The Nature of the Deep Nitrogen Cycle

Lead Research Organisation: University of St Andrews
Department Name: Earth and Environmental Sciences

Abstract

Nitrogen forms an integral part of the main building blocks of life, including DNA, RNA, and proteins. As such, nitrogen geochemistry is fundamental to the evolution of planet Earth and the life it supports. However, even after decades of research, large gaps remain in our knowledge of how the biogeochemical nitrogen cycle has evolved through time. These gaps must be filled in order to understand how Earth became habitable, and to address the origins of life itself.

Furthermore, nitrogen is the dominant gas in Earth's atmosphere, and is stored in all of Earth's geological reservoirs (the crust, mantle, and core). Importantly, these nitrogen reservoirs are in a constant state of disequilibrium due to life's collective metabolisms, chemical weathering, diffusion, and the large-scale geochemical fluxes imposed by plate tectonics.

Importantly, because of plate tectonics, what comes out of the mantle sometimes goes back in, and vice versa. Therefore, the exchange of nitrogen between the surface and interior is governed by volcanism (out-gassing) and subduction (in-gassing), and this interplay ultimately controls atmospheric N2 levels. The further back one looks in time the less data are available, and there is a predictable dearth of data to constrain either the flux of nitrogen over geological time, or the partial pressure of atmospheric nitrogen in the deep past. This means we must use sparse datasets alongside experimental and theoretical thermodynamic models to estimate past nitrogen dynamics.

At present we cannot determine whether the volume of atmospheric nitrogen has increased, or decreased, over geological time. However, there is increasing evidence that a deep nitrogen reservoir exists within the mantle and/or core. This project addresses the volumes of N in the atmosphere, and how they have changed with time by exploring [1] the storage capacity of nitrogen in the mantle, and [2] the flux of nitrogen between the interior and exterior over time.

We will address these problems using an experimental programme where we will determine the pathways followed by nitrogen during subduction, and the physical properties of ammonium at extremely high pressures. The first study will enable us to model the flux of nitrogen returned to the deep Earth at subduction zones, and the second study will enable us to predict the behaviour of ammonium in the deeper, inaccessible parts of the Earth.

Planned Impact

This project is based around curiosity-driven discovery science aimed at investigating fundamental processes that govern how the Earth works as a dynamic planet. Therefore, the main beneficiaries will be academics (see academic beneficiaries section). However, there will also be impact outside of the Earth Sciences. For example, the development of standards and protocols for analysing nitrogen will be of interest to analytical scientists in solid-state chemistry and condensed matter physics, especially those interested in developing oxy-nitride materials with novel, industrially-important properties. Co-I Bromiley will liaise with other members of Centre for Science at Extreme Conditions (CSEC; including Prof. Attfield's group in Chemistry who study oxy-nitrde materials) and commit to giving an inter-departmental CSEC seminar to demonstrate the capability of the Edinburgh Materials and Micro Analysis Centre facility for accurately constraining N contents of materials. Additionally, P-I Mikhail is has set up a joint Earth Sciences-Material Sciences laboratory (piston cylinder press) at the University of St. Andrews for which knowledge exchange between the disciplines is planned (with Professor John Irvine, FRSE, FRSC).

The project members will also make a societal impact through the transfer of knowledge to beneficiaries in industry, HEIs, Research Councils and schools. This will be achieved through a collaboration with a successful, and dedicated outreach programme based at St Andrews, the GeoBus (http://www.geobus.org.uk). The GeoBus is a free educational outreach project for schools developed and run by the Department of Earth and Environmental Sciences at the University of St Andrews. The GeoBus is available to visit all secondary schools in all educational authorities around Scotland. Teaching packages cover broad areas of Earth science and are aimed at supporting STEM subject teaching in general, whilst highlighting career opportunities in Earth science and other STEM subjects. Dr Mikhail has also begun writing popular science articles following several commissions from a fashion magazine (Refinery29.com). This platform will be utilised to communicate the findings of this research to to wider community, importantly, to communities largely outside of sciences (the entertainment and fashion spheres of society).

The topic of planetary habitability captures the imagination of the public, for example a recent contribution from the PI was picked-up by a variety of major news sources in the UK (e.g. Daily Mail), USA (e.g. NBC News), and online outlets (e.g. IFLS.com, popular online news site which regularly reaches 2.1M people) (for links to these media articles, please visit: https://drsamimikhail.wordpress.com/press-releases/). We will play on this natural human curiosity regarding the habitability of the Earth to facilitate the transfer of knowledge to beneficiaries in the general public though a dedicated pedagogical programme. The targeted school pupils are those specifically attending underachieving schools that may otherwise not benefit from exposure to non-curriculum science activities within the UK. The aim is to engage a new generation of talented young people from impoverished backgrounds to fulfill their potential within STEM subjects, and them to pursue the sciences at school, and ideally, into universities and their future careers.

Publications

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Broadley M (2018) Plume-lithosphere interaction, and the formation of fibrous diamonds in Geochemical Perspectives Letters

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Mikhail S (2019) Diamondites: evidence for a distinct tectono-thermal diamond-forming event beneath the Kaapvaal craton. in Contributions to mineralogy and petrology. Beitrage zur Mineralogie und Petrologie