Abstract. The radionuclide Pb is suitable for century-scale dating and has been used to calculate the sedimentation rate in a variety of. The Pb method is used to determine the accumulation rate of sediments in lakes, oceans and other water bodies. In a typical application. The naturally lead isotope Pb may be used to date aquatic sediments and peat bogs. The method is suitable for dating approximately years back.
Analysis of Pb Data The Slope Regression Model In a 'perfect' core, if log [excess Po activity] is plotted as a function of accumulated dry weight of sediment, the line through the data should be a straight line.
Excess Po is the amount of the Po isotope that is in excess to the background Po produced in the sediments by Ra The excess Po is assumed to be from direct atmospheric deposition of Pb plus the import of Pb from the watershed. It is also assumed that the rates of isotope input and sediment input are constant over time. In the real world, cores are often not 'perfect' and they exhibit deviations from the ideal data set: The data may reveal a vertical Po activity profile in the core surface.
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.
The Po activity may peak slightly below the sediment surface. This is commonly seen and may be caused by steep redox gradients across the uppermost few centimeters of sediment.
Dating of Sediments using Lead-210
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.
This will allow the determination of accumulation rate for the mid portion of the core. 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. In case 3where the deepest core sections appear to be above background level, the excess Po activity cannot be calculated because there is no estimate of the background level of Po 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.
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 background activity is not known.
It is based on the assumption that a 'perfect' core will exhibit a linear decrease in log [excess Po activity] vs.
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. The computer model performs a number of linear regressions, each time using a slightly different value of background Po The 'correct' value of background Po is that which provides the best fit i.
R2 closest to 1. One can then calculate the sediment accumulation rate using the best fit line through the data. 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.
Cores which are rich in organics tend to have higher Po activities in their surface sediments, and as a consequence, generally yield better data because the excess Po is much higher than the background Po The situation is quite different, however, in a core taken from a river delta where the rate of inorganic sediment deposition is high.
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. The sediment dating may be supplemented with estimations of the content of alga pigments in the depths required. Thus, it will be possible not only to describe the sediment age, but also to give a picture of the dominant mixture of plankton alga at the same time.
Sediment studies may also be supplemented with determinations of e. Method Decay of uranium in the earth's crust releases the gas radon. This gas produces Pb by decay in the atmosphere. In the water phase Pb is adsorbed to particulate matter and together they are deposited in the sediment.
Understanding the Pb Method
The remaining amount of Pb at a certain depth will reveal the age of the given sediment layer. This dating method covers the past period of years due to the half-life of about 22 years. Scope of application Dating of sediment is used to calculate sedimentation rate and accumulation rates for different substances.
A number of studies have focused on the accumulation of heavy metals during the past years. If the concentration of substances at different depths is compared with the corresponding ages, the accumulation rates of these substances at different times may be determined from the sediment accumulation rate. This gives a better picture of the historical development in a given lake or marine area.