Precambrian/Proterozoic eon

The Proterozoic Eon spanned from 2.5 Billion Years Ago to 538.8 Million Years Ago and comprises 42% of Earth's entire History. The Eon is divided into the following three Eras:

• Paleo-Proterozoic Era - 2.6 Billion Years Ago to 1.6 Billion Years Ago
• Meso-Proterozoic Era - 1.6 Billion Years Ago to 1.0 Billion Years Ago
• Neo-Proterozoic Era - 1.0 Billion Years Ago to 538.8 Million Years Ago

During the Proterozoic Eon, the Earth started to exhibit modern characteristics of plate tectonics, sedimentation and a global climate. Major orogenies occurred throughout the Proterozoic Eon. During the Paleo-Proterozoic Era, the first large continent Laurentia was formed and the Earth had its first ice age. The supercontinet Rodinia was formed during the Meso-Proterozoic ERA and broke apart during the Neo-Proterozoic. The Neo-Proterozoic Era also saw the creation of the modern day Pacific Ocean, and increased glaciation; as well as the proliferation of prokaryotic and eukaryotic organisms and the evolution of multi-cellular organisms.

Paleo-Proterozoic Era - 2.5 Billion Years Ago to 1.6 Billion Years Ago

Laurentia

The first major event of the Paleo-Proterozoic Era was the formation of of Laurentia. Laurentia was the Earth's first large continent, and it formed when many of the separated Precambrian provinces came together. Plate tectonics moved the provinces together and sutured them along orogens as the continental crust collided. Orogens are linear tracts of deformed rocks created by compressional forces and metamorphism that accompany mountain building. The Wopmay orogen in Canada's Northwest Territory is one example of an orogeny and shows a sequence of events called a Wilson Cycle. A Wilson cycle has three steps:

1. The opening of an ocean basin
2. Sedimentation occurs along the margins of the rifting and spreading continents
3. Closing of the ocean basin through plate tectonics

The rifting, plate tectonics and deposition of sediment that occur in the Wilson Cycle are a Proterozoic example of modern geological processes that occurred during the Proterozoic Eon.

Earth's First Ice age

During the Paleo-proterozoic Era, the Earth experienced global glaciation. Evidence of this eustatic glaciation lies North of Lake Huron in the form of alternating layers of varved mudstones and tillites that were deposited between 2.6 billion years ago and 2.1 billion years ago and show that glaciers were once present. A varve is a thin sedimentary layer or pair of layers that represent the depositional record of a single year and usually form in glacial meltwater lakes. Tillites are unsorted glacial drift that has been converted into solid rock. The Paleo-proterozic glaciation was most likely the Earth's first ice age.

Banded Iron Formations(BIF)

Though they started appearing in the Archean Eon, Banded Iron Formations (or BIFs) became prevalent in the Paleo-Proterozoic. BIFs are cherts that show alternating bands of dark red and light gray. The dark red bands are made up of hematite and magnetite (iron oxide minerals). The light gray bands are chemically precipitated quartz. The formations formed on shallow sea floors as underwater hydrothermal vents dissolved iron and other elements. The iron reacted with the oxygen to form compounds that became part of the sediment on the sea floor. The alternating iron oxide bands correlate to the oscillating levels of oxygen in the Proterozoic atmosphere. When atmospheric oxygen levels were high, free oxygen was able to combine with the iron and make the rust-red color. As oxygen levels dropped, there was no free oxygen to combine with the iron and the gray bands were made. A banded iron sequence near Lake Superior contains an interesting rock formation called the Gunflint Chert. The 1.9 billion year old Gunflint Chert contains prokaryotic organisms and cyanobacteria. Cyanobacteria are prokaryotic, photosynthetic microorganisms that possess chlorophyll and produce oxygen.

