Amazing Journey of iron: from hydrothermal vents to the oceans

Dissolved iron (Fe) is essential for phytoplankton growth as it is required for a number of important enzymes that participate in both photosynthesis and respiration. Given that marine photosynthesis is responsible for ~40% of the global atmospheric carbon dioxide (CO2) uptake, Fe plays a key role in the carbon cycle and the climate change. Some parts of the world’s ocean have very low levels of Fe, yet high levels of other major nutrients (nitrate, phosphate and silicate), which means that the operation of the ‘carbon pump’ is highly inefficient and sequestration of atmospheric CO2 is restricted.

The marine phytoplankton consumes carbon dioxide (CO2) from the atmosphere and moves it to sediments in the deep ocean in a process called “biological carbon pump”. (Image credit: LabGrab)

It is therefore critical to determine and quantify the main sources of dissolved iron to the oceans. However, the relative importance of different sources to the global inventory is not well known, and flux estimates from atmospheric dust, oceanic sediments and hydrothermal vents vary by orders of magnitude. The altered seawater, called hydrothermal fluid, is injected back into the ocean at hydrothermal vents and forms hydrothermal plumes when it mixes with near-freezing deep seawater. It has been widely assumed that hydrothermal activity is not a major source of dissolved Fe because of much precipitation of Fe sulfides and hydroxides during the ‘mixing’ process.

A venting black smoker emits jets of particle-laden fluids, and the particles are predominantly fine-grained sulfide minerals. (Image credit: NOAA)

This long-term standing has been challenged by recent researches which show that iron from hydrothermal vents appears to have a much longer lifetime than previously thought and be exported up to 4000 km away from the vent sites. See the figure below- isn’t it amazing? There must be mechanisms that prevent the dissolved Fe from being lost! In fact, Fe is present in a variety of chemical and physical forms that enhance its solubility, facilitate its ‘travelling’ in the water column and ultimately give micro-organisms a chance to access it.

The distribution of dissolved Fe concentration (unit: nano-mole) in the south Pacific Ocean. The transport of hydrothermal iron from the East Pacific Rise several thousand kilometres westward across the ocean is observed. (Image credit: Resing et al., 2015)

Currently I am trying to find out how the dissolved Fe is stabilized and how it is transported throughout the ocean, and my research focus lies in the utility of isotopic tools to fingerprint Fe. Are you interested? Let’s start to explore the iron journey together!

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