Ch 12 Proterozoic Eon

Ch. 12 Proterozoic Eon 

• Proterozoic Eon is ~ 1.95 billion years in 

duration, from 2.5 Ga to 0.543 Ga 

2.5 Ga 1.6 Ga 

1.0 Ga 0.54 Ga 

Paleoproterozoic Era Mesoproterozoic Era Neoproterozoic Era 

PROTEROZOIC EON 

Earth History, Ch. 12 1

Early Proterozoic Life 

• Stromatolites 

continued to increase 

in abundance, 

especially after 2.2 

Ga, probably because 

of the establishment 

of broad continental 

shelves 


Modern 

stromatolite 

structure 


Proterozoic 

stromatolites 



bacterial 

filaments 

(prokaryotes) 


Early Proterozoic life 

• Earliest fossil evidence of eukaryotes is 

single-celled algae (protists), ~2.1 Ga, 

although indirect genetic evidence suggests 

the possibility of Archean eukaryotes 

• Eukaryotic cell probably originated from 

the union of two prokaryotic cells, with one 

engulfing the other (“endosymbiont 

theory”) 


Endosymbiont theory 

• Cytoskeleton (fibers just inside cell 

membrane) allows cell to change shape– 

makes engulfing possible (adaptive 

breakthrough) 

• Simple protists could have engulfed 

chlorophyll-bearing cyanobacteria to 

become plant-like protists (algae) 


Endosymbiont theory 

In modern eukaryotes, DNA 

and RNA of mitochondria 

and chloroplasts differ from 

that in the nucleus, 

suggesting separate origins 


Lynn Margulis 

• National Academy of Sciences 

• Russian Academy of Natural Sciences 

• American Academy of Arts and 

Sciences 

• Proctor Prize 

• National Medal of Science 

• Linnean Society of London 

• Darwin-Wallace Medal 

• Papers archived in Library of Congress 



• Even after the origin of eukaryotes, prokaryotes 

were dominant until about ~1.9 Ga—what 

happened then? 

• Build-up of atmospheric oxygen was achieved 

by ~1.9 Ga, as evidenced by decrease in banded 

iron formations and appearance of “red beds” 

(oxidized sediments) 

• O 2 combined with N 2 to form nitrates (NO 3 ), 

which acted as “fertilizer” to accelerate the 

evolution of eukaryotic algae 


Early 

Proterozoic life 

1.9 Ga prokaryotes from 

the Gunflint Formation 

of Ontario, Canada 

Note small size 



Single-celled, eukaryotic 

algae–acritarchs–became 

abundant after ~1.9Ga 

Acritarchs are relatively 

large and possess a complex 

cell wall. Probably related 

to modern planktonic algae 

(dinoflagellates) 

35µm 

Earth History, Ch. 12 12


acritarchs 

Bar scale = 10 µm 


Neoproterozoic life 

• Transition from Mesoproterozoic to 

Neoproterozoic was marked by an adaptive 

radiation of eukaryotes at ~1.0 Ga 

– Mainly an increase in the diversity of acritarchs 

• But, by the end of Proterozoic time, 

stromatolites had declined in abundance 

and unquestioned multicellular animals 

had spread to many regions of the Earth 



• The evolutionary radiation of animals 

– Beginning at ~570 Ma, rocks contain evidence 

of diverse animal life 

• Trace fossils 

• Animal embryos 

• Impressions of soft-bodied fossils 

• Skeletal fossils 



• Trace fossils 

– Burrows, tracks, trails 

– Not known in rocks 

older than ~570 Ma* 

– Their presence is good 

evidence for animal 

activity 

*hold that thought 

Late Proterozoic feeding 

burrows from Norway 


animal 

embryos 

(570 Ma) 

from China 



• Impressions from the 

Ediacara Hills region of 

Australia (“Ediacara 

Fauna”) 

Primitive 

mollusk 

“Sea pen” 

(cnidarian) 

Worm/arthropod? 

Segmented form 


Neoproterozoic 

life 

• Skeletal fossils 

include the calcitic, 

tube-like Cloudina 



• Thus, by the end of Proterozoic Eon, at least four 

major groups of animals had arisen 

– Segmented worms, arthropods, cnidarians, mollusks 

• What triggered the initial radiation of animals? 

– Critical build-up of atmospheric O 2 ? 

– Evolution of key adaptive features such as muscle cells 

or nerve cells? 

• Answer is still unknown! 


Addendum (2002 news!) 

