Geochemistry of Core Sediments From the Southeast Coast of Bangladesh: Constraints on Chemical Weathering, Paleoenvironmental Conditions, Provenance, and Tectonic Setting

Abstract

Major, trace, and rare-earth elements (REEs) were examined on modern sediments of a 70-m long core collected from the southeast coast of Bangladesh to understand chemical weathering, paleo-redox sensitivity, sediment maturity, source rock composition, and tectonic setting during the deposition of sediments. The core samples contained high SiO₂ (62–91 wt.%) and low Al₂O₃ (~5–17 wt.%) that showed a marked negative correlation (r = −0.98) with linear trends, indicating that the quartz rather than aluminosilicates mainly controlled SiO₂. Substantial depletion of major labile elements (e.g., Na₂O, CaO*, K₂O, Ba, and Sr) compared with the upper continental crust (UCC) indicates the destruction of feldspar during chemical weathering in the source area. The index of compositional variability (ICV) and the SiO₂/Al₂O₃ ratio of the sediments varied from 0.79 to 1.83 and 3.47–22.69, respectively, indicating immature to moderate compositional and mineralogical maturity. The chemical index of alteration (CIA, ~67–81), chemical index of weathering (CIW, ~69–91), and plagioclase index of alteration (PIA, ~71–92) parameters suggest moderate to high chemical weathering intensity in the source area, which was favoured and accelerated by the warm and humid conditions. The elemental ratios of V/Cr, Ni/V, Ni/Co, U/Th, Cu/Zn, and V/V + Ni suggested oxic to sub-oxic paleoenvironmental conditions that prevailed during sediment deposition. Provenance discrimination diagrams and elemental ratios (e.g., Th/Sc, Zr/Sc, La/Sc, Cr/V, and Co/Th) suggest their derivation from felsic source rocks (e.g., granodiorite, rhyolite, and granite). The chondrite-normalised (REEs) distributions show light REE enrichment (La_N/Yb_N, 7.61–14.35), nearly flat heavy REE (Gd_N/Yb_N, 1.33–2.25), and marked Eu anomalies (Eu/Eu*, ~0.58–1.40), suggesting an influx of sediments from felsic provenance. The REE patterns and parameters are similar throughout the sequence, confirming that the overall source composition remained unchanged. The tectonic setting discrimination diagrams show that the core sediments were mainly derived from active and passive margin environments.