Transformation of minerals and mobility of heavy metals during oxidative weathering of seafloor massive sulfide and their environmental significance

Oxidative Dissolution of Sulfide Minerals Tends to Accumulate More Dissolved Heavy Metals in Deep Seawater Environments than in Shallow Seawater Environments

Deep-sea mining magnifies the release of heavy metals into seawater through oxidative dissolution of seafloor massive sulfide (SMS). At present, there is little information about how the metals released into seawater might be affected by the mineral assemblages, seawater conditions, and solid percentages. Here, leaching experiments were carried out to examine the behavior of three sulfides from the Southwest Indian Ridge, under conditions that replicated deep and shallow seawater environments at three solid-liquid ratios. The results demonstrated that sphalerite dissolved rapidly, and the metals released in both experimental conditions were comparable, potentially reflecting galvanic interactions between the sulfide minerals. Large quantities of the released metals were removed from the solutions when hydrous ferric oxides formed, especially for shallow seawater conditions. A comparison of metal concentrations in the leachates with the baseline metal concentrations in natural seawater indicated that most of the released metals, when diluted with seawater, would not have widespread impacts on ecosystems. Based on the obtained unique oxidative dissolution properties of each SMS at variable solid-liquid ratios, targeted wastewater discharge treatments are proposed to minimize impacts from the dissolved metals. This study will support the development of robust guidelines for deep-sea mining activities.

Stressors of emerging concern in deep-sea environments: microplastics, pharmaceuticals, personal care products and deep-sea mining

Although most deep-sea areas are remote in comparison to coastal zones, a growing body of literature indicates that many sensitive ecosystems could be under increased stress from anthropogenic sources. Among the multiple potential stressors, microplastics (MPs), pharmaceuticals and personal care products (PPCPs/PCPs) and the imminent start of commercial deep-sea mining have received increased attention. Here we review recent literature on these emerging stressors in deep-sea environments and discuss cumulative effects with climate change associated variables. Importantly, MPs and PPCPs have been detected in deep-sea waters, organisms and sediments, in some locations in comparable levels to coastal areas. The Atlantic Ocean and the Mediterranean Sea are the most studied areas and where higher levels of MPs and PPCPs have been detected. The paucity of data for most other deep-sea ecosystems indicates that many more locations are likely to be contaminated by these emerging stressors, but the absence of studies hampers a better assessment of the potential risk. The main knowledge gaps in the field are identified and discussed, and future research priorities are highlighted to improve hazard and risk assessment.