| dc.description.abstract | This thesis aims at constraining the impact of estuarine processes on the net riverine fluxes of select metals and their isotopic composition to seawater. I studied the Ganga (Hooghly) River estuary, a monsoon-dominated tropical river characterized by high sediment-to-water ratio, which served as a natural laboratory for studying large-scale estuarine processes. Estuaries, located at the interface of continental and marine setting are characterized by the mixing of freshwater with seawater. Highly dynamic physiochemical gradients (viz. Eh, pH, ionic strength, dissolved oxygen levels, suspended sediment particulate (SPM) concentration, density etc.) in estuaries drive a range of processes such as precipitation/dissolution, adsorption/desorption, colloidal flocculation which result in variable partitioning of metals between dissolved and sediment phases. In addition, benthic boundary fluxes, in the form of groundwater and shallow porewater addition, can also impact the fluid phase composition in estuaries. The collective impact of these processes is the modulation of riverine metal export to the oceans, thus utilising freshwater endmember concentration and water flux to constrain metal export can lead to either over- or under-estimation.
Between the years 2020 and 2024, the Ganga (Hooghly) Estuary was sampled at a high spatio-temporal resolution covering periods of contrasting water discharge, i.e., peak (Aug-Sept) vs lean (Jan-March) discharge. More than 200 samples were quantified for their metal concentrations and isotopic compositions. I quantified the following reservoirs: (i) river and estuarine water; (ii) shallow and deep groundwater; (iii) shallow porewater; (iv) suspended particulate matter (SPM) and benthic sediments; and (v) chemically extracted fractions of sediments (exchangeable, oxide, carbonate, clay), to complete the mass budget. Accordingly, this thesis is divided into four primary sections, each focusing on distinct estuarine process:
(i) Role of Benthic Boundary fluxes: During the low-flow dry season, dissolved Ca concentration ([Ca]d) behaves non-conservatively in the estuary. The [Ca]d demonstrates an “excess” relative to river water-seawater mixing line in low to mid salinity range. Utilizing tracer-based model (Ba–Na), submarine groundwater discharge (SGD) was identified as the primary source of excess calcium. The SGD-derived fluxes of alkaline earth metals (Ba, Sr, Ca, Mg) comprise a non-negligible fraction of global riverine discharges, highlighting their critical contribution to the coastal chemical budget. Our revised SGD flux calculations indicate that the net SGD flux is ~11% of the total Ganga-Brahmaputra riverine flux. This observation significantly refines estuarine chemical mass balance for multiple elements.
(ii) Role of fluid-particle interaction and chemical speciation on micronutrient dynamics: We investigated the role of estuaries in micronutrient cycling by focusing on the export of dissolved copper (Cu) and cadmium (Cd) to coastal ocean. Though rivers are the primary source of these metals to seawater, globally, the estuarine processes can augment their export fluxes by up to 20% for Cu and 271% for Cd. The key finding of this work is that the dissolved Cu and Cd in the mid to high salinity range (4–13‰) exhibit a concentration “excess” relative to conservative river water-seawater mixing. This fluid phase increase is accompanied by a proportionate decrease in the concentration of the metals in the “reactive” solid phase. Through mass balance and metal speciation modelling, remobilization of metals from exchangeable (Cd) and/or amorphous oxide (Cu) phases of the riverine suspended sediments was identified to be the primary modulator of concentration.
(iii) Uptake of lithium by neo-formed authigenic phases in estuaries: Dissolved lithium (Li), in both concentration and isotope ratio space, was observed to demonstrate seasonally contrasting behaviour. The non-conservative behaviour is hypothesised to be driven by variations in water residence time within the estuarine mixing zone. A key observation is that during the dry season, there is significant Li removal from fluid phase via secondary clay formation (alpha : 0.9760±0.0015); whereas during the wet season, the Li removal is largely limited to chemical adsorption (alpha: 0.9832±0.002). Isotope fractionation modelling reveals that approximately 0.6–1.1 microg/g (discharge-weighted) of Li is removed from the dissolved phase into the reactive phases of SPM of the Ganga Estuary. This sedimentary Li removal by estuarine processes is reflected in the net riverine d7Li outflux which increases by up to ~3.4‰ during the dry season and by ~0.5‰ during the wet season. Thus, reactive estuarine sediments serve as an important oceanic Li sink, with ~88–92% of removed Li being sourced from the seawater inventory. Given the strong seasonal dependence of the magnitude of estuarine Li uptake and associated isotope fractionation in present day, I propose that variation in estuarine Li removal efficiency over the late Cenozoic are closely linked to changes in hydrological cycle and sediment flux to ocean and therefore can influence the lower seawater d7Li observed during the wet climatic condition of the past i.e., mid-Miocene climate optimum.
(iv) Rapid Clay Authigenesis in Coastal Delta: Lastly, we provide direct geochemical evidence for rapid authigenic clay formation (reverse weathering) in the Ganga Delta sediments. Shallow porewater chemistry from high salinity regions reveals substantial uptake of alkali metals (Li, K) coinciding with elevated dissolved silica, indicative of reverse weathering mediated by biogenic silica dissolution. Lithium isotope measurements from porewaters display classical signatures of preferential light isotope (⁶Li) incorporation into newly formed clays, confirmed by SEM and elemental mapping analyses of diagenetically altered diatoms in the sediment. These processes signify an important, previously underestimated pathway for seawater alkalinity consumption and atmospheric CO₂ production, potentially influencing global carbon cycling.
In addition, for the accurate and precise quantification of metal concentration from high ionic strength aqueous matrix, I developed a matrix matching analytical strategy which included: (i) gravimetric dilution of samples to [Na]fixed of 50/100/200 mg/L; and (ii) analyses against external calibration standards at identical matrix major ion concentration. I utilized ICP-OES for major and Q-ICPMS in hot plasma Collision-Reaction mode for minor and trace metals. The accuracy and precision of metal concentration, determined by repeat analysis of multiple SRMs (SLRS-6, NIST 1643f, SLEW-3, CASS-6, NASS-6) over multiple analytical sessions were ≤±10%. Whereas, the Li isotope measurements were also done in house, following an established protocol using a Themo® Neptune XT instrument. Long term external precision of 0.5‰ (2s) was achieved, based on the repeat analyses of seawater and a secondary reference material Li-6-N-SRM.
Collectively, this work elucidates the complex interplay of physical, chemical, and geological processes within large river estuaries, significantly revising global estimates of elemental and isotopic fluxes to oceans. The findings highlight the critical importance of estuarine environments, particularly the Ganga estuary, in regulating marine chemical budgets and contributing substantially to global geochemical cycles. | en_US |
