Title
Evidence for the control of the geochemistry of Amazonian floodplain sediments by stratification of suspended sediments in the Amazon
Date Issued
01 November 2014
Access level
metadata only access
Resource Type
journal article
Author(s)
Roddaz M.
Viers J.
Moreira-Turcq P.
Blondel C.
Sondag F.
Moreira L.
Université de Toulouse
Publisher(s)
Elsevier
Abstract
Holocene and historical Amazonian floodplain deposits collected from two cores of the "Varzea do Curuai" flooded area (Brazil) were analysed for major and trace element geochemistry as well as Nd-Sr isotopic compositions (21 samples). The TA11 and TA14 cores (110cm and 270cm in depth, respectively) were collected at different locations in the Varzea, near a channel inlet connecting the Amazon River to the Varzea and at the centre of the Varzea, respectively. The two cores represent records of sedimentation on different time-scales, with TA11 covering the last 100years and TA14 extending back to 5600yearscal BP. Although the sediments are generally coarser in TA11 than in TA14, the major and trace element concentrations, Cr/Th and Th/Sc and Eu anomalies and Nd-Sr isotopic compositions in both cores fail to show any clear variations with depth. However, there are chemical differences between the two analysed cores. The TA14 sediments have higher Al/Si and Chemical Index of Alteration (CIA) values than those of TA11. The TA14 sediments are enriched in Th, U, Y, Nb, REE, Cs, Rb, V and Ni but show slightly depleted MgO, CaO and Sr and more strongly depleted Na2O, Zr and Hf compared to TA11. In addition, the Nd-Sr isotopic compositions of the TA11 sediment core are on the whole similar to the Solimões suspended particulate matter (SPM), whereas TA14 has a similar Nd-Sr isotopic composition compared with the SPM of the Amazon River at Obidos. These differences are best explained by chemical stratification of the SPM of the Amazon River. During flooding of the Amazon River, coarser grained particulates supplied by the Solimões River are deposited in the deepest environments near the channel inlet, as recorded in the TA11 sediment core. By contrast, finer grained suspended sediments derived from the Madeira River are transported into the shallower environments of the Varzea system and deposited as a result of flow expansion and loss of carrying power, as recorded in the TA14 sediment core. Using the Nd isotopic compositions and concentrations of each sediment core, we calculate between ~20 and ~85% of the sediment input to the TA14 core site is derived from the Madeira River, while between 52% and 100% of the sediment input to the TA11 core sample is supplied by the Solimões River. Similar calculation using the Sr isotopic compositions and concentrations of TA11 and TA14 sediments gave similar results with between ~32% and ~49% of TA14 sediments deriving from the Solimões River while between ~70% and ~100% of TA11 sediments originate from the Solimões River. Finally, our findings lead us to question whether particular Amazon deposits that do not integrate the entire range of SPM present in the river are representative of the overall sediment load provided today and in the past by global rivers.
Start page
101
End page
110
Volume
387
Language
English
OCDE Knowledge area
Oceanografía, Hidrología, Recursos hídricos Geoquímica, Geofísica Geología
Scopus EID
2-s2.0-84908329317
Source
Chemical Geology
ISSN of the container
00092541
Sponsor(s)
This research was supported by the French Research Institute for Development (IRD) , through the HyBAm Research Program (Hydrology and Geochemistry of the Amazonian Basin, www.mpl.ird.fr/hybam/ ) in the framework of its cooperation agreement with the Brazilian Research Centre (CNPq process nos. 492685/2004–05 and 690139/2003–09 ). We acknowledge the CNRS/INSU Paleo2 PASCAL project. We thank Michael Carpenter for editing the English style. We thank two anonymous reviewers for their constructive reviews. The Editor is warmly thanked for her helpful editorial handling.
Sources of information: Directorio de Producción Científica Scopus