Le Moigne, F. A. C., S. A. Henson, E. Cavan, C. Georges, K. Pabortsava, E. P. Achterberg, E. Ceballos-Romero, M. Zubkov, and R. J. Sanders (2016), What causes the inverse relationship between primary production and export efficiency in the Southern Ocean?, Geophys. Res. Lett., 43, doi:10.1002/2016GL068480.
“A half tanker of iron”
Tiny algae are a global powerhouse for carbon sequestration. They spend their lives converting CO2 into organic matter through photosynthesis, and when the die they sink and take the carbon with them to the ocean floor, where it can stay for hundreds of years. This process is called the biological pump, and without it constantly shunting CO2 from the atmosphere into the ocean, the planet would be warming even faster than it is today.
If the biological pump could be manipulated to take up more carbon it could be a powerful tool for fighting climate change. For example, many parts of the Southern Ocean have all the ingredients for primary production—nutrients, sunlight, CO2—but algae grow slowly due to a lack of iron. If you add iron, the algae population explodes (Figure 1), which, other things equal should sequester more carbon in the ocean. Oceanographers looking for creative solutions to climate change sensed an opportunity: add iron to these regions of the ocean, watch algae sequester carbon, and stop global warming in its tracks! The exuberance over the potential of this geoengineering technique was best expressed by John Martin in the late 1980s who famously quipped: “Give me a half tanker of iron and I’ll give you an ice age”.
Experiments over the next decade set out to test the great iron hypothesis, but something went wrong: when patches of the ocean were fertilized with iron, the algae bloomed as expected, but they didn’t sink. Instead of increasing carbon sequestration as expected, carbon was returned to the atmosphere just as quickly as it was taken up by the algae. The geoengineers were foiled as the ecosystem changed beneath them!
What went wrong
However, what exactly went wrong is still mostly unknown. A new study by Le Moigne et al. attempts to determine how the ecosystem controls carbon export in the Southern Ocean. To do this, they used a natural iron fertilization experiment: they compared the ecosystem in the waters surrounding the South Georgia Islands off of South America, where algae growth is high due to iron runoff from the islands, with water farther offshore with lower iron and lower growth rates (Figure 2). They observed that the higher the rate of algae growth, the smaller the fraction of produced carbon that sinks and is stored in the deep ocean (Figure 3). This is why increasing algae growth with iron fertilization doesn’t help store more carbon!
One reason that carbon sinking doesn’t keep up with algae growth is that zooplankton, tiny animals that feed on algae, might not keep up with the algae bloom after fertilization. Plankton poop is a very efficient vehicle for carbon export—it effectively packages many slow-sinking algae cells into a larger pellet that sinks like a rock. Fewer zooplankton were found in the highly productive waters surrounding the South Georgia Islands, so more of the organic carbon produced by algae could be decomposed and returned to the atmosphere as CO2 before it has a chance to sink.
In addition, the response of the microbial community to the higher algae growth pushed back against further carbon storage. Productive waters were found to have a larger and more active community of bacteria, breaking down the organic matter back to CO2 and preventing its storage in the deep ocean.
Geoengineering using iron fertilization was an exciting idea for avoiding the worst effects of global warming, but it’s becoming clear that it’s no magic bullet. As this study shows, increasing algae growth is not the same as increasing long-term carbon storage. In addition, even if the organic matter is sinking and storing carbon, the added iron is sinking with it. The algae bloom ends soon after we stop adding iron, so to have a significant impact on climate, tons of iron would have to be continuously added over the course of many years. John Martin’s half tanker wouldn’t cut it! At the end of the day, reducing fossil fuel emissions is still the only 100% safe and effective way to fight climate change.
I recently completed a PhD in Marine Science at the University of South Carolina and am now a postdoc at Memorial University of Newfoundland. I research the effects of climate change on soil organic matter in boreal forests and peatlands. I spend my free time picking berries and exploring “The Rock” (Newfoundland).