Tracking 'geo-respiration' of fossil rock carbon using trace metals and their isotopes

使用痕量金属及其同位素追踪化石岩石碳的“地球呼吸”

• 批准号: 2598394
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2598394
• 关键词: Tracking; geo; respiration; fossil; rock

The chemical weathering of rocks has profound impacts on Earth's biogeochemistry, influencing surface processes from nutrient cycling to the carbon cycle and climate. The canonical view of the weathering control on climate has been that the dissolution of silicate minerals by carbonic acid moves carbon from the atmosphere to the geosphere, providing a negative feedback for atmospheric CO2 and stabilisingclimate over geological time. However, recent work shows that oxidative weathering - particularly of shales - may release comparable volumes of carbon to the atmosphere as is consumed via silicate mineral dissolution, raising questions for the conventional weathering thermostat. Shale-derived CO2 release can occur through two key pathways i) oxidation of 'fossil' organic carbon contained within rocks, and ii) release of carbon from carbonate rocks via dissolution by sulfuric acid derived from sulfide oxidation. Together these processes facilitate 'georespiration' of rock carbon, transferring carbon tothe atmosphere, and potentially generating a positive weathering feedback for atmospheric CO2. The global CO2 fluxes from these processes and the factors affecting them remain poorly constrained, butare essential to quantify in order to assess the net carbon budget of weathering and its role in mediating Earth climate. Key questions include the relative roles of erosion and climate in driving fluxes, as well as how oxidative fluxes behave under contrasting geomorphic conditions, and how this impacts net weathering CO2 fluxes. Further work is needed in order to understand how oxidative weathering reactions and their climate impacts are controlled in the present day, how they have changed in the past, and how they might change in the future.This knowledge gap can be addressed through application of the Rhenium (Re) and Vanadium (V) trace metal and isotope systems as proxies for oxidative weathering reactions. Rhenium and Vanadium may be effective tools to track oxidative reactions due to their enrichment in petrogenic organic matter and sulfides - the materials undergoing oxidative weathering. Rhenium loss has been recorded in shalesoil profiles undergoing weathering, and has been connected to accompanying loss of petrogenic organic carbon. Stable isotope fractionation of Re (187Re and 185Re) has been predictedduring redox, and first observations of isotope fractionation between the solid and dissolved products of weathering have been made. Less is understood about the behaviour of V during weathering. Asa redox-sensitive element V is expected to interact with oxidative weathering processes, however there are only limited measurements from soils and rivers. The existing data suggest V is hosted insilicates and sulfides, but its cycling behaviour following release during weathering is not well known. Vanadium stable isotopes (51V and 50V) are of interest because the vanadate-vanadyl redox coupleoperates across the natural range of redox conditions at Earth's surface, and has showed some utility in tracking redox processes in magmatic systems. By further developing and combining Re and V and their isotopes, there is potential for a powerful tool to track the full suite of oxidative reactions and concomitant CO2 release occuring in the present day and through geological time.

岩石的化学风化对地球的地球化学有着深远的影响，影响着从养分循环到碳循环和气候的表面过程。风化作用对气候控制的经典观点是，碳酸溶解硅酸盐矿物，将碳从大气层转移到地圈，为大气中的CO2提供负反馈，并在地质时期稳定气候。然而，最近的研究表明，氧化风化-特别是页岩-可能会释放相当数量的碳到大气中，因为通过硅酸盐矿物溶解消耗，这对传统的风化恒温器提出了问题。页岩衍生的CO2释放可以通过两个关键途径发生：i）岩石中含有的“化石”有机碳的氧化，以及ii）通过硫化物氧化产生的硫酸溶解从碳酸盐岩中释放碳。这些过程共同促进了岩石碳的“地质呼吸”，将碳转移到大气中，并可能对大气CO2产生积极的风化反馈。来自这些过程的全球CO2通量和影响它们的因素仍然没有得到很好的限制，但为了评估风化的净碳收支及其在调节地球气候中的作用，量化是必不可少的。关键问题包括侵蚀和气候在驱动通量中的相对作用，以及在对比地貌条件下氧化通量的表现，以及这如何影响净风化CO2通量。需要进一步的工作，以了解如何氧化风化反应和它们的气候影响是控制在今天，他们如何在过去发生变化，以及他们如何可能在未来发生变化。这方面的知识差距可以通过应用铼（Re）和钒（V）微量金属和同位素系统作为氧化风化反应的代理。铼和钒可能是追踪氧化反应的有效工具，因为它们富集成岩有机物和硫化物-经历氧化风化的材料。铼的损失已记录在经历风化的页岩油剖面中，并与伴随的成岩有机碳的损失有关。在氧化还原过程中，Re（187 Re和185 Re）的稳定同位素分馏已被预测，并首次观察到风化的固体和溶解产物之间的同位素分馏。对风化过程中V的行为了解较少。阿萨氧化还原敏感元素V预计与氧化风化过程，但只有有限的测量土壤和河流。现有的数据表明，钒是托管在硅酸盐和硫化物，但其循环行为后，在风化过程中释放并不为人所知。钒酸盐稳定同位素（51 V和50 V）之所以令人感兴趣，是因为钒酸盐-钒基氧化还原偶在地球表面氧化还原条件的自然范围内起作用，并且在跟踪岩浆系统中的氧化还原过程中显示出一定的实用性。通过进一步开发和结合Re和V及其同位素，有可能成为一种强大的工具来跟踪当今和整个地质时期发生的全套氧化反应和伴随的CO2释放。