Passive sampling of labile dissolved trace metals in the deep sea: a suitable monitoring tool for marine mining activities?

Analytical challenges such as low concentrations and a saline matrix, contamination risks during sampling and analysis, and the accessibility of remote ocean areas limit available methodologies for the investigation of trace elements in the open ocean and call for innovative, easy to use and robust investigation tools. Furthermore, the application of methodologies for the monitoring of e.g., deep-sea mining activities as part of environmental impact assessments will become of increasing importance for future marine research. We here evaluate an in situ passive sampling method using the technique of diffusive gradients in thin films (DGT), focussing on Mn, Cu, Ni, V, Cd, As, Sb, and rare earth elements and yttrium. Sampling was performed in bottom seawater above the BGR contract area for polymetallic nodule exploration in the Clarion Clipperton Zone in the NE Pacific. We present data from different passive sampling setups at depths between 0.3 m and 550 m above the seafloor accumulating trace metals for periods of 3 days up to 2 years, and discuss the effects of deployment time and the binding membrane type. Derived concentrations represent the labile fraction of the metals and hence that fraction that is (bio)available for an uptake through cell membranes. Results are compared to point sampling of seawater providing trace metal concentrations for different size fractions, which allows to constrain the degree of lability of dissolved trace metals. We further present results from the monitoring of a collector mining test of polymetallic nodules with respect to trace metal mobilization at the seafloor.