Geobiological Impacts of PalaeozoicLand Plant Evolution

Sammanfattning: For two centuries, questions about the origin of terrestrial plants and their impacts on the Earth systemhave occupied palaeobotanists. This essay attempts to synthesise the state of research to date, and outlineareas where major questions remain. Fossil evidence for land plants first appears in rocks of MiddleOrdovician age (~ 470 Ma), but it was not until the Devonian that vascular plants became the dominantgeobiological agents on the continents. Plants began as small organisms, lacking any vascular tissues,and essentially confined to wetlands. Key developments in their reproductive biology and the evolutionof mycorrhizal symbiosis subsequently enabled early plants to exploit a broader range of environments,enhancing water uptake and absorption of nutrients. In turn, the evolution of plant roots has significantlyimpacted terrestrial landscapes. The Devonian rise of plants led to a modification of the weathering rateand a sharp increase in the rate of mudrock production. This was driven by the impacts that plants exerted on watercourses, with the creation of meandering rivers and deltas that retained fine siliciclasticmaterials. This increase in weathering rate, combined with the development of leaves and the intensification of photosynthesis also had consequences for the carbon cycle and atmosphere, reducing the levelof CO2 and increasing that of O2 in the atmosphere. The increased proportion of oxygen and creation ofcombustible material is also thought to have led to the planets first wildfires, whilst the decrease in CO2lowered global temperatures. Via a complex set of feedbacks, these modifications may even have drivena series of anoxic events in the oceans, generating one of the five major mass extinctions at the end ofthe Devonian.