Archean Eon

What happened during this time?

Geophysical

• Beginning of Archean dated to the oldest rocks found on Earth

• Recent discoveries have pushed back the earliest dated rocks to about 4.0 billion years old

• Earth moderately warm

  • Sun 30% cooler than today, 

  • Earth's geological activity was much greater, creating a warmer climate

• Earth was hotter than it is today, its mantle may have contained less water because mantle minerals hold onto less water at higher temperatures (Dong et al. 2021)

  • Assuming that the mantle currently has more than 0.3-0.8 times the mass of the ocean, a larger surface ocean might have existed during the early Archean. 

  • At that time, the mantle was about 1,900-3,000 degrees Kelvin (2,960-4,940 degrees Fahrenheit), compared to 1,600-2,600 degrees Kelvin (2,420-4,220 degrees Fahrenheit) today.

• If early Earth had a larger ocean than today, that could have altered the composition of the early atmosphere and reduced how much sunlight was reflected back into space (Dong et al. 2021) 

  • These factors would have affected the climate and the habitat that supported the first life on Earth.

• Formation of proto-continents 

  • When continents appear is under debate, but there is evidence of continental drift

  • These original proto-continents no longer exist; remnants found in cratons 

    • Cratons are areas of ancient rock that survive on some continents 

    • Appear when the overlying rock (mostly volcanic igneous rock) is buried deep but not deep enough to be re-melted. 

    • Heat and pressure convert it into metamorphic rock.

    • These are areas where the crust has thickened, with fresh igneous rock on top and metamorphic rock beneath (though folding of the crust can obscure this relationship).

  • The complexity of life in the ocean hints at a greater continental mass during this time

    • "...if there was life in the ocean, you need some amount of continental weathering taking place to deliver phosphorus so the organisms can live." (Satkoski, 2016)

• Earth has a reducing atmosphere 

• The Great Oxidation Crisis event 

  • Oxygen produced by bacteria (see below) oxidized rocks and iron in the oceans, 

  • Increase in atmospheric oxygen for a very long time. Atmospheric oxygen did not begin to rise significantly until billions of years after photosynthesis first began.

Biological

• Origin of prokaryotic organisms, probably bacteria

  • Oldest direct evidence found in Western Australia (3.43 Ga)

  • First organisms were likely non-photosynthetic, utilizing methane, ammonia, or sulfates for their energy needs

• The beginning of bioenergetic processes, such as photosynthesis (2.7 Ga, possibly as early as 3.5 Ga)

  • Photosynthesis became common with the cyanobacteria, perhaps as early as 3.5 billion years ago. 

  • The oxygen produced by these bacteria went into oxidizing rocks on the Earth and the iron in the oceans,

  • Increase in atmospheric oxygen occurs during Siderian (2.5-2.3 Ga)

• Origin of the archaea and then their decline toward the end of the eon.

• Colonial stromatolites appear and proliferate