Integrated source apportionment and geochemical background assessment of organic and inorganic components in Amazonian upland lakes: insights from an extensive basin-wide geochemical database

Purpose This study established reliable natural geochemical backgrounds for Amazonian upland lakes and evaluated how lateritic catchment materials are transferred and transformed in lake sediments across Carajás, Araguaia, and Xingu. Methods Ferruginous duricrusts and lake sediments were analyzed using a robust median + 2×median absolute deviation (mMAD) approach to define regional inorganic and organic backgrounds. Source-to-sink changes were quantified with Hf-referenced mass-transfer coefficients. Sediment composition was further assessed by principal component analysis, redox-sensitive element ratios (Mo/U and S/Fe), rare earth element patterns, and MixSIAR modeling of organic matter sources in surface sediments. Results Region-specific mMAD backgrounds captured lithological controls while minimizing outlier influence. Mass-transfer coefficients indicated net silica enrichment and ferruginous loss from catchments to sediments, expanding to multi-element enrichment in organic-rich infilled basins. Principal component analysis revealed a dominant gradient from silica dilution to iron-phosphate carrier enrichment, with Al co-varying with Si and Ba, indicating limited suitability of Al normalization and supporting the use of high-field-strength elements such as Hf and Th. Redox proxies showed prevailing non-sulfidic anoxia with episodic micro-sulfidization sufficient for Mo retention, especially in basins with greater accommodation space and organic loading. MixSIAR indicated that bacterial organic matter dominated sediments in all sub-regions, followed by macrophytes and phytoplankton, with clear spatial differences among lake districts. Conclusion The integrated framework provides defensible geochemical backgrounds, supports high-field-strength element-based normalization, and offers process-based guidance for monitoring and management of tropical lateritic watersheds and lake sediments.