GoKawiilNever in a million years would the craziest of the mad geoengineers dream of adding a fraction of the iron the Sahara dumps on the North Atlantic every day.
Adding nutrients to the ocean so tiny ocean plants remove carbon dioxide from the atmosphere is an idea that people aren’t used to. Unfamiliar ideas make us uncomfortable. That gut level “I’m not so sure about this” is natural enough. It comes out as an inchoate sense that such a thing must be risky, that we’ve never done it before and so couldn’t possibly know what the consequences would be.
It’s not true, though. We have quite a precise understanding of what happens when you add nutrient-rich minerals to nutrient-poor waters. Dozens of natural experiments have settled the question. Because nature puts minerals into the ocean all the time, all around the world, through many different mechanisms. And it’s been doing so for millions of years.
Volcanoes do it. Wildfires do it. Even educated whales do it. Dust storms and icebergs do it too. And because satellites have been measuring the ocean’s color with minute accuracy for over twenty years, and research ships have been chasing these events with sensors and sample bottles, we have a large and growing record to show what happens when nutrient-starved water suddenly gets fed. It’s one of the best-documented cause-and-effect relationships in ocean science.
Remember, most of the open ocean is a desert. The top layer of water gets plenty of sunlight, but as you get farther from land the tiny drifting plants that form the base of the marine food web — phytoplankton — run short of one or two key ingredients, usually iron, sometimes nitrogen or phosphorus. Give them what they’re missing and they multiply, sucking carbon out of the air in the process. Take the nutrient source away, and within weeks the effect fades. The pattern has been observed over and over, in different waters, driven by different processes, sort of everywhere.
Tonga on my mind
One case of particular interest to me comes from the subtropical South Pacific. It had long been hypothesized, but never proven, that shallow undersea vents near Tonga might be leaking iron-rich fluid close enough to the surface to reach sunlit water and feed nitrogen-fixing bacteria.
Then, in 2019, a research cruise went and found it.
A single string of undersea vents feeds a patch of ocean the size of Germany with enough iron to fuel a bloom of nitrogen-fixing bacteria that grew two to eight times faster than those in the surrounding water, and pulled two to three times more carbon down into the deep ocean. That work was published in Science in 2023. Follow-up cruises mapped exactly how sharply the iron levels spiked near the vents and faded away with distance, tracked the plankton response cell by cell, and even built a nitrogen budget showing that in some seasons, nearly all of the region’s biological productivity runs on vent iron.
The Tonga-Kermadec range isn’t unique, either. Similar iron-fed blooms have now been traced to volcanic ridges south of Africa, near Antarctica, and scattered across the Pacific seafloor — some fed by iron plumes that drift over a thousand kilometers before they’re used up. One recent study even linked the strength of a Southern Ocean bloom, year by year, to how many small earthquakes had recently shaken the nearby volcanic ridge: more quakes appear to be related to more iron release, which leads to more phytoplankton growth.
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