This satellite image shows a large phytoplankton bloom, about 150 miles in diameter, that occurred in the Northwest Pacific Ocean off the coast of Hokkaido, Japan in May 2009. Researchers have assumed that such blooms were driven by iron in the post-Ice Age ocean, but a new study by WHOI scientists suggests that a “perfect storm” of light and nutrients spurred an explosion of phytoplankton and other tiny creatures in the North Pacific 14,000 years ago. (Credit: Photo by Norman Kuring, MODIS Ocean Color Team/NASA) Enlarge

July 14, 2013 — At the end of the last Ice Age, as the world began to warm, a swath of the North Pacific Ocean came to life. During a brief pulse of biological productivity 14,000 years ago, this stretch of the sea teemed with phytoplankton, amoeba-like foraminifera and other tiny creatures, who thrived in large numbers until the productivity ended — as mysteriously as it began — just a few hundred years later.

Researchers have hypothesized that iron sparked this surge of ocean life, but a new study led by Woods Hole Oceanographic Institution (WHOI) scientists and colleagues at the University of Bristol (UK), the University of Bergen (Norway), Williams College and the Lamont Doherty Earth Observatory of Columbia University suggests iron may not have played an important role after all, at least in some settings. The study, published in the journal Nature Geoscience, determines that a different mechanism — a transient "perfect storm" of nutrients and light — spurred life in the post-Ice Age Pacific. Its findings resolve conflicting ideas about the relationship between iron and biological productivity during this time period in the North Pacific — with potential implications for geo-engineering efforts to curb climate change by seeding the ocean with iron.

"A lot of people have put a lot of faith into iron — and, in fact, as a modern ocean chemist, I've built my career on the importance of iron — but it may not always have been as important as we think," says WHOI Associate Scientist Phoebe Lam, a co-author of the study.

Because iron is known to cause blooms of biological activity in today's North Pacific Ocean, researchers have assumed it played a key role in the past as well. They have hypothesized that as Ice Age glaciers began to melt and sea levels rose, they submerged the surrounding continental shelf, washing iron into the rising sea and setting off a burst of life.

Past studies using sediment cores — long cylinders drilled into the ocean floor that offer scientists a look back through time at what has accumulated there — have repeatedly found evidence of this burst, in the form of a layer of increased opal and calcium carbonate, the materials that made up phytoplankton and foraminifera shells. But no one had searched the fossil record specifically for signs that iron from the continental shelf played a part in the bloom.

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