Virginia Institute of Marine Science researchers Dr. James Bauer and Sasha Tozzi are co-authors of an article in today's issue of the prestigious journal Science.
The article presents evidence that silica plays an unexpected role in the ocean's response to iron enrichment. Sprinkling iron onto the ocean surface has been touted as one way to help curb global warming—based on the idea that this iron "fetilizer" can boost the rate at which marine plants remove carbon dioxide from the atmosphere.
Bauer and Tozzi's work was part of SOFeX (the Southern Ocean Iron Experiment), one of the largest oceanographic experiments ever mounted. This two-year collaborative effort brought 3 ships, 45 tons of equipment and supplies, and 17 leading U.S. oceanographic institutions to the waters around Antarctica. VIMS researchers Drs. Walker Smith and Hugh Ducklow and their graduate students were also involved in the project.
SOFeX was designed to test the "iron hypothesis." This is the idea that a shortage of iron—a minor yet crucial ingredient for phytoplankton growth—may help to explain lower-than-expected productivity in certain ocean regions. These vast stretches of open ocean support few phytoplankton despite ample stores of nitrogen and phosphorous, two major nutrients that these tiny, floating plants require to grow.
Add iron to these regions, the thinking goes, and you can make the ocean bloom. Early experiments supported the idea-sprinkling just 1,000 pounds of iron across a patch of the equatorial Pacific produced a verdant bloom of phytoplankton.
The idea of iron fertilization quickly captured the attention of policymakers. That's because phytoplankton take up carbon dioxide from seawater during photosynthesis, allowing the ocean surface to absorb more of this gas from the atmosphere. Because carbon dioxide is the main greenhouse gas, any reduction in its atmospheric concentration would help to curb global warming.
But many oceanographers view large-scale iron fertilization projects with apprehension. One concern is how the Antarctic ecosystem might respond to iron enrichment and the ensuing increase in organic matter. The Southern Ocean around Antarctica is the most likely site for any future large-scale iron-fertilization projects (it's the world's largest iron-poor ocean region and outside busy shipping lanes), but it might respond to added iron very differently than the equatorial waters where early iron experiments were done.
The Science article focuses on a second unknown in the Antarctic iron equation-the role of silica. Earlier experiments suggest that single-celled plants called diatoms are likely to grow the fastest when iron is added to polar waters, even though they are relatively uncommon there. But diatoms build their skeletons out of silica—another substance that limits phytoplankton growth in some parts of the Southern Ocean.
To better understand silica's role in iron fertilization, the researchers created their first iron patch in a silica-poor area north of Antarctica, and the second in a silica-rich area nearer the continent.
Measurements by Tozzi and others showed that addition of iron to the silica-rich patch produced a bloom of diatoms that quickly began to remove carbon dioxide from the surrounding seawater. "There were a bunch of happy phytoplankton out there," says Tozzi. The silica-poor patch supported fewer diatoms, but other plankton took up some of the slack.
Bauer helped to document how much of the carbon from the two patches sank to the ocean depths when the plankton died or were eaten. Carbon in the deep sea contributes nothing to global warming and can remain there for hundreds or thousands of years. But if marine animals and bacteria transform plankton-derived carbon back into carbon dioxide gas before it sinks to the depths, the promise of a quick-fix to global warming may prove groundless.
The SOFeX team was somewhat surprised when they compared the amount of sinking carbon beneath the two patches. Because diatoms are relatively large and heavy, they had expected that more carbon would descend beneath the southern patch where abundant iron and silica supported a dense diatom bloom. Instead, they found that carbon "export" beneath the two patches was similar.
"Although the northern, silica-poor patch supported fewer diatoms, we also measured significant carbon export there," says Bauer.
"Together, these results show that iron truly is one of the key limiting trace nutrients in these waters," adds Bauer, "at least in the short term." The report cautions, however, that silica would likely become limiting in silica-poor waters if iron enrichment continued for a prolonged period.
The study was supported by grants from the U.S. National Science Foundation and Department of Energy.