The Devonian mass extinction: cataclysm or death by a thousand cuts?

复杂的多细胞生命在大约8亿年前首次出现在地球上，随后通过令人眼花缭乱的复杂的物种起源/灭绝模式而多样化。然而，这并不是一个稳定和不间断的过程。地球上的生物群至少有五次被一场灾难摧毁，这场灾难消除了相当大比例的总生物多样性--包括整个生物群(较高的类群)。这些所谓的“大灭绝”从根本上改变了地球上生命的本质，将进化带入了一个完全不同的轨道。在“五大”大灭绝中，最不为人所知的是大约3.7亿年前发生的泥盆纪大灭绝。关于这一事件的时间和性质都存在广泛的辩论，导致对其原因完全缺乏共识。这项提议试图从一个新的角度来研究泥盆纪的大灭绝，重点放在碳循环的变化上。地球目前有两个类似规模的碳循环：一个是基于光合作用的浮游生物的海洋碳循环，一个是基于光合作用的陆地植物的陆地碳循环。由于陆地植被性质的戏剧性变化，泥盆纪发生了碳循环的根本变化。在泥盆纪之初，陆地植物有厘米高，植根于非常浅的土壤，覆盖了大陆的有限区域。到泥盆纪末期，大陆的大片地区被几十米高的树木森林所覆盖，这些树木深深扎根于成熟的土壤中。这些重大的植被变化导致了陆地碳循环的深刻变化(由于化学风化和生物量埋藏造成的碳固定)。我们假设，正是陆地碳循环的戏剧性变化扰乱了地球系统，导致了泥盆纪的大灭绝。然而，我们认为这不是一个单一的灾难性事件(如造成白垩纪末期大灭绝的火球撞击)，而是随着离散的形态/解剖学创新导致植物大小和覆盖率的变化而顺序发生的，从而导致陆地碳循环的阶梯变化。研究项目将集中在西班牙北部的泥盆纪层序。它们是理想的，因为：(I)它们非常完整，包含了在弗拉斯尼亚-法门期和泥盆纪-石炭纪界线的已知灭绝事件；(Ii)在一个大的微大陆上孤立地积累，因此不受物种迁徙/迁出和栖息地追踪的影响；(Iii)包含海洋浮游生物(无尾类和几丁虫)和陆地植被(植物孢子/花粉)的良好化石记录。我们将通过物种起源/灭绝模式的研究来研究海洋浮游生物和陆地植被的进化动力学。这一生物多样性概况将与地球化学分析相结合，以确定地球碳循环中的扰动(除了养分循环、氧化还原条件和火山活动之外)。这些数据将被输入到我们使用反向建模技术生成的泥盆纪碳循环的地球系统模型中。该模型还将纳入主要植物群和新奇生物(例如第一森林)的出现数据。这些数据将揭示初级生产者中泥盆纪物种灭绝事件的性质和时间，并将其与碳循环的变化联系起来。我们的研究将阐明泥盆纪大灭绝的许多方面(自然和时间)，并将其与陆地植被的戏剧性进化带来的碳循环的巨大变化联系起来。这也将是对当今世界的一个警告，即人类通过砍伐森林、土壤侵蚀和其他有害活动而导致的地球碳循环变化的后果。

Complex multicellular life first appeared on Earth some 800 million years ago and subsequently diversified through a bewilderingly complex pattern of species originations/extinctions. However, this was not a steady and uninterrupted process. On at least five occasions the biota of the planet was devastated by a catastrophe that eliminated a considerable proportion of total biodiversity--including entire groups of organisms (higher taxa). These so-called 'mass extinctions' fundamentally changed the nature of life on Earth by steering evolution into a completely different trajectory.Of the 'Big Five' mass extinctions by far the least understood is the Devonian mass extinction that occurred ca.370 million years ago. There is widespread debate regarding both the timing and nature of this event, which has led to a complete lack of consensus regarding its causes. This proposal seeks to investigate the Devonian mass extinction from a fresh perspective focussing on changes in carbon-cycling.The Earth currently has two carbon-cycles of similar magnitude: a marine one based on photosynthetic plankton and a terrestrial one based on photosynthetic land plants. Fundamental changes in carbon-cycling took place during the Devonian due to dramatic changes in the nature of terrestrial vegetation. At the start of the Devonian land plants were centimetres tall, rooted in very shallow soils and covered a limited area of the continents. By the end of the Devonian vast swathes of the continents were shrouded in forests of trees tens of metres tall that deep-rooted into mature soils. These major vegetation changes caused profound changes in the terrestrial carbon-cycle (due to carbon sequestration from chemical weathering and biomass burial). We hypothesise that it was dramatic changes to the terrestrial carbon-cycle that disrupted the Earth system and caused the Devonian mass extinction. However, we believe that it was not a single catastrophic event (such as the bolide impact that caused the end Cretaceous mass extinction) but rather it occurred sequentially as discrete morphological/anatomical innovations led to changes in plant size and coverage causing step-changes in the terrestrial carbon-cycle.The research project will focus on the Devonian sequences of northern Spain. These are ideal because they: (i) are remarkably complete and incorporate known extinction events at the Frasnian-Famennian and Devonian-Carboniferous boundaries; (ii) accumulated in isolation on a large microcontinent and as such are not influence by species immigration/emigration and habitat tracking; (iii) contain an excellent fossil record of both marine plankton (acritarchs and chitinozoans) and terrestrial vegetation (plant spores/pollen).We will study the evolutionary dynamics of both the marine plankton and terrestrial vegetation through a study of species origination/extinction patterns. This biodiversity profile will be integrated with geochemical analyses that will identify perturbations in the Earth's carbon-cycle (in addition to nutrient cycling, redox conditions and volcanic activity). These data will be fed into an Earth Systems model for the Devonian carbon-cycle that we generate using inverse modelling techniques. The model will also incorporate data on the appearance of major plant groups and novelties (e.g. first forests).Together these data will shed light on the nature and timing of Devonian extinction events among primary producers and link them to changes in the carbon-cycle. Our research will clarify many aspects of the Devonian mass extinction (nature and timing) and link it to the monumental changes in carbon-cycling brought about by the dramatic evolution of terrestrial vegetation. This will also serve as a warning for the present day regarding consequences of human induced changes to the Earth's carbon-cycle bought about by deforestation, soil erosion and other detrimental activities.