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Chemistry

Deep Iron: Good for the Ocean’s Bones

Paper: Fitzsimmons, J.N., et al., 2017. Iron persistence in a distal hydrothermal plume supported by dissolved–particulate exchange. Nature Geoscience, v.10: 195–201.

Phytoplankton blooms off the Atlantic coast. Iron is a limiting nutrient for blooms like this one, and iron can spur growth. (Credit: Wikicommons)

Iron is a vital nutrient for your bones and your growth. But did you also know it’s good for the ocean? In its dissolved form, iron is what’s known as a ‘limiting nutrient’ on the ocean surface because the element is necessary for the process of photosynthesis in phytoplankton. Because it is highly insoluble (hard to be dissolved) in sea water, it is considered a trace element that can limit or stimulate primary production. Algal blooms can be created by supplying iron to iron-deficient ocean water, which can act as a CO2 sink and in turn help other organisms grow. In fact, experiments with iron fertilization, in which iron is purposefully introduced to iron-poor areas of the ocean surface to stimulate phytoplankton production, have been conducted as a possible means to fight rising CO2 levels and global warming.

Where can iron (Fe) come from in the ocean? Traditionally, studies have focused on atmospheric dust and continental margins as primary Fe sources. Although dissolved iron (dFe) rarely sinks through the seawater in which it is dissolved, it can bind to other particles that sink through the water column. Another important source of iron in the ocean interior is hydrothermal vents (check out this article to learn more about these vents!). The hot, metal-heavy fluids these vents spout deep in the ocean can contain dFe and dissolved manganese (dMn) in concentrations a million times higher than the background deep ocean. In theory, this iron could make its way to the surface and stimulate phytoplankton growth.

Hydrothermal Vent. (Credit: NOAA)

Historically, hydrothermally-sourced Fe was expected to precipitate and settle into sediments close to vent sources, and thus the vents were considered to supply little dFe to the global ocean. However, it has been observed that the hot fluids from vents create plumes that are capable of transporting and dispersing dFe and dMn across vast distances. In the East Pacific Rise, for example, the world’s largest hydrothermal plume disperses dissolved metals more than 4,000 kilometers. It has also been observed that microscopic minerals and organic ligands (ions or molecules that bind to a central metal atom) that bind with Fe comprise a fraction of Fe in seawater, and these colloids (small inorganic nanoparticles) could protect dFe from precipitation and gravitational settling.

Some researchers explored weather dFe sinks in conjunction with particulate iron (pFe; not a form that is used as nutrient) in a ~4,300 km transect starting at the Southern East Pacific Rise ridge axis and extending along the core of the largest known hydrothermal plume globally. Fitzsimmons and colleagues measured dFe and pFe compared to dMn and pMn, and independent tracers of the plume (He3+), along a 4,300 km transect. They found that dissolved iron seemed to sink within the plume steadily and relatively quickly, by up to 5–10 meters per year (yes, this is considered quick for particles of this tiny size). They used X-ray microscopy to confirm the presence of particulate iron oxyhydroxide minerals that are coated with organic matter. In the distant plume, iron was a smaller and more unevenly distributed component inside lower-density particles made from carbon, which are assumed to settle slowly. The authors argue that this slow settling accounts for the persistence of particulate iron in the plume.

Helium (3He) and iron (Fe) are injected into the Pacific Ocean by hydrothermal venting along the mid-ocean ridge. (Credit: Nature Geoscience)

The results show complementary pFe and pMn distributions across the section. They highlight the previously unreported deepening of dFe relative to isopycnals (lines connecting points of a specific density or potential density in the water column), and infer from isotope and synchrotron speciation techniques that this vertical descent of dFe is mediated by reversible exchange with a sinking pFe phase that they believe is likely facilitated by Fe associations with organic matter. Mn, in contrast, does not sink across isopycnals, due to the association of pMn with low-density materials as well as the lack of organic and colloidal speciation for dMn, which may inhibit exchange with sinking particle phases.

So what does this all mean? In short, water movement alone cannot readily account for the sinking of iron: as it moved across the transect, away from the vent, dFe was found to cross layers of ocean density and become offset from other chemical tracers of plume dispersal such as dMn. The researchers hypothesize that comparatively rapid and reversible exchanges with the slowly sinking Fe particles must be responsible for the descent of dFe. Thus a dynamic process could take place in which dissolved iron, which itself sinks much more slowly (if it sinks at all), is bound and released from sinking particles. However, they do not know the processes that could be responsible for these reversible exchanges, how representative they are of other ocean regions, or their wider significance for biogeochemical cycles. Whatever the processes driving these exchanges, they could be occuring throughout the entire ocean, with implications for other processes important to the global carbon budget.

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