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

地球的全球气候是由无机碳循环在数百万年的时间尺度上调节的，因此大气中的二氧化碳浓度受火山供应和变质脱气的控制，并受硅酸盐岩石中碱性阳离子(例如钙和镁)的化学风化作用的影响。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，在海洋环境中，最终通过碳酸盐沉淀和深层沉积进行清除。该循环由全球硅酸盐风化速率的温度依赖性产生的负反馈回路稳定。植物演化中的两个主要轴线被广泛假设为通过促进硅酸盐矿物风化速率来增强大气中二氧化碳的长期清除：(1)从志留纪到泥盆纪(416-359Myr前)，深生根的维管植物在整个高地地区的进化、多样化和蔓延；(2)从白垩纪开始的更高级的被子植物取代裸子植物。然而，这些拟议的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史具有重要意义。我们的多学科项目制定了一个突破性的研究计划，通过实验研究解决这两种植物进化趋势的假设影响。一项关键的实验进展是我们独特的能力，可以在受控的实验室条件下进行定量的生物风化实验，同时量化根际反应矿物表面的光合作用通量。我们将利用新的实验技术，以完全重复的方式进行这些研究，以便能够量化风化作用下的元素通量。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树状植物的进化梯度，以及落叶和常绿的活化石裸子植物和白垩纪代表性的“早期被子植物”分类群。在受控环境中，植物将被培养在两种浓度的大气CO2中，以确定CO2施肥对风化速率的反馈。我们的研究将集中在玄武岩和花岗岩的风化。风化速率将通过几种互补的方法进行量化，并与无植物对照进行比较。主要标准是反应堆系统中选定的固体样品在纳米尺度上从单个岩石颗粒与未反应样品相比所测定的矿物体积损失。在整个多因素实验范围内，对风化溶质通量的更广泛测量将是反应堆排水和生物吸收的溶质的质量和通量平衡。该项目将利用与由谢菲尔德SAB领导的NERC资助的相关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，即通过开发生物风化过程的定量通用数学模型来严格整合实验结果。这些活动将通过将谢菲尔德大学的一名学生分配给拟议的项目来促进。这将致力于植物对矿物风化的影响的数学模拟，以及它们在长期碳循环的地球化学模型中的包含。总体而言，我们的项目将有助于对生物群在数百万年来调节地球系统中的作用的基本知识和理解。它解决了国家环境研究中心提出的关键问题，即如何将关键界面(例如，地圈-生物圈临界区)的生物地球化学循环与重点放在进化时间尺度上结合起来。

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.