VIZUALIZING EARTH HISTORY
By 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 System
Understand 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 System
Explain 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 System
Banded Iron Formations
Oxygenation of the Atmosphere-Ocean System
Redbeds 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 Glaciations
Snowball 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 Forms
The 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 Forms
The 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 Forms
Proterozoic 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 Forms
Proterozoic prokaryotes
Proterozoic Life Forms
Early 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 Forms
Early 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 Forms
Early 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 Forms
Early eukaryotes
Proterozoic Life Forms
Early 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 Forms
The 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 Forms
The Ediacaran biota and the radiation of animals
Proterozoic Life Forms
The 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 Forms
The 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 Forms
The 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 Forms
The 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.