Iron and Life and Volcanic Ash
In 2010, there was a large volcanic eruption spewing tons of ash into the atmosphere and into the sea. The ash caused major flight delays as well as posing potential health hazards. Nevertheless, the Icelandic volcano's ash plume resulted in the oceans absorbing more carbon dioxide (CO2) than usual, say scientists. In about a third of the global ocean, the abundance of life is limited by a lack of biologically available iron. The supply of iron to a region that is depleted in this important nutrient can stimulate algal productivity, and can result in a temporary boom in biological activity. For much of the surface ocean, the wind-borne transport of iron-rich dust and the upwelling of nutrient-filled water are the normal major sources of iron.
The 2010 eruption of Eyjafjallajökull were relatively small for volcanic eruptions. The eruption was declared officially over in October 2010, when snow on the glacier did not melt. From April 14—20, ash covered large areas of northern Europe when the volcano erupted.
Though satellite observations and modeling work suggest that volcanic ash could seed life in such a way, there have been only a limited number of direct observations of the effects of ash deposition on surface ocean waters. Thanks to a bit of slucky scheduling, Achterberg et al. conducted a series of research cruises in the Iceland Basin region of the North Atlantic Ocean both during and after the month-long eruption of Iceland's Eyjafjallajökull volcano in the spring of 2010.
Three cruises allowed the authors to undertake measurements of surface ocean iron concentration before, during, and after the eruption in a region directly affected by the towering ash plume. Beneath the plume, the authors found peak dissolved iron concentrations up to 10.2 nanomolar, compared to 0.23 to 0.45 nanomolar detected before ash deposition.
The types of minerals present in volcanic ash are dependent on the chemistry of the magma from which it was erupted. Considering that the most abundant elements found in magma are silica and oxygen, the various types of magma (and therefore ash) produced during volcanic eruptions are most commonly explained in terms of their silica content. Low energy eruptions of basalt produce a characteristically dark colored ash containing ~45 - 55% silica that is generally rich in iron (Fe) and magnesium (Mg).
Using a model of the ash plume trajectory and ash deposition rates, along with measurements of iron dissolution, the authors calculated that up to 570,000 square kilometers (220,000 square miles) of North Atlantic waters could have been seeded with at least 0.2 nanomolar of iron. In controlled biological incubation experiments, the authors added volcanic ash collected under the plume to sea water, and find that iron leached from the ash could drive an increase in biological productivity and a draw-down of nutrient levels.
In the oceans south of Iceland there isn't usually enough iron for phytoplankton to bloom for more than a few weeks before it runs out. This latest study reveals that the volcanic ash column supplied enough iron that the phytoplankton were able to bloom for longer, and absorb more CO2 than they would typically have been able to.
'In normal years the iron levels are very low in the Iceland basin as the system runs out of this nutrient during the annual spring bloom. But in 2010 the iron supply was so high that demands were met. But then the phytoplankton stripped the nitrogen out of the surface waters so they became limited by that instead,' says Achterberg.
The research, published in Geophysical Research Letters, found even with the added iron from the volcano and the longer blooming period, the phytoplankton were only able to absorb about 15-20 per cent more CO2 than in other years before the nitrogen in the water ran out.
For further information see Article or Ash.
Eruption image via Wikipedia.