Introduction

• Hot
 
• Why? - Primordial heat, collisions and compression during accretion, decay of short-lived radioactive elements
• Consequences - Constant volcanism, surface temperature too high for liquid water or life as we know it, molten surface or thin, unstable basaltic crust.
• Atmosphere - early atmosphere probably completely different in composition (H₂, He)
• Cooling
 
• Primordial heat dissipated to space
• Condensation of water (rain), accumulation of surface water.
• Accumulation of new atmosphere due to volcanic out gassing
• Conditions appropriate for evolution of life

Evolution of the Atmosphere

• Chemical Composition Today - Nitrogen (N₂)- 78%, Oxygen (O₂)- 21%, Carbon Dioxide (CO₂) - 0.03 %, plus other miscellaneous gases (H₂O for one).

First Atmosphere

• Composition - Probably H₂, He
• These gases are relatively rare on Earth compared to other places in the universe and were probably lost to space early in Earth's history because
• Earth's gravity is not strong enough to hold lighter gases
• Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's magnetic field (magnetosphere = Van Allen Belt) which deflects solar winds.
• Once the core differentiated the heavier gases could be retained

Second Atmosphere

 
• Gases produced were probably similar to those created by modern volcanoes (H₂O, CO₂, SO₂, CO, S₂, Cl₂, N₂, H₂) and NH₃ (ammonia) and CH₄ (methane)
• No free O₂ at this time (not found in volcanic gases).

• Ocean Formation - As the Earth cooled, H₂O produced by out gassing could exist as liquid in the Early Archean, allowing oceans to form.
• Evidence - pillow basalts, deep marine seds in greenstone belts.

Addition of O₂ to the Atmosphere

• Oxygen Production
 
• Photochemical dissociation - breakup of water molecules by ultraviolet
• Produced O₂ levels approx. 1-2% current levels
• At these levels O₃ (Ozone) can form to shield Earth surface from UV
• Photosynthesis - CO₂ + H₂O + sunlight = organic compounds + O₂ - produced by cyanobacteria, and eventually higher plants - supplied the rest of O₂ to atmosphere. Thus plant populations
• Oxygen Consumers
 
• Chemical Weathering - through oxidation of surface materials (early consumer)
• Animal Respiration (much later)
• Burning of Fossil Fuels (much, much later)

 During the Proterozoic the amount of free O₂ in the atmosphere rose from 1 - 10 %. Most of this was released by cyanobacteria, which increase in abundance in the fossil record 2.3 Ga. Present levels of O₂ were probably not achieved until ~400 Ma.

Evidence from the Rock Record

• Iron (Fe) i s extremely reactive with oxygen. If we look at the oxidation state of Fe in the rock record, we can infer a great deal about atmospheric evolution.
• Archean - Find occurrence of minerals that only form in non-oxidizing environments in Archean sediments: Pyrite (Fools gold; FeS₂), Uraninite (UO₂). These minerals are easily dissolved out of rocks under present atmospheric conditions.
• Banded Iron Formation (BIF) - Deep water deposits in which layers of iron-rich minerals alternate with iron-poor layers, primarily chert. Iron minerals include iron oxide, iron carbonate, iron silicate, iron sulfide. BIF's are a major source of iron ore, b/c they contain magnetite (Fe₃O₄) which has a higher iron-to-oxygen ratio than hematite. These are common in rocks 2.0 - 2.8 B.y. old, but do not form today.
• Red beds (continental siliciclastic deposits) are never found in rocks older than 2.3 B. y., but are common during Phanerozoic time. Red beds are red because of the highly oxidized mineral hematite (Fe₂O₃), that probably forms secondarily by oxidation of other Fe minerals that have accumulated in the sediment.

Biological Evidence

• Chemical building blocks of life could not have formed in the presence of atmospheric oxygen. Chemical reactions that yield amino acids are inhibited by presence of very small amounts of oxygen.
• Oxygen prevents growth of the most primitive living bacteria such as photosynthetic bacteria, methane-producing bacteria and bacteria that derive energy from fermentation. Conclustion - Since today's most primitive life forms are anaerobic, the first forms of cellular life probably had similar metabolisms.
• Today these anaerobic life forms are restricted to anoxic (low oxygen) habitats such as swamps, ponds, and lagoons.

Atmospheric oxygen built up in the early history of the Earth as the waste product of photosynthetic organisms and by burial of organic matter away from surficial decay. This history is documented by the geologic preservation of oxygen-sensitive minerals,
deposition banded iron formations, and development of continental "red beds" or BIFs. Figure from the University of Michigan's Introduction to Global Change web site.
 

Atmospheric Structure
Not only does the atmosphere have a relatively stable composition, but it also has a structure to it.  Below are two figures from Lutgens and Tarbuck (The Atmosphere, 8th edition) showing vertical profiles of the atmosphere.  Note that the first major break, or boundary in the atmosphere as we ascend is the tropopause.