VIZUALIZING EARTH HISTORYBy Loren E. Babcock

Chapter 9

Proterozoic World

Evolution of the Cratons and Proterozoic Supercontinents

About 2.5 billion years ago the Earth entered a new

phase of its history, which is why this period was

chosen as beginning of the Proterozoic Eon.

During the Archean, the Earth experienced its mobile crust

phase. Between 2.7 and 2.3 billion years ago, most small

crustal bodies were amalgamated through orogenesis into

larger cratons, which are the continental cores.

Evolution of the Cratons and Proterozoic Supercontinents

Evolution of the Cratons and Proterozoic Supercontinents

Contrast the nature of the crust in the Proterozoic Eon

with that of the Archean Eon.

Platform - The part of a continent covered by flat-lying or gently

tilted, mostly sedimentary strata.

Platform phase - Interval of Earth history, beginning

with the Proterozoic Eon, characterized by relatively

stable, amalgamated continental cores.

Evolution of the Cratons and Proterozoic Supercontinents

Contrast the nature of the crust in the Proterozoic Eon

with that of the Archean Eon.

Evolution of the Cratons and Proterozoic Supercontinents

Rodinia - An early supercontinent, assembled in the Mesoproterozoic and separated in the

Neoproterozoic.

This supercontinent was considerably different in its

configuration from Pangea, which would become assembled more than a billion years later

Evolution of the Cratons and Proterozoic Supercontinents

Orogenic belt - A linear or arcuate region subjected to

folding and other deformation during a mountain building cycle. Also known as an orogen.

Grenville orogenic belt - An arcuate orogenic region that

developed 1.3 to 1.0 billion years ago and that affected an

extensive area of present-day North America and adjacent regions.

Evolution of the Cratons and Proterozoic Supercontinents

Supercontinent cycle - A tectonically driven cycle

defined by the assembly of a supercontinent and later fragmentation and dispersal of its

pieces.

Pannotia - A hypothesized late Neoproterozoic supercontinent.

A supercontinent cycle begins with the collision and welding

of tectonic plates to form an enormous mass of continental

crust. It ends with the breakup and dispersal of fragments

of the supercontinent.

The Proterozoic Eon witnessed at least two supercontinent cycles.

Formation of Paleozoic continents from Neoproterozoic supercontinents

Gondwana - The Paleozoic to mid-Mesozoic

landmass that included South America, the Falkland Islands,

Africa, Madagascar, India, Australia, and Antarctica.

Continental breakup stimulated a substantial rise in global sea

level during the Neoproterozoic as high rifting rates produced

a large volume of warm, relatively expanded, oceanic

lithosphere.

The eustatic rise lasted into the Cambrian Period.

Oxygenation of the Atmosphere-Ocean SystemUnderstand the ways that free oxygen can be

released to the atmosphere.

Since early in Earth’s history, free oxygen (O2) was released

in small amounts from the breakdown of water vapor in the

upper atmosphere by the Sun’s ultraviolet radiation. By

about 3.5 billion years ago, photosynthetic prokaryotes

(especially cyanobacteria) also began releasing oxygen to

the atmosphere-ocean system.

Oxygenation of the Atmosphere-Ocean SystemExplain banded iron formations and what they signify about oxygen levels in the atmosphere

and ocean.

Banded iron formations (or BIFs), which are composed of

iron minerals interlayered with silica, offer further evidence

of oceanic oxygen levels. Most banded iron formations were

deposited between 3.6 and 1.9 billion years ago. A few

Neoproterozoic and Phanerozoic examples exist but they tend

to be small in size compared to Archean and Paleoproterozoic examples. Banded iron

formations serve as the world’s major sources of iron ore.

Oxygenation of the Atmosphere-Ocean SystemBanded Iron Formations

Oxygenation of the Atmosphere-Ocean SystemRedbeds and what they tell us about atmospheric oxygen levels.

Redbeds, which contain well-oxidized, iron-bearing sediments, show a clear relationship to atmospheric oxygenation. The occurrence of redbeds has an inverse relationship to band on formations— they are almost non-existent in strata older than 2 billion years, and are prevalent in strata younger than 1.9 billion years.

Proterozoic Glaciations

Multiple episodes of glaciation occurred during the

Paleoproterozoic and Neoproterozoic eras.

Glacial conditions are so much a part of the sedimentary record

of the middle part of the Neoproterozoic that the interval

between 850 and 630 million years ago has been named

the Cryogenian Period (from Greek, kryos, ice, and genesis, birth).

Proterozoic GlaciationsSnowball Earth hypothesis

During the Neoproterozoic, the Earth experienced at least two

episodes of global glaciation. One glacial episode was in the

Cryogenian Period, between 745 and 725 million years ago,

and the other was in the Ediacaran Period, between 590

and 550 million years ago.

Cryogenian and Ediacaran periods are associated with

banded iron formations and capped by carbonate rocks.

Snowball Earth hypothesis - The concept that during the

Proterozoic Eon, the entire surface of the Earth was

repeatedly plunged into freezing conditions.

Proterozoic Life FormsThe most important steps in the history of life

on Earth.

