Applicability of Magnetic and Geochemical Characterization Techniques to Assess the Evolution of Estuarine Systems: A Case Study of Gad River Estuary Sediments, Maharashtra

Abstract

River estuarine samples are a complex repository of terrestrial and nearshore geophysical changes. Sediment core raised from the Gad river estuary, situated in Maharashtra state of India, was analyzed to understand their rock magnetic, particle size and geochemical characteristics. These three parameters are closely related to one another signifying detrital model of deposition, devoid of soft diagenetic changes. The magnetic grain size and particle grain size constraining the domain structure and physical structure respectively, are both in consonance with chemical elements that are amenable largely to physical comminution. The integrated approach of this nature has been tried for the first time on estuarine samples of India. Mineral magnetism utilises the magnetic behaviour of a material to interpret environmental processes acting upon it. The study unraveled two-tier climate-induced modifications to erosional activity, suggesting abrupt change at 20 cm depth in a 45 cm core. The magnetic characteristics are seen to be controlled by the presence of ferrimagnetic minerals. Rock magnetic results (χlf, ARM, SIRM) and Curie temperature reveal that low-coercivity magnetite (and/or titanomagnetite) is the dominant magnetic mineral. χlf is more at the bottom and less at the top. Magnetic grain size is coarse at the top and fine at the bottom. Clay and silt proportion is more at the top of the core and that of sand is more at the bottom. Concentration of detrital geochemical elements like Al, Ti and Cr are less at the top and more at the bottom of the core, corroborated by chemical index of weathering and chemical index of alteration. Thus, it is clear from χlf that magnetic minerals are more at the bottom and so are the detrital minerals like Al, Ti and Cr, along with sand. This reveals energy conditions were vigorous at the time of the deposition of these entities at the bottom, which eased out at the top and are corroborated by the presence of clay and silt, having correspondingly low Al, Ti and Cr with low χlf. Micromorphological features revealed abrasion pits, grooves and bumped edges on sub-angular grains, which indicate moderate transport distance under moderate to low energy conditions revealing terrestrial origin of this material. Thus, similar studies in other estuarine complexes can be used as a proxy to understand monsoonal or environmental changes initiated by neotectonic activity.