PhD defence: Fredrik Sønderholm

Fredrik Sønderholm defends his thesis,

Constraints on Past Atmospheric Oxygen Levels from Fossil Land Plants
An overlooked coupling between land plants and atmospheric oxygen

Supervisor:
Professor MSO Christian J. Bjerrum, IGN

Assessment Committee:
Professor Claire Belcher, University of Exeter – UK
Professor Lee Kump, The Pennsylvania State University – USA
Associate Professor Arne Thorshøj Nielsen (chair), IGN

Summary:
This thesis explores the interaction between land plants and atmospheric oxygen on a geological scale. Atmospheric oxygen rose to near modern levels during the Paleozoic, but exactly when and how is still unresolved. It has been argued that land plants, which appeared and evolved simultaneously with this atmospheric oxygenation, played a central role. The appearance of land plants increased atmospheric oxygen levels through increased net primary productivity and organic carbon burial, and the effect of land plants has therefore been included in atmospheric oxygen models. However, these oxygen models currently do not take into account that insufficient oxygen levels, especially in the root system, results in impeded plant growth or death. This coupling between atmospheric oxygen and land plants, which has until now been overlooked in a geological context, is investigated in the present thesis through three studies including growth experiments and modelling.
Our growth experiments on liverworts, which resemble the earliest land plants, suggest that both growth and reproduction would have been severely impaired if the plants grew in an atmosphere low in oxygen. This challenges the idea that early and widespread colonization by land plants alone increased atmospheric oxygen pressures to near modern levels.
Constraints on atmospheric oxygen levels through geological time can be obtained from the minimum oxygen requirements of fossil plants. We model the minimum requirements of two different Devonian fossil root systems in three locations, using two different root models: One for aerated soils and one for wetlands. The results are used to constrain existing atmospheric oxygen models.
Based on the coupling between land plants and atmospheric oxygen, we propose a new positive feedback mechanism: Increasing atmospheric oxygen results in increased net primary produc-tivity through more effective root systems, which increases carbon burial and atmospheric oxygen levels futhermore. Combined with previously described negative feedback mechanisms, the new positive mechanism advocates for the existence of an oxygen optimum for plant growth.

A digital version of the PhD thesis can be obtained from the PhD secretary Anne Marie Faldt anmf@ign.ku.dk