Evolution of the Atmosphere, Oceans, Continents

1. Evolution of the Atmosphere, Oceans, Continents

2. Evolution of Atmosphere, Ocean, & Life • We will address the following topics.... • Evolution of Earth’s atmosphere, continents, and oceans • Early Earth had small continents, no ocean and a thin, inhospitable primordial atmosphere. How did the modern atmosphere and ocean come about, and what role did life play? • What was the Timing of Life and what was its impact on the composition of the Earth?

3. State of Early Earth Very Early Earth…a vision of Hell? • Hot: from primordial heat, impacts, decay of radioactive elements • Violent: frequent impacts • Unstable: constant volcanism; thin, unstable basaltic crust • Inhospitable: scalding atmosphere devoid of oxygen

4. MECHANISMS FOR CREATING FELSIC CONTINENTAL CRUST Fig. 13.05 a, b W. W. Norton

5. How did we date the age of Continents When did continents form? • Ratio of Nb/U tracks the creation of continental crust • U is preferentially extracted during the creation of continental crust • Causes the mantle Nb/U ratio to increase • Today the ratio is 47 • Examining past Nb/U ratio of mid-ocean ridge basalts provides evidence of continental crust production Hoffman et al. (1997)

7. AGE OF CONTINENTAL CRUST -- CRATONS Fig. 13.08 W. W. Norton

8. The Growth of the Continents By investigating the Nb/U ratio, geologists have found: • 4.5-4.0 Ga: Slow production of continental crust • 4.0-2.5 Ga: Rapid growth of continents, 70% of continental volume was achieved by 3.0 Ga • 2.5-0 Ga: Slow production of continental crust

9. Primordial atmosphere: Earth’s first early atmosphere • Primordial atmosphere • Composed of H2 and He gas from protoplanetary disk • Gravity on the terrestrial planets was too low to retain these light gases • Also driven off by planetary heat, solar wind, and violent impacts SO WHERE DID THE ATMOSPHERE COME FROM?

10. HOW DO YOU GET GAS??

11. IF YOU’VE GOT GAS, HOW DO YOU GET RID OF IT?

12. State of Early Earth Secondary atmosphere: a new atmosphere formed early in Earth’s history • Secondary atmosphere • Initially composed of CO2, N2O, and H2O and ?CH4? • Impact degassing: vaporization of planetesimals during period of heavy bombardment would have contributed CO2, H2O, NH3 • Volcanic outgassing: output of gases by volcanic eruptions (H2O, CO2, N2, HCl, other volatiles) Gas compositions from 3 volcanoes

13. Composition of Early Atmosphere Modern • Secondary atmosphere composition • No O2: No photosynthetic organisms to produce free O2 • Some N2: Inert gas, so all N2 from volcanic and impact degassing would have remained in atmosphere • Lots of CO2: Chemical weathering rates would have been lower because continents would have been smaller– 30,000x present value! • Lots of H2O: Due to vaporization of oceans • Global warming! • - Due to high CO2, surface temperatures may have been 80-90°C O2 N2 ~4.56 Ga H2O N2 CO2

14. How long have we had Oceans? Oceans: formed soon after Earth’s temperature fell to levels where liquid water was stable • Oceans may have condensed and then been vaporized many times as impacts bombarded early Earth • Size of impactor matters • - Diameter of ~100km will vaporize photic zone (upper 100m) • - Diameter >440 km will vaporize entire ocean • - Last ocean-vaporizing event probably occurred at 4.1-4.3 Ga

15. Elemental Composition of the Ocean

16. Rise of Oxygen Rise of oxygen: essential to the rise of multicellular eukaryotic organisms- organisms whose cells have nuclei • Rise of oxygen • - Requires O2 source > O2 sink • Earth Earth had a reducing atmosphere • Reduced gases from volcanic eruptions (H2 and CO) reacted with free oxygen (O2) to form H2O and CO2 • Result: early atmosphere had low oxygen concentrations • Sink of oxygen

17. Redox Conditions Redox conditions: whether environment is conducive to oxidation or reduction • Oxidation: loss of electrons by a molecule or atom • Reduction: gain of electrons by a molecule or atom • E.g., Fe2+ Fe3+ = oxidation • Oxygen is a Great “oxidizing” agent

18. Rise of Oxygen Prebiotic atmosphere: oxygen levels were very low • Source of O2 • Photochemical reactions: chemical reactions induced by light • Photolysis of CO2 and H2O leads to production of H and O2 • H escapes to space • In reducing atmosphere, O2 source < O2 sink, no accumulation of atmospheric O2 Photolysis But what about me??

