Earth’s Nitrogen and Carbon Cycles

Abstract

Understanding the Earth’s geological nitrogen (N) and carbon (C) cycles is fundamental for assessing the distribution of these volatiles between solid Earth (core, mantle and crust), oceans and atmosphere. This Special Communication about the Earth’s N and C cycles contains material that is relevant for researchers who are interested in the Topical Collection on planetary evolution “Reading Terrestrial Planet Evolution in Isotopes and Element Measurements”. Variations in the fluxes of N and C between these major reservoirs through geological time influenced the evolution and determined the unique composition of the Earth’s atmosphere. Here we review several key geological aspects of the N and C cycles of which our understanding has significantly advanced during the last decade through field-based, experimental and theoretical studies. Subduction zones are the most important pathway of both N and C from the Earth’s surface into the deep Earth. A key question in the flux quantification is how much of the volatile elements is stored in the downgoing slab and introduced into the mantle and how much is returned back to the surface and the atmosphere through arc magmatism. For N, the retention of N as $\text{NH}_{4}^{+}$ NH 4 + in minerals has a major influence on fluxes between reservoirs. The temperature-dependent stability of $\text{NH}_{4}^{+}$ NH 4 + -bearing minerals determines whether N is predominantly retained in the slab to mantle depths (in subduction zones with a low geothermal gradient) or devolatilized (in subduction zones with a high geothermal gradient). Several lines of evidence suggest that the mantle is regassing with respect to N due to a net influx of subducted N over time, but this issue is highly debated and evidence to the contrary also exists. Nevertheless, there is consensus that the majority of the planetary N budget is stored in the Earth’s mantle, with the continental crust also constituting a significant N reservoir. For C, release from the subducting slab occurs through decarbonation reactions, dissolution and formation of carbonatitic liquids, but reprecipitation of C in the slab or the forearc mantle wedge may limit the effectiveness of direct return of C into the atmosphere. Carbon release through regional metamorphism in collision zone orogens also has potentially profound effects on C release into the atmosphere and consensus has emerged that such orogens are sources rather than sinks of atmospheric CO2. On shorter timescales, contact metamorphism through interaction of mantle-derived magmas with C-bearing country rocks, and the resulting release of large quantities of CH4 and/or CO2, has been linked to global warming events.

Behavior of Nitrogen Contents and Isotopes during Thermal Evolution and Hydrocarbon Generation in Shale: Insights from Hydrothermal Experiments in a Semiopen System

“…22 Previous studies have shown that a series of nitrogen biochemical processes are involved in the sedimentary evolution of organic matter. First, almost all the nitrogen in sedimentary rocks was originally captured by living organisms that either fixed N 2 or assimilated dissolved NO 3 − , NH 3 , or NH 4 + with a characteristic isotopic composition. 9 As these organisms died and settled on the seafloor, their isotopic composition was archived in the sedimentary record.…”

The iron spin transition of deep nitrogen-bearing mineral Fe3N1.2 at high pressure

“…It is also one of the most poorly studied elements under the Earth's deep interior pressure and temperature (P-T) conditions. Surface and deep nitrogen are largely connected via subduction and volcanic degassing [Halama et al, 2012;Mao and Mao, 2020;Halama and Bebout, 2021]. Notably, the nitrogen concentration in the present-day bulk silicate Earth may be only tens of ppm, severely depleted with respect to other volatiles like carbon and sulfur [Marty, 2012;Bergin et al, 2015;Yoshioka et al, 2018].…”

Nitrogen Retention in Feldspar: Implications for Nitrogen Transport in Subduction Zones

“…To better constrain the contribution of subducting slab to deep Earth's nitrogen, it is critical to know not only how much nitrogen can be incorporated into minerals in subducting slabs, but also the thermal stabilities of nitrogen, which determine nitrogen retention during subducting (Halama & Bebout, 2021). Recent experimental and theoretical studies have reported that subduction minerals, such as biotite, phlogopite, muscovite, tobelite, stishovite, and feldspar, have large storage capacities of nitrogen (e.g., Bos et al., 1988; Förster et al., 2019; Fukuyama et al., 2020; Y. Li et al., 2021).…”

“…Therefore, ammonium may be carried by feldspar to a great depth along cold subduction. which determine nitrogen retention during subducting (Halama & Bebout, 2021). Recent experimental and theoretical studies have reported that subduction minerals, such as biotite, phlogopite, muscovite, tobelite, stishovite, and feldspar, have large storage capacities of nitrogen (e.g., Bos et al, 1988;Förster et al, 2019;Fukuyama et al, 2020;.…”