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Ocean’s Iron Impact Could Alter Climate Predictions

By Alex Kirby, Climate News Network

LONDON – British scientists say estimates of the amount of iron dissolving into seawater around some of the world’s coasts may be drastically wrong.

They say there is no standard, one-size-fits-all way to measure how much iron enters the water in different parts of the globe. Instead, they say, the amounts may vary by up to ten thousand times between one area and another, with profound implications for the impact of the iron on the oceanic carbon cycle.

This uncertainty, they say, has probably led to iron’s impact being both exaggerated and underplayed. It is compounded by another discovery: that the iron enters the water by two mechanisms, not the one thought so far to be solely responsible.

Phytoplankton is the foundation of the oceanic food chain and the key to the absorption of CO2.
Credit: NOAA MESA Project

Iron is key to the removal of carbon dioxide from the atmosphere as it promotes the growth of microscopic marine plants (phytoplankton), which mop up the greenhouse gas and lock it away in the oceans.

But the new study, led by researchers based at the National Oceanography Centre Southampton, UK, has found that the amount of dissolved iron released into the oceans from continental margins – the zone of the ocean floor that separates the thin oceanic crust from thick continental crust – varies in ways not currently captured by ocean-climate prediction models.

This, they say, could alter predictions of future climate change, because iron plays a key role in the global carbon cycle.

The study found that the amount of iron leaking from continental margin sediments varies between regions because of local differences in weathering and erosion on land. The results of the study are published in Nature Communications.

Adding sugar to tea

Iron acts like a giant lever on marine life storing carbon,” says Dr. Will Homoky, lead author and postdoctoral research fellow at University of Southampton Ocean and Earth Science, which is based at the Centre. “It switches on growth of microscopic marine plants, which extract carbon dioxide from our atmosphere and lock it away in the ocean.”

Continental margins are a major source of dissolved iron entering the oceans. But until now measurements have been taken only in a limited number of regions across the globe, all with low oxygen levels and high sedimentation rates. The Southampton study focused on a region with contrasting environmental conditions – in Atlantic waters off the coast of South Africa.

“We were keen to measure iron from this region because it is so different from areas studied before. The seawater here contains more oxygen, and sediments accumulate much more slowly on the seafloor because the region is drier and geologically less active”, says Professor Rachel Mills, co-author of the study.

The team found substantially smaller amounts of iron being supplied to seawater than measured anywhere before, challenging preconceptions of global iron supply.

They also found two different mechanisms by which rocks are dissolving on the seafloor, by measuring the isotopic composition of the iron using a technique developed with co-authors based at the University of South Carolina.

Iron switches on the growth of microscopic marine plants, which extract CO2 from our atmosphere and lock it away in the ocean.
Credit: flickr/Milan Boers

“We already knew that microbial processes dissolve iron in rocks and minerals,” says Dr. Homoky. “But now we find that rocks also dissolve passively and release iron to seawater, a bit like sugar dissolving in a cup of tea.”

 “The fact that we have found a new mechanism makes us question how much iron is leaking out from other areas of the ocean floor. If certain rocks are going to dissolve irrespective of microbial processes, suddenly there are whole regions that might be supplying iron that are presently unaccounted for.”

“Model simulations indicate that the presence or absence of iron supply from continental margins may be enough to drive Earth’s transition between glacial and interglacial periods.”

“Therefore these findings could certainly have implications for global climate modeling – to what extent is yet to be determined.”

“Our study shows that the amount of iron coming off different margins might vary by up to ten thousand times. In some regions we are probably over-estimating – and in others under-estimating – the influence of sedimentary iron supply on the ocean’s carbon cycle.”

The study is highly topical now as debate continues over where the heat caused by greenhouse gas emissions is going. Some claim that climate change is at a virtual standstill, because atmospheric heating has slowed a little. Others say the heat is going into the oceans. Intriguingly, it remains unclear which group can claim the study supports it.

The study formed part of GEOTRACES, an international program designed to improve understanding of biogeochemical cycles and large-scale distribution of chemical elements and their isotopes in the marine environment. 

Alex Kirby, a former BBC environment correspondent, is a founding journalist of Climate News Network. Climate News Network is a news service led by four veteran British environmental reporters and broadcasters. It delivers news and commentary about climate change for free to media outlets worldwide.


By dan_in_illinois
on July 28th, 2013

Hey, Doomsters.

I thought the science was already settled and that the “experts” had determined once and for all time what the correct climate model was.  Now I’m reading that there are additional aspects to the climate model that haven’t been included and that could create considerable variance in the predictions.  What gives?

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By Gerald Wilhite (austin, texas 78703)
on July 29th, 2013

It seems to me that all this article is telling us is we don’t know what the heck we are doing yet. Our models indicate that the role of iron in the global carbon cycle could be very important. However, our studies also make it clear that our models are obviously not reliable on this issue.

That’s all.

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By dan_in_illinois
on July 29th, 2013


You say:

“It seems to me that all this article is telling us is we don’t know what the heck we are doing yet….our studies also make it clear that our models are obviously not reliable on this issue.

That’s all.”

That seems like a lot to me.  Basically you seem to be saying that the models the “experts” have developed aren’t worth much of anything.  I agree.

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By Vincenzo Palumbo (Valle Agricola, Italy)
on July 30th, 2013

So, what happens to the CO2 when phytoplankton die/are consumed by other organisms?  Is it released into the oceans, thus suffocating sea life, does it bubble up to the surface and rejoin the thinner atmosphere of the air, or what?

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By Herb Curl (Seattle/WA/98105)
on July 31st, 2013

Interesting article but irrelevant to climate modelling. But, as usual, it’s complicated.

The mass-wasting of geological formations is geographically dependent and dissolved iron is more abunant in some parts of the coastal oceans than others. Iron can be a limiting element for phytoplankton production because it’s mostly in the form of ferric hydroxide, a gel, not readily available to algae except by a slow process that occurs on cell surfaces that converts it to a soluble form. The experiments involving dumping iron solutions in the ocean creates momentary blooms which disappear as the dissolved iron is converted to the hydroxide gel because the ocean water is slighly alkaline. So it’s a momentary effect.

If you want to include phytoplankton production in a model you measure the production directly and don’t worry about most of the factors controlling the production, except for physical phenomena like coastal upwelling. But there are some possible feedback mechanisms.

The oceans are becoming acidified by the incorporation of increasing atmospheric CO2. The effect were observing now is the dissolution of the carbonate shells of both of some phytoplankton and of very young shellfish such as oysters. If an oyster can form a carbonate shell it can’t attach to a solid object and dies. We don’t know yet what the overall effect on phytoplankton will be, but one possibility is the species with calcified cell walls will decline but the increasing acidification will make iron more available increasing production.

(I conducted research on iron availability as an oceanographer before retiring.)

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