Dynamics of iron and manganese in the Black Sea and Baltic Sea

Phytoplankton in the oceans are responsible for about half of the photosynthetic fixation of carbon on Earth. Oceanic primary production can be limited by the availability of nutrients in the photic zone. In many areas, especially in coastal zones, phosphorus (P) or nitrogen (N) is the limiting nutrient. However, there are large areas further off-shore that are characterized by high concentrations of P and N and low primary productivity. In these so called "high nutrient low chlorophyll zones", primary production is often limited by the availability of micro-nutrients. Iron (Fe) and manganese (Mn) are examples of such micro-nutrients. Especially Fe limits primary production in large parts of the ocean and, in these areas, an increased supply of Fe could lead to enhanced CO2 uptake and, ultimately, an impact on climate. Iron and manganese are delivered to the ocean through different pathways. While dust was long thought to be the major source of both Fe and Mn, recently a more complex picture of the various sources of Fe and Mn has emerged. Besides dust, hydrothermal vents, subglacial runoff and continental shelves were discovered to be important sources of Fe and Mn to the open ocean. In particular, benthic release of Fe and Mn and subsequent lateral transport, which is termed "shuttling", was found to be a major source of dissolved Fe and Mn to the ocean. The shuttling of Fe and Mn over continental shelves depends on the external supply of Fe and Mn, usually from rivers, release of Fe and Mn from the sediment and on transport processes in the water column. Mobilization of Fe(II) and Mn(II) in marine sediments is typically high in sediments with a high input of both Fe and Mn (oxyhydr)oxides (henceforth termed Fe and Mn oxides) and organic matter. In marine sediments, this organic matter is remineralized by microorganisms that use a range of electron acceptors. Oxygen is energetically the most favorable electron acceptor for microbial respiration of organic matter. Therefore, oxygen is typically depleted within the first cm of organic-rich sediments. In the absence of oxygen, organic matter is subsequently remineralized by denitrification, Mn and Fe oxide reduction, sulfate reduction and methanogenesis. Remineralization leads to the release of nutrients (e.g. ammonium, nitrate and phosphate) and, reduced species (e.g. Mn(II), Fe(II) and sulfide (H2S)) to the porewater in distinct zones of the sediment. Subsequently, Fe(II) and Mn(II) can diffuse towards the sediment-water interface. Both Fe(II) and Mn(II) are highly reactive towards oxygen and thus will typically become oxidized and precipitate in the oxic layer of the sediment. As a consequence, release of Fe(II) and Mn(II) from sediments underlying oxic bottom waters is expected to be limited. However, biological activity by macrofauna may contribute to release of Fe(II) and Mn(II) from such sediments.