Use of multi-proxy approaches to determine the origin and depositional processes in modern lacustrine sediments: Carajás Plateau, Southeastern Amazon, Brazil

Abstract

Geochemical and isotopic compositions of surficial sediments from a plateau lake in Carajás, Southeastern Amazon region, were investigated to understand the spatial distribution of major and trace elements, δ13C, δ15N, and C/N ratio, depositional processes, and the origin of inorganic and organic fractions. The δ13C, δ15N, and C/N ratio indicate mainly an autochthonous source, with siliceous sponge spicules and algae being the major source of organic matter in the center of the lake (Sector 3), while an allochthonous source, mainly derived from C3 vascular forest plants dominates in the shallowest portion of the lake (Sector 1). Consequently, there was an apparent dilution of C4 plants (montane savanna) in the sediment/water interface. Among major elements, Fe2O3 is highly enriched in Sector 3, which is controlled by the erosion of catchment laterites and underwater topography, while Al2O3 and P2O5 enrichment near the northern border of the lake is controlled by the weathering of mafic rocks. Similar spatial distribution of SiO2 with total organic carbon (TOC) and isotopic evidence indicate that Si distribution is partially controlled by organic components such as siliceous sponge spicules and algae. The occurrence of most of the trace and rare earth elements (REE) is independent of Fe2O3 and TOC, but controlled by detrital aluminum silicates and heavy minerals, indicating a lack of post-depositional diagenetic control on their distribution. The distribution of As and Mo are possibly controlled by organic matter mineralization during early diagenesis and subsequently precipitation of Fe phases. The values of the chemical index of alteration (CIA) in the sediments are very high (94–99) and similar to those of the source rocks, suggesting that sediment composition is mainly controlled by mechanical weathering, rather than chemical weathering. Geochemical indices (Al/K, Ti/K, Al/Ti, La/Th, Ti/Zr, Zr/Hf, Hf/Nb, La/Al, Co/Th, Ba/Sr, and Th/Sc), together with the A–CN–K plot, suggest that the provenance of the inorganic sediments remained relatively uniform or constant during the depositional period, and that they were mainly derived from laterite crusts and subordinately from mafic soils. This inference was further substantiated by the chondrite-normalized REE patterns and discrimination plots. Geochemical indices such as U/Th, authigenic-U, Mo/Al, and V/Cr indicate that the sediments were deposited under an oxic environment.