Contribution of resuspended sedimentary particles to dissolved iron and manganese in the ocean: An experimental study

A number of trace metals play essential roles in marine ecosystem structure and biological productivity. Until recently, it has been argued that phytoplankton access primarily dissolved iron, while particulate iron was considered a refractory material with little use biologically and limited interaction with the dissolved pool. In order to assess the transfer mechanisms between sediment-sourced particulate trace metals and the dissolved pool, we conducted a 14-month incubation that reacted resuspended sediments with natural seawater, both originating from the Kerguelen area (KEOPS cruises; Southern Ocean), in the dark, and at concentrations replicating natural conditions. Three types of sediments were investigated (named BioSi, BioSi + Ca, and Basalt), mostly composed of (i) biogenic silica (bSiO ₂ ), (ii) bSiO ₂ and calcite, and (iii) basaltic fragments, respectively. The release of dissolved silicon (dSi), iron (dFe) and manganese (dMn) was monitored regularly throughout the incubation, as well as living bacteria density and Fe organic ligands. Depending on the origin and composition of the sediment, unique dFe and dMn fluxes were observed, including a strong decoupling between dFe and dMn. The basaltic sediment released up to 1.09 ± 0.04 nmol L ⁻¹ of dFe and 0.28 ± 0.09 nmol L ⁻¹ of dMn, while the biogenic sediments released a higher 3.91 ± 0.04 nmol L ⁻¹ and 8.03 ± 0.42 nmol L ⁻¹ of dFe and dMn, respectively. Several factors influencing the release and removal of dFe and dMn were discernable at the temporal sampling resolution of the incubation, including the structural composition of the sediment, bacterial abundance, and the formation of manganese oxides. The regular sampling over short timescales and the extended sampling over one year proved to be critical to constrain the processes and exchanges that govern the contribution of the particulate to the dissolved pools. Overall, this incubation provides a strong basis for reassessing the role of resuspended sedimentary particles in the marine biogeochemical cycles of Fe and Mn. Indeed, we show that biogenic silica, calcite-rich and basaltic particles can contribute substantial dissolved Fe and Mn to the overlying water column. In the future, the global extent of this previously overlooked external metal source should be quantified through further process studies and biogeochemical models. This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.