Before 380 million years (Ma) ago, the continents had only patches of mosses and algae with no tree cover. The effect of the evolution of trees (large vascular plants with deep, extensive roots) changed the world for ever, according to Dr Robert Berner (Yale University). He presents his findings at Earth Systems Processes, a multidisciplinary meeting in Edinburgh, UK, hosted jointly by the Geological Society of London (GSL) and the Geological Society of America (GSA).
The first trees soaked up nutrients from rocks at a rate never before seen. This enhanced the weathering of calcium (Ca) and magnesium (Mg) silicate minerals, which in turn removed carbon dioxide from the atmosphere as Ca and Mg became locked together with carbonate ions in lime-rich sediments in the worlds oceans. The removal of CO2 from the atmosphere by this method and by increased photosynthesis (fixation) led to atmospheric CO2 stabilising at lower levels than the world had known for most of its previous 4200Ma history.
Trees also produced, for the first time, large amounts of lignin - the major constituent of wood. Resistant to decay, this substance led to increased amounts of organic carbon being buried with sediments, representing a large excess of global photosynthesis (carbon fixation) over global respiration (carbon oxidation). As atmospheric carbon dioxide was progressively reduced and photosynthesis reached unprecedented levels, the atmosphere became extremely rich in oxygen much richer than the air we know today.
Oxygen-enriched air was directly responsible for the evolution of very large insects - much larger than could live in todays relatively oxygen-poor air (insects rely upon more or less passive diffusion of O2 into their tissues, so large bodies become impossible at lower concentrations).
As well as the formation of huge coal deposits well known from Upper Carboniferous (Pennsylvanian) rocks worldwide, this radical shift in the Carbon Cycle also low
Contact: Ann Cairns
Geological Society of America