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

岩石的化学风化作用对地球生物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等地表过程。风化作用控制气候的典型观点是，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了负反馈，并在地质时期稳定了气候。然而，最近的研究表明，氧化风化作用——尤其是页岩的氧化风化作用——可能会向大气中释放出与硅酸盐矿物溶解消耗相当数量的碳，这对传统的风化恒温器提出了质疑。页岩衍生的二氧化碳释放可以通过两个关键途径发生：1)岩石中含有的“化石”有机碳氧化；2)由硫化物氧化产生的硫酸溶解碳酸盐岩中的碳释放。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，并可能对大气中的二氧化碳产生积极的风化反馈。这些过程产生的全球二氧化碳通量和影响它们的因素仍然知之甚少，但为了评估风化的净碳收支及其在调解地球气候方面的作用，对它们进行量化是必不可少的。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CO2通量。为了了解氧化风化反应及其气候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如何变化，还需要进一步的工作。这一知识缺口可以通过应用铼（Re）和钒(V)微量金属和同位素系统作为氧化风化反应的代用物来解决。铼和钒在产岩有机质和硫化物（氧化风化物质）中富集，可能成为跟踪氧化反应的有效工具。风化作用下的页岩油剖面记录了铼的损失，并与相应的成岩有机碳的损失有关。对Re （187Re和185Re）在氧化还原过程中的稳定同位素分馏进行了预测，并首次观测了固体风化产物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。氧化还原敏感元素V预计与氧化风化过程相互作用，但只有有限的测量从土壤和河流。现有的数据表明，V以硅酸盐和硫化物为主，但其在风化过程中释放后的循环行为尚不清楚。钒的稳定同位素（51V和50V）引起了人们的兴趣，因为钒酸盐-钒基氧化还原偶对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些效用。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，用于追踪当今和整个地质时代发生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.