Over time the uranium-238 very slowly decays into uranium-234 (half-life = 248,000 ).
Radium-226 in the soil exhibits the same level of radioactivity as uranium-238 from which it was originally derived, because of a natural phenomenon called secular equilibrium.
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.
An iterative best fit computer model was designed to process the data in cases where the Po-210 background activity is not known. Since the excess Po-210 activities depend directly on the value of the background Po-210 activity, it is evident that only one level of background will yield a perfectly linear fit of the data.
In the case of (1) and/or (2), the data may still form a straight line on a log [excess Po-210 activity] vs.
cumulative dry sediment plot if the upper part of the core data is disregarded.
When applying the Pb-210 technique, we assume that lake and ocean sediments are receiving a constant input of Pb-210 from the atmosphere.
While it may not be very satisfying to learn that the sediment accumulation rate in your core is between 200 - 600 mg dry weight/cm Cores which are rich in organics tend to have higher Po-210 activities in their surface sediments, and as a consequence, generally yield better data because the excess Po-210 is much higher than the background Po-210.
For example, Canadian Shield lakes can have surface Po-210 activities of 200 DPM/g dry and background of 3 DPM/g.
In case (3), where the 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.
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.