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 AtmosphereEarth'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 Early Anoxic AtmosphereThe atmosphere 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.

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

5. Banded iron formations (BIF) appear during Precambrian (1.8 - about 3 b.y.).Red cherts with alternating laminations of dark oxidized iron (mainly magnetite).

Formed as precipitates on shallow sea floor. Some iron probably came from weathering of iron-bearing rocks on continents. Most iron was probably from submarine volcanoes and hydrothermal vents (hot springs).

Great economic importance: major source of iron mined in the world.

Banded Iron Formation

Banded Iron Formations

Additional Evidence for an Anoxic Atmosphere

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 Proterozoic, which began 2.5 billion years ago, at the end of Archean.

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

1. Photochemical dissociation - Breaking up of water molecules into H and O in the upper atmosphere, caused by ultraviolet radiation from the Sun (a minor process today)

2. Photosynthesis - The process by which photosynthetic bacteria and plants produce oxygen (major process).

Formation of an Oxygen-rich Atmosphere

Evidence of Oxygen in Proterozoic Atmosphere 1. Red beds - Sedimentary rocks with iron

oxide cements (including shales, siltstones, and sandstones), appear in rocks younger than 1.8 billion years old. This occurred during Proterozoic, after the disappearance of the banded iron formations (BIFs).

2. Carbonate rocks (limestones and dolostones) appear in the stratigraphic record at about the same time that red beds appear. This indicates that CO2 was less abundant in the atmosphere and oceans so that the water was no longer acidic.

Evidence of Oxygen in Proterozoic Atmosphere

PrecambrianPrecambrian covers about 4 billion years (and

87%) of Earth history. Precambrian is divided into 2 eons:

– Proterozoic Eon 2.5 - 0.542 billion years ago (or 2500 - 542 million years ago)

– Archean Eon 4.6 - 2.5 billion years ago (lower limit not defined)

Table of time divisions of

Precambrian

Precambrian is not well known or completely understood. Why?

• Precambrian rocks are often poorly exposed. • Many Precambrian rocks have been eroded or

metamorphosed. • Most Precambrian rocks are deeply buried beneath

younger rocks. • Many Precambrian rocks are exposed in fairly

inaccessible or nearly uninhabited areas. • Fossils are seldom found in Precambrian rocks; only

way to correlate is by radiometric dating.

Areas where Precambrian rocks are exposed are shown in yellow, as well as in the red areas in orogenic belts.

Shields and Cratons Most of what we know about Precambrian is

based on studies of rocks from cratons - large portions of continents which have not been deformed since Precambrian or Early Paleozoic.

Shields and Cratons• The most extensive exposures of

Precambrian rocks are in geologically stable regions of continents called shields.

• Example = Canadian shield in North America. Mostly igneous and metamorphic rocks; few sedimentary rocks. Overlying sedimentary rocks were scraped off by glaciers during last Ice Age.

Shields and Cratons• Stable regions of the craton where shields

are covered by sedimentary rocks are called platforms.

• Precambrian rocks are often called basement rocks because they lie beneath a covering of fossil-bearing sedimentary strata.

North American craton, shield, platform, and orogenic belts.

Precambrian Provinces Various Precambrian provinces can be

delineated within the North American continent, based on radiometric ages of rocks, style of folding, and differences in trends of faults and folds.

– Oldest (Archean) rocks are shown in orange.

– Younger (Proterozoic) rocks are shown in green.

Precambrian provinces in North America

Origin of Plate Tectonics • By about 4 b.y. ago, the Earth had probably

cooled sufficiently for plate formation. • Once plate tectonics was in progress, it

generated crustal rock that could be partially melted in subduction zones and added to the continental crust.

Origin of Plate Tectonics• Continents also increased in size by addition

of microcontinents along subduction zones. • Greater heat in Archean would have caused

faster convection in mantle, more extensive volcanism, more mid-oceanic ridges, more hot spots, etc.

• Growth of volcanic arcs next to subduction zones led to formation of greenstone belts.

Granulites and Greenstones The major types of Archean rocks on the

cratons are: – Granulites – Greenstones

Granulites• Granulites - Highly metamorphosed

gneisses (metamorphosed tonalites, granodiorites, and granites) and anorthosites (layered intrusive gabbroic rocks).

Granulites formed from partially melted crust and sediments in subduction zones. Metamorphism altered the rocks to form granulites.

Greenstones• Greenstones - Metamorphosed volcanic

rocks and sediments derived from the weathering and erosion of the volcanic arc rocks.

Greenstone volcanic rocks commonly have pillow structures, (called pillow basalts), indicating extrusion under water.The green color is the result of low-grade metamorphism, producing green minerals such as chlorite and hornblende.

Greenstones• Mostly found in trough-like or synclinal belts. • Sequence of rock types :

– Ultramafic volcanic rocks near the bottom (komatiites)

– Mafic volcanic rocks (basalts) – Felsic volcanic rocks (andesites and rhyolites) – Sedimentary rocks at the top (shales, graywackes,

conglomerates, and sometimes BIF), deposited in deep water environments adjacent to mountainous coastlines.

Earth's Earliest Glaciation By 2.8 billion years ago, Earth had cooled

sufficiently for glaciation to occur. Earth's earliest glaciation is recorded in 2.8 billion year-old sedimentary rocks in South Africa.

Earliest Evidence of LifeThe earliest evidence of life occurs in

Archean sedimentary rocks. – Stromatolites – Microscopic cells of prokaryotes– Algal filaments – Molecular fossils

Stromatolites • An organo-sedimentary structure built by

photosynthetic cyanobacteria or blue-green algae.

• Stromatolites form through the activity of cyanobacteria in the tidal zone. The sticky, mucilage-like algal filaments of the cyanobacteria trap carbonate sediment during high tides.

