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Evolution of the Atmosphere, Oceans, Continents. Evolution of Atmosphere, Ocean, & Life. We will address the following topics.... Evolution of Earth’s atmosphere, continents, and oceans
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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?
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
MECHANISMS FOR CREATING FELSIC CONTINENTAL CRUST Fig. 13.05 a, b W. W. Norton
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)
AGE OF CONTINENTAL CRUST -- CRATONS Fig. 13.08 W. W. Norton
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
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?
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
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
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
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
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
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??
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
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
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
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
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
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
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
BANDED IRON FORMATION -- BIFsCOMPOSED OF REDUCED IRON MINERALS
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
Oxygen Levels and BIF Deposits Fig. 13.12 W. W. Norton
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
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
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
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
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!