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

Highlights: • Distribution of elements in lake sediments depicts the depositional process. • Geochemical proxies reflect the weathering and provenance of sediments. • δ¹³C and δ¹⁵N values have best used to trace organic sediments. • CIA values of sediment are mainly controlled by physical erosion process. - 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