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Deep-sea floors act as reservoir for various marine mineral deposits rich in strategic metal resources important to growth of nation’s economy, national security and the emerging blue economy. Central Indian Ocean Basin (CIOB) is a region with vast marine mineral deposits with a congregation of metals in the form of nodules known as polymetallic nodules (PMN). Government of India signed a 15-year contract with International Seabed Authority (ISA) for exploration of PMN from CIOB. Extensive survey and other scientific developmental activities have been carried out in 75,000km2 area retained by India in CIOB. To conduct mining activities prior to assessment of impact on environment would remark an unwise undertaking. Deep sea mining operation will inevitably impact the surrounding ecosystem due to removal and separation of nodules from soft sediments and increases in suspended finer particles from the altered deep-sea sediment. Disturbances to sediments will have impact on the speciation (labile/inert complexes) of sedimentary metal complexes and may increase metal mobility and bioavailability in marine ecosystem. Study on the possible impact of deep-sea PMN mining requires systematic scientific investigation.This study presents kinetic speciation of metal in surface sediments and their downcore profile.To understand the stability and lability of metals, kinetic speciation study is carried out using ethylenediaminetetraacetic acid (EDTA) as the extracting reagent. Kinetic fractionation studies were performed from two sediments core collected from Test mining site (TMS) and Reference mining site (RMS) in CIOB to determine the metal-sediment complexes and their fate, mobility and bioavailability. Concentration of labile metals complexes and their dissociation rate constant in deep sea sediment can be an index for their bioavailability. It was found that total concentration of Cu and Co gradually decreases, whereas Ni increases in the study area The observed results suggest that the stability of metals-sediment complexes for Cu, Ni, and Co with respect to the depth of the sediment cores gradually increased in the study area. This study suggests that disturbances of deep-sea sediments during PMN mining may not increase labile metals complexes at water-sediment interfaces in the surrounding marine ecosystem. However, results obtained from two sediment cores from Test mine site and Reference mine site respectively may not satisfy representation of the entire 75,000km2 CIOB. Further study is recommended to provide better understanding of the impact of mining activities on marine ecosystem.

Sedimentary organic matter (SOM) plays an important role in hosting and reducing HgII in marine/estuarine sediment. This study provides a better understanding on the influence of nature of SOM, in regulating sedimentary mercury (Hg) and elemental mercury (Hg0) distribution, and speciation in the Zuari and Mandovi Estuaries that are representative of monsoon fed tropical estuaries, located in the central west coast of India. Salinity of the overlying water column controlled the physical and chemical characteristics of SOM in the estuarine systems. The high molecular weight (MW) SOM dominated at the mid and upstream (low salinity region) of the estuaries, whereas, the low MW SOM prevailed at the downstream (high salinity region). Sediment Hg showed more affinity towards the SOM of high MW. Increasing MW of SOM increased total sedimentary HgT in both the estuaries. SOM with low MW in the estuarine sediment displayed a negative relationship with the sediment Hg concentration. Distribution of Hg0 concentration in the estuarine sediment suggests that reduction of HgII in presence low MW SOM was a dominant process. It was also found that distribution and speciation of Hg0 in the estuarine sediment depends on the quantity, quality of the SOM, and the total sediment Hg loading. This study demonstrated that the competition between Hg-SOM complexation and HgII reduction by SOM controls HgII/Hg0 distribution in tropical estuarine sediment systems.

There is limited knowledge regarding seawater intrusion on Hg distribution in monsoon fed tropical estuarine systems during dry season (February to May). This study examined the influence of resuspension of estuarine sediment and intrusion of Hg bound SPM (HgSPM) (by the tide) from the outside of an estuary in controlling distribution, mobility, and bioavailability of Hg within the estuarine systems during the dry season. This investigation was carried out in the Mandovi estuary, a monsoon fed tropical estuary from the central west coast of India. Total Hg concentrations in the water column showed an increasing trend from upstream to downstream of the estuary. The concentration of Hg in the water column of the estuary was much below the concentration recommended by the EPA for aquatic life ambient water quality. Dissolved Hg (HgDissolved) was found to associate mainly with the higher molecular weight fraction of dissolved organic matter (DOM). A significant portion of the total Hg in the water column was present as HgSPM. The average bio-accumulated Hg concentration in edible oyster was high (collected from the estuary) during the dry season compared to the wet season (June to September). This study reveals that resuspension of Hg associated finer sediment particles and intrusion of HgSPM from the outside of the estuary may increase bioavailability of Hg in the Mandovi estuarine systems during the dry season. It is suggested that Hg bioaccumulation in commercially important biological species from different tropical estuarine system may increase during the dry season. The outcome of this research can be useful for policy making and to take proper decision to reduce and control Hg/toxic metals pollution (if any) in tropical estuarine system.

This is the first study to comprehend copper (Cu)-dynamics in a monsoon fed Indian estuarine system (the Mandovi estuary from the central west coast of India). Distribution and speciation of Cu in estuarine sediment, pore water, suspended particulate matter (SPM) and water column was used to understand geochemical cycling of Cu in the estuary. Geochemical fractionation study reveals that sedimentary organic carbon (Corg) was the major hosting phase for non-residual Cu in the sediments. Experimental analysis and chemical speciation modelling suggests that leaching of sedimentary Cu2+, CuCO3 and a fraction of Cu-Corg complexes increased Cu-concentrations in the pore water towards the downstream of the estuary. Dissolved Cu concentration in overlying water column was observed to increase with increasing Cu concentrations in the pore water. This study suggests that chemical speciation of sedimentary Cu play key role in controlling its distribution and dynamics in the tropical estuarine system during dry period.

Sedimentary organic matter (SOM) plays an important role in hosting and reducing Hg