19. Role of Early Life and Atmosphere Evolution Earliest know life is ~3.8 billion years old • Source of O2? • Evidence of early life • Microfossils: preserved remains of single-celled prokaryotic organisms (3.5 Ga) Microfossils from 3.5 Ga Warrawoona Formation in Australia

20. Early Life Earliest know life is ~3.8 billion years old Modern stromatolites, Australia • Source of O2? • Evidence of early life • Microfossils: preserved remains of single-celled organisms (3.5 Ga) • Stromatolites: layered structures formed by trapping, binding, and cementation of sediments by cyanobacteria (3.2 Ga). Blue-green algae • Organic carbon in ancient sedimentary rocks (3.8 Ga) Ancient stromatolites

21. Rise of Oxygen Great Oxidation Event: rise in atmospheric oxygen levels between 2 and 2.2 Ga • Cyanobacteria (prokaryotes) develop ability to photosynthesize • Appeared 1 billion years before rise of oxygen • Increase O2 source • Oxidation of mantle • - Decrease O2 sink • Switch from mainly submarine to subaerial volcanoes • Due to development of thick continental crust • Decrease O2 sink Great Oxidation Event

22. Rise of Oxygen Great Oxidation Event: rise in atmospheric oxygen levels between 2 and 2.2 Ga • Cyanobacteria (prokaryotes) develop ability to photosynthesize • Appeared 1 billion years before rise of oxygen • Increase O2 source • Oxidation of mantle • - Decrease O2 sink • Switch from mainly submarine to subaerial volcanoes • Due to development of thick continental crust • Decrease O2 sink Cyanobacteria- first organisms to produce O2 by photosynthesis Photosynthesis: CO2 + H2O --> CH2O + O2 Preferentially uses 12C CO2 + H2O --> 12CH2O + O2 Results in an shift in 13C/12C preserved in limestones

23. The Oxygen Cycle (Bio)geochemical cycle: pathway through which a molecule moves through compartments of the natural world (including biotic and abiotic) • Geochemical cycles • Reservoir: compartment where chemical species resides • Flux: rate of transfer of chemical species between reservoirs • Source: origin of chemical species in reservoir • Sink: destruction of chemical species in reservoir • Carbon cycle, water cycle, oxygen cycle, nitrogen cycle, phosphorus cycle

24. Rise of Oxygen Great Oxidation Event: rise in atmospheric oxygen levels between 2 and 2.2 Ga • Cyanobacteria (prokaryotes) develop ability to photosynthesize • Appeared 1 billion years before rise of oxygen • Increase O2 source • Oxidation of mantle • - Decrease O2 sink • Switch from mainly submarine to subaerial volcanoes • Due to development of thick continental crust • Decrease O2 sink Oxidation of mantle changed composition of volcanic outgassing-- to less reducing

25. Archaean Rise of Oxygen Great Oxidation Event: rise in atmospheric oxygen levels between 2 and 2.2 Ga • Cyanobacteria (prokaryotes) develop ability to photosynthesize • Appeared 1 billion years before rise of oxygen • Increase O2 source • Oxidation of mantle • - Decrease O2 sink • Switch from mainly submarine to subaerial volcanoes • Due to development of thick continental crust • Decrease O2 sink Switch from mainly submarine to combination of submarine/subaerial volcanoes

26. BANDED IRON FORMATION -- BIFsCOMPOSED OF REDUCED IRON MINERALS

27. Evidence for Rise of Oxygen Evidence from Rock Record of Low O2 until 2.2 Ga • Rocks provide evidence of the oxidation state of the atmosphere/ocean • Presence of detrital minerals, uraninite and pyrite • These minerals are insoluble (can’t be dissolved) in absence of oxygen • Uraninite and pyrite disappeared after 2.3 Ga • Banded iron formation • Marine sedimentary rocks consisting of layers of iron-rich minerals and chert • Iron is only soluble in seawater in its reduced form (Fe2+)- indicating low O2 • BIFs become scarce after ~2.2 Ga BIF

28. Oxygen Levels and BIF Deposits Fig. 13.12 W. W. Norton

29. Formation of Ozone Shield Rise of ozone (O3): critical to the evolution of life • Ultraviolet radiation is harmful to eukaryotes (cells w/nucleus) • Ozone absorbs ultraviolet (UV) radiation, providing a protective shield to life • Ozone • Absent in early earth • Formed by the interaction of UV and O2 • As atmospheric O2 rose, ozone layer would have accumulated

30. Structure of Earth’s Atmosphere Earth’s atmosphere is divided into layers based on the lapse rate • Lapse rate: change in temperature with altitude • Troposphere: temperature decreases with height • Stratosphere: temperature increases with height • Mesosphere: temperature decreases with height

31. Evidence for Rise of Oxygen Evidence from Rock Record of High O2 after 2.2 Ga • Rocks provide evidence of the oxidation state of the atmosphere/ocean • Presence of red beds • Reddish-colored sedimentary rocks • Red color comes from oxidation of iron (rusting) • Iron-rich paleosols • Prior to 1.9 Ga, iron in soils was in reduced form (Fe2+), soluble, and weathered away • After 1.9 Ga, iron in soils was in oxidized form (Fe3+), insoluble, and retained in soil Red Beds

32. Rise of Oxygen Rise to modern atmospheric levels Rise to modern levels • Modern oxygen levels (21%) were not reached until about 400 Ma • Reasons for slow rise • Oxidation of mantle • Evolution of higher plants and increase in photosynthesis

33. Decline of CO2 and H2O Earth’s early atmosphere had high levels of CO2 and H2O. Where did they go? • Atmospheric H2O would have declined as Earth’s atmosphere cooled • Atmospheric CO2 would have declined due to chemical (silicate) weathering • CO2 + H2O  H2CO3 (carbonic acid) • CaSiO3 + 2H2CO3 Ca2+ + 2HCO3- + SiO2 + H2O (silicate weathering) • Ca2+ + 2HCO3-  CaCO3 + H2CO3 (carbonate precipitation) • Net: CaSiO3 + CO2  CaCO3 + SiO2 Conversion of CO2 gas to CaCO3 mineral!