Wednesday, May 06, 2009

Plankton Blooms Don't Sequester Atmospheric Carbon

Oceanographers Jim Bishop and Todd Wood of the U.S. Department of Energy's Lawrence Berkeley National Laboratory have measured the fate of carbon particles originating in plankton blooms in the Southern Ocean, using data that deep-diving Carbon Explorer floats collected around the clock for well over a year. Their study reveals that most of the carbon from lush plankton blooms never reaches the deep ocean.

The surprising discovery deals a blow to the simplest version of the Iron Hypothesis, whose adherents believe global warming can be slowed or even reversed by fertilizing plankton with iron in regions that are iron-poor but rich in other nutrients like nitrogen, silicon, and phosphorus. The Southern Ocean is one of the most important such regions.

"Just adding iron to the ocean hasn't been demonstrated as a good plan for storing atmospheric carbon," says Bishop, a member of Berkeley Lab's Earth Sciences Division and a professor of Earth and planetary sciences at the University of California at Berkeley. "What counts is the carbon that reaches the deep sea, and a lot of the carbon tied up in plankton blooms appears not to sink very fast or very far."

The reasons, while complex, are most likely due to the seasonal feeding behavior of planktonic animal life, and specifically to the effects of the dark Antarctic winter on plant and animal growth and the mixing of surface and deep waters by winter storms. Phytoplankton blooms in the spring may indicate that much of the zooplankton (animal) population essential for carbon sedimentation has starved during the winter.

The Carbon Explorers involved in the study were launched in January, 2002, as part of the Southern Ocean Iron Experiment (SOFeX), a collaboration led by scientists from Moss Landing Marine Laboratory and the Monterey Bay Aquarium Research Institute. SOFeX was meant to test the Iron Hypothesis in waters between New Zealand and Antarctica during the Antarctic summer. The Berkeley Lab Carbon Explorers were originally intended to monitor the iron-fertilization experiment for 60 days, but they continued to report by satellite throughout the Antarctic fall and winter and on into the following year.

"We would never have made these surprising observations if the autonomous Carbon Explorer floats hadn't been recording data 24 hours a day, seven days a week, at depths down to 800 meters or more, for over a year after the experiment's original iron signature had disappeared," Bishop says.

He explains that "assumptions about the biological pump – the way ocean life circulates carbon – are mostly based on averaging measurements that have been made from ships, at intervals widely separated in time. Cost, not to mention the environment, would have made continuous ship-based observations impossible in this case. Luckily one Carbon Explorer float costs only about as much as a single day of ship time."

Bishop and Wood report their results in a forthcoming issue of the journal Global Biogeochemical Cycles. Preprints are now available to subscribers at

Woooo! Go Colabbies! That said, if this bares out, this means any plan on using iron seeding is dead in the water.

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