Iron fertilization cools ice age climate

Plankton remove the greenhouse gas carbon dioxide from the atmosphere during growth and transfer it to the deep ocean when their remains sink to the bottom. Iron fertilization has previously been suggested as a possible cause of the lower carbon dioxide levels that occur during ice ages. Iron fertilization has also been suggested as one way to draw down the rising levels of carbon dioxide associated with the burning of fossil fuels. Improved understanding of the drivers of ocean carbon storage could lead to better predictions of how the rise in manmade carbon dioxide will affect climate in the coming years.

The role of iron in storing carbon dioxide during ice ages was first proposed in 1990 by the late John Martin, an oceanographer at Moss Landing Marine Laboratories who made the landmark discovery that iron limits plankton growth in large regions of the modern ocean. Based on evidence that there was more dust in the atmosphere during the ice ages, Martin hypothesized that this increased dust supply to the Southern Ocean allowed plankton to grow more rapidly, sending more of their biomass into the deep ocean and removing CO2 from the atmosphere. Martin focused on the Southern Ocean because its surface waters contain the nutrients nitrogen and phosphorus in abundance, allowing iron to fertilize plankton without them running low on these necessary nutrients.

Previous efforts to test Martin¡¯s hypothesis established a strong correlation of cold climate, high dust and productivity in the Subantarctic region, a band of ocean encircling the globe between roughly 40 and 50 degrees south latitude. However, it was not clear whether the elevated ice age productivity was due to iron fertilization or the northward shift of a zone of naturally occurring productivity that today lies to the south of the Subantarctic. This uncertainty was made more acute by the finding that ice age productivity was lower in the Antarctic Ocean, which lies south of the Subantarctic region.

To settle the matter, research groups of ETH Zurich and Princeton University teamed up to use a new method developed at Princeton to analyze fossils in a deep sea sediment core that captured sediments deposited during the last ice age in the Subantarctic region, with the goal of reconstructing past changes in the nitrogen concentration of surface waters and combining the results with side-by-side measurements of dust-borne iron and productivity performed at ETH Zurich. If the dust-borne iron fertilization hypothesis was correct, then nitrogen would have been more completely consumed by the plankton, leading to lower residual nitrogen concentrations in the surface waters. In contrast, if the productivity increases were in response to a northward shift in ocean conditions, then nitrogen concentrations would have risen. The investigators found that nitrogen concentrations indeed declined during the cold periods when iron deposition and productivity rose, in a manner uniquely consistent with the dust-borne iron fertilization theory.

¡°Our study confirms Martin's hypothesis, and shows that the associated increase in the efficiency of carbon sequestration into the deep ocean can explain a significant part of the atmospheric CO2 decrease observed during ice ages¡±. said Alfredo Mart¨ªnez-Garc¨ªa, research associate working in the group of ETH Professor Gerald Haug, and first author of the study.

Although Martin had proposed that purposeful iron addition to the Southern Ocean could reduce the rise in atmospheric carbon dioxide, Daniel Sigman, Princeton's Dusenbury Professor of Geological and Geophysical Sciences and a co-leader of the study noted that the amount of carbon dioxide removed though iron fertilization is likely to be minor compared to the amount of carbon dioxide that humans are now pushing into the atmosphere. "The dramatic fertilization that we observed during ice ages should have caused a decline in atmospheric carbon dioxide over hundreds of years, which was important for climate changes over ice age cycles," Sigman said. "But for humans to duplicate it today would require unprecedented engineering of the global environment, and it would still only compensate for less than 20 years of fossil fuel burning.