210Pb, a natural radionuclide, has become a nuclide of growing importance in the environmental science. In addition, being the subject of numerous impact assessment studies, it is having been used for dating, tracing and modelling biogeochemical cycling in ecosystems, to study climate and environmental changes during the last 100-150 years (Polikarpov, 1966;16Villa et al., 2007; Barlas Simsek and Cagatay, 2014; Mabit et al., 2014). Since the 1970s, 210Pb measurements have been used extensively for dating sediments deposits in a range of the sedimentary environments, including lakes, reservoirs, flood plains, wetlands, and coastal marine environments (Mabit et al., 2014).
210Pb is a natural product of the 238U decay series, with a half-life of 22.3 years. It is derived from the decay of gaseous 222Rn, the daughter of 226Ra, which naturally occurs in soils and rocks and generates 210Pb that can be in equilibrium with its parent (“supported” 210Pb).
Diffusion of a small quantity of the 222Rn from the soils introduces 210Pb into the atmosphere and its subsequent fallout provides an input of this radionuclide to the soil surface that is not in equilibrium with its parent 226Ra (“unsupported” or “excess” 210Pb). As a fallout radionuclide 210Pb is rapidly and strongly adsorbed by the surface soil and is redistributed within lacustrine sediments.The 210Pb fallout flux is essentially continuous through time because of its natural origin.
However, weekly, monthly, seasonal and long-term variations of the 210Pb concentration in the air and in atmospheric fallout (rain, snow and dry deposition) have been documented by long-term measurements (Baskaran et al., 1993; Pfenning et al., 1998). The global pattern of 210Pb fallout is characterized by a high spatial variability due to predominant west to east movement of air masses (Mabit et al., 2014).
This commonly results in low 210Pb fallout in the western areas of the continents and much higher in the eastern areas, where the air masses will have passed over the continental interiors.In practice, the supply of 210Pb fallout to the bottom sediments may be influenced by many factors, including (Appleby, 2008):i. the atmospheric flux;ii. the rate of transport from the catchment;iii.
the water residence time;iv. the fraction of the radionuclide attached to settling particles;v. the mean particle setting velocity;vi. post-depositional transport processes (sediment focusing, mixing).
The key feature of 210Pb behaviour in the soil is its strong binding to soil particles and its chemical stability in soil environments, making possible the fundamental assumption that Pb moves only with soil particles and that the major processes causing its redistribution in the landscape are mechanical processes such as water, wind and erosion (Mabit et al., 2014). When reaching the soil surface as wet or dry fallout from the atmosphere, 210Pb is therefore rapidly and strongly absorbed by organic matter (Trivedi et al., 2003) and clay mineral particles (Strawn and Sparks, 2000).17There are two major sources of 210Pb in sediments, why two fractions are commonly considered (Sternbeck et al, 2006):i. supported 210Pb which is formed in situ by decay of 226Ra;ii.
excess 210Pb which is formed by decay of 222Rn in the atmosphere.In the uppermost sections, excess 210Pb generally constitutes the majors fraction of 210Pb. Because excess 210Pb is introduced at the sediment surface, but is not formed within the sediments, its activity will decay with a half-life of 22.26 years.
The concentration of unsupported (excess) 210Pb decrease as a function of depth because of radioactivity decay, providing the basis of 210Pb dating. It is possible by applying the law of radioactivity decay to decreasing concentration of 210Pb with depth, to calculate the ages of the sediment at any horizon. In order to do this, certain assumptions describing the delivery of 210Pb to the sediment must be made which determine the validity of the chronology.
226Ra is also present in the sediment as a part of the erosive input of particulate material from the catchment. 210Pb formed within the sediment by decay of 226Ra is referred to as supported 210Pb. which is normally assumed to be in equilibrium with the 226Ra (Barlas Simsek and Cagatay, 2014). The activity of supported lead generally shows less vertical variations, and can be estimated from measurements of 226Ra assuming secular equilibrium.