Multiple Equilibria in Atmospheric Oxygen: Archean, Proterozoic, Phanerozoic. Tom Laakso & Dan Schrag Goldschmidt Geochemistry June 13, 2014

Multiple Equilibria in pO2 Kump 2008

Multiple Equilibria model: Proterozoic / Phanerozoic 3-box ocean/atmosphere model oxygen, carbon, sulfur, iron cycles first order kinetics oxygen-sensitive organic carbon burial efficiency oxygen-sensitive recycling of sedimentary P oxygen-sensitive riverine P flux

Multiple Equilibria model: Proterozoic / Phanerozoic Laakso & Schrag 2014

The Great Oxidation Event Kump 2008

Archean redox budget: prebiotic world hydrogen outgassing serpentinization Archean redox budget: prebiotic world

Archean redox budget: prebiotic world mantle hydrogen source Archean redox budget: prebiotic world

Archean redox budget: prebiotic world hydrogen escape mantle hydrogen source Archean redox budget: prebiotic world

Archean redox budget: early life hydrogen escape mantle hydrogen source chemoautotrophy: 2 H2 + CO2 H2O + CH2O Archean redox budget: early life

Archean redox budget: early life hydrogen escape mantle hydrogen source chemoautotrophy: 2 H2 + CO2 H2O + CH2O organic burial (< nutrient input) Archean redox budget: early life

Archean redox budget: oxygenic photosynthesis hydrogen escape mantle hydrogen source oxygenic photosynthesis CH2O + O2 CO2 + H2O chemoautotrophy: 2 H2 + CO2 H2O + CH2O organic burial (< nutrient input) Archean redox budget: oxygenic photosynthesis

Archean redox budget: oxygenic photosynthesis hydrogen escape oxidation: atmosphere mantle hydrogen source oxygenic photosynthesis CH2O + O2 CO2 + H2O chemoautotrophy: 2 H2 + CO2 H2O + CH2O oxidation: seafloor, aqueous, continental organic burial (< nutrient input) Archean redox budget: oxygenic photosynthesis

Archean equilibrium redox budget Hydrogen: Oxygen: mantle H2 source = nutrient source = hydrogen escape + aqueous & crustal oxidation + atmospheric chemistry atmospheric chemistry

Is this model consistent with the Proterozoic? Hydrogen: Oxygen: mantle H2 source = nutrient source = hydrogen escape + aqueous & crustal oxidation + atmospheric chemistry atmospheric chemistry Across the GOE:

Is this model consistent with the Proterozoic? Hydrogen: Oxygen: mantle H2 source = nutrient source = hydrogen escape + aqueous & crustal oxidation + atmospheric chemistry atmospheric chemistry Across the GOE: 1. crustal oxidation rises => atmospheric sink must decrease

Is this model consistent with the Proterozoic? Hydrogen: Oxygen: mantle H2 source = nutrient source = hydrogen escape + aqueous & crustal oxidation + atmospheric chemistry atmospheric chemistry Across the GOE: 1. crustal oxidation rises => atmospheric sink must decrease 2. Slower atmos. chemistry, increasing O2 => decreasing H2

Is this model consistent with the Proterozoic? Hydrogen: Oxygen: mantle H2 source = nutrient source = hydrogen escape + aqueous & crustal oxidation + atmospheric chemistry atmospheric chemistry Across the GOE: 1. crustal oxidation rises => atmospheric sink must decrease 2. Slower atmos. chemistry, increasing O2 => decreasing H2 3. Decreasing H2 => decreased H2 escape

Is this model consistent with the Proterozoic? Hydrogen: Oxygen: mantle H2 source = nutrient source = hydrogen escape + aqueous & crustal oxidation + atmospheric chemistry atmospheric chemistry Across the GOE: 1. crustal oxidation rises => atmospheric sink must decrease 2. Slower atmos. chemistry, increasing O2 => decreasing H2 3. Decreasing H2 => decreased H2 escape 4. Hydrogen budget cannot be balanced in the Proterozoic!

Is this model consistent with the Proterozoic? Hydrogen: Oxygen: mantle H2 source = nutrient source = hydrogen escape + aqueous & crustal oxidation + atmospheric chemistry atmospheric chemistry Across the GOE: 1. crustal oxidation rises => atmospheric sink must decrease 2. Slower atmos. chemistry, increasing O2 => decreasing H2 3. Decreasing H2 => decreased H2 escape 4. Hydrogen budget cannot be balanced in the Proterozoic! 5. …unless hydrogen escape increases, despite falling H2

Oxygen-sensitive hydrogen escape H2 escape depends on the temperature of the thermosphere. Temperature depends on O2 absorption of UV radiation.

Oxygen-sensitive hydrogen escape H2 escape depends on the temperature of the thermosphere. Temperature depends on O2 absorption of UV radiation. Hydrodynamic model with Jeans boundaries Temperature boundary related to O2 through thermosphere energy balance model (Bougher & Roble 1991)

Oxygen-sensitive hydrogen escape

Hydrogen escape and the Great Oxidation Archean: high H2, low O2 Oxygen controlled by reaction with free H2 Hydrogen controlled by oxidation and escape

Hydrogen escape and the Great Oxidation Archean: high H2, low O2 Oxygen controlled by reaction with free H2 Hydrogen controlled by oxidation and escape Great Oxidation: large perturbation in pO2 Thermosphere warms, increasing H2 escape pH2 falls, slowing reaction with O2 Oxygen remains elevated

Hydrogen escape and the Great Oxidation Archean: high H2, low O2 Oxygen controlled by reaction with free H2 Hydrogen controlled by oxidation and escape Great Oxidation: large perturbation in pO2 Thermosphere warms, increasing H2 escape pH2 falls, slowing reaction with O2 Oxygen remains elevated 3. Proterozoic: higher O2, lower H2 Oxygen controlled by weathering and respiration Hydrogen controlled by escape from a hot thermosphere

Archean / Proterozoic biogeochemical model Escape: Hydrodynamic model with Jeans boundaries Temperature boundary related to O2 through thermosphere energy balance model Atmospheric chemistry: Photochemical model of Pavlov et al. (2001) Solid phase oxidation: Pyrite: linear in pO2 up to 10-4 PAL Organic carbon: linear in pO2 Inputs: P: 20% modern H2: 1010 cm2 s-1

Archean / Proterozoic biogeochemical model

Archean / Proterozoic biogeochemical model

Glaciation and oxidation Hoffman & Schrag 2002

Glaciation and oxidation

Glaciation and oxidation

Glaciation and oxidation

Glaciation and oxidation

Glaciation and oxidation

Glaciation and oxidation

Multiple equilibria: a history of pO2

Multiple equilibria: a history of pO2

Multiple equilibria: a history of pO2

Summary and Conclusions The hydrogen escape rate are not diffusion limited for less-than-modern levels of pO2, but are in the Jeans regime. The escape rate varies strongly with pO2. In our simple model, the Archean atmosphere is stabilized at low oxygen levels by the reaction kinetics between O2 and H2 in the atmosphere. Escape from the cold thermosphere is a secondary term in the H2 budget. Hydrogen levels are suppressed at high pO2 by efficient escape from a hot thermosphere. This allows for a second equilibrium at high pO2: the weathering-dominated regime of the Proterozoic and Phanerozoic. Transient slowing of atmospheric reactions during a >250,000 year glaciation pumps enough oxygen into the atmosphere to flip the atmosphere between its low- and high-oxygen states.