Privileged Planet: the role of iron (Introduction)

by David Turell , Monday, June 20, 2022, 15:53 (666 days ago) @ David Turell

A large role in climate:

https://knowablemagazine.org/article/physical-world/2019/ocean-iron-fertilization?utm_s...

"Proposed in 1990 by the late oceanographer John Martin, the hypothesis suggests that flurries of dust — swept from cold, dry landscapes played a crucial role in the last major ice age. When this dust landed in the iron-starved Southern Ocean, Martin argued, the iron within it would have fertilized massive blooms of diatoms and other phytoplankton. Single-celled algae with intricate silica shells, diatoms photosynthesize, pulling carbon from the atmosphere and transforming it to sugar to fuel their growth. Going a step further, Martin proposed that using iron to trigger diatom blooms might help combat global warming.

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"Although it’s now well-established that an uptick in iron fertilization occurred in the Southern Ocean during the last major ice age, for example, scientists still argue about how much it reduced carbon dioxide levels in the atmosphere. And while Martin’s hypothesis inspired 13 large iron fertilization experiments that boosted algae growth, only two demonstrated removal of carbon to the deep sea; the others were ambiguous or failed to show an impact, says Ken Buesseler, a marine radiochemist at the Woods Hole Oceanographic Institution in Massachusetts.

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"Three billion years ago the ocean was chock-full of iron, ancient mineral deposits show. Iron was plentiful when life first evolved, and the metal was incorporated into a long list of essential cellular functions. Animals need iron to transport oxygen in their blood and to break down sugar and other nutrients for energy. Plants need iron to transfer electrons during photosynthesis and to make chlorophyll. Phytoplankton need it to “fix” nitrogen into a usable form.

"Despite being the fourth most abundant element in the Earth’s crust, iron is vanishingly scarce in the modern ocean. It started disappearing from the seas more than 2.4 billion years ago, when cyanobacteria evolved and started to breathe in carbon dioxide and exhale oxygen. When this happened, dissolved iron rapidly linked up with the newly plentiful oxygen atoms, forming iron oxides such as hematite, a common mineral that contains a form of the element known as iron(III). Most phytoplankton and other living organisms can’t use iron in this state. They require a different form, iron(II), which more readily dissolves and is absorbed by cells.

"Hematite has another downside: It sinks. Over billions of years, layer upon layer fell to the sea floor, forming iron ore deposits hundreds to thousands of feet deep. Meanwhile, iron in the waters above diminished to barely detectable levels — an average liter of seawater contains roughly 35 grams of salt, but only on the order of a billionth of a gram of iron.

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"Winds blowing off the Sahara are one of the most important sources of iron dust in the ocean, supplying more than 70 percent of dissolved iron to the Atlantic, another group has found. But there are several other paths by which iron(II) makes its way to the oceans, including rivers, hydrothermal vents, volcanoes and glacial outwash plains

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"Shoenfelt Troein fed the iron(II)–rich, glacial sediment to a common species of diatom, Phaeodactylum tricornutum, and the diatoms reproduced 2.5 times as fast as they did on weathered sediment, the team reported in Science Advances in 2017. This would translate into a roughly fivefold increase in carbon uptake compared with the non-glacial sediment, the team calculated.

"When the team looked at marine sediment cores from several glacial and interglacial periods spanning 140,000 years, Winckler, Shoenfelt Troein and colleagues found that dust from the glacial periods contained 15 to 20 times more iron(II) than did dust from the current interglacial period. That suggests that the potency of glacial sediment led to a self-reinforcing cycle, in which higher rates of iron fertilization in the oceans reduced carbon in the air, leading to colder temperatures, which in turn, grew glaciers, the team reported in the Proceedings of the National Academy of Sciences in 2018. It also suggests that not all iron is equal when it comes to fertilization, and that freshly mined, fine-ground iron might be more effective than other forms, Winckler says.

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"They’ve found that most naturally produced iron dust blows off the Sahara and other deserts, but large amounts are also released in plumes of hot dissolved minerals from hydrothermal vents. Volcanoes, which can spew thousands of kilograms of iron into the atmosphere in a single eruption, are another important source. Although the evidence is circumstantial, iron fertilization from volcanic ash may have contributed to the brief hiatus in carbon buildup in the atmosphere after the 1991 eruption of the Philippines’ Mt. Pinatubo, says Emerson.

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"in June 2019, Schmittner and colleagues...calculating that cooler temperatures and iron fertilization were responsible for most of the decrease, and ocean circulation and sea ice had “close to zero” impact, he says. Iron fertilization alone accounted for a 25 to 35 ppm decrease in atmospheric carbon during that period, “a larger effect than we expected,” he says."

Comment: Iron played a huge role in climate c hange on the early Earth. But it is obvious how much role it plays today is not known.