How iron gets into the North Pacific

The wind-blown-dust theory of iron fertilization had no direct evidentiary support until Jim Bishop himself made the first observation of dust in action. In the spring of 2001, two robotic Carbon Explorer floats recorded the rapid growth of phytoplankton in the upper layers of the North Pacific Ocean after a passing storm had deposited iron-rich dust from the Gobi Desert. The Carbon Explorers had been designed by Bishop with colleagues at the Scripps Institution of Oceanography; their measurements, radioed back to him by satellite, marked the first time wind-blown terrestrial dust had been recorded fertilizing the growth of aquatic plant life.

"But the plankton blooms the two Carbon Explorers saw lasted only two weeks," Bishop says, "which raised questions about how important this transport mechanism really is."

From a site at 47 degrees north latitude and 160 degrees east longitude in the Western North Pacific (marked X), iron and manganese found at depths of 100-200 meters originated... Click here for more information.

Further doubts arose when, beginning in 2001, Lam analyzed samples that Bishop had collected five years earlier well out in the Eastern North Pacific. In February, 1996, a rare break in the winter weather had allowed Bishop to deploy a Multiple Unit Large Volume Filtration System (MULVFS), an array of collectors lowered over the side of the research vessel by cable. What MULVFS brought back were samples that contained iron plus evidence of a vigorous plankton bloom -- in the middle of the Eastern Subarctic Pacific, in the cold, dark days of midwinter.

There was no evidence dust storms could have carried terrestrial iron to the North Pacific that February, nor was the chemistry of the iron in the samples characteristic of iron from upwelling or past deposited dust. As the source of the iron, only the continental margins were logical.

Lam and Bishop and their colleagues published their studies in 2006, concluding that the iron had indeed come from the continental margins of the Aleutian Islands, 900 kilometers to the northwest of the site where the midwinter plankton bloom had been found. Iron particles and soluble iron had been carried there along a layer of denser water roughly 100 to 150 meters deep (the pycnocline), and the iron had been stirred up by storms that made it available to near-surface plankton in the dead of winter.