[1] Iron (Fe) is a critical nutrient for phytoplankton. In the open ocean, this demand coupled with scarce supply often makes Fe the limiting factor in phytoplankton growth. The largest source, by mass, of Fe to the open ocean is windblown soil dust, but this Fe is much less soluble than Fe from other aerosol sources. Therefore, to fully understand how Fe reaches this ecosystem, it is necessary to understand the range of sources of aerosol Fe. To do this, we collected sizeâ€segregated aerosol samples from Bermuda and analyzed them to determine their Fe isotope composition. From this analysis, we found clear evidence in the fine size fraction (< 2.5 µm) of an important nonâ€soilâ€dust Fe source. Our isotope analysis of multiple oil and coal fly ashes shows that those materials cannot explain our finding. We suggest biomass burning as the most likely source.
