Geochemistry of core sediments from the southeast coast of Bangladesh: Implications for provenance and chemical weathering intensity

Geochemical analyses of major oxides, trace, and rare-earth elements (REE) were examined on the ~70 m core sediments collected from the southeast coast of Bangladesh to determine sediment provenance, maturity, and chemical weathering conditions. The sediment samples contained high SiO₂ (62-91 wt.%) and low Al₂O₃ (~5-17 wt.%) contents and showed a marked negative correlation (r = -0.99) with strong linear trends, indicating that SiO₂ was mainly controlled by the quartz content rather than aluminosilicates. Substantial depletion of major labile elements (Na₂O, CaO, K₂O, Ba, and Sr) compared to the upper continental crust (UCC) indicates the destruction of feldspar during chemical weathering in the source area. The chondrite-normalized REE patterns show LREE enrichment (La_N/Yb_N, 7.61-14.35), nearly flat HREE (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. Numerous provenance discrimination diagrams and elemental ratios (Th/Sc, La/Sc, Zr/Sc, Cr/Th, Th/Co, Eu/Eu*, and Gd_N/Yb_N) show that the core sediments were derived from felsic source rocks mostly granodiorites, rhyolites, and granites. The REE patterns and parameters are very similar throughout the sequence studied, indicating that the overall source composition in the basin remained unchanged. The Index of Compositional Variability (ICV) values of the sediments varied from 0.79 to 1.83, which indicates immature to moderate compositional maturity. The Chemical Index of Alteration (CIA, ~67 to 81), Chemical Index of Weathering (CIW, ~69 to 91), and Plagioclase Index of Alteration (PIA, ~71 to 92) parameters suggest moderate to high chemical weathering intensity in the source area, which was favored and accelerated by the warm and humid climatic conditions. The elemental ratios (V/Cr, Ni/Co, Cu/Zn, and V/V+Ni) suggested oxic to sub-oxic depositional environment for the accumulation of sediments in the studied Bengal coast. However, the variation of weathering patterns and proxies in the core sediments could be influenced by the strength of South Asian monsoon circulation over the Himalaya-Tibetan Plateau.