deepest core sections appear to be above background level, the excess Po-210 activity cannot be calculated because there is no estimate of the background level of Po-210. We analyzed one sediment core from Hendrix Lake in southwestern Arkansas for excess (210)Pb and (137)Cs. A number of subsequent radioactive decays occur over a period of minutes and Pb-210 (half-life.3 yr) is produced. The 'correct' value of background Po-210 is that which provides the best fit.e. Certainly in some cores this is not true because two or more distinct slopes can be seen in the Po-210 activity profiles. This logic can be extended to calculate the age of sediments at other depths in the sediment column and/or the rate of sediment accumulation. Cumulative dry sediment plot if the upper part of the core data is disregarded. It is possible to indirectly estimate the background Po-210 by measuring the Ra-226 (via Rn-222) in the sediments but this is often omitted because of the additional analytical costs. Analysis of Pb-210 Data, the Slope Regression Model, in a 'perfect' core, if log excess Po-210 activity is plotted as a function of accumulated dry weight of sediment, the line through the data should be a straight line.
The Po-210 is much diluted in this location and the surface activity may be less than 10 DPM/g dry weight, while at the same time background may be 3 DPM/g. Often the fits are very good over a large range of background Po-210 values tested, and in these cases, only lower and upper boundaries for the sediment accumulation rate can be expressed.
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In lieu of these analyses, it is necessary to make an assumption that the background level is less than the lowest activity measured in the core but greater than zero. Four independent evidences (two-marker events based on (137)Cs and two marker events based on Hg mining activity) result in about the same sedimentation rates and thus, we endorse earlier suggestion that (210)Pb profile always needs to be validated with at least one another independent method. Less dramatic changes in sediment accumulation rates (or Pb-210 deposition) may not be detected and could yield accumulation rates which are somewhat in error. Last Updated Jan 7, 2009 Flett Research Ltd. Thus, in a Shield core, a 50 error in background determination makes very little difference when calculating a sediment accumulation rate. The Pb-210 method is used to determine the accumulation rate of sediments in lakes, oceans and other water bodies. Cumulative dry weight of sediment. In cores that are too short to include background, it generally should not be used.
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