Trace metal mobility in intertidal sediments is strongly controlled by microscale biogeochemical processes. In this study, LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) combined with the DGT technique (Diffusive Gradients in Thin-films) was used to investigate the two-dimensional (2D) distribution of redox-sensitive and trace metals in intertidal sediments. The results revealed distinct microscale heterogeneity of Fe, Mn, Co, Ni, Cu, and Cd. These patterns indicate that the metal mobilization mechanism was controlled by redox conditions, biological activity, and sediment heterogeneity. Comparisons between different time periods (May and December) and different selected DGT zones showed that metal behavior varied temporally and spatially. A common co-mobilization of Mn and Co, Ni, Cd and Cu was found for both periods and zones, while Fe was associated with Co, Ni, Cu and partially Cd in December. The metal ratios of Co, Ni, Cd, and Cu relative to Fe and Mn were calculated and were comparable worldwide.
The total dissolved and labile dissolved metal concentration gradients across the sediment–water interface (SWI) were further used to estimate diffusive fluxes, with and without turbulent effects, i.e., using turbulent and molecular diffusion coefficients. The turbulent diffusive fluxes were approximately two orders of magnitude higher than the molecular ones, indicating that the tide currents exert a significant impact on metal exchange at the SWI. The turbulent diffusive fluxes were comparable to those reported in the previous study in the vicinity of the study area. Using turbulent diffusive fluxes, the potential metal release quantities from sediment porewater to the water column were estimated during one tidal cycle. Using 2D DGT results, horizontal metal diffusive fluxes and vertical distributions of fluxes were also calculated. The local maxima in the vertical flux distribution indicated that SWI is highly heterogeneous and strongly influenced by localized mobilization zones that would be overlooked by conventional one-dimensional profiles.
This study demonstrates that LA-ICP-MS combined with the DGT technique provides valuable insight into the spatial distribution and diffusive transport of trace metals in sediments. The approach improves our understanding of sedimentary metal cycling and offers a useful framework for assessing metal mobility and potential fluxes under changing biogeochemical conditions.