Abstract
Abstract. 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 large-scale eruptions and fine ash may stay aloft for days to weeks, thereby reaching even the remotest and most iron-starved 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 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 CO2 and other gases relevant for climate in the Earth's history.
Highlights
1.1 Purpose and structure of the review paperIt is widely recognised that soils formed on volcanic materials are highly fertile (Schmincke, 2004)
Based on iron-release data, Duggen et al (2007) calculated that the deposition of 1 mm of subduction zone-related ash over the ocean may raise the surface ocean iron concentrations by a few nM. Such input would be enough to cause significant marine primary productivity (MPP) response in an iron-limited (HNLC) oceanic area, as >2 nM Fe-increase were shown in mesoscale ironenrichment experiments to be sufficient to stimulate a massive phytoplankton bloom (Wells, 2003)
There are two principal mechanisms by which iron in volcanic ash can be mobilised into soluble forms upon contact with seawater and become bio-available in the surface ocean (Fig. 3): (i) dissolution of iron-containing salt coatings found on the surface of ash particles in the volcanic plume, and (ii) dissolution of the silicate glass and silicate and non-silicate mineral components of the ash
Summary
It is widely recognised that soils formed on volcanic materials are highly fertile (Schmincke, 2004). In a more recent contribution, Langmann et al (2010) provided new satellite-based evidence that volcanic ash fall-out can be a significant source of available iron in HNLC areas These authors argue that the ash from the Kasatochi volcano in the Aleutian subduction zone caused a large-scale phytoplankton bloom in August 2008 in the northeast subarctic Pacific. Based on iron-release data, Duggen et al (2007) calculated that the deposition of 1 mm of subduction zone-related ash over the ocean may raise the surface ocean iron concentrations by a few nM Such input would be enough to cause significant MPP response in an iron-limited (HNLC) oceanic area, as >2 nM Fe-increase were shown in mesoscale ironenrichment experiments to be sufficient to stimulate a massive phytoplankton bloom (Wells, 2003). If the ash deposited in sea ice has maintained its full potential to release soluble and bio-available iron, natural iron addition during ice melting may occur in the open ocean far away from the ash fall-out area of the volcano
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