Pacific Sediments Cast Doubt On Iron Fertilization Plans

Steve Hovan of the Indiana University of Pennsylvania and Allison Jacobel of Lamont-Doherty Earth Observatory cut a Pacific sediment core into sections for analysis. Pratigya Polissar/Lamont-Doherty Earth Observatory

A study of sediments from the depth of the Pacific Ocean has bad news for those who think we can solve our climate problems by fertilizing the largest ocean. Deposits laid during the last ice age indicate that the Pacific did not play the role in cooling the planet that has been suggested, raising doubts about its capacity to do something similar in future.

During recent ice ages, atmospheric carbon dioxide levels fell, amplifying the cooling effects of orbital changes. This has led to hopes that if we can replicate the processes that drew CO2 out of the atmosphere, we might be able to diminish the effects of global warming. First, however, we need to know where the carbon dioxide went.

Kassandra Costa, a doctoral student at Columbia University, set out to test the theory that much of this carbon ended up on the floor of the Pacific Ocean. According to proponents of this idea, plants declined in the cold Ice Age climate, allowing more dust to be blown from the continents to the oceans. Iron in the dust fertilized the growth of photosynthetic algae, many of which sank to the sea floor, taking billions of tonnes of carbon with them.

However, when Costa studied 17,000 to 26,000-year-old sediments cores from the central Pacific Ocean seabed, collected at depths of around 3,000 meters (10,000 feet), she found the theory is only half right. The era was indeed dusty, two to three times as much so as the last 10,000 years. However, there was no sign of increased marine plant growth as a result.

A researcher checks the mud from a sample collected from the bottom of the Pacific Ocean. Pratigya Polissar/Lamont-Doherty Earth Observatory

Costa has reported her findings in Nature. She concluded the problem is that iron is not the only nutrient the marine algae need to flourish. "There's only a limited amount of total nutrients in the oceans. So if there's greater use in one area, it seems you'd have lesser concentrations in other areas," Costa said in a statement

The central Pacific is supplied by waters from the Southern Ocean with a steady dose of nitrates and phosphates. At the same time as dust was settling on the Pacific, it was also landing in the Southern Ocean at rates 20-30 times the rate. Costa told IFLScience that during the last ice age, Patagonia was the major source of Southern Ocean dust. Patagonian dust was probably more biologically available than that coming off arid lands to the Pacific. Costa reasoned that on receiving the extra iron Southern Ocean algae bloomed, leaving insufficient amounts of nutrients for the Pacific.

"This shows how different parts of the system are connected," said coauthor Professor Jerry McManus. "If you push hard in one place, the system pushes back somewhere else."

Fertilizing the Southern Ocean with iron may be viable, and possibly already happening in nature, Costa and McManus argue, although concerns remain about the effects on local ecosystems. However, Pacific fertilization would require additional phosphates and nitrates in quantities orders of magnitude greater than the iron that advocates have hoped would be sufficient.

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