Nitrogen Exposed as a ‘Chameleon’ Element

Nitrogen is the main component of Earth’s atmosphere, but the element also cycles through marine and terrestrial environments and eventually intoEarth’s interior. Despite research into various components of the nitrogen cycle, scientists still don’t know whether the element’s movements over geological timescales have trapped it in the mantle, or simply concentrated it into the atmosphere. Nitrogen bears many similarities to carbon, but the deep nitrogen cycle still remains a mystery.

A trio of DCO members, Sami Mikhail (University of St. Andrews, UK), Peter Barry (University of Oxford, UK), and Dimitri Sverjensky (Johns Hopkins University, USA) teamed up to investigate the behavior of nitrogen in the deep subsurface. They used geochemical modeling techniques to study how the pH of water in the mantle affects whether nitrogen exists as molecular nitrogen, ammonia, or ammonium at a range of temperatures, pressures, and oxygen reactivities. The researchers find that the presence of different minerals can greatly alter the pH of water in the mantle, which impacts nitrogen’s form, which in turn alters its behavior in the mantle. The group published their findings in a new paper in the journal Geochimica et Cosmochimica Acta [1].

Mikhail and Sverjensky began working on nitrogen together after Sverjensky heard Mikhail give a talk at a biweekly deep carbon meeting at Carnegie Institution for Science in Washington, DC. Their initial collaboration resulted in a paper in Nature Geoscience [2] on how Earth’s nitrogen-rich atmosphere evolved. Later, Barry joined the collaboration after he and Mikhail happened to share a room at the second DCO Early Career Scientist workshop in the Azores, where they engaged in hours of discussion about deep nitrogen.

“Nitrogen and carbon have so many similar properties and are found in so many of the same places,” said Mikhail. “They’re both atmospheric elements that control the climate system and are essential for life, but so much more is known about deep carbon than is known about nitrogen, despite the obvious linkages.”

The researchers used Sverjensky’s Deep Earth Water Model (DEW) to calculate how nitrogen dissolved in water would behave at pressures ranging from 1–5 GPa and temperatures of 600–1000 ºC in equilibrium with different minerals. They included mineral compositions to represent the upper mantle, the mantle beneath a volcanic arc, and oceanic crust pushed into the mantle through subduction, the downward movement of the edge of a tectonic plate into Earth’s mantle.

The model shows that the type of minerals present can alter the pH of the fluids by up to 4 pH units. The pH, combined with the temperature, pressure, and oxygen reactivity of the fluids, dictates what form the element will take. “The chemistry of the fluids has a strong control on nitrogen speciation,” said Barry. “Whether you’re going to see ammonium or ammonia is strongly controlled by the pH, which is quite variable, depending on the mineral assemblage.”

The researchers acknowledge that their model is a simplified version of the mantle, and does not take into account common ions that exist in the fluids, such as potassium. The model, however, enables them to demonstrate the role of pH in controlling nitrogen speciation, and its behavior as a “chameleon” element, reacting to the environment around it. Molecular nitrogen, ammonia, and ammonium all behave differently in mantle fluids, which helps determine whether nitrogen stays trapped in the mantle or returns to the atmosphere through volcanic activity.

In future work, Mikhail will simulate high temperature and pressure conditions in his lab to observe how nitrogen behaves in association with different minerals. Barry plans to study how isotopes can be used to better understand how subduction transports and modifies nitrogen during transport into the deep mantle.

The researchers hope that these insights into the effects of mantle pH on element speciation will inspire others to consider pH in their work. “A lot of the elements in the periodic table are sensitive to pH, for example, carbon,” said Mikhail. “For those of us who work in high-temperatures geochemistry it’s like finding a new dimension to explore!”

Dimitri Sverjensky demonstrates thermodynamic calculations. Credit: Sami Mikhail
 
Sami Mikhail and Peter Barry in the Azores while attending the Second DCO Early Career Scientist workshop held in São Miguel, Portugal in 2015. Credit: Sami Mikhail

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