Charcoal, Forests, and Earth's Palaeozoic Geochemical Oxygen Cycle
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i UNIVERSITY OF SOUTHAMPTON FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES Ocean and Earth Science CHARCOAL, FORESTS, AND EARTH’S PALAEOZOIC GEOCHEMICAL OXYGEN CYCLE by David Kevin Carpenter Thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy September 2016 ii iii UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES Ocean and Earth Sciences Thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy CHARCOAL, FORESTS AND EARTH’S PALAEOZOIC GEOCHEMICAL OXYGEN CYCLE By David Kevin Carpenter It is widely assumed that the Devonian transition to a forested planet, and subsequent massive expansion of coal-swamp environments during the Carboniferous, significantly increased the production and retention of atmospheric oxygen by fuelling increased organic carbon burial, fundamentally altering the biotic regulation of Earth's long-term oxygen cycle. Modelling approaches to the reconstruction of Phanerozoic pO2 are hampered by the unavoidable complexity of the models, and the difficulty in testing their inherent assumptions. This has led to a wide variety of predictions for atmospheric O2 during this critical 120 million-year interval. The abundance of microscopic charcoal (‘inertinite’) in coals has previously been used as the basis for a direct pO2 reconstruction, on the assumption that fire activity correlates with oxygen supply; however, coals are scarce prior to the Viséan. A high-resolution Devonian–Carboniferous dataset charting inertinite abundance in dispersed organic matter indicates that O2 could not have fallen below c. 16.5% vol. at any stage, and that a previously identified Mid-Devonian ‘charcoal gap’ is most likely an artefact of low sampling density. Thus, models predicting intervals of profound hypoxia cannot be correct, and the hypothesis that the Tournaisian tetrapod diversity crisis (‘Romer’s Gap’) was the result of global hypoxia is not supported. Both coal and DOM charcoal records indicate gently declining fire activity during the Mississippian, followed by a sharp rise in the Mid-Pennsylvanian; this is consistent with the GEOCARBSULF palaeoatmosphere model, and suggests a substantial lag between afforestation and pO2 increase. This is turn suggests it was not the rise of forests per se but the formation of very large wetland basins, which was tectonically driven, that led to enhanced coal formation and a late Palaeozoic oxygen pulse. An interesting corollary of this interpretation is that high pO2 was not the main driver of arthropod gigantism during the Palaeozoic. iv v Table of Contents List of tables...................................................................................................................................... vii List of figures ..................................................................................................................................... ix List of supplementary information ................................................................................................ xvii DECLARATION OF AUTHORSHIP ..................................................................................................... xix Acknowledgements ......................................................................................................................... xxi List of definitions and abbreviations ............................................................................................ xxiii 1. Introduction .................................................................................................................................... 1 1.1: The evolution of atmospheric O2 through geological time ...................................................... 3 1.2: Modelling Phanerozoic O2 ........................................................................................................ 7 1.3: Atmospheric O2 and wildfire activity ..................................................................................... 10 1.4: Is all inertinite charcoal? ........................................................................................................ 18 1.5: Testing the models: establishing a new Silurian–Carboniferous charcoal dataset ............... 28 2. Identifying and quantifying charcoal in the fossil record: materials, methods, and rationale .................................................................................................................................... 30 2.1: Sample preparation and analysis ........................................................................................... 30 2.2: Data analysis .......................................................................................................................... 34 2.3: Relationship between fusinite and total inertinite ................................................................ 45 2.4: Reflectance, fire intensity, and pO2 ....................................................................................... 48 2.5: Maceral oxidation .................................................................................................................. 49 2.6: Taphonomic considerations ................................................................................................... 54 2.7: Summary ................................................................................................................................ 59 3. The Devonian charcoal gap .......................................................................................................... 61 3.1: The Catskill Delta complex in New York and Pennsylvania .................................................... 63 3.2: Sampling localities – Devonian, USA ...................................................................................... 67 3.3: Sampling localities – Silurian, USA ....................................................................................... 109 3.4: The Catskill Delta complex in eastern Canada ..................................................................... 112 3.5: The Devonian in Gondwana: the Malvinokaffric realm ....................................................... 116 3.6: Results .................................................................................................................................. 123 3.7: Discussion ............................................................................................................................. 131 3.8: Summary .............................................................................................................................. 134 4. Atmospheric O2 during Romer’s Gap ......................................................................................... 135 4.1: Sampling localities – Northumberland, Fife, and the Scottish Borders ............................... 139 4.2: Sampling localities – Central East Greenland ...................................................................... 147 vi 4.3: Sampling localities – Spitsbergen ........................................................................................ 150 4.4: Results .................................................................................................................................. 154 4.5: Discussion ............................................................................................................................ 158 4.6: Summary .............................................................................................................................. 160 4.7: Tetrapod World: early evolution and diversification .......................................................... 161 5. Late Palaezoic hyperoxia ........................................................................................................... 176 5.1: Sampling localities ............................................................................................................... 179 5.2: Results .................................................................................................................................. 187 5.3: Discussion ............................................................................................................................ 192 5.4: Summary .............................................................................................................................. 196 6. Conclusions ................................................................................................................................ 197 7. Appendix 1: high-vitrinite samples ........................................................................................... 203 7.1: Tynemouth (DOM) ............................................................................................................... 203 7.2: Tynemouth (coals) ............................................................................................................... 205 7.3: Willie’s Hole (DOM) ............................................................................................................. 207 7.4: Fife (DOM) ........................................................................................................................... 209 7.5: Fife (coals) ............................................................................................................................ 211 7.6: Fife (carbonaceous shales) .................................................................................................. 213 8. Appendix 2 – keys for