Comprehensive Notes on the Archean Eon

History of the Earth through Deep Time

Archean Eon

• Lasted from 4 to 2.5 Ga (Billion years ago), representing 32.6% of geologic time.
• Rocks from this period are difficult to interpret due to metamorphism, deep burial, and lack of abundant complex fossils.

Hadean Eon

• Precedes the Archean Eon.
• Origin of life is hypothesized to have occurred during the Hadean Eon.
• Possible ocean-vaporizing impacts and a Moon-forming impact occurred.
• Late heavy bombardment.

Precambrian Eon

• Encompasses the Hadean, Archean, and Proterozoic Eons.
• Includes the Archean Eon.

Proterozoic Eon

• Follows the Archean Eon.
• Marked by the oxidizing atmosphere and ozone layer formation.
• Great Oxidation Event, $~2.4$ to $2.1-2.0$ Ga
• O2 overshoot?
• Worldwide glaciations $2.4-2.2$ Ga
• Loss of $CH<em>4$ & rise of $O</em>2$

Cratons

• Stable interiors of continents, serving as "nuclei" for further rock deposition.
• Started forming in the Archean Eon and continued into the Proterozoic.
• Exposed cratons are called shields, while buried ones are platforms.
• Small cratons formed and grew by accretion along margins due to plate tectonics.
• By the end of the Archean Eon, approximately 30-40% of present-day continental crust had formed.
• Faster plate movement and magma formation due to radiogenic and residual heat.
• Komatiite (ultramafic) lavas mark edges of cratons and suture zones.
• Rapid continental accretion.
• Deformation belts indicate collisions of cratons.

Atmosphere

• Modern Earth atmosphere is composed of 78% $N<em>2$ and 21% $O</em>2$.
• Early atmosphere was very different, with hydrogen and helium rapidly lost to space.
• Following core and magnetic field formation, volcanic gases were retained, leading to atmosphere buildup.
• Early atmosphere lacked $O<em>2$ and was rich in $CO</em>2$, $NH<em>3$, $CH</em>4$. Gases from modern volcanoes (water vapor, $CO<em>2$, $N</em>2$, etc.) are similar to the Archean atmosphere.
• By the end of the Archean Eon ($~2.5$ Ga), the atmosphere probably had about 1% of present-day $O_2$.
• $O_2$ becomes more common in the Proterozoic.
• High $CO_2$ levels resulted in a greenhouse effect.

Geological Evidence for Archean Atmosphere

• Few oxidized iron minerals; iron sulfides (pyrite) were more common.
• Rocks older than $1.8$ Ga show no oxidation (red stain).
• Archean rocks are dark in color due to unoxidized carbon.
• Limited carbonate ($CO<em>3$) rocks due to low $O</em>2$ and abundant iron.
• Banded Iron Formations (BIFs) forming in the Proterozoic Eon are the first sign of increased atmospheric $O_2$.

Hydrosphere

• Sources of Earth’s surface waters include outgassing of Earth’s interior and extraterrestrial sources (icy bodies).
• Relative importance of each source is unknown.
• Archean oceans existed, but their volume and extent are unknown.
• Early oceans were likely salty and in chemical equilibrium.
• Formed after the atmosphere cooled, with water vapor condensing and precipitating to fill low-lying areas.
• Comets (icy) supplied additional water.
• Early oceans could have been highly acidic due to high atmospheric $CO_2$.
• Water recycled over time by the sun and gravity, creating the water cycle.

Life

• Life today is divided into 3 domains and several kingdoms, all originating from a single common ancestor.
• Oldest fossils date back to approximately $3.5$ Ga; possible organic carbon found in $3.8$ Ga rocks.
• Abiogenesis: origin of life from non-living matter.
• Life, such as a bacterium or a complex organic molecule, did not form fully developed from non-living materials but through many small steps.