emission and transport of dust and other important aerosols to the Southern Ocean on Dec. 30, 2006
March 22, 2014

Iron Fertilization Helped Plankton Thrive During The Last Ice Age

April Flowers for redOrbit.com - Your Universe Online

In a region of the Southern Ocean, iron fertilization caused plankton to thrive during the last ice age, according to a new study from Princeton University and the Swiss Federal Institute of Technology in Zurich.

The findings, published in Science, confirm a longstanding theory that wind-borne dust carried iron to this region of the Antarctic. This iron dust drove plankton growth and eventually led to the removal of carbon dioxide from the atmosphere.

As plankton grows, it removes carbon dioxide (CO2) from the atmosphere. As their remains sink to the bottom, the CO2 is transferred to the deep ocean. Scientists have previously suggested iron fertilization as a possible cause of the lower CO2 levels that occur during ice ages. These decreases are, in turn, believed to have amplified the ice ages by making them much colder. Some researchers think that there would have been no ice ages at all without the CO2 depletion.

Other research has also suggested that iron fertilization could be one way to draw down the rising levels of CO2 associated with the burning of fossil fuels. Having a better understanding of the driving factors of ocean carbon storage could help scientists to make better predictions about how the rise in manmade carbon dioxide will affect climate in the coming years.

The late John Martin, an oceanographer at Moss Landing Marine Laboratories, first proposed the theory of the role of iron in carbon dioxide storage in 1990. Martin made the groundbreaking discovery that iron limits plankton growth in large regions of the modern ocean.

Martin based his hypothesis that the increased dust supply to the Southern Ocean allowed plankton to grow more rapidly on evidence that there was more dust in the atmosphere during the ice ages. The rapid growth of plankton would send more of their biomass 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.

Daniel Sigman, Princeton's Dusenbury Professor of Geological and Geophysical Sciences, says that the current study's results confirm Martin's theory. "I was an undergraduate when Martin published his 'ice age iron hypothesis,'" he told Princeton's Catherine Zandonella. "I remember being captivated by it, as was everyone else at the time. But I also remember thinking that Martin would have to be the luckiest person in the world to pose such a simple, beautiful explanation for the ice age CO2 paradox and then turn out to be right about it."

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 — was established by prior efforts to test Martin's theory. These studies, however, did not make it 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. The uncertainty over productivity was made more acute by the finding that ice age productivity was lower in the Antarctic Ocean, which lies south of the Subantarctic region.

Sigman's research team collaborated with Gerald Haug and Tim Eglinton at ETH Zurich to settle the matter using a new method developed at Princeton. Fossils found in deep sea sediment — deposited during the last ice age in the Subantarctic region — were analyzed with the goal of reconstructing past changes in the nitrogen concentration of surface waters. The results were combined with side-by-side measurements of dust-borne iron and productivity. The nitrogen should have been completely consumed by the plankton, leading to lower residual nitrogen concentrations in the surface waters, if the dust-borne iron fertilization theory was correct. On the other hand, if the productivity increases were the result of a northward shift in ocean conditions, then the concentration of nitrogen would have risen.

The ratio of nitrogen isotopes, which have the same number of protons but differing numbers of neutrons, preserved within the carbonate shells of a group of marine microfossils called foraminifera were measured. The analysis revealed 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, demonstrate that iron fertilization of plankton in the Southern Ocean can account for roughly half the CO2 decline during peak ice ages.

Martin's theory proposed that the purposeful addition of iron to the Southern Ocean could reduce the rise in atmospheric CO2. However, the amount of human generated CO2 that is being pushed into the atmosphere would render the CO2 removed by iron fertilization to a relatively minor amount, according to Sigman.

"The dramatic fertilization that we observed during ice ages should have caused a decline in atmospheric CO2 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."

Edward Brook, a paleoclimatologist at Oregon State University who was not involved in the research, said, "This group has been doing a lot of important work in this area for quite a while and this an important advance. It will be interesting to see if the patterns they see in this one spot are consistent with variations in other places relevant to global changes in carbon dioxide."