The open Southern Ocean is 8 million square miles of nutrient. rich water, fed by the most Vigorous upwelling currents in the world, with temperatures no colder and sunlight no fainter than near the coast, but for lack of iron the phytoplankton do not bloom. Yet as Martin soon found out, that was not always true in the past. Not long before Martin had gotten interested in the subject, French and Russian researchers had extracted a two-kilometre-deep core from the Antarctic ice a frozen record of Earth’s atmosphere through the last ice age and beyond. That core showed, as had previous ice cores, that the carbon dioxide level in the atmosphere had been much lower at the peak of the last glaciation, 18,000 years ago: only 200 parts per million, compared with 280 parts per million in the preindustrial nineteenth century and 360 parts per million today. At the same time, though, the amount of dust falling onto Antarctica was between 10 and 20 times greater. The world was drier during the ice age, with vast tropical deserts and stronger winds that carried dust off the deserts and out over the sea. Along the whole length of the ice core, Martin noticed, atmospheric carbon dioxide and atmospheric dust fluctuated in counterpoint; one went up when the other went down.
For Martin the explanation was simple: whenever a lot of ironrich dust had fallen out of the atmosphere, the Southern Ocean phytoplankton had been cured of their anaemia, and the massive blooms that ensued had drawn carbon dioxide out of the atmosphere. That is what made the world colder during the ice age: the phytoplankton did it. By blooming to their full potential and converting all the available nitrate into organic matter, they drew two billion tons of carbon dioxide out of the surface water, which drew it out of the atmosphere.
Mapping the Deep - Robert Kunzig