Source area and seasonal⁸⁷Sr/⁸⁶Sr variations in rivers of the Amazon basin

Abstract

We report the results of a detailed study of dissolved Sr isotopes in the Solimões and Beni‐Madeira Rivers of the Amazon basin. This study developed data collected over 8 years indicating large spatial and temporal variations in dissolved Sr isotopes among the rivers of the Amazon basin. The large 87Sr/86Sr 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 87Sr/86Sr ratio and large 87Sr/86Sr 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 Solimões 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 Óbidos. 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 87Sr/86Sr ratios, such rivers as the Negro and Tapajós play a minor role in the total Sr budget of the Amazon basin. Copyright © 2013 John Wiley & Sons, Ltd.

Movement patterns of adult peacock bass Cichla temensis between tributaries of the middle Negro River basin (Amazonas – Brazil): an otolith geochemical analysis

“…Santos et al . ). However, core chemistry from this individual clusters closely with those of adults caught from the adjacent Padauari River, and therefore, it is also possible that this fish was spawned there and emigrated to the Preto River.…”

“…) and outside the narrow range of 87 Sr/ 86 Sr variation reported from the Solimões River basin (Santos et al . ). Such variation was often contrasted with adult otolith records from the same or adjacent collection locations showing only minor variation (+/‐ 0.005) in 87 Sr/ 86 Sr values over their entire life history.…”

“…Work by Santos et al . () documented that 87 Sr/ 86 Sr can vary with hydrological cycle in some sub‐basins of the Amazon system, indicating that this is an area in need of further consideration for otolith study applications.…”

“…; Santos et al . ). In such river networks, otolith chemistry can be used to examine large‐scale movements (between rivers) over long periods of time by capturing an individual's complete movement history among river habitats.…”

“…Studies using fish otolith geochemical signatures have demonstrated the utility of this approach for reconstructing natal origins and movement history of small and large fish in marine (Brazner et al 2004;Humphreys et al 2005;Walther & Thorrold 2006, 2010 and freshwater habitats (Humston et al 2010;Garcez et al 2014). In river basins with sufficiently heterogeneous bedrock geology, tributaries may be distinguished by differences in elemental and/or isotopic water chemistry (Humston & Harbor 2006;Barnett-Johnson et al 2008;Bataille & Bowen 2012;Hegg et al 2013) and this has been specifically documented in the Amazon basin (Gaillardet et al 1997;Garcez et al 2014;Pouilly et al 2014;Hegg et al 2015;Santos et al 2015). In such river networks, otolith chemistry can be used to examine large-scale movements (between rivers) over long periods of time by capturing an individual's complete movement history among river habitats.…”