Minimum levels of atmospheric oxygen from fossil tree roots imply new plant-oxygen feedback.

The appearance and subsequent evolution of land plants is among the most important events in the earth system. Plant resulted in a change of earth surface albedo and the hydrological cycle, as well as increased rock weatherability thereby causing a persistent change in atmospheric CO2 and O2 . Land plants are, however, themselves dependent on O2 for respiration and long-term survival, something not considered in current geochemical models. In this perspective, we highlight two aspects of land plants' dependency on O2 relevant for the geobiological community: (a) fossil root systems can be used as a proxy for minimum levels of past atmospheric O2 consistent with a given fossil root depth; and (b) by identifying a positive feedback mechanism involving atmospheric O2 , root intensity, terrestrial primary production and organic carbon burial. As an example, we consider archaeopterid fossil root systems, resembling those of modern mature conifers. Our soil-plant model suggest that atmospheric O2 with 1 SD probably reached pressures of 18.2 ± 1.9 kPa and 16.8 ± 2.1 kPa by the Middle and Late Devonian, respectively, that is 86 ± 9% and 79 ± 10% of the present-day 21.2 kPa.