Radioactivity Measurements Laboratory
Using Pb-210
Introduction
Since its introduction in the sevienties, the 210Pb method has been widely used in dating of sediments (lake, peat etc) and soil. 210Pb in sediments consists of authigenic 210Pb coming from the decay of 226Ra, which is a natural component of terrestrial rocks, and of allochthonous 210Pb produced from 222Rn which was liberated from the Earth's surface to the atmosphere. Allochthonous 210Pb is deposited on the Earth's surface bound to aerosols or directly to the rain droplets. To lakes, additionally 210Pb is supplied directly from the atmosphere or from lake surroundings (e.g. due to soil erosion). Concentration of authigenic 210Pb in sediments is constant, as the radioactive decay of 210Pb is counterbalanced by its production from 226Ra. On the other hand, allochthonous 210Pb is not bound to 226Ra, and its concentration decreases after burial in sediments. For that reason, measurments of 210Pb concentration enable dating of sediments, and determination of their deposition rate. Due to the short half-life of 210Pb (22.26 year), the 210Pb method is suitable for dating sediments not older than 150-200 years.
Specific activity of 210Pb may be related to the age of sediments by the simple law of radioactive decay if: a) the specific activity of 210Pb in the sediment at the moment of its deposition is known, b) neither vertical mixing of sediments nor vertical migration of 210Pb within sediments occur. To derive 210Pb activity at the time of deposition, 210Pb activity is measured in modern sediments, and one assumes either that the specific activity 210Pb (expressed e.g. in Becquerels per gram of sediment) at the moment of sedimentation was constant in the past, or that the flux of 210Pb supplied to the lake (expressed e.g. in Becquerels per year per square meter of sediment area) was constant over time. Both models give the same results if the sedimentation rate was constant.
In the measurement, total activity of 210Pb (allochthonous plus authigenic) is determined. To derive the amount of allochthonous component, one usually assumes uniform concentration of 226Ra and authigenic 210Pb in the whole profile. This can be determined by the measurement of 210Pb in a sufficiently old layer of sediment.
