Abstract
Nitrogen forms an integral part of the main building blocks of life, including DNA, RNA, and proteins. N2 is the dominant gas in Earth's atmosphere, and nitrogen is stored in all of Earth's geological reservoirs, including the crust, the mantle, and the core. As such, nitrogen geochemistry is fundamental to the evolution of planet Earth and the life it supports. Despite the importance of nitrogen in the Earth system, large gaps remain in our knowledge of how the surface and deep nitrogen cycles have evolved over geologic time. Here, we discuss the current understanding (or lack thereof) for how the unique interaction of biological innovation, geodynamics, and mantle petrology has acted to regulate Earth's nitrogen cycle over geologic timescales. In particular, we explore how temporal variations in the external (biosphere and atmosphere) and internal (crust and mantle) nitrogen cycles could have regulated atmospheric pN2. We consider three potential scenarios for the evolution of the geobiological nitrogen cycle over Earth's history: two in which atmospheric pN2 has changed unidirectionally (increased or decreased) over geologic time and one in which pN2 could have taken a dramatic deflection following the Great Oxidation Event. It is impossible to discriminate between these scenarios with the currently available models and datasets. However, we are optimistic that this problem can be solved, following a sustained, open‐minded, and multidisciplinary effort between surface and deep Earth communities.
Highlights
Understanding the nitrogen cycle is part of a dynamic geobiological puzzle, for which the ultimate goal is illuminating the mechanics responsible for the development of habitability on Earth and on other planets, in our solar system and beyond
Sometime in the mid- to late Archean (e.g., Farquhar, Zerkle, & Bekker, 2011), microbial life developed the ability to perform oxygenic photosynthesis, which uses energy from the sun and raw materials extracted from the geosphere (CO2 + H2O) to generate energy, construct essential building materials, and releases oxygen in a gas phase (O2) as a waste product
We suggest that there are three plausible scenarios to reconcile these issues within the geobiological nitrogen cycle over Earth’s 4.6-billion-y ear history, notably that atmospheric pN2 has changed unidirectionally over geological time (Figure 5, conceptual model 1), or the direction of the nitrogen abundance of the atmosphere took a dramatic deflection following the Great Oxidation Event (Figure 5, conceptual model 2)
Summary
Understanding the nitrogen cycle is part of a dynamic geobiological puzzle, for which the ultimate goal is illuminating the mechanics responsible for the development of habitability on Earth and on other planets, in our solar system and beyond. Sometime in the mid- to late Archean (e.g., Farquhar, Zerkle, & Bekker, 2011), microbial life developed the ability to perform oxygenic photosynthesis, which uses energy from the sun and raw materials extracted from the geosphere (CO2 + H2O) to generate energy, construct essential building materials, and releases oxygen in a gas phase (O2) as a waste product Over time this biological revelation cumulatively oxygenated Earth’s. The combination of a uniquely reactive- gas-rich atmosphere and hydrosphere, coupled with subduction zone plate tectonics, means that Earth injects oxidizing material into a relatively reduced mantle (Frost & Mccammon, 2008; Kelley & Cottrell, 2009). We compare and contrast current ideas for how this unique interaction of biological innovation, geodynamics, and mantle petrology could have acted to regulate the geobiological nitrogen cycle over Earth history
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