Unraveling the sedimentation environment of Marajó island: Insights from geochemical studies and implications for the origin of potentially toxic element in soils

Abstract

Marajó Island, located at the mouth of the Amazon and Tocantins rivers, is the largest fluvial maritime island on Earth. Its unique location within the Amazon biome and its complex sedimentary dynamics makes it a region of great ecological and socio-economic importance. However, the island is facing significant challenges due to climate change, with projections suggesting permanent flooding of its floodplains in the coming decades. Despite numerous studies on soil geochemistry in the Amazon biome, there is a lack of research specifically focused on Marajó Island. Our study aims to investigate the soil's geochemical properties on the major parent materials in the eastern portion of Marajó Island, seeking to unravel the nature of its complex sedimentary environment, elemental relationships, origin and implications of potentially toxic elements. Samples from seven soil profiles representative of the eastern portion island were collected in a chronosequence, to conduct physical and chemical analyzes: selective dissolution of Fe oxides by sodium citrate-bicarbonate-dithionite and ammonium acid oxalate; quantification of major, trace and rare earth elements (REE) performed by multi-acid digestion and determination in ICP-OES/MS; characterization of weathering intensity by the chemical index of alteration (CIA), chemical weathering index (CWI), plagioclase index of alteration (PIA), chemical proxy of alteration (CPA), and index of compositional variability (ICV); and assessment of potentially toxic elements using enrichment factor and geoaccumulation index. The results indicated that the geochemical variation of soils reflected particularities of the parent material influenced by marine transgression events, and by the current conditions of water table oscillation and humid tropical climate. Holocene Sediments (HS) revealed enrichment by light REE. In soils on Post-Barreiras (PB), Laterite Formation (LF) and Lateritic Debris (LD), the main elements were Al, Fe, and Ti. Due to intense weathering and leaching, negative anomalies were observed in nearly all elements, including the REE. Arsenic enrichment in HS soils was due to the past estuarine influence and the current hydromorphic conditions, while soils on PB, LF and LD are naturally enriched by the parent material.