CONICET | Buscador de Institutos y Recursos Humanos

The events affecting the natural ecosystem in the last 200 years, such as the increase in the population, massive deforestation, dessertification, dam construction, introduction of exotic species, extensive use of agrochemicals, etc. have modified and accelerated the natural evolution processes. Most of these changes have occurred without planning or effective control and, in many cases, there are not records available of these alterations. The study of the historical evolution of water bodies and sedimentation rates can provide useful information to study the trophic evolution of the environment and the impact of anthropic activities, such as introduction and alteration of sources of contamination, changes of habits in human settlements, etc. The reconstruction of both natural and historical events can be performed by means of the study of sedimentary sequences of such water bodies, mainly lakes and estuaries. In these studies, it is of central importance the establishment of the chronology of the sedimentary sequence. One of the dating methods based on the measurement of natural radioactive isotopes for this time scale is by means of 222Rn decay product 210Pb (half life = 22.3 years). 222Rn is a noble gas (daughter product of 238U) which escapes from the surface rocks and, once in the atmosphere, it decays with a disintegration period of 3.8 d. 210Pb is the last radioactive product of a serie of 222Rn short-lived daughter products. It returns to the earth surface within a few weeks as dry or wet deposition where it is adsorbed in the sediments [El-Daoushy, 1998]. For the time scale involved, the generation of 210Pb can be considered as simultaneous to the generation of the sediments. 210Pb activity can be determined by two different techniques; namely, by measuring the Ñ radiation emitted by 210Pb after radiochemical separation, or by measuring the 46 keV gamma-ray. The second alternative has the advantages of allowing the simultaneous determination of the 210Pb and 226Ra activities [Joshi, 1987] which are necessary for the estimation of the age and, on the other side, it is non-destructive, the sample conditioning is simple and the material can be re-used if necessary [Appleby et al., 1986; Joshi, S. R., 1989]. 226Ra activity must be known to evaluate the contribution of the 210Pb activity originated from its decay inside the sediment matrix (supported 210Pb). The main technical drawbacks are related to the difficulties of measuring the weak 46 keV 210Pb gamma-ray peak emitted by a volumetric low activity source, which is also present in the natural background. 222Rn decay product 210Pb (half life = 22.3 years). 222Rn is a noble gas (daughter product of 238U) which escapes from the surface rocks and, once in the atmosphere, it decays with a disintegration period of 3.8 d. 210Pb is the last radioactive product of a serie of 222Rn short-lived daughter products. It returns to the earth surface within a few weeks as dry or wet deposition where it is adsorbed in the sediments [El-Daoushy, 1998]. For the time scale involved, the generation of 210Pb can be considered as simultaneous to the generation of the sediments. 210Pb activity can be determined by two different techniques; namely, by measuring the Ñ radiation emitted by 210Pb after radiochemical separation, or by measuring the 46 keV gamma-ray. The second alternative has the advantages of allowing the simultaneous determination of the 210Pb and 226Ra activities [Joshi, 1987] which are necessary for the estimation of the age and, on the other side, it is non-destructive, the sample conditioning is simple and the material can be re-used if necessary [Appleby et al., 1986; Joshi, S. R., 1989]. 226Ra activity must be known to evaluate the contribution of the 210Pb activity originated from its decay inside the sediment matrix (supported 210Pb). The main technical drawbacks are related to the difficulties of measuring the weak 46 keV 210Pb gamma-ray peak emitted by a volumetric low activity source, which is also present in the natural background.

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