Carbon Storage Stymied in Twilight Zone

Every year, microscopic phytoplankton turn about 50 billion metric tons of carbon into plant life. Much of that carbon comes straight out of the atmosphere. On the surface of things, that sounds pretty good – but a paper in today’s Science reports that below the surface it’s rather more complicated.

The study – called VERTIGO, in one of oceanography’s acronymic triumphs – involved 17 authors and more than 40 scientists from seven countries. They sailed the seas off Hawai’i and Japan, chucking recently invented, free-diving samplers overboard to follow what happens to all that carbon after it becomes phytoplankton. The short answer is, it gets recycled. And while recycling is a good thing to do with bottles and cans, doing it with carbon is counterproductive.

When phytoplankton decomposes near the ocean surface – between 100 and 1,000 meters depth, in a literally gray area called the twilight zone – it results in no net carbon storage. It’s the same reason that burning biodiesel creates no net emissions (the french-fry-scented carbon coming out your tailpipe is just going back where it had been during the last growing season).

Before VERTIGO, hopes had been high that most of those gigatons of phytoplankton sank to the bottom of the ocean, far from the atmosphere, where they could start their million-year conversion to more oil. Evidence from the project suggests that 50 to 80 percent of the carbon never sinks past 500 meters. The amount varied between the tropical and temperate sampling sites. Extrapolate those two estimates across the globe, and that’s a difference of 3 billion tons of carbon reaching the deep ocean. For perspective, that uncertainty is equal to half the world’s current fossil-fuel emissions.

How does the plankton decompose? That’s ecology at work for you. Even though diminishing light shuts out plant life below about 100 meters depth, zillions of intrepid zooplankton squirt around in the twilight zone scavenging falling detritus. The recycling happens over and over as well-fed zooplankton excrete marine snow (one of the most delicate euphemisms ever invented), to be scavenged by deeper, even more intrepid creatures. The result is that surprisingly little carbon makes it from surface waters into the depths.

If you’ve ever heard of a global-warming solution involving fertilizing the ocean with iron, this is what people have been talking about. Dump iron in the surface waters and phytoplankton multiply like crazy, pulling extra carbon dioxide out of the air, dying, and sinking. Oceanographers were once hopeful about this, but actual experiments – involving 100 square kilometers near the Galapagos and in the Southern Ocean – made them suspect that very little carbon made it down to the depths. The VERTIGO results indicate their skepticism was warranted, but might also suggest that some parts of the ocean are better places to try than others.

Image (detail): Woods Hole Oceanographic Institution. Cheers to Woods Hole wunderkinder Carl Lamborg and Phoebe Lam.