Geochemical and mineralogical controls on organic carbon retention in estuarine sediments and water systems: Insights from the Ashtamudi Wetland, India

Clay minerals play a critical role in controlling sediment geochemistry, organic matter stabilization, and depositional processes in estuarine environments; however, their influence in tropical Indian wetlands remains insufficiently characterized. This study investigates the mineralogical and geochemical properties of surface sediments in the Ashtamudi Wetland, a major estuarine system along the southwest coast of India, using an integrated approach combining physicochemical analysis and X-ray diffraction (XRD). Key parameters including sediment pH, electrical conductivity (EC), and organic carbon were analysed alongside XRD-derived mineral indices such as quartz, feldspar, clay content, and mineralogical diversity (peak density). Correlation analysis revealed that clay content is strongly positively associated with organic carbon (r = 0.55), feldspar (r = 0.85), and mineral peak density (r = 0.91), indicating enhanced organic matter retention, and increased mineralogical complexity in fine-grained sediments. In contrast, quartz shows a strong negative correlation with clay (r = −0.72), reflecting coarse-grained sediments with lower geochemical reactivity. A composite sediment quality index was developed using weighted contributions of mineralogical and geochemical parameters, including modifying effects of pH and EC, to assess spatial variability in sediment characteristics. The results demonstrate a clear gradient from clay-rich, organic-bearing sediments with higher geochemical quality to quartz-dominated sandy deposits with limited capacity for organic matter stabilization. Overall, the study highlights the dominant role of clay minerals and mineralogical diversity in governing sediment geochemistry and provides a robust mineralogy-based framework for sediment quality assessment in tropical estuarine environments.