Iron-manganese concretions of the Estonian marine area: distribution, formation mechanisms and economic potential

Fe-Mn-concretions occur on the seafloor over large regions of the Gulf of Finland, particularly in the areas where there is no modern sediment deposition and the seafloor is affected by seepage of chemically reduced fluids derived from underlying strata. The economic potential of Fe-Mn-concretions and their genesis are however poorly understood, but these aspects are currently being assessed in industry-driven projects in several areas of the Baltic Sea. Fe-Mn-concretions are effective scavengers of phosphorous with concentrations in precipitates exceeding the seafloor sediments by an order of magnitude. Given the sensitivity of Fe-Mn-concretions to redox changes, their dissolution can readily occur during episodes of an expansion of anoxic bottom water conditions or physical disturbance of the seafloor (i.e. ships anchoring) that can change the nutrient budgets and cause eutrophication. Hence any mining project of Fe-Mn precipitates needs to account for environmental consequences.