northwestern Ontario, were inconsistent with sediment chronologies derived from 210Pb distributions. Peaks in the 37Cs profiles were sometimes shallower than the depth corresponding to the 1963 fallout maximum (based on 210Pb chronology) and sometimes 137Cs peaks were not observed at all. Expected peaks in the profiles of experimentally added nuclides were similarly absent in three of four lakes. The absence of these peaks at depths corresponding to their time of maximum input to the lakes probably resulted from the initial uptake of the nuclides by littoral sediments followed by the winnowing over periods of several years of nuclide-tagged fine particles to the deeper parts of the basins where the cores were collected. Sediment focussing is manifested by 210Pb sediment inventories at the coring sites greatly exceeding inventories that can be supported by atmospheric deposition of 210Pb in this area.137Cs and the experimentally added nuclides were found much deeper in the cores than the depths corresponding to the time of their initial inputs to the lakes based on 210Pb chronologies. Diffusion of the nuclides in the pore waters could explain some of their downward penetration. However, in some cases, the profiles could only be fit if the effective diffusion coefficient increased with depth. This situation might reflect mixing of these high porosity sediments during core collection or extrusion.Distributions of 137Cs and of the experimentally added nuclides do not provide valid chronologies which can be used to date other features recorded in the sediments of these lakes. Although unsupported 210Pb profiles in these lakes provided excellent fits to classical steady-state sediment accumulation models, 210Pb profiles too may have been affected by sediment mixing or core disturbance. Further studies are necessary to test the reliability of 210Pb-based sediment chronologies in high porosity sediments such as those studied at ELA.