Core samples of sediment from the bottom of the Pacific Ocean have provided evidence that could change scientists' understanding of how the oceans' nutrient systems work.
Published on Wednesday as a letter in Nature, a study led by researchers from Lamont-Doherty Earth Observatory provides evidence that iron deposited in the equatorial Pacific during the last ice age did not enhance algae growth as expected.
Results from the study also suggest that potential technology using deposited iron to fight climate change might not be as straightforward as initially thought.
Iron deposition via atmospheric dust into oceans has been linked to the climate since the 1980s, when it was proposed as a method of reducing the greenhouse effect that leads to global warming. This mechanism is a result of iron allowing phytoplankton—microscopic algae that live in the ocean—to increase their productivity.
According to Jerry McManus, a professor of geochemistry and co-author of the study, iron usually acts as a limiting agent.
"Nitrate and phosphate—those are the big macronutrients. That's what everything on Earth needs," he said. "When they're depleted, that's the end of productivity. The idea of a micronutrient like iron is to help explain why there are some places in the ocean where the nutrients just don't go to zero."
However, when higher levels of iron are present, it can act like any other nutrient for phytoplankton.
"At a basic level, it's a nutrient like nitrogen and phosphorus," lead author and geochemistry Ph.D. student Kassandra Costa said. "They [phytoplankton] need it to build their bodies and respire and photosynthesize. So the idea with iron fertilization is that if you give them more iron, they'll grow more and take more carbon dioxide out of the atmosphere."
With more carbon dioxide out of the atmosphere, the greenhouse effect is reduced, causing cooling. Direct evidence of iron's effect on phytoplankton was seen following Mt. Pinatubo's eruption, which launched iron dust into the ocean, causing enough algae growth to create a global reduction of carbon dioxide.
In recent years, scientists from around the world have been investigating high levels of iron dust deposited in the oceans during the last ice age, which occurred between about 20,000 and 30,000 years ago.
Studying iron fertilization from tens of thousands of years ago in the middle of the Pacific is far from easy. To get a picture of conditions back then, researchers had to take core samples of sediments at the bottom of the ocean.
According to Costa, the work isn't quite glamorous.
"Life at sea was 12-hour days hauling mud up from the bottom of the ocean, running it through the machines, and sawing the cores open with bandsaws," Costa said.
These cores are filled with fine silt that contains a geochemical record of tens of thousands of years. Using radiocarbon dating of small shells left by tiny, amoeba like creatures, the researchers precisely measured the time periods in the samples.
Costa and her colleagues probed six core samples for three factors: iron dust, phytoplankton productivity, and nitrate consumption.
The researchers found that the levels of iron dust during the ice age were two to three times higher than levels today, a result consistent with past findings. However, they also found that phytoplankton productivity decreased and nitrate consumption remained the same. Both factors are generally expected to increase with an increase of iron dust.
Although the scientists do not know for sure why phytoplankton productivity decreased and nitrate consumption was the same in the equatorial Pacific, they do have a theory: Iron fertilization was working really, really well in the South Pacific.
The researchers said that their hypothesis is compatible with a study from 2014 that showed iron fertilization happening in the South Pacific during the last ice age.
"The South Pacific is where water originates and upwells in equatorial pacific. There's more iron fertilization in the south, with phytoplankton consuming more nutrients down there," Costa said. "By the time the water gets to the equatorial pacific, there's less for the algae to consume."
With other nutrients acting as limiting agents, even increased levels of iron couldn't improve productivity.
However, this hypothesis is as yet untested.
"Changing dust deposition but not changing biological productivity is based on evidence of the core samples," Sarah Gille, a professor of physical oceanography at Scripps Institution of Oceanography, said. "But the speculation about what that means for nutrient cycles in the global ocean is speculation."
Gille said this did not mean the researcher's hypothesis was wrong, but that more samples would need to be taken and more thorough analyses made before it could be considered more seriously.
If the hypothesis is correct, it could complicate the use of iron fertilization to fight global warming.
"The global inventory of nutrients is a finite quantity. There's only so much in the ocean," Costa said. "It's not going to be that simple. You can't just go out to the coast and dump in your iron and expect to see an impact on climate."
For McManus, there's an even bigger, potentially more exciting picture than geoengineering techniques.
"If we're right here, it says something about the connectivity of the ocean," he said. "[Our study] didn't solve the issue for once and for all, but it made a really important point, and I think that will send many people exploring."