Unlocking the C and N budget of the Earth

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Earth's carbon and nitrogen cycles, which on short timescales involve all life, the oceans, and the atmosphere, are ultimately rooted in Earth's deep mantle. The release of carbon and nitrogen from Earth's mantle occurrs through volcanism, and their return to the mantle occurs through subduction. Over our planet's history these cycles have built a nitrogen-rich atmosphere, and sustained the carbon cycle. Although we have abundant carbon and nitrogen at Earth's surface, vastly more may reside in its mantle. Understanding the habitability of Earth's surface environment therefore lies in understanding volatiles in the deep Earth.In this proposal we will study magmas from two ocean island locations, Iceland and the Canary Islands. The mantle beneath these islands is known to represent both ancient deep portions of Earth's interior and subduction recycled material. Our new observations of these magmas will allow us to address three questions central to understanding carbon and nitrogen cycles on the Earth:1. How much carbon and nitrogen does Earth contain?A huge community effort over the last decade has improved our picture of carbon abundances in the shallow mantle. A key innovation has been the analysis of carbon abundances in magmas at small spatial scales. These 'microanalytical' methods allow pockets of magma trapped inside growing crystals, melt inclusions, to be analysed. Melt inclusions preserve information from earlier in a magma's life, before all carbon and nitrogen are lost to a gas phase, and before magma mixing destroys variability. From microanalytical studies we now have an unparalleled picture of carbon's abundance in the shallow mantle, but we know far less about its abundance in the deep mantle, whilst for nitrogen, the microanalytical revolution is still in its early stages. Consequently, the deep mantle may contain three or more times the carbon content of all shallower reservoirs combined, whereas for nitrogen the deep mantle may be an even greater reservoir; but we cannot yet be certain. Our preliminary work has shown that magmas from Iceland with deep mantle chemical signatures, contain melt inclusions with exceptional relative-enrichment in carbon. In this project we will use these carbon-enriched inclusions to identify if they have corresponding enrichments in nitrogen, enabling improved constraints on deep mantle volatile budgets. 2. Does subduction return atmospheric carbon and nitrogen to the mantle?Earth's climate is regulated by the removal of carbon dioxide from the atmosphere by rock weathering. The product of weathering is carbonate sediments, the utility of which as a carbon sink depends on their longevity. If carbonate sediments were subducted back into the mantle that would represent near-permanent carbon storage. However, whether atmospheric carbon can be returned to the mantle depends on how it behaves during subduction, with estimates ranging from all atmospheric carbon being subducted every thirty thousand years, to every three hundred million years. In this project we will investigate the recycling efficiency of carbon and nitrogen by reconstructing their abundances in magmas sampling subduction recycled material.3. Where in the solar system did Earth's carbon and nitrogen come from?Estimates of Earth's carbon and nitrogen inventory and carbon isotope composition allow its comparison to solar system sources of these volatiles. These comparisons suggest Earth is compositionally anomalous compared to most plausible solar system sources of its volatile elements. We will use our improved abundance estimates of these elements in the deep mantle and our carbon isotope measurements of deep mantle carbon to understand what solar system source provided the growing Earth with carbon and nitrogen.