Mineralogy and Sr–Nd isotopes of SPM and sediment from the Mandovi and Zuari estuaries: influence of weathering and anthropogenic contribution

Abstract

Clay minerals and Sr–Nd isotopes of suspended particulate matter (SPM) and bottom sediment were investigated along transect stations of the Mandovi and Zuari estuaries, western India to determine the provenance and role of estuarine processes on their distribution. Kaolinite and illite, followed by minor goethite, gibbsite and chlorite were present in SPM and bottom sediment at all stations, both during monsoon and pre-monsoon. Smectite occurred in traces at river end stations but its contents increased downstream in both estuaries. Smectite contents were much higher in Zuari than in Mandovi estuary. The ⁸⁷Sr/⁸⁶Sr ratios and ɛNd of SPM were higher than those in hinterland rocks and laterite soils. The Sr ratios were highest at river end stations of both estuaries and decreased sharply seaward. The Sm/Nd ratios of SPM and sediment were close to that of iron ore material flushed into the estuaries. The mean ɛNd of SPM and sediment were similar in both estuaries. It is suggested that the smectite is formed in coastal plains and its distribution downstream is controlled by lithology and drainage basin of rivers. Abundant kaolinite and high Sr ratios reflect chemical weathering and lateritization of source rocks. Sr isotopic ratios along transects are influenced by changes in salinity, organic matter and turbidity. High and near identical ɛNd values along transect stations of both estuaries suggest that the Nd isotopic compositions are influenced by the lateritization of source rocks and anthropogenic contribution of ore material.