Source area and seasonal Sr-87/Sr-86 variations in rivers of the Amazon basin

Abstract

We report the results of a detailed study of dissolved Sr isotopes in the Solimoes and Beni-Madeira Rivers of the Amazon basin. This study developed data collected over 8years indicating large spatial and temporal variations in dissolved Sr isotopes among the rivers of the Amazon basin. The large Sr-87/Sr-86 variations were found to be correlated with the geology of the source areas of the suspended sediments. The Beni-Madeira River displays a high average Sr-87/Sr-86 ratio and large Sr-87/Sr-86 fluctuations during the hydrological cycle. This large average value and fluctuations were related to the presence of Precambrian rocks and Ordovician sediments in the source area of the suspended sediment of the river. In contrast, the Solimoes River displays a narrow range of Sr isotope ratio variations and an average value close to 0.709. This river drains mostly Phanerozoic rocks of northern Peru and Ecuador that are characterized by low Sr isotope ratios. The isotopic fluctuations in the Beni-Madeira River were observed to propagate downstream at least as far as bidos. This signal is characterized by an inverse relationship between the concentration of elemental Sr and its isotopic ratios. We further demonstrate that the Sr isotopic composition and content in the Beni-Madeira River is controlled by suspended sediments derived from the Andes. Despite draining areas underlain by Precambrian rocks and having high Sr-87/Sr-86 ratios, such rivers as the Negro and Tapajos play a minor role in the total Sr budget of the Amazon basin