Dust in the wind drove iron fertilization during ice age

Plankton remove the greenhouse gas carbon dioxide (CO2) 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 CO2 levels that occur during ice ages. These decreases in atmospheric CO2 are believed to have "amplified" the ice ages, making them much colder, with some scientists believing that there would have been no ice ages at all without the CO2 depletion.

Iron fertilization has also been suggested as one way to draw down the rising levels of CO2 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 in California 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 plankton to be fertilized by iron without 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 that lies in the path of the winds that blow off South America, South Africa and Australia. However, it was not clear whether the 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, Garcia et al. analyzed fossils found in deep sea sediment —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. 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 researchers measured the ratio of nitrogen isotopes, which have the same number of protons but differing numbers of neutrons, that were preserved within the carbonate shells of a group of marine microfossils called foraminifera. The investigators found that nitrogen concentrations indeed declined during the cold periods when iron deposition and productivity rose, in a manner consistent with the dust-borne iron fertilization theory. Ocean models as well as the strong correlation of the sediment core changes with the known changes in atmospheric CO2 suggest that this iron fertilization of Southern Ocean plankton can explain roughly half of the CO2 decline during peak ice ages.

The paper, "Iron Fertilization of the Subantarctic Ocean During the Last Ice Age," was published in the March 21, 2014, issue of the journal Science.