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

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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.