Eighth Edition
by Harold L. Levin

Chapter 8 - page 6

The Earliest Earth: 2,100,000,000 Years of the Archean Eon

Evolution of the Atmosphere and Hydrosphere

Earth's atmosphere has evolved or changed over time.
Earth's first, primitive atmosphere lacked free oxygen.
The primitive atmosphere was derived from gases associated with the comets and meteorites which formed the Earth during accretion.
The gases reached the Earth's surface through a process called outgassing.

Volatiles = substances easily driven off by heating.

Primitive Atmosphere

Earth's gases were originally derived from impacts of comets and meteorites.

Comets are made of frozen gases, ice, and dust.
Analysis of Halley's comet showed the following composition:

• 80% water ice
• Most of the rest is carbon dioxide ice (dry ice)
• Hydrogen cloud surrounds comet
• Dust near the nucleus contains iron, oxygen, silicon, magnesium, sodium, sulfur, and carbon

Meteorites called carbonaceous chondrites are mainly composed of silicate minerals, but also contain:

• Nitrogen
• Hydrogen
• Water
• Carbon in the form of complex organic molecules (proteins and amino acids)

Water and gaseous elements would have been released by the heat associated with bombardment and accretion, or by melting and volcanism accompanying later differentiation.

Volcanic Outgassing

Outgassing = the release of water vapor and other gases from the Earth through volcanism.

Analysis of samples from Hawaiian eruptions include the following gases:

• 70% water vapor (H₂O)
• 15% carbon dioxide (CO₂₎
• 5% nitrogen (N₂₎
• 5% sulfur (in H₂S)
• chlorine (in HCl)
• hydrogen
• argon

Most of the water on the surface of the Earth and in the atmosphere was outgassed in the first billion years of Earth history. We know this because there are 3.8 billion-year-old marine sedimentary rocks, indicating the presence of an ocean by 3.8 billion years ago.

Formation of the Hydrosphere

Once at the Earth's surface, the gases and other volatile elements underwent a variety of changes.

• Water vapor condensed and fell as rain.
• Liquid water probably began to fall on the Earth's surface as early as 4.4 billion years ago.
• Rain water accumulated in low places to form seas. The seas were originally freshwater (rain).
• Carbon dioxide and other gases dissolved in the rain made the water more acidic than today. Carbon dioxide and water combine to form carbonic acid.
• Acid waters caused rapid chemical weathering of the exposed rocks, adding sodium, calcium, potassium, and other ions to seawater.
• A change to more alkaline water may have occurred rapidly as large amounts of calcium, sodium, and iron were introduced by submarine volcanism, neutralizing the acid.
• Ions accumulated in the water, increasing the salinity. Ocean salinity is relatively constant today because surplus salts are precipitated at about the same rate at which they are supplied to the sea. Sodium remains in sea water due to its high solubility.
• Much later, when the seas became less acidic, calcium ions bonded with carbon dioxide to form shells of marine organisms and limestones.
• The presence of marine fossils suggests that sodium has not varied appreciably in sea water for at least the past 600 million years.

Today Earth's water is continuously recirculated through the hydrologic cycle (evaporation and precipitation, powered by the sun and by gravity).

Evolution of the atmosphere. Note gases released by volcanoes, condensation of water vapor, precipitation, and accumulation of liquid water, photochemical reactions in the atmosphere, and formation of carbonate rocks (limestones) later, after the seas became less acidic.

The Early Anoxic Atmosphere

Earth's early atmosphere was strongly reducing and anoxic (lacked free oxygen or O₂ gas) and probably consisted primarily of:

• Water vapor (H₂O)
• Carbon dioxide (CO₂)
• Nitrogen (N₂)
• Carbon monoxide (CO)
• Hydrogen sulfide (H₂S)
• Hydrogen chloride (HCl)

The composition would have been similar to that of modern volcanoes, but probably with more hydrogen, and possibly traces of methane (CH₄) and ammonia. If any free oxygen had been present, it would have immediately been involved in chemical reactions with easily oxidized metals such as iron.

Evidence for a Lack of Free Oxygen in Earth's Early Atmosphere

1. Lack of oxidized iron in the oldest sedimentary rocks. (Instead, iron combined with sulfur to form sulfide minerals like pyrite. This happens only in anoxic environments.)
2. Urananite and pyrite are readily oxidized today, but are found unoxidized in Precambrian sedimentary rocks.
3. Archean sedimentary rocks are commonly dark due to the presence of carbon, which would have been oxidized if oxygen had been present.
4. Archean sedimentary sequences lack carbonate rocks but contain abundant chert, presumably due to the presence of an acidic, carbon dioxide-rich atmosphere.
   In an acidic environment, alkaline rocks such as limestone do not form.
5. Banded iron formations (BIF) appear in the Precambrian (1.8 - about 3 by).
   They are cherts with alternating laminations of red oxidized iron and gray unoxidized iron.
   BIF formed as precipitates on the floors of shallow seas.
   Some of the iron probably came from weathering of iron-bearing rocks on the continents, but most was probably from submarine volcanoes and hydrothermal vents (hot springs) on the sea floor. Great economic importance; major source of iron mined in the world.

Banded iron formation, Egypt.	Polished specimen of banded iron from Australia. A common name for this type of banded iron is "Tiger Iron". Metric ruler for scale. Photo courtesy of Pamela Gore.

6. The simplest living organisms have an anaerobic metabolism. They are killed by oxygen.
   Includes some bacteria (such as botulism), and some or all Archaea, which inhabit unusual conditions.
7. Chemical building blocks of life (such as amino acids, DNA) could not have formed in the presence of O₂.

Formation of an Oxygen-rich Atmosphere

The change from an oxygen-poor to an oxygen-rich atmosphere occurred by the Proterozoic, which began 2.5 billion years ago at the end of the Archean.

The development of an oxygen-rich atmosphere is the result of:

• Photochemical dissociation - The breaking up of water molecules into hydrogen and oxygen in the upper atmosphere caused by ultraviolet radiation from the Sun (a minor process today)
• Photosynthesis - The process by which photosynthetic bacteria and plants produce oxygen (major process).

Evidence for Free Oxygen in the Proterozoic Atmosphere

1. Red beds, or sedimentary rocks with iron oxide cements, including shales, siltstones, and sandstones, appear in rocks younger than 1.8 billion years old. This is in the Proterozoic Eon, after the disappearance of the BIF.
2. Carbonate rocks (limestones and dolostones) appear in the stratigraphic record at about the same time that red beds appear. This indicates that carbon dioxide was less abundant in the atmosphere and oceans so that the water was no longer acidic.