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 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

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

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)

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

The Growth of the Continents By investigating the Nb/U ratio, geologists have found: Ga: Slow production of continental crust Ga: Rapid growth of continents, 70% of continental volume was achieved by 3.0 Ga Ga: Slow production of continental crust

Primordial atmosphere: Earth’s first early atmosphere Primordial atmosphere - Composed of H 2 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?

HOW DO YOU GET GAS??

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

State of Early Earth Secondary atmosphere: a new atmosphere formed early in Earth’s history Secondary atmosphere - Initially composed of CO 2, N 2 O, and H 2 O and ?CH 4 ? - Impact degassing: vaporization of planetesimals during period of heavy bombardment would have contributed CO 2, H 2 O, NH 3 - Volcanic outgassing: output of gases by volcanic eruptions (H 2 O, CO 2, N 2, HCl, other volatiles) H2OH2OCO 2 SO 2 H2SH2SHCl Gas compositions from 3 volcanoes

Composition of Early Atmosphere Modern ~4.56 Ga CO 2 N2N2 H2OH2O N2N2 O2O2 Secondary atmosphere composition - No O 2 : No photosynthetic organisms to produce free O 2 - Some N 2 : Inert gas, so all N 2 from volcanic and impact degassing would have remained in atmosphere - Lots of CO 2 : Chemical weathering rates would have been lower because continents would have been smaller– 30,000x present value! - Lots of H 2 O: Due to vaporization of oceans Global warming! - Due to high CO 2, surface temperatures may have been 80-90°C

Oceans: formed soon after Earth’s temperature fell to levels where liquid water was stable How long have we had Oceans? 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 Ga

Elemental Composition of the Ocean

Rise of Oxygen Rise of oxygen: essential to the rise of multicellular eukaryotic organisms- organisms whose cells have nuclei Rise of oxygen - Requires O 2 source > O 2 sink Earth Earth had a reducing atmosphere Reduced gases from volcanic eruptions (H 2 and CO) reacted with free oxygen (O 2 ) to form H 2 O and CO 2 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., Fe 2+  Fe 3+ = oxidation Oxygen is a Great “oxidizing” agent

Rise of Oxygen Prebiotic atmosphere: oxygen levels were very low Source of O 2 Photochemical reactions: chemical reactions induced by light - Photolysis of CO 2 and H 2 O leads to production of H and O 2 - H escapes to space - In reducing atmosphere, O 2 source < O 2 sink, no accumulation of atmospheric O 2 Photolysis But what about me??

Role of Early Life and Atmosphere Evolution Earliest know life is ~3.8 billion years old Source of O 2 ? Evidence of early life - Microfossils: preserved remains of single-celled prokaryoticorganisms (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 Ancient stromatolites Source of O 2 ? 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)

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 O 2 source Oxidation of mantle - Decrease O 2 sink Switch from mainly submarine to subaerial volcanoes - Due to development of thick continental crust - Decrease O 2 sink Great Oxidation Event

Rise of Oxygen Great Oxidation Event: rise in atmospheric oxygen levels between 2 and 2.2 Ga Cyanobacteria- first organisms to produce O 2 by photosynthesis Photosynthesis: CO 2 + H 2 O --> CH 2 O + O 2 Cyanobacteria (prokaryotes) develop ability to photosynthesize - Appeared 1 billion years before rise of oxygen - Increase O 2 source Oxidation of mantle - Decrease O 2 sink Switch from mainly submarine to subaerial volcanoes - Due to development of thick continental crust - Decrease O 2 sink Preferentially uses 12 C CO 2 + H 2 O --> 12 CH 2 O + O 2 Results in an shift in 13 C/ 12 C 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 Oxidation of mantle changed composition of volcanic outgassing-- to less reducing Cyanobacteria (prokaryotes) develop ability to photosynthesize - Appeared 1 billion years before rise of oxygen - Increase O 2 source Oxidation of mantle - Decrease O 2 sink Switch from mainly submarine to subaerial volcanoes - Due to development of thick continental crust - Decrease O 2 sink

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 O 2 source Oxidation of mantle - Decrease O 2 sink Switch from mainly submarine to subaerial volcanoes - Due to development of thick continental crust - Decrease O 2 sink Switch from mainly submarine to combination of submarine/subaerial volcanoes Archaean

BANDED IRON FORMATION -- BIFs COMPOSED OF REDUCED IRON MINERALS

Evidence for Rise of Oxygen Evidence from Rock Record of Low O 2 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 (Fe 2+ )- indicating low O 2 - BIFs become scarce after ~2.2 Ga BIF

Fig W. W. Norton Oxygen Levels and BIF Deposits

Formation of Ozone Shield Rise of ozone (O 3 ): 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 O 2 - As atmospheric O 2 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 O 2 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 (Fe 2+ ), soluble, and weathered away - After 1.9 Ga, iron in soils was in oxidized form (Fe 3+ ), insoluble, and retained in soil Red Beds

Rise of Oxygen Rise to modern atmospheric 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 Rise to modern levels

Decline of CO 2 and H 2 O Earth’s early atmosphere had high levels of CO 2 and H 2 O. Where did they go? Atmospheric H 2 O would have declined as Earth’s atmosphere cooled Atmospheric CO 2 would have declined due to chemical (silicate) weathering - CO 2 + H 2 O  H 2 CO 3 (carbonic acid) - CaSiO 3 + 2H 2 CO 3  Ca HCO SiO 2 + H 2 O (silicate weathering) - Ca HCO 3 -  CaCO 3 + H 2 CO 3 (carbonate precipitation) - Net: CaSiO 3 + CO 2  CaCO 3 + SiO 2 Conversion of CO 2 gas to CaCO 3 mineral!