Sediment dating with 210Pb

210Pb is analysed by extracting the grand-daughter 210Po and counting it in an alpha spectrometer. However a portion of the 210Pb that is supported by 226Ra within the sediment is also extracted and this background must be deducted from the total 210Pb concentration values.

sediment dating pb graph

The background is estimated by extending the depth of sampling to determine the region in which the curve flattens out, thus showing supported 210Pb only. When the average 210Pb concentration of at least two deep samples is subtracted from all data, a semi‑log plot of unsupported 210Pb against depth or cumulative weight (to allow for compacting) will be a straight line if the sedimentation rate has been constant. Least squares regression is used to determine the slope of the decay curve, from which the sedimentation rate can be calculated. The method is most reliable when unsupported 210Pb and the fission product 137Cs are analysed and interpreted together, i.e. the 137Cs provides absolute tie points for the 210Pb profile.

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