Control of source-to-sink processes on the dispersal, fractionation, and deposition of magnetic minerals in a tropical mesotidal estuarine system

Abstract

This study aims to elucidate the control of source-to-sink processes on the sediment dynamics of a complex mesotidal tropical estuarine system using rock magnetic, sedimentological, mineralogical, and geochemical methods. Integration of multiproxy data of catchment rocks, riverbank and hinterland soils, and bedload sediments from various depositional environment of Mandovi estuary revealed a marked change in the magnetic mineral composition and accumulation pattern. The magnetic mineralogy mainly comprised ferri- (magnetite, titanomagnetite) and antiferromagnetic (hematite, goethite) minerals. Higher accumulation of coarser magnetic particles in the catchment area followed by a systematic trend of progressive fining of magnetic grain size in the down reaches of the estuary can be attributed to the decrease in transport energy. Enhanced accumulation of coarser magnetic particles in the estuarine region can be reconciled with the strong tidal and wave-induced bottom currents, which preferentially favored mineral-density selective fractionation resulting in higher deposition of heavy (magnetic) particles. A clear trend of loss of fluvial-derived fine silt-size clastic sediments and selective retention of coarser and heavy magnetic particles within the estuary can be attributed to the hydrodynamic driven sediment partitioning regime, which inhibited the permanent settling of fine sediment fraction and are therefore frequently flushed out to the sea. A marked drop in S-ratio in different zones of the estuary can be linked to the increased input of high coercive magnetic particles derived from pedogenic soils along eroding riverbanks. We further demonstrate that the S-ratio is an effective parameter to track the riverbank erosion in an estuarine system.