A research team led by scientists at Columbia's Lamont-Doherty Earth Observatory has documented for the first time that an infusion of wind-blown iron particles in Antarctic waters triggered a population explosion of marine plants that may have significantly reduced the amount of heat-trapping carbon dioxide in earth's atmosphere during the ice age.
The research, reported in the Dec. 14 issue of Nature, confirms the principles underlying the "iron hypothesis," first postulated by the late John Martin of the Moss Landing Marine Laboratories in the late 1980s.
Building on earlier theories, Martin suggested that a dustier ice-age world naturally fertilized Antarctic Ocean regions that are starved for iron today. He became controversial for speculating that fertilizing the Southern Ocean surrounding Antarctica with iron could stimulate a "biological pump" of microscopic marine plants, which would draw large amounts of carbon dioxide from the atmosphere into the deep ocean, counteracting the buildup of a greenhouse gas that threatens to warm the planet.
But until now, no evidence existed to confirm that Antarctic blooms ever occurred--even though Antarctic ice cores showed that earth's atmosphere had high levels of iron-rich particles and low levels of carbon dioxide during the last ice age between about 75,000 and 12,000 years ago.
Now a seven-member team of scientists studying the Atlantic sector of the Southern Ocean reported in Nature that large quantities of organic matter fell from the ocean surface to the seafloor during the ice age. The scientists used a new method to track blooms of marine plants by measuring naturally occurring radioactive isotopes, whose levels rise in the sediments when more organic matter fell to the ocean bottom.
During the ice age, the presence of large ice sheets lowered global sea levels and vast tracts of now-submerged land at the southern tip of South America became exposed. Strong, prevailing winds blew particles of soil from this land to the Southern Ocean. The particles were rich in iron that plants need for many biological reactions.
The research was conducted by Niraj Kumar, Robert Anderson, Richard Mortlock and Philip Froelich, at Lamont-Doherty, Columbia's earth science research institute in Palisades, N.Y., and by Peter Kubik, Beate Dittrich-Hannen and Martin Suter, at the Institute for Particle Physics of ETH in Zurich, Switzerland.
Scientists have noted blooms in modern times in small areas near the edges of oceans where winds, rivers or currents provide a steady supply of iron. Last year, an experiment to enrich a 20-square-miles patch of the Pacific with iron also demonstrated that iron fertilization could stimulate the productivity of oceanic plants.
The new research by the Lamont-Doherty/ETH team documents for the first time that the iron fertilization scenario did occur on a large scale during the ice age in the Southern Ocean. Writing in Nature, the team said: "Our results demonstrate that the biogeochemical principles embodied within Martin's 'iron hypothesis' find validation in nature." When the Southern Ocean received a sustained supply of wind-blown iron, they said, "the result was an explosion of biological productivity" that eventually sent large amounts of organic matter, including carbon, down to the depths.
However, the team also found that the ice-age blooms did not spread throughout the Southern Ocean, as Martin had originally theorized, and therefore less carbon dioxide was taken out of the atmosphere than he had postulated.
The scientists estimated that the Antarctic blooms accounted for no more than one-fifth to one-third of the 30 percent reduction in atmospheric carbon dioxide during the ice age.
Previous research failed to recognize the enhanced fertility of the ice-age Southern Ocean. Earlier studies evaluated past changes in the burial in seafloor sediments of opal, a form of silica used by plants to build their shells, which sank to the ocean bottom. They assumed that more marine plants blooming near the sunlit ocean surface would produce more shell material in the sediments below.
But such records are hard to interpret because nearly all of the shell materials are "remineralized," or re-dissolved in seawater, before they are ever buried.
The new results from the Columbia/ETH team show explicitly that the burial rate of this shell material in Southern Ocean sediments failed to record reliably past changes in the ocean's biological productivity.
In the new research, the scientists used a newly developed approach to measure past changes in the rain of organic material from ocean's surface to the seafloor.
They analyzed six Southern Ocean sediment cores from Lamont-Doherty's world-renowned repository of ocean sediment cores, the world's largest.
The research was supported by the U.S. National Science Foundation.
Columbia University Record -- February 9, 1996 -- Vol. 21, No. 16