Use of multi-proxy approaches to determine the origin and depositional processes in modern lacustrine sediments: Carajás Plateau, Southeastern Amazon, Brazil Prafulla Kumar Sahoo a,⇑ , Pedro Walfir Martins Souza-Filho a,b , José Tasso Felix Guimarães a,b , Marcio Sousa da Silva b , Francisco Ribeiro Costa b , Carmem-Lara de Oliveira Manes a , Douglas Oti a , Renato Oliveira Silva Júnior a , Roberto Dall’Agnol a,b a Vale Institute of Technology, Rua Boaventura da Silva 955, Nazaré, 66055-090 Belém, Pará, Brazil b Universidade Federal do Pará, Geosciences Institute, Programa de Pós-graduação em Geologia e Geoquímica, Av. Augusto Correa 1, Guamá, 66075-110 Belém, Pará, Brazil article info Article history: Available online 18 November 2014 Editorial handling by M. Kersten abstract Geochemical and isotopic compositions of surficial sediments from a plateau lake in Carajás, Southeast- ern Amazon region, were investigated to understand the spatial distribution of major and trace elements, d 13 C, d 15 N, and C/N ratio, depositional processes, and the origin of inorganic and organic fractions. The d 13 C, d 15 N, 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 sedi- ment/water interface. Among major elements, Fe 2 O 3 is highly enriched in Sector 3, which is controlled by the erosion of catchment laterites and underwater topography, while Al 2 O 3 and P 2 O 5 enrichment near the northern border of the lake is controlled by the weathering of mafic rocks. Similar spatial distribution of SiO 2 with total organic carbon (TOC) and isotopic evidence indicate that Si distribution is partially con- trolled 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 Fe 2 O 3 and TOC, but controlled by detrital aluminum silicates and heavy minerals, indicating a lack of post-depositional diagenetic control on their distribu- tion. 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 sed- iment 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 uni- form 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. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Lake sediments are complex heterogeneous systems that result from mixing of inorganic and biogenic/organic components (Schnurrenberger et al., 2003). The inorganic components (major, trace and REEs) are mainly allochthonous (i.e., externally pro- duced), depending on the natural and anthropogenic inputs into the basin. Biogenic components, mainly organic matters, can be autochthonous (i.e., internally produced) such as aquatic algae and macrophytes, and allochthonous, such as degraded plant materials and soil organic matter derived from the vegetations of the catchment area (Wei et al., 2010). After deposition or re-work- ing, the detritus can continue to move as both particulate and dis- solved form by selective sorting and weathering process (Nyakairu and Koeberl, 2001). In addition, post depositional processes can also influence the distribution of elements depending on environ- mental conditions (Nyakairu and Koeberl, 2001; El Bilali et al., http://dx.doi.org/10.1016/j.apgeochem.2014.11.010 0883-2927/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +55 9132135565. E-mail address: prafulla.sahoo@itv.org (P.K. Sahoo). Applied Geochemistry 52 (2015) 130–146 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem