Environmental controls on the distribution of metals in porewater and their diffusion fluxes at the sediment-water interface of the western Pacific

Abstract

The intermountain basin of the western Pacific is rich in polymetallic nodules with commercial exploitation potentialities. Regarding the potential impacts of metals during deep sea mining operations, the regeneration process of metals with environmental controls and their diffusion fluxes at the sediment water interface (SWI) are of great importance. Our results demonstrated that the deep-sea environment in the western Pacific seafloor was characterized as organic carbon-starved sediments, with a total organic carbon content (TOC) between 0.2%–0.5%. Sedimentary Eh (SEH) ranged from 400 mV to 570 mV, indicating suboxidized to oxidized conditions. The vertical profiles of dissolved metals in porewater showed an enrichment of nodule-related metals and conservative metals (Mo, Li, Rb, Cs) in the uppermost suboxidized zone. They were related to desorption with labile particles, as well as remobilization by the reduction of Mn- and Fe-oxides under suboxidized condition. The diffusion fluxes of those nodule-related metals (Co, Ni, Mn, Pb, Zn) to the overlying water were low and fluctuated within the study area. The relatively high concentrations in the overlying water also confirmed a continuum with equivalent contributions to nodule formation from the bottom water as well as from the pore water. The principal components analysis (PCA) results further indicated that environmental gradients in Eh, TOC, δ13Corg, and mean grain size of sediment as well as spatial gradients in distance from the nearest sea mountain, latitude, and water depth posed a significant impact on the diffusion fluxes of different metals. Despite many uncertainties in sediment disturbance during nodule mining activities, this baseline information supports further estimation of the potential release of metals under sediment disturbance.