The fine-scale remobilization of metals in the surface sediment of the North-East Atlantic

Vertical profiles of trace metals were measured at 1-mm intervals by deploying DGT probes in sediment cores collected from the deep Atlantic Ocean (Feni Drift) before and after the spring phytoplankton bloom. DGT (diffusive gradients in thin-films) accumulates metals on a chelating resin after their diffusive transport through a layer of hydrogel. The mean concentration of metal in the porewaters at the surface of the device during its deployment was derived from the measured mass of metal. Well-defined laboratory systems were used to establish the reproducibility and accuracy of the DGT technique for measuring metals at 1-mm intervals. The Fe and Mn profiles showed the expected redox sequence with depth, with additional fine-scale (2–5mm) features. The close correspondence between the Mn and Co profiles, on both coarse and fine scales, showed that their chemistry is closely coupled in sediments as well as water columns. Changes in the background concentrations of Cd, Cu, Ni and Zn with depth were well correlated with Mn and Co, but the four metals also displayed mm-scale maxima that were largely independent of Mn, Co or Fe. Two-dimensional measurements at a 100-μm resolution on a DGT probe, deployed in situ with an autonomous lander, showed that the maxima were most likely associated with near-spherical, highly localised sources. The results were consistent with release of Cd, Cu, Ni and Zn from organic and planktonic skeletal material. Where there was associated release of Mn and/or Fe, through their oxides acting as electron acceptors in the decomposition of the organic matter, trace metals could also be supplied by release from the oxides. The microniches responsible for high local concentrations of metals in the porewaters were hypothesised to be due to downward mixing of material by burrowing organisms, known to be prevalent in these sediments. Much of this material appeared to be present as faecal pellets of various origins, with sizes in the range of 50μm to 1cm.