The different diffusive transport behaviours of some metals in layers of Peru Basin surface sediment

Abstract

Abstract Diffusion experiments with a duration of 90 days were carried out with radio tracers in an oxic and a suboxic sediment layer from two deep-sea sediment cores. Diffusion coefficients, breakthrough and time-lag values were determined and the results checked by modelling. Only small differences of diffusive transport were observed for Na and Cs during the four experiments; Na behaved as a perfect conservative tracer, and the somewhat stronger uptake of Cs in one sample was caused by preferential absorption by illite that was enriched in this sample. The significant differences between the four experiments for the diffusion parameters of Mn, Zn, Co, and Cd correlate with the differing chemical compositions of the sediment layers; higher Mn oxide concentrations in the sample led to slower diffusion through the sediment sample and to significant sorption of the diffusing heavy metals. This result confirms the dominating role of Mn oxide phases in controlling the diffusive exchange of heavy metals between deep-sea sediment and bottom water. Sediment resuspension of the oxic surface layer, such as during potential ferromanganese nodule mining in the deep-sea, would be followed by increased diffusive flux of dissolved heavy metals, especially Mn, from suboxic sediment layers to the new disturbed sediment–bottom-water interface. According to our modelling, the Mn flux from the oxic sediment surface of 0.3 mg m−2 yr−1 in the undisturbed state would increase to up to several mg m−2 yr−1 within the first few weeks after disturbance. Depending on the time necessary until a stable oxic and Mn oxide rich layer is again re-established, a total Mn2+ release of 54 mg m−2 was calculated if the original state is reached within 100 yr, in contrast to only 3.5 mg m−2 for 5 yr of re-establishment. The fluxes of the other heavy metals would be smaller, corresponding to their lower pore water concentrations in the suboxic layer compared to Mn.