Probable trace fossils 

> 1.2 Ga from SW 

Australia 

Discovery suggests a 

much earlier origin of 

active organisms than 

previously thought 

Rasmussen et al. 2002, Science 


Proterozoic life (review) 

oldest evidence for animals * 

adaptive radiation of acritarchs * 

* oxygenation of atmosphere 

eukaryotes 

? 

prokaryotes 

2.5 Ga 2.1 1.9 

1.2 1.0 0.57 


Oxygenation of the atmosphere 

• Most iron-bearing sedimentary deposits younger 

than 1.9 billion are highly oxidized (red beds) 

• Cessation of Banded Iron Formation deposition 

– BIFs are alternating bands of iron-rich layers and 

chert layers 

– Very few BIFs younger than 1.9 billion years old (when 

atmospheric O 2 increased) 

– Principal source of world’s iron ore 

Earth History, Ch. 11 23

Banded iron formations 


Oxygenation of the atmosphere 

Fe ++ was dissolved in oceans before 

oxygenation of the atmosphere 

Once O 2 became plentiful in atmosphere 

and oceans, Fe ++ became oxidized and was 

precipitated as banded iron formations 

Fe ++ oxidation 

Fe +++ + e - 

soluble 

(ferrous) 

insoluble 

(ferric) 

Earth History, Ch. 11 25

Proterozoic continents 

• Major cratons were assembled by the 

suturing of protocontinents 

• Once cratons formed, incremental growth of 

continents occurred through continental 

accretion 


Continental 

accretion 

• Lateral growth of a 

continental margin by 

suturing of a microplate 

and/or compression and 

metamorphism of 

sediments 


Assembly of North America 

• Laurentia = Proterozoic 

combined landmass of 

North America and 

Greenland 

– Core of Laurentia was 

North American craton 

(now exposed in Canadian 

Shield) 

• Throughout Proterozoic 

time, Laurentia grew by 

continental accretion 


Assembly of Laurentia 

• First step in assembly of Laurentia was the 

fusion of six Archean protocontinents to form 

the stable craton 

– Suturing of protocontinents occurred from 1.95– 

1.85 Ga 

– Trans-Hudson belt is metamorphosed suture 

zone between Superior protocontinent and 

Wyoming-Hearne protocontinent 





• Second major step was continental 

accretion of central and southern U.S. (1.8- 

1.6 Ga) 

• Third step was Grenville Orogeny 

– Collision of eastern Laurentia with northern 

South America, ~1.2–1.0 Ga 

– Resulted in continental accretion along entire 

eastern margin of Laurentia 




Midcontinent rift 

•Midcontinent Rift formed at about same time as 

Grenville orogeny (~1.2 to 1.0 Ga), but relationship 

is unclear 

–Split in continental crustrifting filling of elongate 

basins with Keweenawan basalts 


Continental rift sequence 



Midcontinent 

rift system 


Gravity 

survey 




Assembly of the supercontinent 

Rodinia 

• Grenville orogenic belt 

was connected to 

orogenic belts along 

margins of southern 

Africa, India, Australia 

and Antarctica 

• By ~1.0 Ga all these 

continental areas had 

merged to form the 

supercontinent Rodinia 


Breakup of Rodinia 

• Rodinia split in half 

at ~800–700 Ma 

• This rift created the 

Pacific Ocean 


Breakup of Rodinia—another 

failed rift 


End of Precambrian 

• By end of Precambrian time, Laurentia, 

Baltica, and Siberia were distinct 

continents 

• Pan-African suturing resulted in the 

formation of the supercontinent 

“Gondwanaland” (Africa, South America, 

India, Australia, Antarctica) 



Gondwana 

Laurentia 

Siberia 


Iowa’s Precambrian record 

• Only Precambrian rocks exposed at surface are in 

northwestern corner of state in Gitchie 

Manitou State Preserve 

– Sioux Quartzite (~1.7 Ma) (deposited after Trans- 

Hudson Belt but before Midcontinent Rift) 

• Best record of Precambrian rocks in subsurface is 

Amoco Eischeid well (Carroll County) 

– ~15,000 ft of late Proterozoic sedimentary rocks that 

filled the Midcontinent Rift valley above Keweenawan 

basalts 



Sioux Quartzite 

southern Minnesota ~1900 


“Pipestone” figurines 


Snowball Earth?? 

Locations of known glacial till deposits of Late Proterozoic age 


Tillite overlain by limestone 


Snowball Earth? 


570 Ma 

Earliest evidence of 

multicellular animals 

is immediately after 

“Snowball Earth” period

Ch 12 Proterozoic Eon

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