The Nature of the Deep Nitrogen Cycle

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.