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

岩石的化学风化对地球的生物地球化学有深远的影响，影响着从营养循环到碳循环和气候的地表过程。气候风化控制的经典观点是，碳酸对硅酸盐矿物的溶解将碳从大气转移到地圈，为大气中的二氧化碳提供负反馈，并在地质时间内稳定气候。然而，最近的研究表明，氧化风化--尤其是页岩--可能会释放出与硅酸盐矿物溶解消耗相当的碳到大气中，这对传统的风化恒温器提出了质疑。页岩释放二氧化碳的途径有两条：一是岩石中含有的“化石”有机碳的氧化，二是硫化物氧化生成的硫酸溶解碳酸盐释放碳。这些过程共同促进了岩石碳的“地理呼吸”，将碳转移到大气中，并可能对大气中的二氧化碳产生积极的风化反馈。来自这些过程的全球二氧化碳通量及其影响因素仍然没有得到很好的约束，但必须量化，以评估风化作用的净碳收支及其在调节地球气候中的作用。关键问题包括侵蚀和气候在驱动通量方面的相对作用，以及氧化通量在不同地貌条件下的表现，以及这如何影响净风化二氧化碳通量。为了了解氧化风化反应及其对气候的影响在当今是如何控制的，它们在过去是如何变化的，以及它们在未来可能会如何变化，还需要进一步的工作。这一认识差距可以通过应用Re(Re)和V(V)痕量金属和同位素体系作为氧化风化反应的替代物来解决。Re和V可能是跟踪氧化反应的有效工具，因为它们富含成岩有机物和硫化物--这些材料经历了氧化风化。Re的损失已被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。预测了Re(187Re和185Re)在氧化还原过程中的稳定同位素分馏，并首次观测到了固体和溶解风化产物之间的同位素分馏。关于V在风化过程中的行为，人们知之甚少。作为氧化还原敏感的元素V，预计会与氧化风化过程相互作用，然而，只有有限的土壤和河流测量结果。现有的数据表明，V是硅酸盐和硫化物的宿主，但它在风化过程中释放后的循环行为并不是很清楚。钒稳定同位素(51V和50V)之所以令人感兴趣，是因为钒-钒氧化还原对在地球表面氧化还原条件的自然范围内工作，并在追踪岩浆系统中的氧化还原过程方面显示出一些有用的作用。通过进一步开发和组合Re和V及其同位素，有可能成为一种强大的工具来跟踪发生在当今和整个地质时代的全套氧化反应和伴随的二氧化碳释放。

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.