RESULTS AND DISCUSSION Recent sedimentation rates determined by 210Pb method in sediment cores from Nhecolândia Pantanal, MS, Brasil Instituto de Pesquisas Energéticas e Nucleares April 6 – 9, 2014 São Paulo, Brazil 1 Sandra R. DAMATTO, 1 Levi F. dos SANTOS, 2 Laurent BARBIERO, 3 Sônia M. FURIAN, 4 Ary T. REZENDE-FILHO 1 Instituto de Pesquisas Energéticas e Nucleares (IPEN) – Laboratório de Radiometria Ambiental, 2 Centro de Energia Nuclear na Agricultura (CENA-USP), Laboratório de Ecologia Isotópica, 3 Universidade de São Paulo (USP) – Departamento de Geografia , 4 Universidade Federal do Mato Grosso do Sul, Campus de Nova Andradina damatto@ipen.br INTRODUCTION Pantanal, located in southwest part of Brazil, eastern Bolivia and northeastern Paraguay, is one of the world's largest freshwater wetlands, with an area that ranges between 140,000 km2 to 210,000 km2, depending on the season time. It is a huge alluvial plain depression of slight declivity along the Paraguay River and its tributaries that becomes flooded during the rainy season. The rains during the wet season, cause widespread flooding in the basin. This results in a landscape of river channels, seasonally isolated swamps, and lakes. The Pantanal is a complex hydrological system that supports multitudes of aquatic plants, invertebrates, fish, and other species, and it is highly dependent on the flood cycle. Pantanal was recognized as National Inheritance by the Brazilian Constitution of 1988 and as Wet Area of International Value by Ramsar Convention. In 2000 it was designated as Biosphere Reserve by UNESCO and as Humanity Natural Inheritance. These organizations were created because several wetlands like Pantanal, are under threat in all over the world from a variety of man-induced impacts and technological developments such as urban contamination, irregular use of the land, excessive agricultural defensive utilization, tourism installations, leisure activities and other forms of human intervention. Sediment cores were collected with the objective of to determine the sedimentation rate using the 210Pb dating method and the grain size analyses. RESULTS AND DISCUSSION Figures 1, 2, 3 and 4 show the results of grain size analysis and dating for cores from Salina AL, Salina 6S , Salina 6M and Salina Verde, respectively. Grain size analysis for all cores shows that the predominant fraction is sand, with a high percentage, more or equal to 50%, exception only for core from Salina AL where the samples presented higher values of silt + clay. The sedimentation rates obtained were 1,16 cm y-1 Salina AL; 0,70 cm y-1 Salina 6S; 0,43 cm y-1 Salina 6M and 0,20 cm y-1 Salina Verde. Figure 1 – Grain size analysis and dating for the core from Salina Al Figure 2 – Grain size analysis and dating for the core from Salina 6S MATERIAL AND METHODS  Four sediment cores were collected, in 2010, in four small ponds in Nhumirim farm, Salina AL, Salina 6S, Salina 6M and Salina Verde. The cores were collected in the central part of the ponds with a manual sampler using a polyethylene tube of two-inch diameter; all ponds had approximately 40 cm of water column. The cores were sliced every 2 cm, dried at 60oC for two days in a ventilated oven and homogenized in a glass mortar for the analysis. Grain-size analysis of the sediments samples were performed by sieving about 10g of each slice using sieves of mesh number 60, 230 and 635 that allow separate the samples in sand, silt and clay. The radionuclides 226Ra and 210Pb were used to determine the dates and sedimentation rate in each slice of the core in the fration silt + clay. The samples were dissolved in mineral acids, and H2O2 30%, in a microwave digestor and submitted to the radiochemical procedure for the determination of Ra and Pb. This procedure consists of an initial precipitation of Ra and Pb with 3M H2SO4, dissolution of the precipitate with nitrilo-tri-acetic acid at basic pH, precipitation of Ba(226Ra)SO4 with ammonium sulphate and precipitation of 210PbCrO4 with 30% sodium chromate. The 226Ra concentration was determined by gross alpha counting of the precipitate and the 210Pb concentration through its decay product, 210Bi, by measuring the gross beta activity of the precipitate. The chemical yields, both radionuclides, were determined by gravimetric analysis and they were determined in a low background gas flow proportional detector. The dates were calculated by the Constant Rate of Supply (CRS) model. r= 0,924 Figure 3 – Grain size analysis and dating for the core from Salina 6M Figure 4 – Grain size analysis and dating for the core from Salina Verde CONCLUSION The four sediment cores provided different sedimentation rates, mainly due to the sediment grain size. These different sedimentation rates probably indicate periods that the Salinas were almost dry getting sediment deposition by the action of the winds; lower rates, and the highest rates were associated with floods occurring in the Pantanal carrying a large load of sediment to Salina. Two profiles showed very dark coloring of the sediment probably indicating a high concentration of organic matter. In the event of a drought the 210Pb present in the sediment can migrate from one layer to another especially when the rains start which might cause variations in sedimentation rates.