Contribution of Resuspended Sediments to the Dissolved Trace Metal Pool (Fe, Mn): An Experimental Study

A number of trace metals are thought to play essential roles in marine ecosystem structure and biological productivity. Until recently, it was argued that phytoplankton access primarily dissolved iron, while particulate iron was considered as refractory material with little use biologically and little interaction with the dissolved pool. To assess the transfer mechanisms between the dissolved and particulate pools of sediment-sourced trace metals we have conducted a 14-month experiment reacting suspended sediments originating from the Kerguelen area (KEOPS2 cruise; Southern Ocean) with natural Southern Ocean seawater, in the dark and at concentrations replicating natural conditions.

Three types of sediments, namely A3, C11, C1, that were composed for the most part of respectively (i) biogenic silica, (ii) biogenic silica and calcite and (iii) basaltic fragments were investigated. Release of dissolved silicon (dSi), iron (dFe) and manganese (dMn) were regularly monitored throughout the experiment, as well as Fe organic ligands and living bacteria concentrations. These additional parameters improved our understanding of the mechanisms involved in the dissolution processes observed during the experiment. In particular, depending on the origin and composition of the sediment, specific release mechanisms were revealed, including a strong decoupling between Fe and Mn. The basaltic sediment C1 released 1.09 ± 0.01 nmol kg^-1 of dFe and 0.28 ± 0.02 nmol kg^-1 of dMn, while the biogenic sediment A3 released a maximum 3.91 ± 0.01 nmol kg^-1 and 8.23 ± 0.36 nmol kg^-1 of dFe and dMn, respectively, in the 11 L reactors. In a broader context, these results strengthen the hypothesis of natural Fe fertilization by Kerguelen Plateau sediments. This experiment also provides a strong basis for reassessing the role of resuspended sedimentary particles in the marine biogeochemical cycles of Fe and Mn.