The findings of an international team of researchers, reported in Nature this week, support the hypotheses that adding iron to some high-nutrient regions of the ocean can stimulate the growth of phytoplankton (microscopic algae), which may then take up additional carbon from the atmosphere and transport it to the deep ocean.
In much of the global ocean iron is an important factor in regulating phytoplankton growth, which is measured by chlorophyll levels. Until now, however, clearly demonstrating and quantifying the increase in deep ocean carbon storage in response to overcoming iron limitation has proved difficult.
Raymond Pollard of the National Oceanography Centre Southampton, UK, and colleagues present data from the Southern Ocean that show that carbon export fluxes to the deep from waters naturally fertilized with iron were two to three times greater than those fluxes from an adjacent high-nutrient low-chlorophyll area not fertilized by iron. The efficiency of carbon export was somewhat greater than that reported in experiments in which iron was added artificially to promote a phytoplankton bloom — a possible consequence of large losses of the artificially added iron. However, this efficiency was smaller compared to that reported from another naturally induced bloom. The authors also point out that the estimate of carbon sequestration for a given iron supply is some 15–50 times less than some geo-engineering estimates.
Pollard and colleagues note that their findings have “significant implications for proposals to mitigate the effects of climate change through purposeful addition of iron to the ocean.”
- Southern Ocean deep-water carbon export enhanced by natural iron fertilization (Letter p577, doi: 10.1038/nature07716)
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