This is why you want to be careful with the geoengineering proposals. Some links after the jump.
From Hakai Magazine, January 21:
A new study uncovers the origins of iron-rich dust that contributed to the last ice age’s coldest temperatures.
Eleven years ago, in the Southern Ocean off the coast of Antarctica, Katharina Pahnke stood on the deck of a large research vessel. Rocking back and forth on the ocean waves, Pahnke watched as a piston corer, a long, heavy tube, was lowered off the side of the ship and plunged through the frigid water to pierce the soft seabed below. Six hours later, the tube—and the long cylinder of mud contained within—was hauled back up to the surface.
On this 2010 research expedition, Pahnke and others scoured the Southern Ocean between Antarctica, New Zealand, and Chile—collecting sediment cores over an area roughly the size of Russia. These tubes of mud have since been used for a wide range of scientific studies. For Pahnke, a geochemist who studies the ocean’s history, the dust trapped within these sediment cores would eventually allow her to answer questions about the coldest period of the last ice age, which happened around 20,000 years ago.
In 1990, 20 years before Pahnke trekked out to the Southern Ocean, oceanographer John Martin proposed that iron-rich dust greatly contributed to the last ice age. Known as the iron hypothesis, Martin argued that flurries of this dust traveled on the wind from cold, barren landscapes to the Southern Ocean. The dust fertilized the marine ecosystem, triggering massive blooms of phytoplankton. These photosynthesizing marine algae sucked carbon dioxide out of the air to produce sugar and oxygen. Once the phytoplankton died, they fell to the bottom of the ocean, taking the sequestered carbon with them, which dramatically cooled the planet.
Today, it is widely accepted that there was an increase in iron fertilization and dust in the Southern Ocean during the last ice age. Yet where this iron-rich dust came from has remained unknown—until now. Using the same sediment cores collected years before, Pahnke and other scientists from the University of Oldenburg in Germany discovered that the dust captured in the tubes traveled up to 20,000 kilometers east from northwestern Argentina—all the way around the world—to where it was deposited in the Southern Ocean....
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One of the reasons we ran Plankton Week last October—no, not as counterprogramming to Shark Week—was to refresh memories of one of the topics of conversation at all the better salons and soirées circa 2007.
Plankton Week: “Give me a half tanker of iron, and I will give you an ice age.”
The headline quote is from oceanographer John Martin during a 1988 lecture at Woods Hole Oceanographic Institution. Here's NASA's Earth Observatory archive page on the statement.
It is a bit of an exaggeration, you may need ten of those Valemax bulk carriers, currently the second largest ships in the world at 400,000 dwt (Euronav's two TI oil tankers at 441,000 dwt are bigger), to make an environmental change but what a change it would be. The orders of magnitude of carbon the iron-fed plankton would sequester are almost mind-boggling:
...Martin gathered the results of the incubation experiments and laid out the evidence in support of the Iron Hypothesis together with some back‐of‐the‐envelope calculations and presented his findings at a Journal Club lecture at Woods Hole Oceanographic Institution in July of 1988. He estimated that using a conservative Fe : C ratio that 300,000 tons of iron in the Southern Ocean induce the growth of phytoplankton that could draw down an estimated two billion tons of carbon dioxide. Then, putting on his best Dr. Strangelove accent, he suggested that “with half a ship load of iron….I could give you an ice age.” The symposium broke up with laughter and everyone retired to the lawn outside the Redfield Building for beers (from Chisholm and Morel, Editors, preface to: What controls phytoplankton production in nutrient‐rich areas of the open sea? Limnology and Oceanography, 36, 8 December 1991).
As repeated in "John Holland Martin: From Picograms to Petagrams and Copepods to Climate"
—Bulletin of Limnology and Oceanography, Wiley. 25 March 2016
....Coming up tomorrow, the Pope, and a Vancouver stock promoter.
Our series thus far:
October 27
Plankton Week: "Metal deposits from Chinese coal plants end up in the Pacific Ocean, research shows"
October 26
"Plankton Bloom Heralded Earth’s Greatest Extinction"