There may be a bright side to glacier melt
Release of iron into the ocean may fertilise phytoplankton that can trap carbon dioxide. Phil Dooley reports.
One of the alarming harbingers of global warming has been the melting of glaciers, but it turns out that there may be a silver lining, albeit a small one. The glaciers are releasing iron into the ocean and fertilising microscopic single-celled marine plants known as phytoplankton. Geoengineers have long suggested adding iron to the ocean to fertilise plankton. It seems nature is doing it on its own.
Jon Hawkings from Bristol University in the UK led the team studying the melt waters that pool beneath west Greenland’s glaciers and found them rich in iron. The researchers calculated the total amount of iron entering the world’s oceans this way would average around a million tonnes per year – the weight of 125 Eiffel Towers.
It’s unclear what proportion of the plankton’s trapped carbon
remains bound up when the creatures die.
Iron is the fourth most abundant element in the Earth’s crust, and plenty accumulates in glaciers as they gouge their way across the landscape. Nevertheless iron is very scarce in the oceans as it reacts with oxygen to form iron oxides. Once fully oxidised, it forms insoluble crystals that sink to the bottom of the ocean, leaving the phytoplankton hungry.
However, the iron in the glacial melt water has a different story, says Hawkings.
“We think the isolated environment under the glaciers might have no oxygen, it’s all been used up by chemical reactions.” The result is that iron carried into the sea is only partially oxidised, in a form that phytoplankton can access. Once out in the ocean the iron begins to oxidise further, but not so fast that the phytoplankton can’t snack on some first. This surge of nutrients enables them to multiply exponentially, in the process trapping large amounts of carbon dioxide. As the phytoplankton die they carry some of that carbon down to the ocean floor, where it remains.
The discovery that the glacial melt water trickling into the ocean carries bioavailable iron solves a previous puzzle. “You can observe large algal blooms in satellite images, sometimes stretching for hundreds of kilometres. People have struggled to explain why they’re there,” says Hawkings, whose research was published in Nature Communications in May. The iron provides the explanation.
So what might be the impact on global warming of this iron feast? It depends on where it goes, says marine biogeochemist Philip Boyd at the University of Tasmania. “Coastal waters are some of the most iron-rich places in the ocean, there’s probably a surfeit of iron there already,” he says. “But if the buoyancy of the freshwater melt carries it reasonable distances to where the phytoplankton are iron-poor it might have some impact.”
Hawkings agrees there are a lot of unknowns. “This is an expanding field,” he says. “There are a whole host of things that plankton and other micro-organisms need: molybdenum, vanadium, cadmium, manganese, even some vitamins.” It’s also unclear what proportion of the plankton’s trapped carbon remains bound up when the creatures die and sink to the bottom of the ocean.
Past climate records suggest the silver lining from glacier melt may be nothing to get too excited about. As glaciers melted during past ice age cycles “there was a lot more iron going into the oceans for several thousand years”, says Boyd. But all that extra iron only decreased atmospheric carbon dioxide by about 30 parts per million over this time. “The anthropogenic increase of atmospheric CO2 in my lifetime has been several-fold higher than that.”