Meso-Proterozoic Era - 1.6 Billion Years Ago to 1.0 Billion Years Ago

Rodinia

The Meso-Proterozoic Era brought about the creation of the new supercontinent Rodinia. Rodinia was formed when Laurentia collided with other smaller land masses. Its size rivaled that of the supercontinent Pangea. The continental collisions that occurred during the formation of Rodinia created numerous orogenies, including the Greenville Orogeny in present day North America. Greenville rocks originally started out as carbonates and sandstones, but the orogenic forces placed upon them as land masses collided turned them into metamorphic rocks that contained many igneous intrusions. The figure to the right shows how the Earth’s land masses assembled to form Rodinia. Continents during this time were oriented in a far different manner than they are today. For instance, the east coast of North America was next to Antarctica and Australia. Rodinia was fully assembled by the end of the Meso-Proterozoic.

Neo-Proterozoic Era - 1.0 Billion Years Ago to 538.8 Million Years Ago

Rodinia Breaks Up

About 750 million years ago, during the Neo-Proterozoic Era, Rodinia began to break apart. The proto-Pacific Ocean (also called the Panthalassa) was created as rifting occurred, land masses separated and ocean water filled rift valleys. The aptly named proto-Pacific ocean eventually became the modern day Pacific Ocean.

Ice Age

During the Neo-Proterozoic, comprehensive global glaciation occurred again. Evidence of the Varangian glaciation is seen in tillites, dropstones and striations found in Neo-Proterozoic rocks on virtually all of Earth's landmasses. There are two prevalent hypotheses as to the cause of the glaciation. One is that highly reflective land masses reflected the Sun's energy away from the Earth's surface. As the ice caps grew, the new ice reflected even more energy and the Earth's temperature continued to drop and glaciers formed. The second hypothesis is that carbon dioxide levels in the Earth's atmosphere dropped and decreased the greenhouse effect. The decrease of carbon dioxide and the increase of oxygen were further perpetuated by the proliferation of photosynthetic organisms. The lack of the greenhouse gasses resulted in cooling the Earth and formation of glaciers.

Proterozoic Life

While Prokaryotes appeared on the Earth during the Archean, they flourished during the Proterozoic Eon. Prokaryotes are any cellular organisms that have no nuclear membrane, no organelles in the cytoplasm except ribosomes, and have genetic material in the form of single continuous strands forming coils or loops. Stromatolites, which still exist today in Sharks Bay Australia are one example of a Prokaryotic organism. Prokaryotes became more structurally complex and evolved into Eukaryotes. Eukaryotes are organisms that have as its fundamental structural unit a cell, that contains specialized organelles in the cytoplasm, a membrane-bound nucleus enclosing genetic material organized into chromosomes, and an elaborate system of division by mitosis or meiosis. Some examples are stromatolites, Animeka, Kakabekia, Eosphaera, and Acritarchs. Evidence of multi-cellular animals (Metazoans) from approximately 570 million years ago was discovered in South China and Australia. Some examples of Proterozoic Metazoans are Cyclomedusa, Tribrachidium, Cloudina, and Charniodiscus. Eukaryotes are incredible important because they are the first cells in Earth history to reproduce sexually. Acritarchs are one example of a Proterozoic Eukaryote. Just before the end of the Proterozoic Eon, there was one last major jump in evolutionary diversification. Eukaryotes evolved into multi-cellular animals, or metazoans. Metazoans are multi-cellular animals that possess more than one kind of cell and have their cells organized into tissues and organs. Kimberella was a jellyfish like Proterozoic metazoan of special note because it was the first organism to contain a coelum, or body cavity in which the digestive tract and internal organs were suspended. Metazoans were a major building block in the evolution of much of life on earth today.

References

• Cloud, Preston. Oasis in Space: Earth History From the Beginning. New York: W. W. Norton & Company Inc., 1988. Print
• Fedonkin, Mikhail A., James G. Gehling, Kathleen Grey, Guy M. Narbonne,and Patricia Vickers-Rich. The Rise of Animals: Evolution and Diversification of the Kingdom Animalia. Maryland: The John Hoplins University Press, 2007. Print.
• Levin, Harold. The Earth Through Time. New Jersey: John Wiley & Sons, Inc., 2006. Print.
• Lunine, Jonathan L. Earth: Evolution of a Habitable World, New York: Cambridge University Press, 1999. Print.
• Walker, Gabriel. Snowball Earth. New York: Crown Publishers, 2003. Print.