NITRO-MET: REACTIVE NITROGEN SUPPLY DRIVEN BY OROGENIC METAMORPHISM

Nitrogen is one of six elements that form the building blocks of all life on Earth. The nitrogen cycle includes species that act as greenhouse gases, while its role as an essential nutrient for plants during photosynthesis on land and in the ocean means it has both direct and indirect effects on climate change. Biogeochemical and geological processes act together to distribute nitrogen amongst the Earth’s atmosphere, oceans, and biosphere, although the vast majority at the surface (N2 gas) is unavailable for life. While nitrogen-fixing organisms can produce reactive nitrogen as a major pathway into ecosystems, metamorphic reactions and mountain building also liberate reactive nitrogen from rocks and supply it to the surface over geological timescales. Hints from modern ecosystems and assessments of global rock weathering suggest that metamorphic rocks can be a an important reactive nitrogen source, with a flux of 20-30 TgN/yr, which is double pre-industrial rates of reactive nitrogen atmospheric deposition. Indeed, while a growing number of studies recognise the importance of mountain belts in the geological carbon cycle, the role of metamorphism and weathering on nitrogen redistribution remains poorly constrained. To move forward, NITRO-MET will assess how the continents spatially and temporally control reactive nitrogen supply to the biosphere over timescales relevant to the evolution of marine and terrestrial life. To deliver this outcome, the team will quantify the rates and magnitudes of reactive nitrogen production during mountain building, its storage in metamorphic minerals, and the release of nitrogen species from the continental crust during erosion and weathering. By doing so we will be able to make the first assessment of this poorly studied reactive nitrogen flux at Earth’s surface and its long-term impacts on atmospheric and oceanic chemistry, climate, and biological productivity.
Our central hypothesis is that tectonic processes associated with supercontinent formation and breakup – namely, (1) orogenic degassing driven by metamorphic reactions and (2) uplift, weathering and erosion of exhumed metamorphic and granitic rocks – are key processes that deliver reactive nitrogen to Earth’s biosphere over geological timescales. Clues for the importance of this overlooked driver are found in the variability of nitrogen isotopic compositions documented in marine sediments over the past ~700 Myr. Metamorphism affects the isotope composition of nitrogen-bearing minerals in rocks, and several large changes in the isotopic ratio of nitrogen in the marine sedimentary record coincide with major plate tectonic assembly events and transitions between icehouse and greenhouse climate states. Together this suggests previously unrecognised links between plate tectonic processes and the Earth’s climate and biosphere. Indeed, while a growing number of studies recognise the importance of mountain belts in the geological carbon cycle, the role of metamorphism and weathering on nitrogen redistribution remains underexplored. However, we lack constraints on the production of reactive nitrogen during metamorphism, and how much is subsequently released to the atmosphere or leached into the oceans due to weathering of uplifted metasedimentary rocks and associated granites over 10-to-100-Myr timescales.
NITRO-MET will combine field study, geochemical and isotopic analyses, petrological modelling, and planetary-scale box models to achieve this goal, specifically taking advantage of recently developed, high-precision analytical procedures at world-leading labs that can measure parts-per-million concentrations of nitrogen and its isotopes in individual minerals. This will deliver important new data that constrain dynamic interactions within the nitrogen cycle between the solid Earth, atmosphere, and oceans.