Iron is a key micronutrient for phytoplankton growth in the surface ocean. Yet the significance of volcanism for the marine biogeochemical iron-cycle is poorly constrained. Recent studies, however, suggest that offshore deposition of airborne ash from volcanic eruptions is a way to inject significant amounts of bio-available iron into the surface ocean. Volcanic ash may be transported up to several tens of kilometers high into the atmosphere during largescale eruptions and fine ash may stay

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... t for days to weeks, thereby reaching even the remotest and most ironstarved oceanic regions. Scientific ocean drilling demonstrates that volcanic ash layers and dispersed ash particles are frequently found in marine sediments and that therefore volcanic ash deposition and iron-injection into the oceans took place throughout much of the Earth's history. Natural evidence and the data now available from geochemical and biological experiments and satellite techniques suggest that volcanic ash is a so far underestimated source for iron in the surface ocean, possibly of similar importance as aeolian dust. Here we summarise the development of and the knowledge in this fairly young research field. The paper Correspondence to: S. Duggen (svend duggen@skoleforeningen.de) covers a wide range of chemical and biological issues and we make recommendations for future directions in these areas. The review paper may thus be helpful to improve our understanding of the role of volcanic ash for the marine biogeochemical iron-cycle, marine primary productivity and the ocean-atmosphere exchange of CO 2 and other gases relevant for climate in the Earth's history. Introduction Purpose and structure of the review paper It is widely recognised that soils formed on volcanic materials are highly fertile (Schmincke, 2004) . This is due to the relative ease with which a range of macro-and micronutrients are released from the silicate glass and minerals upon weathering. In contrast, the fertilising potential of tephra for the marine environment is not well understood. There is, however, evidence from ocean sediment cores that large amounts of volcanic ash (tephra with size less than 2 mm) produced during explosive events can enter the marine environment. Volcanic ash plumes can travel considerable distance before the ash is being deposited onto the surface of open oceanic regions (Fig. 1) . Whilst the process Published by Copernicus Publications on behalf of the European Geosciences Union.