At the end of the last ice age, part of the Pacific ocean suddenly saw a burst of biological activity. Strangely, and as suddenly as it started, the activity suddenly stopped just a few hundred years later. Now scientists know why.


14,000 years ago, after the last ice age, part of the north Pacific Ocean suddenly burst with life. Phytoplankton, amoeba-like foraminifera and other microorganisms multiplied like crazy and thrived in large numbers. Oddly though, a few hundred years later, as suddenly as the activity began, it suddenly ended. The reasons for the strange phenomenon have eluded us until now, when scientists finally believe they have the answer.

Previously, the best bet was that the biological activity came from a sudden increase in iron. A new study by scientists from Woods Hole Oceanographic Institution, the University of Bristol, the University of Bergen, Williams College and the Lamont Doherty Earth Observatory however, suggests that iron may have had very little to do with it.

“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 Phoebe Lam, a co-author of the study.
This is plankton. It probably ends up in your ear every time you go swimming.
Iron plays a big part in the biological activity in modern oceans and scientists have always assumed that this was true in the past as well. Melting glaciers were believed to lead to higher ocean levels, which submerged land and washed iron deposits into the sea. The new research suggests the true reason for the biological burst, is a cocktail of light and nutrients.
The research team examined core samples from the Pacific ocean near Russia and found that iron had decreased in concentration at the end of the ice age: “We saw the flux of iron was really high during glacial times, and that it dropped during deglaciation,” says Lam, “we didn’t see any evidence of a pulse of iron right before this productivity peak.”
What the sediment sample does suggest, is that the changing climate induced deep mixing in the Pacific, stirring up nutrients from the sea bed into the shallow parts of the ocean, where plankton could thrive on them. The stirring then stopped as a result of melting glacial water entering the ocean, and that essentially trapped the nutrients and microorganisms in a thin, bright, nutrient-rich strip of water.
“We think that ultimately this is what caused the productivity peak—that all these things happened all at once,” Lam says. “And it was a transient thing, because the iron continued to drop and eventually the nutrients ran out.”


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