Differences between the methods are greatest where uncertainties exist in assigning a rate by one or both methods (i.e., 210Pb trends and/or "possible" horizon assignments).
Both methods show the same relative rates, with greater sediment accumulation more common in the upper and middle estuary and less toward the mouth.
Whenever possible, two methods should be used for dating sediments because a rate, albeit erroneous, can be obtained isotopically in sediments that are mixed; accurate sedimentation rates are also difficult to determine where the time boundary is a zone rather than a horizon, where the historical record does not provide a precise date for the pollen horizon, or where scouring has removed some of the sediment above a dated pollen horizon.
The study of sedimentation rates and age of marine sediments has been performed through many years using natural and artificial radioactive isotopes.A finite difference model (SEDIMIX) was used to find the combination of sedimentation and Fickian redistribution that provided the best fit to the 210Pb sediment data.Comparisons of sedimentation rates obtained by 210Pb and pollen analyses of 1-m cores collected throughout the Potomac Estuary show good agreement in the majority of cores that can be analyzed by both methods.Spectra are analysed for different environmental radioisotopes, including 210Pb, 226Ra, 137Cs, 134Cs, 241Am, 7Be and 40K.The focus of this work was geochemical cycling of 210Pb in lakes, including the water column, sediments, and their interactions with each other.The results indicate that geochronologic studies of estuarine sediments should be preceded by careful observation of sedimentary structures, preferably by X-radiography, to evaluate the extent of mixing of the sediments.Time horizons, whether paleontologic or isotopic, are generally blurred where mixing has occurred, precluding precise identification. Anoxic conditions caused remobilization of reduced iron, which reprecipitated at the oxycline and returned to the bottom via settling.210Pb followed the same pattern except that, at the interface, it was scavenged rather than precipitated.Below the epilimnion, vertical eddy diffusion was calculated by the heat flux gradient method including corrections for both radiant heating and heat loss to sediments.Vertical mixing was very low because of stability imparted by a steep temperature/density gradient extending right to the sediment water interface.