Stromatolites Modern stromatolites, Shark Bay, western Australia

Stromatolites• More abundant in Proterozoic rocks than in

Archean rocks. Examples: – Oldest are 3.5 b.y. old, Warrawoona Group,

Australia's Pilbara Shield – 3 b.y. old Pongola Group of southern Africa – 2.8 b.y. old Bulawayan Group of Australia

Stromatolites• Stromatolites are scarce today because

microorganisms that build them are eaten by marine snails and other grazing invertebrates.

• Stromatolites survive only in environments that are too saline or otherwise unsuitable for most grazing invertebrates.

• The decline of stromatolites is associated with the evolutionary appearance of new groups of marine invertebrates during Early Paleozoic.

Oldest direct evidence of life• Microscopic cells and filaments of prokaryotes.• Associated with stromatolites• Similar to cyanobacteria living today, which produce

oxygen.• Fossiliferous chert bed associated with the Apex

Basalt• Found in Warrawoona Group, Pilbara Supergroup,

western Australia• 3.460-3.465 billion years old

Other evidence of Archean life• Algal filament fossils

Filamentous prokaryotes preserved in stromatolites.Found at North Pole, western Australia;3.4-3.5 b.y. old.

• Spheroidal bacterial structuresFound in rocks of the Fig Tree Group, South Africa (cherts, slates, ironstones, and sandstones).Prokaryotic cells, showing possible cell division; 3.0 - 3.1 b.y. old.

Other evidence of Archean life• Molecular fossils

Preserved organic molecules that only eukaryotic cells produce.

• Indirect evidence for eukaryotes.In black shales from northwestern Australia; 2.7 b.y.

• Origin of eukaryotic life is pushed back to 2.7 b.y.

Life requires these elements: »Carbon »Hydrogen »Oxygen »Nitrogen »Phosphorus »Sulfur

Each of these is abundant in the Solar System.

Four essential components of life: 1. Proteins - Chains of amino acids. Proteins are used to

build living materials, and as catalysts in chemical reactions in organisms.

2. Nucleic acids - Large complex molecules in cell nucleus. 1. DNA (carries the genetic code and can replicate itself) 2. RNA

3. Organic phosphorus compounds - Used to transform light or chemical fuel into energy required for cell activities.

4. Cell membrane to enclose the components within the cell.

• The earliest organisms developed in the presence of an atmosphere which lacked oxygen. The organisms must have been anaerobic (i.e., they did not require oxygen for respiration).

• Organic molecules could not assemble into larger structures in an oxygenated environment. Oxidation and microbial predators would break down the molecules.

• Because the atmosphere lacked oxygen, there was no ozone shield to protect the surface of the Earth from harmful ultraviolet (UV) radiation.

Where Did Life Originate? Early life may have avoided UV radiation by

living: – Deep beneath the water – Beneath the surface of rocks (or below sediment -

such as stromatolites) Life probably began in the sea, perhaps in

areas associated with submarine hydrothermal vents or black smokers.

Feeding Life on Earth – Obtaining Nutrients Examples of types of feeding modes: 1. Fermenters - digest chemicals, such as sugar, in the

absence of oxygen, to obtain energy. Produce CO2 and alcohol. Example: Yeast

2. Autotrophs - manufacture their own food.Examples: sulfur bacteria, nitrifying bacteria, and photoautotrophs (such as plants and photosynthetic bacteria) that use photosynthesis

3. Heterotrophs - can't make their own food, so they must find nutrients in the environment to eat. Example: Animals.

Evolution of Early Life • The earliest cells had to form and exist in anoxic

conditions (in the absence of free oxygen).Likely to have been anaerobic bacteria or Archaea.

• Some of the early organisms became photosynthetic, possibly due to a shortage of raw materials for energy. Produced their own raw materials. Autotrophs.Photosynthesis was an adaptive advantage.

• Oxygen was a WASTE PRODUCT of photosynthesis.

Consequences of Oxygen in Atmosphere 1. Ozone layer which absorbs harmful UV radiation,

and protected primitive and vulnerable life forms. 2. End of banded iron formations which only formed

in low, fluctuating O2 conditions 3. Oxidation of iron, leading to the first red beds.4. Aerobic metabolism developed. Uses oxygen to

convert food into energy. 5. Development of eukaryotic cell, which could cope

with oxygen in the atmosphere.

Prokaryotes vs. Eukaryotes • Prokaryotes reproduce asexually by simple cell

division. This restricts their genetic variability. Prokaryotes have shown little evolutionary change for more than 2 billion years.

• Eukaryotes reproduce sexually through the union of an egg and sperm. This combines chromosomes from each parent and leads to genetic recombination and increased variability. Many new genetic combinations. Led to a dramatic increase in the rate of evolution.

Prokaryotes vs. Eukaryotes

Prokaryotes vs. Eukaryotes

The Earliest Eukaryotes Earliest large cells that appear to be eukaryotes appear

in the fossil record about 1.6 - 1.4 b.y. ago (during Proterozoic).

Eukaryotes diversified around the time that the banded iron formations disappeared and the red beds appeared, indicating the presence of oxygen in the atmosphere.

Origin of eukaryotic life was probably around 2.7 b.y., based on molecular fossils.

Endosymbiotic Theory for Origin of Eukaryotes • Billions of years ago, several prokaryotic cells came

together to live symbiotically within a host cell as protection from (and adaptation to) an oxygenated environment.

• These prokaryotes became organelles. • Evidence for this includes the fact that mitochondria

contain their own DNA.• Example - a host cell (fermentative anaerobe) +

aerobic organelle (mitochondrion) + spirochaete-like organelle (flagellum for motility).

Endosymbiotic Origin of

Eukaryotes