Controls on the geochemistry of suspended sediments from large tropical South American rivers (Amazon, Orinoco and Maroni)

Abstract

We report mineralogical, elemental (major and trace elements) and SrNd isotopic data for suspended particulate matter (SPM) samples from the Amazon, Orinoco and Maroni Rivers collected on a monthly basis over a one-year long hydrological cycle. The aim of this study was i) to characterize the mineralogical and geochemical composition of major South American tropical rivers and ii) to evaluate the effect of seasonal hydroclimate variations and changes in sediment discharge on the composition of these SPM. In addition to displaying particular mineralogical and geochemical signatures (e.g. Al/Si ratios, weathering indices), the Amazon, Orinoco and Maroni SPM are characterized by marked differences in ⁸⁷Sr/⁸⁶Sr (0.7213 ± 0.0030, 0.7288 ± 0.0018 and 0.7335 ± 0.0019, respectively), and εNd values (−10.6 ± 0.6, −14.1 ± 0.3 and −23.7 ± 1.2), which reflect differences in source rock lithology. While we find no effect of the hydrological cycle on the geochemistry of Orinoco SPM, particulate εNd and Cr/Th signatures fluctuate with the hydrological cycle in the Maroni basin, indicating that they are controlled by variation in rainfall distribution linked to the latitudinal migration of the Intertropical Convergence Zone (ITCZ). In contrast to Maroni and Orinoco SPM, the Amazon SPM are characterized by significant Sr isotope annual variability correlated with suspended sediments discharge and a small but significant Nd isotopic variability over the year. This latter variation is related to seasonal changes in the rainfall distribution patterns across the Amazon basin, associated with latitudinal migrations of the ITCZ. This suggests that the geochemical composition of the SPM exported over the year from the Amazon Basin faithfully responds to hydroclimate changes related associated with the migration of the rain belt over regions of contrasted geochemical signatures.

These findings have implications for the application of Sr and Nd isotopes as provenance proxies in sedimentary archives and paleoclimatic studies. The Sr isotopic composition of exported SPM appears to be mostly controlled by weathering processes and/or mineralogical sorting, rather than being indicative of sediment provenance. In contrast, the relationship documented between Nd isotopes and the hydrological variability indicates that their application to archives of past river sediment discharges can provide unique insights on paleo-hydroclimate changes over tropical South America.