Ocean Iron Differs Substantially Between Locations

Ocean Iron Study Means Climate Rethink

The amount of dissolved iron in oceans bordering the world’s continental shelves varies far more between regions than realised, researchers say, with implications for future climate prediction.

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.

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.

Like 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.

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

“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 modelling – 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 programme designed to improve understanding of biogeochemical cycles and large-scale distribution of chemical elements and their isotopes in the marine environment. – Climate News Network

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