Wind-blown iron deposits spurs plankton growth and regulates carbon-dioxide levels
FRISCO — Wind-transported dust has long been identified as a key ingredient in ocean nutrient cycles, and new research now shows that, during the last ice age, iron fertilization caused plankton to thrive in a region of the Southern Ocean.
The study by researchers with Princeton University and the Swiss Federal Institute of Technology in Zurich confirms a longstanding hypothesis that wind-borne dust drove plankton growth around Antarctica eventually leading to the removal of carbon dioxide from the atmosphere.
Plankton remove heat-trapping 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 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.
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 the 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.
“I was an undergraduate when Martin published his ‘ice age iron hypothesis,”said Daniel Sigman, Princeton’s Dusenbury Professor of Geological and Geophysical Sciences, and a co-leader of the study.
“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,” Sigman said.
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.
Although Martin had proposed that purposeful iron addition to the Southern Ocean could reduce the rise in atmospheric CO2, Sigman said that the amount of CO2 removed though iron fertilization is likely to be minor compared to the amount of CO2 that humans are now pushing into the atmosphere.
“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.”