Biotic regulation of the inorganic carbon cycle: Quantifying the impact of plant evolution and CO2 on mineral weathering

地球的全球气候在数百万年的时间尺度上受到无机碳循环的调节，其中大气中的二氧化碳浓度由火山和变质脱气提供的二氧化碳以及硅酸盐岩石中碱性阳离子（如Ca和Mg）的化学风化作用所去除的二氧化碳控制。这种离子通量对于陆地向海洋环境输入碱度和溶解的无机碳至关重要，而海洋环境最终通过碳酸盐沉淀和深层沉积进行去除。这个循环是由一个负反馈回路稳定的，这个负反馈回路是由全球硅酸盐风化速率的温度依赖性所产生的。植物进化的两个主要轴被广泛假设为通过促进硅酸盐矿物的风化速率而增强了对大气中二氧化碳的长期清除：(1)从志留纪到泥盆纪（416-359万年前）在整个高地地区的深根维管陆地植物的进化、多样化和传播；(2)从白垩纪开始，裸子植物被更先进的被子植物取代。然而，这些陆地植物和地圈之间相互作用的定量性质仍然是一个完全被忽视的实验研究领域，尽管它对理解地球动态地球化学历史至关重要。我们的多学科项目设定了一个开创性的研究计划，通过实验调查来解决这两种植物进化趋势的假设影响。一项重要的实验进展是我们在受控实验室条件下进行定量生物风化实验的独特能力，同时量化根际中反应矿物表面的光合通量。我们将利用新的实验技术以完全复制的方式进行这些研究，从而可以量化风化过程中的元素通量。我们先进的实验方法将应用于“活化石”植物分类群，以代表从苔藓植物到小根植物和早期乔木植物的进化梯度，以及落叶和常绿活化石裸子植物和具有代表性的白垩纪“早期被子植物”分类群。植物将在两种浓度的大气二氧化碳的受控环境中种植，以确定二氧化碳施肥对风化率的反馈。我们的调查将集中在玄武岩和花岗岩的风化上。风化率将通过几种互补方法量化，并与无植物对照进行比较。对于反应器系统中选定的固体样品，主要标准是在纳米尺度上测定的单个岩石颗粒与未反应样品的矿物体积损失。在整个多因子实验范围内，对风化溶质通量的更广泛测量调查将是反应器排水和生物吸收的溶质的质量和通量平衡。该项目将利用由谢菲尔德的SAB领导的相关nerc资助的风化财团的协同效应，但重点是完全不同的问题。这两个项目都有一个共同的理念，即通过发展生物风化过程的定量广义数学模型来严格整合实验结果。这些活动将通过分配谢菲尔德大学的一名学生来促进拟议的项目。这将致力于植物对矿物风化影响的数学建模，并将其纳入长期碳循环的地球化学模型。总的来说，我们的项目将有助于了解数百万年来生物群在调节地球系统中的作用。它解决了NERC提出的关键问题，即如何在关键界面（例如，地圈-生物圈关键带）整合生物地球化学循环，重点关注进化时间尺度。

Earth's global climate is regulated on a multi-million year timescale by the inorganic carbon cycle, whereby the atmospheric CO2 concentration is controlled by its supply from volcanoes and metamorphic degassing, and removal by the chemical weathering of base cations (e.g, Ca and Mg) from silicate rocks. This ion flux is crucial to the terrestrial input of alkalinity and dissolved inorganic carbon to the marine environment where removal ultimately occurs by carbonate precipitation and deep sedimentation. The cycle is stabilized by a negative feedback loop created by the temperature-dependence of the global rate of silicate weathering. Two major axes in plant evolution are widely hypothesized to have enhanced the long-term removal of CO2 from the atmosphere by promoting silicate mineral weathering rates: (1) the evolution, diversification and spread of deep-rooting vascular land plants throughout upland areas from the Silurian to the Devonian (416-359 Myr ago) and (2) the replacement of gymnosperms by the more advanced angiosperms from the Cretaceous onward. However, the quantitative nature of these proposed interactions between land plants and the geosphere remains a totally neglected experimental research field, in spite of its central importance to understanding Earth's dynamic geochemical history. Our multidisciplinary project sets out a groundbreaking programme of research for addressing the hypothesized influences of these two plant evolutionary trends with experimental investigations. A crucial experimental advance is our unique capability to carry out quantitative biological weathering experiments under controlled laboratory conditions while quantifying photosynthate flux to reacting mineral surfaces in the rhizosphere. We will utilize novel experimental techniques to undertake these investigations in a fully replicated manner that allow quantification of element fluxes from weathering. Our advanced experimental approach will be applied to 'living fossil' plant taxa selected to represent an evolutionary gradient from bryophytes through to small rooted plants and early arborescent forms, and deciduous and evergreen living fossil gymnosperms and representative 'early angiosperm' Cretaceous taxa. Plants will be cultivated at two concentrations of atmospheric CO2 in controlled environments to determine the feedback of CO2-fertilization on weathering rates. Our investigations will focus on the weathering of basalt and granite. Weathering rates will be quantified by several complementary methods and compared with plant-free controls. The primary standard is the volumetric loss of mineral determined at nanometric scale from individual rock grains compared with unreacted samples, for selected solid samples in the reactor systems. The wider measurement survey of weathering solute fluxes across the entire range of multi-factorial experiments will be mass and flux balance of solutes from reactor drainage and taken up biologically. The project will exploit synergies with the related NERC-funded weathering consortium led by SAB in Sheffield, but with an emphasis on quite separate questions. Both projects share a philosophy of rigorous integration of the experimental results through development of quantitative generalized mathematical models of biotic weathering processes. These activities will be promoted by the allocation of one of their University of Sheffield studentships to the proposed project. This will be devoted to the mathematical modelling of plant impacts on mineral weathering and their inclusion in geochemical models of the long-term carbon cycle. Overall, our project will contribute fundamental knowledge and understanding on the role of biota in regulating the Earth system over millions of years. It addresses key questions posed by NERC on how to integrate biogeochemical cycles at critical interfaces (e.g., geosphere-biosphere critical zone) with a focus on evolutionary timescales.