The Proterozoic Eon witnessed some of the most pivotal

changes in the history of life on Earth. During the preceding

Archean Eon, life was dominated by prokaryotes. As early as

3.5 billion years ago, organisms had adopted three major

strategies for acquiring nutrients:

chemosynthesis (in archeans),

photosynthesis (in cyanobacteria), and

heterotrophy (predation, scavenging, herbivory, and

breakdown of detritus in eubacteria).

Proterozoic Life FormsThe most important steps in the history of life

on Earth.

Photosynthetic activity eventually led to evolution

of an oxygenated atmosphere-ocean system.

Heterotrophy gained in importance as the probable

means by which the eukaryotic cell initially evolved

(through the symbiotic association of predator and

undigested prey). Later, heterotrophy became a major driving force in the evolution of

skeletons and behavioral strategies in animals.

Proterozoic Life FormsProterozoic prokaryotes

At the start of the Proterozoic, prokaryotes were

the dominant life forms on Earth.

Molecular clock evidence suggests that origination times of

eubacteria and archaebacteria were before the end of

the Archean Eon, although we do not have much supporting

data from fossils.

The earliest prokaryotic specimens are preserved in chert, and

comprise small rounded cells and filaments formed of linked

cells. They have been reported from Archean strata as old as

3.5 billion years. Biogenic-sedimentary structures constructed

by cyanobacteria include stromatolites and thrombolites.

Proterozoic Life FormsProterozoic prokaryotes

Proterozoic Life FormsEarly eukaryotes

Eukaryotes so far are known from rocks as old as the

Proterozoic, although molecular clock evidence suggests

they originated during the Archean. Distinguishing the cells

of simple, single-celled eukaryotic organisms from

similar-appearing prokaryotic organisms is not always

easy. Size and structure are the basic means of identifying cell type.

Prokaryotic cells typically range up to 10 micrometers (μm)

in diameter, whereas eukaryotic cells are usually larger than 10 μm.

Proterozoic Life FormsEarly eukaryotes

Inside eukaryotic cells are distinct masses presumed to be

remains of cell nuclei or other organelles.

Eukaryotic cell - A cell type having a true nucleus.

Symbiosis - Condition in which two or more dissimilar

organisms live together.

Proterozoic Life FormsEarly eukaryotes

Eukaryotic cells evolution - All eukaryotic cells contain

mitochondria, (extractor of energy from food). Mitochondrial

precursors could have been independent prokaryotic

organisms captured by other cells but resistant to

digestion inside the predator cells. Minor alteration

allowed captured cells to adapt to a symbiotic life,

transforming into cell organelles.

Proterozoic Life FormsEarly eukaryotes

Proterozoic Life FormsEarly eukaryotes

The oldest known multicellular eukaryotes are algae resembling

modern seaweed. Coiled, ribbonlike Grypania fossils, some as

much as 50 cm long, occur in rocks estimated to be 1.4 to

perhaps 2.1 billion years old.

Accompanying the multicellular condition is the implication of sexual reproduction, although

algae can also reproduce asexually.

Proterozoic Life FormsThe Ediacaran biota and the radiation of

animals

Until the Ediacaran Period, most organisms were microscopic.

That changed about 570 million years ago with the appearance

of a remarkable collection of Neoproterozoic organisms that

mark an important step toward the multicellular eukaryote-dominated world of the Phanerozoic.

Ediacaran biota - Fossils dating from the Ediacaran Period,

including the earliest putative animals.

Metazoan - A multicellular animal.

Proterozoic Life FormsThe Ediacaran biota and the radiation of

animals

Proterozoic Life FormsThe Ediacaran biota and the radiation of

animals

The Ediacaran Period derives its name from a conspicuous

assortment of fossils having flattened zipper-like, concentric,

frondlike, radial, and other miscellaneous shapes. Most are

between 1 and 30 cm in size, but a few reach lengths of

50 cm or more. These fossils, collectively referred to as the

Ediacaran biota, are so named for an early discovery site

in the Ediacara Hills of South Australia.

Today Ediacaran-type fossils are known from localities

on all modern continents except Antarctica.

Proterozoic Life FormsThe Ediacaran biota and the radiation of

animals

Today microbial mat-stabilized sediments are relatively

restricted in their distribution, but during the

Neoproterozoic microbial mats may have stabilized sedimentary surfaces

across the continental shelves.

Microbial mat - Layer of microscopic bacteria and

fungi growing at the sediment surface.

Proterozoic Life FormsThe Ediacaran biota and the radiation of

animals

The beginning of a trace fossil record coincides with

the beginning of a body fossil record of animals at

about 570 million years ago.

Scratches on the mud surface, near-surface horizontal traces, and simple tubular burrows are about all that Ediacaran strata have to

offer as evidence of animal behavior.

Proterozoic Life FormsThe Ediacaran biota and the radiation of

animals

Most Ediacaran animals had relatively soft, pliable, but fairly

durable external coverings, useful for locomotion but offered

little protection from predators.

Before the end of the Ediacaran Period, the cloudiniids,

evolved small conical skeletons of hard calcium carbonate,

to reduce the threat of predation.

One of the most remarkable indications of animal life during

the Ediacaran comes from carbonate strata deposited in

China and elsewhere just before the end of the period.