Unlocking the C and N budget of the Earth

地球的碳氮循环在很短的时间内涉及所有生命、海洋和大气，最终植根于地球深部地幔。碳和氮从地幔释放出来是通过火山作用发生的，它们通过俯冲返回地幔。在我们星球的历史上，这些循环建立了一个富含氮的大气层，并维持了碳循环。尽管我们在地球表面有丰富的碳和氮，但地幔中可能蕴藏着更多的碳和氮。因此，了解地球表面环境的宜居性取决于了解地球深处的挥发物。在这个建议中，我们将研究冰岛和加那利群岛这两个海洋岛屿上的岩浆。众所周知，这些岛屿下面的地幔既代表了地球内部古老的深层部分，也代表了俯冲回收材料。我们对这些岩浆的新观察将使我们能够解决理解地球碳和氮循环的三个核心问题：1.地球含有多少碳和氮？过去十年来，社区的巨大努力改善了我们对地幔浅层碳丰度的认识。一项关键的创新是在小空间尺度上分析岩浆中的碳丰度。这些“微量分析”方法可以对生长中的晶体、熔融包裹体中捕获的岩浆进行分析。熔融包裹体保存了岩浆生命早期的信息，在所有的碳和氮都损失到气相之前，在岩浆混合破坏可变性之前。从微观分析研究中，我们现在对浅地幔中碳的丰度有了一幅无与伦比的图景，但我们对其在地幔深处的丰度知之甚少，而对于氮，微观分析革命仍处于早期阶段。因此，深部地幔的含碳量可能是所有浅层储集层碳含量总和的三倍或更多，而对于氮，深部地幔的含碳量可能更大；但我们还不能确定。我们的初步工作表明，来自冰岛的岩浆具有深部地幔化学特征，包含具有异常相对富碳的熔体包裹体。在这个项目中，我们将使用这些富碳包裹体来确定它们是否具有相应的氮富集，从而改善对地幔深处不稳定预算的限制。2.俯冲作用会将大气中的碳和氮返回地幔吗？地球的气候是由岩石风化作用从大气中去除二氧化碳来调节的。风化的产物是碳酸盐沉积物，其作为碳汇的效用取决于它们的寿命。如果碳酸盐沉淀物被俯冲回地幔，那将是近乎永久的碳储存。然而，大气中的碳能否回到地幔取决于它在俯冲过程中的表现，估计从每三万年一次到每三亿年一次。在本项目中，我们将通过重建岩浆样品中碳和氮的丰度来研究碳和氮的回收效率。地球的碳和氮来自太阳系的哪里？对地球碳和氮库存和碳同位素组成的估计使其可以与太阳系这些挥发物的来源进行比较。这些比较表明，与大多数可能的太阳系挥发性元素来源相比，地球在组成上是反常的。我们将使用我们对深地幔中这些元素的改进丰度估计和对深地幔碳的碳同位素测量来了解太阳系来源为不断增长的地球提供了碳和氮。

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.