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 CO(2) and O(2). Land plants are, however, themselves dependent on O(2) 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 O(2) relevant for the geobiological community: (a) fossil root systems can be used as a proxy for minimum levels of past atmospheric O(2) consistent with a given fossil root depth; and (b) by identifying a positive feedback mechanism involving atmospheric O(2), 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 O(2) 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.