EVIDENCE FOR THE GREAT MID-PALEOZOIC TRANSITION LINKED TO THE COLONIZATIONS OF LAND (Invited Presentation)

The Paleozoic emergence of land plants is theoretically predicted to have forever increased O₂ levels in the atmosphere and cooled the planet by drawing down CO₂. Vegetation affects the Earth system and the global biogeochemical cycles in two principle ways: enhanced organic carbon preservation by recalcitrant biopolymers (e.g. lignin) and changing the continental weathering regime that govern nutrient supply for new biological production in the oceans. Over 'shorter' (<1 My) time scales, enhanced weathering by plants might fertilize the oceans and intensify the oxygen minimum zones, but productive oceans would eventually become self-limiting as oxygen is released to the atmosphere and surface ocean. We have tested these ideas over both shorter and longer time scales and found supporting evidence that vegetation might indeed have profound effects on the Earth system. In this talk, I will review some of the geochemical evidence for increasing O₂ levels and declining CO₂ and highlight an apparent discrepancy between the timing of atmospheric O₂ rise and CO₂ decline, suggesting the Mid-Paleozoic transition took place in multiple steps. To resolve this, I suggest that the invasions of ecologically distinct flora had disproportionate effects on Earth's climate and oxygenation state and that the geological record may illuminate the global impact of early plants.