GEOL 102 Historical Geology

Spring Semester 2018
The Proterozoic Eon

The Proterozoic Earth, by Andrey Atuchin

Proterozoic Eon: 2.5 - 0.542 Ga

Global and Regional Geology of the Proterozoic

Proterozoic is distinct from Archean in:

The 2.5 Ga boundary is somewhat arbitrary: shift from "Archean" to "Proterozoic"-style crust begins at 2.95 Ga in southern Africa, but not until 2.6 in North Ameirca, and after 2.45 in some other regions.

Lithologies of the Proterozoic:

The Great Oxidation Event: Prokaryotic photosynthesizers, included the newly-evolved cyanobacteria (and eventually eukaryotic algae) release more and more oxygen into atmosphere. Between 2.7 and 2.4 Ga, most of this oxygen got absorbed by the copious levels of dissolved iron in the ocean water. This produced "rust", which accumulated on the sea floors as Banded Iron Formations (or BIFs).

When the dissolved iron was all used up, BIF production stopped and the oxygen began to add to the atmosphere (and as dissolved oxygen in the water). Atmospheric levels of oxygen rises to about 10% of modern levels. The Great Oxidation Event produced the modern oxygen-nitrogen atmosphere:

The Great Oxidation Event would have devastated anaerobic organisms, which from that point onward would survive only in "extreme" environments.

Paleoproterozoic saw the suturing together of many small Archean cratons to form much larger continents: the formation of the large cratons of today:

Recent work has suggested the following Paleoproterozoic continents and supercontinents had formed by accretion and suturing of Archean provinces:

The Paleoproterozoic Era has recently been subdivided into the Siderian ("iron", after banded iron: 2.50 - 2.30 Ga), Rhyacian ("streams of lava": 2.30 - 2.05 Ga), Orosirian ("mountain building": 2.05 - 1.80 Ga), and Statherian ("stabilization": 1.80 - 1.60 Ga) Periods. The boundaries are arbitrarily defined, but their names are derived from prominent geologic processes occurring at that time.

Recently the Mesoproterozoic Era has been divided into the following Periods (with arbitrarily defined boundaries): Calymmian ("covering", after the development of platforms: 1.60 - 1.40 Ga), Ectasian ("extension", both of platforms and of the beginnings of the rifting events: 1.40 - 1.20 Ga), and Stennian ("narrowing", after narrow belts of intense metamorphism and deformation, such as the Grenville Orogeny: 1.20 - 1.00 Ga).

Mesoproterozoic saw high amounts of igneous activity. It may be due to large-scale mantle "superplumes" forming underneath the first continental or supercontinental (not yet observed) granitic masses.

Belt Supergroup: a huge (16 km thick!) sequence of terrestrial mudstones, shallow marine sandstones & limestones & turbidites in western North America, deposited from 1.45 to 0.85 Ga. A huge filled-in down-warped basin of western Laurentia.

During late Mesoproterozoic, assembly of the first well-known supercontinent Rodinia (newer reconstruction; requires online access to Lyell collection):

[Someone has created the Rodinian National Anthem]

(Despite all this, the period from 1.8 to 0.8 Ga (basically the Mesoproterozoic Era and Tonian Period) are nicknamed "The Boring Billion", because much less major change in Earth's geology, atmosphere, and biology than the time before or the time afterwards).

During Neoproterozoic, Rodinia rifts apart around 750 Ma: birth of Pacific Ocean (although generally we call it the Panthalassic Ocean until the break up of Pangaea in the Mesozoic).

From this point onward, western Laurentia never has contact with continental-size masses (although much of the far western part of North America was accreted during the Phanerozoic).

The Pan-African Orogeny (and related orogenies):

The Neoproterozoic Era has recently been divided into three Periods: Tonian ("stretching", after continued expansion of the platform covers": 1000 - 720 Ma), Cryogenian ("ice origins", after the Snowball Earth glaciations: 720 - 635 Ma), and Ediacaran (After Ediacara Springs, Australia: 635 - 542 Ma). The boundaries of the Tonian are arbitrarily defined, while the uppermost boundary of the Cryogenian is the the end of the Marinoan glaciation, and the uppermost boundary of the Ediacaran is the first appearance of the trace fossil Trichophycus (or Treptichnus) pedum (and thus the oldest biostratigraphically-determined boundary).

Climates and Atmospheres of the Proterozoic

Good evidence of rising levels of oxygen in atmosphere (and hence oceans):

Suggests atmosphere of about 1% of atmosphere was oxygen at end of Archean, rising to 10% (or half of modern level) by end of Proterozoic.

During Paleoproterozoic, good evidence of widespread glaciation (tillites, striations, carbon shifts).
During Neoproterozoic (in particular, from c. 720-580 Ma), evidence for superglaciations:

Strong evidence for a Neoproterozoic Snowball Earth

The reason Neoproterozoic superglaciations stop and haven't been seen again may be due to the rise of complex animals, which were able to liberate the carbon in the sediment through bottom-feeding (previously was simply trapped in sediment). However, another factor may be the ever-increasing brightness of the Sun.

Life in the Proterozoic

Stromatolites still common, becoming even more common c. 2.2 Ga, becoming more complex c. 1.2 Ga.

Origin of eukaryotes by endosymbiosis:

Eukaryotes remain unicellular for most of the Proterozoic. Life remains entirely aquatic, but food chains get more complex with diversifying levels of heterotroph consumers and detritivores and phototroph producers.

Fossils from Bitter Springs Formation of Australia (1 Ga) looks as if they show cells dividing.

Oldest known acritarchs (fossils of uncertain origin: some likely cysts of some kind of fossil photosynthesizers, possibly dinoflagellates, others are eggs of animals) at 1.4 Ga; become complex around 600 Ma; the main index fossils for the Proterozoic

Metabionts: multicellular organisms include:

Some of these groups have records back to the Proterozoic:

Animal record is somewhat better, but still much debate:

The Ediacaran fauna is preserved only as impressions in sediment. Indicate a variety of organisms. Originally were pigeon-holed into modern groups, but new evidence suggests that a number of types of animals are present:

None of the Ediacaran fauna apparently burrowed into the sediment, nor had preservable hard parts.

Interestingly, almost all the evidence for multicellular organisms comes AFTER the Gaskiers Glaciation. Some speculate that selective pressures from these hard times led to development of complex life; others that the appearance of creatures able to mobilize the carbon in the sea sediments kept atmospheric carbon dioxide from getting low enough to trigger the super ice ages.

Recent studies suggest that the size (and complexity) increases seen with the rise of endosymbiosis and of multicellularity both coincide with major increases in the amount of oxygen available.

Some videos reviewing some key issues of Proterozoic geology and paleontology:
The Great Oxidation Event, the rise of green chlorophyll:

The "Green Holocaust", when oxygen from photosynthesis almost killed everything:

How Two Microbes Changed History (Endosymbiosis and the rise of the Eukaryotes):

Here is a brief video about the Ediacaran biota. It uses the not-yet-standard phrase "Avalon Explosion", and its ID of some of the creatures are decidedly old-fashioned, but still interesting:

To Lecture Notes.

Last modified: 7 March 2018

Detail of figure of Ediacaran biota by D.W. Miller