Evolution of the Atmosphere, Oceans, Continents

Evolution of Atmosphere, Ocean, & Life

We will address the following topics....

• Evolution of Earth’s atmosphere, continents, and oceansEarly 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. 13.05 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. 13.08

W. W. Norton

AGE OF CONTINENTAL CRUST -- CRATONS

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?

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 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)

H2O CO2 SO2 H2S HCl

95 1.1 1.5 0.07 0.006

96 1.9 2.3 0.08 0.004

97 1.1 1.5 0.07 0.006

Gas compositions from 3 volcanoes

CO2

N2

H2O

Other

O2

Composition of Early Atmosphere

CO2

N2

H2O

Other

O2

Modern

~4.56 Ga

CO2

N2

H2O

N2

O2

• 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

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 4.1-4.3 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 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 oldModern stromatolites, Australia

Ancient stromatolites

• 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)

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- first organisms to produce O2 by photosynthesis

Photosynthesis:CO2 + H2O --> CH2O + O2

• 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

Preferentially uses 12CCO2 + H2O --> 12CH2O + O2

Results in an shift in 13C/12Cpreserved 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 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

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

Archaean

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

Fig. 13.12

W. W. Norton

Oxygen Levels and BIF Deposits

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

• 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 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!