The deep, clear open ocean appears empty at first glance. These blue waters are so bereft of nutrients, or oligotrophic, that oceanographers refer to them as “deserts.” With none of the hustle and bustle that characterizes coastal kelp forests or coral reefs, it is easy to fall prey to the illusion that these seas are practically lifeless. Peer at them through a microscope, however, and a different picture emerges.
Microorganisms dominate the world’s oligotrophic ocean gyres, those vast, slowly swirling bodies of water between continents set in motion by the Coriolis effect. Phytoplankton in particular are ubiquitous here.
Phytoplankton, carbon, and climate change
These single-celled ancestors of plants grow in the upper, sunlit surface layer of the ocean, taking in carbon dioxide for photosynthesis. When these cells die, they eventually sink to the deep ocean, taking that sequestered carbon with them.
This is good news for humans concerned about climate change, since carbon that sinks to the deep won't resurface and re-enter the atmosphere for hundreds of years. However, phytoplankton are significantly less abundant in ocean gyres than in productive, nutrient-rich zones like coasts and river deltas; their growth is restricted by the low supply of nutrients.
Giving phytoplankton an iron supplement
The oceanographer John Martin posited in 1990 that iron was the chief factor limiting phytoplankton growth in unique parts of these marine deserts called high-nutrient low-chlorophyll zones, where most nutrients are plentiful but chlorophyll-containing phytoplankton remain scarce.
The idea was initially an academic hypothesis, one that could help other ocean scientists understand what drives phytoplankton blooms today and in the ancient seas. Research cruises began to test Martin's hypothesis that ocean iron fertilization (OIF), or adding dissolvable iron to low-iron areas of the open ocean, could stimulate such algal blooms.
The relative success of these small-scale experiments soon caught the attention of climate change activists and policy makers. After all, if a little iron could stimulate a large bloom--some of the induced blooms had even been visible from space--couldn't a lot of iron spark a gigantic bloom that would suck up significant quantities of carbon dioxide?
Even Martin himself recognized the potential climate change relevance of his idea. He once joked, "Give me half a tanker of iron and I'll give you the next ice age."
Issues with ocean iron fertilization
Martin's joking comment touched on a real concern of oceanographers and climate scientists: we do not yet know the full scope of ecological consequences that could result from artificially fertilized blooms. Our understanding of ocean food webs, community structure, and marine carbon sequestration remains incomplete, and fertilizing the ocean with iron could alter or disrupt these dynamics in unpredictable ways.
The amount of carbon, if any, that sinks to the deep sea following an OIF-induced bloom varies widely depending on poorly understood factors, including the particular nutrient dynamics and biogeochemical and physical properties of the location being fertilized. And any carbon that does sink out of the surface waters will eventually resurface, though not for several generations.
Another worry is that OIF might affect ocean biogeochemistry, particularly in that these blooms could release greenhouse gases like methane and nitrous oxide. Clearly, such emissions have the potential to offset or even nullify the intended sequestration of carbon. Few experimental iron additions have registered production of these gases, but it is possible that they are emitted outside of the study area or after the ship has returned to port.
Perhaps the most pressing worry, though, is that the amount of carbon sequestered by OIF would be but a drop in the bucket compared to the amount currently being produced each year. Furthermore, the net benefits of iron fertilization attempts must exceed the carbon costs of mining and refining the iron, fueling the ship, and the diverse costs of running the experiment or operation.
Given these concerns, it is dubious whether OIF could be an effective carbon mitigation strategy. This method would be impractical and potentially environmentally irresponsible on a commercial level. Rather than implementing carbon sequestration plans that would likely have unintended, unwanted consequences, we would do better to actively focus on reducing our greenhouse gas emissions.
Sources
- Boyd, Philip, et al. "Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and Future Directions." Science 315 (2007): 612-617.
- Buesseler, Ken, et al. "Ocean Iron Fertilization—Moving Forward in a Sea of Uncertainty." Science 319 (2008): 162.
- Kintisch, Eli. "Should Oceanographers Pump Iron?" Science 318 (2007): 1368-1370.
- Trugillo, Al. The Iron Hypothesis. Palomar College, Feb. 2011. Web. 12 April 2011.
Amy Hansen - Amy Hansen is a biologist-turned-writer currently living in the San Francisco Bay Area. Though science writing is her true passion, she'll ...
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