Sediment Disturbance during PMN Mining and Its Impact on Metal Speciation of Sedimentary Cu, Ni, Co in Central Indian Ocean Basin

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.