2007 International Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-02-1 210 Pb GEOCHRONOLOGY OF STREAM SEDIMENTS FROM THE GUAMA RIVER AND GUAJARA BAY, BELEM - AMAZON REGION Fabiana F. Dias 1*, Maria Helena T. Taddei 1, Suziane M. Nascimento 2, and Jean M. Lafon 2 1 Brazilian Nuclear Energy Commission - CNEN Poços de Caldas Laboratory Rodovia Poços de Caldas Andradas Km 13 37701-970 Poços de Caldas, MG - Brazil *fdias@cnen.gov.br mhtaddei@cnen.gov.br 2 Para Federal University Campus Universitário Rua Augusto Correa, 1 - Bairro do Guama 66075-110 Belem, PA - Brazil lafonjm@ufpa.br engsuzi@yahoo.com.br ABSTRACT This study was able to date three sediment profiles collected at the mouth of the Amazon river using the 210 Pb geochronology method. All sediment profiles were sliced into layers of 5 cm and each layer was analyzed for radionuclides by Gamma Spectrometry. The results obtained dated the sediments as far back as 65 years. In addition, the sedimentation rate was also determined. A subsequent interpretation of the results can provide information on pollutants present in the sediment layers and infer possible contamination patterns by operating industries and anthropogenic activities in the area of the Amazon Hydrographic Basin. 1. INTRODUCTION The 210 Pb dating method became the most important tool in sediment studies aimed at determining the absolute ages of modern sediments up to 100 years. The method is dependent on fractionation of nuclides from the 238 U decay chain (primarily 222 Rn) in rocks and soils next to the surface [1]. 222 Rn, an inert gas with a half life of 3.8 days, produced from the decay of 226 Ra, escapes from the earth surface to the atmosphere. Radon decay products (for which the longest half life son is 210 Pb, 22 years) are removed from the atmosphere composing a measurable source in the earth surface. The atmospheric 210 Pb is first removed from the atmosphere by precipitation through rain or dry deposition. The 210 Pb is then removed from the water column in aquatic systems by resident particles and deposited in sedimentation zones. By measuring 210 Pb and 226 Ra concentrations in sediment samples from lakes, reservoirs, estuarine and protected coastal
environments, it is possible to estimate the excess or non-supported concentration of 210 Pb in the sediment which is primarily derived from atmospheric flow. In areas where the excess 210 Pb deposition rate has been kept constant during the last century and there was not any sediment disturbance, the radionuclide concentration within the sediment core follows an exponential decrease with the depth that results from the radioactive decay. It is also considered that total 210 Pb present in the sediment is a sum of the fractions of 210 Pb atmospheric and 210 Pb in radiological equilibrium with 226 Ra [3]. 2. DISCUSSION Considering that the 210 Pb initial activity can be measured or estimated and considering that this activity is kept constant during the studied period, the residence time or the age of a certain deposit can be determined. Figure 1. Principles of 210 Pb dating [2]. In the CRS dating model (Constant Rate of Supply), also called the CF model (constant flux), the sedimentation rates are variable in time but the flux of 210 Pb to sediments remains constant. The excess 210 Pb profile vertically integrated to a depth x (or alternatively, cumulative dry-mass) will equal the flux (constant) integrated over the corresponding time interval [1]. The activity of 210 Pb atmospheric, after t years, is given by the radioactive decay equation: A = A 0. e -λ (210Pb). t (mbq g -1 ) [1]
In this work, the geochronology of the sediment from the Guama river and Guajara bay in Belem, at the mouth of the Amazon river, was obtained in order to determine the age of the sediment over a period of time of less than 100 years. Three sediment cores were taken from the studied area and sliced into 5 cm layers. Each layer was then analyzed for 210 Pb and 226 Ra by Gamma Spectrometry, in order to obtain 210 Pb atmospheric used as a dating tool. Table 1 below shows the corrected depth for each sediment layer and its associated age. Table 1. Data for Sediment Samples from the Guama river and Guajara bay. Sediment Core 1 Guajara Bay Corrected Depth (cm) Age (Years) DATE (Initial Date: 2006) 225-02 10 14 1992 225-03 15 21 1985 225-04 20 28 1978 225-05 25 35 1970 225-06 30 43 1963 Sediment Core 2 Guama river Corrected Depth (cm) Age (Years) DATE (Initial Date: 2006) 225-07 5 9 1997 225-08 10 18 1988 225-09 16 28 1978 225-10 22 38 1968 225-11 27 47 1959 225-12 32 57 1949 225-13 37 65 1941 Sediment Core 3 Guama river Corrected Depth (cm) Age (Years) DATE (Initial Date: 2006) 225-22 4 7 1999 225-21 8 13 1993 225-20 12 20 1986 225-19 16 27 1979 225-18 20 33 1973 225-17 24 40 1966 225-16 28 48 1960 225-15 33 55 1955 225-14 38 63 1949
The Figures below show the geochronology method applied for the three sediment cores collected from the Guama river and Guajara bay. 3,6 3,4 3,2 3,0 Ln 210 Pb Atm 2,8 2,6 2,4 2,2 2,0 Linear Regression Y = A + B * X Parameter Value A 3,70215 B -0,04514 5 10 15 20 25 30 Corrected Depth (cm) Figure 2. Depth versus Ln 210 Pb atmospheric for Sediment Core 1. 4,5 4,0 3,5 Ln 210 Pb Atm 3,0 2,5 2,0 Linear Regression Y = A + B * X Parameter Value A 4,47727 B -0,05723 1,5 0 5 10 15 20 25 30 35 40 Corrected Depth (cm) Figure 3. Depth versus Ln 210 Pb atmospheric for Sediment Core 2.
4,5 4,0 3,5 Ln 210 Pb Atm 3,0 2,5 2,0 1,5 Linear Regression Y = A + B * X Parameter Value A 4,28504 B -0,06479 1,0 0 5 10 15 20 25 30 35 40 Corrected Depth (cm) Figure 4. Depth versus Ln 210 Pb atmospheric for Sediment Core 3. 3. CONCLUSIONS It was possible to verify in this study that the geochronology method was adequate, useful, and very important to determine the age of sediment slices. Studies regarding sediments reflect an interest in understanding and finding solutions for present environmental problems as well as prediction of future problems; all of this based on knowledge of a recent past. Trace contaminant concentration profiles in sediment columns offer a natural archive with which pollution addition can be reconstructed. Immobilized species in the sedimentary environment don t necessarily remain in this condition; they can return to the water column as a result of chemical and physical alterations of the medium, making them available again. The reconstruction of historic addition of anthropogenic products or chemical elements such as heavy metals (potentially toxic metals) and organic pollutants is important for the improvement of management strategies and evaluation of the success of recent pollution control measures.
ACKNOWLEDGMENTS The authors would like to thank the technicians at the CNEN Poços de Caldas Laboratory and CNPq for sponsorship of the main author. REFERENCES 1. J. Carroll and I. Lerche. Sedimentary Processes: Quantification using Radionuclides. Radioactivity in the Environment. Volume 5. Elsevier. Kidlington, UK. (2003). 2. R. Cazotti. Geocronologia de Sedimentos Recentes com 210 Pb: Metodologia e Critérios para sua Aplicação em Lagos e Represas. Tese de Doutorado. Centro de Ciências Exatas e de Tecnologia. Departamento de Química. Universidade Federal de São Carlos. (2003). 3. E. D. Goldberg. Geochronology with Lead-210. Radioactive Dating - IAEA. Vienna. pp. 121-131. (1963).