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The overall result is that radium is found at low and essentially unchanging levels in soils everywhere. Radium in the soil exhibits the same level of radioactivity as uranium from which it was originally derived, because of a natural phenomenon called secular equilibrium. When applying the Pb technique, we assume that lake and ocean sediments are receiving a constant input of Pb from the atmosphere. The deepest sections analyzed may still appear to be above background levels of Po, as evidenced by a non vertical profile in the deepest part of the core. Pb is a naturally occurring radioactive element that is part of the uranium radioactive decay series.
From the accumulation rate, the age of sediment from a particular depth in the sediment column can be estimated. The data may reveal a vertical Po activity profile in the core surface. It is also assumed that the rates of isotope input and sediment input are constant over time.
Understanding the Pb Method
If one assumes that the accumulation rate has remained constant in the upper, more recent sediments, then the age of the sediments can be calculated for any depth in the core. This will allow the determination of accumulation rate for the mid portion of the core. For additional information, contact Dr.
The Po activity may peak slightly below the sediment surface. The excess Po is assumed to be from direct atmospheric deposition of Pb plus the import of Pb from the watershed. Robert Flett at flett flettresearch. Often the fits are very good over a large range of background Po values tested, and in these cases, only lower and upper boundaries for the sediment accumulation rate can be expressed.
Since the excess Po activities depend directly on the value of the background Po activity, it is evident that only one level of background will yield a perfectly linear fit of the data. 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. Although the concentration of uranium varies from location to location, it is present in essentially all soils and sediments, at least at some low level. Certainly in some cores this is not true because two or more distinct slopes can be seen in the Po activity profiles.
In other cores it appears as though a sudden deposition of low activity sediment has occurred on top of higher activity sediments, perhaps as a consequence of dredging or sediment transport. This is often seen and is usually interpreted to mean mechanical mixing of the surface sediments by benthic organisms or by hydrodynamic activity of the overlying water. It is possible to indirectly estimate the background Po by measuring the Ra via Rn in the sediments but this is often omitted because of the additional analytical costs. The situation is quite different, however, in a core taken from a river delta where the rate of inorganic sediment deposition is high.
The Pb which falls into a lake or ocean tends to end up in the sediments over the next few months and becomes permanently fixed on the sediment particles. This is commonly seen and may be caused by steep redox gradients across the uppermost few centimeters of sediment. In spite of these potential difficulties, researchers who use the Pb technique tend to continue with it because the results often make sense when corroborated with other information. This model does not require that the rate of sediment accumulation be constant over time. In a typical application, the average accumulation rate over a period of - years is obtained.
Understanding the Pb Method The Pb method is used to determine the accumulation rate of sediments in lakes, oceans and other water bodies. It is important to note that, in all of the foregoing discussions, the rate of sediment accumulation and Pb deposition have been assumed to be constant.
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