Fast-growing, shallow-water ferro-manganese nodules from the western Baltic Sea: origin and modes of trace element incorporation

Ferro-manganese nodules of the western Baltic Sea reveal enhanced heavy metal concentrations since the end of the 19th century and are therefore potentially useful for retrospective monitoring of the heavy metal pollution history. However, only Zn shows a significant rise, even though Cu is also introduced to the western Baltic Sea by natural and anthropogenic sources. This study focused on geochemical considerations to evaluate the mechanism responsible for the predominance of Zn in ferro-manganese nodules compared to Cu. The particulate and dissolved metal fractions in the water column, in the fluffy layer (an approximately 2-cm-thick sediment layer situated at the sediment–water interface) and in the pore water adjacent to ferro-manganese nodules were studied. The analysis of the dissolved fraction and speciation studies were carried out using cathodic stripping voltammetry (CSV). The particulate fraction and the pore water were analyzed via graphite furnace–atomic absorption spectroscopy (GF–AAS) and inductively coupled plasma–optical emission spectroscopy (ICP–OES). The fluffy layer provides high quantities of Mn, Fe, Zn, and Cu to the dissolved fraction of the bottom water during the stratification period. Hence, the signal manifested in the ferro-manganese nodules consists of newly emitted elements on top of the amount collected in the fluffy layer over many years and remobilized throughout the stratification. Trace elements with the tendency to form weak inorganic complexes in the water column, like Zn, are more likely to be incorporated in ferro-manganese nodules and therewith document the anthropogenic metal input, while elements which tend to form strong organic complexes such as Cu are scavenged by particulate organic matter in the water column and the fluffy layer.