Evidence for the control of the geochemistry of Amazonian floodplain sediments by stratification of suspended sediments in the Amazon

- We analysed 21 Amazonian floodplain sediments from two cores for their geochemistry.
- Proximal core sediments have similar chemical characteristics to Solimões sediments.
- Distal core sediments have Amazon River chemical characteristics.
- Stratification of Amazon SPM explains these differences.

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 (110 cm and 270 cm 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 100 years and TA14 extending back to 5600 years cal 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.