Lead 210 (210Pb) is a radioactive form of lead, having an atomic weight of 210. It is one of the last elements created by the radioactive decay of the isotope uranium-238 (238U). 210Pb forms naturally in the sediments and rocks that contain 238U, as well as in the atmosphere, a by-product of radon gas. Within 10 days of its creation from radon, 210Pb falls out of the atmosphere. It accumulates on the surface of the earth where it is stored in soils, lake and ocean sediments, and glacial ice. The 210Pb eventually decays into a non-radioactive form of lead. 210Pb has a half-life of 22.3 years, which means that after 22.3 years, only half of the original amount is undecayed. If the sediment layers are undisturbed, then as the sediment ages it slowly loses its radioactivity. We can determine how old a sediment layer is by how much 210Pb it contains. It takes about 7 half-lives, or 150 years for the 210Pb in a sample to reach near-zero radioactivity.
The table at left shows the uranium-238 decay series and the time it takes for each product to reach only half it's original concentration (half-life). Uranium-238 is the first in the series; it decays into thorium, which decays into protactinium, which decays into uranium 234, and so on. Note that the time it takes for each to decay varies from minutes to billions of years. The decay emission is either an electron (b-) or a proton and neutron (a). Since protons are relatively heavy, when one is lost out of an atomic nucleus it changes the atomic weight. There are several things that can create uncertainty in our 210Pb dates, including changes in the deposition of atmospheric 210Pb and changes in the concentration of sedimentary 210Pb. Fortunately, we often have written records documenting major events during the last 150 years. We can look for evidence of major changes in these records and use them as time-markers for our 210Pb date estimates. For example, we know from written records that nuclear bomb tests in the 1960's left measurable concentrations of cesium-137 (137Cs) in the atmosphere. The cesium rained out and entered the sediments in much the same way that atmospheric lead does. This influx of (137Cs) is visible as a distinct peak in many recent sediments. We also know that before the Envronmental Protection Agency started regulating the amount of lead additives to gasoline in 1973, atmospheric concentrations of non-radioactive lead was relatively high because of automobile emissions. After 1973, atmospheric lead levels started to decrease and by 1988 were considerably lower. By measuring (137Cs) and lead concentrations, alongside 210Pb, we can often determine the accuracy of our (210Pb)dates. Peter Schwarzenski has completed (210Pb) and (137Cs)analyses for the 1998 short-cores from Bear Lake. Results indicate that our cores contain sediments from the 20th Century and that sedimentation rates increased when the Bear River was diverted into the lake. To learn more about how (210Pb)is used for dating visit these sites:
Short-lived isotopic chronometers
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